[Mechanism underlying spatial vision deficit of monocular amblyopia based on the theory of Magnocellular and Parvocellular (M-P) pathways].

PubMed

Song, Feng-wei; Sun, Zhao-hui; Yang, Yi; Wang, Li-ping; Tang, Xia-jing; Chen, Bin-bin; Yu, Xiao-ning

2014-01-01

To investigate the relationship between the characteristics of spatial vision deficit and the degree of amblyopia in monocular amblyopes, and to analyze its mechanism with the theory of Magnocellular and Parvocellular pathways. One hundred and eleven patients with monocular amblyopes aged 7-34 were included in this study. Distance best corrected visual acuity (BCVA) in logMAR units and contrast sensitivity function test were performed on both eyes in all patients with ETDRS digital visual chart and functional test system OPTECR 6500. The spatial vision of amblyopic and non-amblyopic eyes was evaluated by the AULCSF, Smax, Frmax and cutSF derived from the curve of contrast sensitivity function. The degree of amblyopia was significantly correlated with the difference of AULCSF between the amblyopic and non-amblyopia eyes (r=-0.83, P<0.01). BCVA of amblyopic eyes was significantly correlated with AULCSF, CutSF, Smax, Frmax(r=-0.68, -0.80, -0.73, -0.56, respectively; P<0.01). In amblyopic eyes, significant difference in BCVA, AULCSF, Smax, Frmax and CutSF was seen among different amblyopic groups (P<0.01), which was defined by the degree of amblyopia. In non-amblyopic eyes,no significant difference in BCVA, AULCSF, Smax, Frmax and CutSF was noted among different amblyopic groups (P>0.05). In mild amblyopes, no significant difference in AULCSF and Frmax was found between the amblyopic eyes and non-amblyopic eyes (P>0.05), while Smax and CutSF were significantly different. However, in moderate and severe amblyopes, significant differences in BCVA, AULCSF, Smax, Frmax and CutSF was seen between the amblyopic and non-amblyopic eyes (P<0.01). In amblyopic eyes, significant difference in contrast sensitivity was noted in all kinds of spatial frequencies among different amblyopic groups (P<0.01), and in non-amblyopic eyes, significant differences in contrast sensitivity was not seen in all kinds of spatial frequencies among different amblyopic groups. The AULCSF, CutSF, Smax and Frmax are accorded with visual acuity for evaluation of the spatial vision of amblyopia. As the severity of amblyopia increases, the overall function of spatial vision in amblyopic eyes gradually decreases, the resolution ability of high spatial frequency is gradually weaken, the peak of contrast detection function gradually descends, and the optimal spatial frequency for contrast detection offsets toward low level of spatial frequency. Mild monocular amblyopia produces spatial contrast sensitivity loss in high spatial vision, suggesting there may be decreased sensitivity of the Parvocellular pathway, and no significant anomalous processing of Magnocellular Pathway. Whereas, in moderate and severe amblyopes, a generalized loss of sensitivity is observed at each spatial frequency. This result shows that both Magnocellular and Parvocellular pathways are damaged in different degrees, especially in Parvocellular pathway.

Window of visibility - A psychophysical theory of fidelity in time-sampled visual motion displays

NASA Technical Reports Server (NTRS)

Watson, A. B.; Ahumada, A. J., Jr.; Farrell, J. E.

1986-01-01

A film of an object in motion presents on the screen a sequence of static views, while the human observer sees the object moving smoothly across the screen. Questions related to the perceptual identity of continuous and stroboscopic displays are examined. Time-sampled moving images are considered along with the contrast distribution of continuous motion, the contrast distribution of stroboscopic motion, the frequency spectrum of continuous motion, the frequency spectrum of stroboscopic motion, the approximation of the limits of human visual sensitivity to spatial and temporal frequencies by a window of visibility, the critical sampling frequency, the contrast distribution of staircase motion and the frequency spectrum of this motion, and the spatial dependence of the critical sampling frequency. Attention is given to apparent motion, models of motion, image recording, and computer-generated imagery.

X-ray spatial frequency heterodyne imaging of protein-based nanobubble contrast agents

PubMed Central

Rand, Danielle; Uchida, Masaki; Douglas, Trevor; Rose-Petruck, Christoph

2014-01-01

Spatial Frequency Heterodyne Imaging (SFHI) is a novel x-ray scatter imaging technique that utilizes nanoparticle contrast agents. The enhanced sensitivity of this new technique relative to traditional absorption-based x-ray radiography makes it promising for applications in biomedical and materials imaging. Although previous studies on SFHI have utilized only metal nanoparticle contrast agents, we show that nanomaterials with a much lower electron density are also suitable. We prepared protein-based “nanobubble” contrast agents that are comprised of protein cage architectures filled with gas. Results show that these nanobubbles provide contrast in SFHI comparable to that of gold nanoparticles of similar size. PMID:25321797

Visual resolution and contrast sensitivity in two benthic sharks.

PubMed

Ryan, Laura A; Hart, Nathan S; Collin, Shaun P; Hemmi, Jan M

2016-12-15

Sharks have long been described as having 'poor' vision. They are cone monochromats and anatomical estimates suggest they have low spatial resolution. However, there are no direct behavioural measurements of spatial resolution or contrast sensitivity. This study estimates contrast sensitivity and spatial resolution of two species of benthic sharks, the Port Jackson shark, Heterodontus portusjacksoni, and the brown-banded bamboo shark, Chiloscyllium punctatum, by recording eye movements in response to optokinetic stimuli. Both species tracked moving low spatial frequency gratings with weak but consistent eye movements. Eye movements ceased at 0.38 cycles per degree, even for high contrasts, suggesting low spatial resolution. However, at lower spatial frequencies, eye movements were elicited by low contrast gratings, 1.3% and 2.9% contrast in H portusjacksoni and C. punctatum, respectively. Contrast sensitivity was higher than in other vertebrates with a similar spatial resolving power, which may reflect an adaptation to the relatively low contrast encountered in aquatic environments. Optokinetic gain was consistently low and neither species stabilised the gratings on their retina. To check whether restraining the animals affected their optokinetic responses, we also analysed eye movements in free-swimming C. punctatum We found no eye movements that could compensate for body rotations, suggesting that vision may pass through phases of stabilisation and blur during swimming. As C. punctatum is a sedentary benthic species, gaze stabilisation during swimming may not be essential. Our results suggest that vision in sharks is not 'poor' as previously suggested, but optimised for contrast detection rather than spatial resolution. © 2016. Published by The Company of Biologists Ltd.

Spatial frequency dependence of target signature for infrared performance modeling

NASA Astrophysics Data System (ADS)

Du Bosq, Todd; Olson, Jeffrey

2011-05-01

The standard model used to describe the performance of infrared imagers is the U.S. Army imaging system target acquisition model, based on the targeting task performance metric. The model is characterized by the resolution and sensitivity of the sensor as well as the contrast and task difficulty of the target set. The contrast of the target is defined as a spatial average contrast. The model treats the contrast of the target set as spatially white, or constant, over the bandlimit of the sensor. Previous experiments have shown that this assumption is valid under normal conditions and typical target sets. However, outside of these conditions, the treatment of target signature can become the limiting factor affecting model performance accuracy. This paper examines target signature more carefully. The spatial frequency dependence of the standard U.S. Army RDECOM CERDEC Night Vision 12 and 8 tracked vehicle target sets is described. The results of human perception experiments are modeled and evaluated using both frequency dependent and independent target signature definitions. Finally the function of task difficulty and its relationship to a target set is discussed.

Spatial Contrast Sensitivity in Adolescents with Autism Spectrum Disorders

ERIC Educational Resources Information Center

Koh, Hwan Cui; Milne, Elizabeth; Dobkins, Karen

2010-01-01

Adolescents with autism spectrum disorders (ASD) and typically developing (TD) controls underwent a rigorous psychophysical assessment that measured contrast sensitivity to seven spatial frequencies (0.5-20 cycles/degree). A contrast sensitivity function (CSF) was then fitted for each participant, from which four measures were obtained: visual…

Spatial-frequency spectrum of patterns changes the visibility of spatial-phase differences

NASA Technical Reports Server (NTRS)

Lawton, T. B.

1985-01-01

It is shown that spatial-frequency components over a 4-octave range affected the visibility of spatial-phase differences. Contrast thresholds were measured for discrimination between two (+45- and -45-deg) spatial phases of a sinusoidal test grating added to a background grating. The background could contain one or several sinusoidal components, all in 0-deg phase. Phase differences between the test and the background were visible at lower contrasts when test and background frequencies were harmonically related than when they were not, when test and background frequencies were within 1 octave than when they were farther apart, when the fundamental frequency of the background was low than when it was high, and for some discriminations more than for others, after practice. The visibility of phase differences was not affected by additional components in the background if the fundamental and difference frequencies of the background remained unchanged. Observers' reports of their strategies gave information about the types of attentive processing that were used to discriminate phase differences. Attentive processing facilitated phase discrimination for multifrequency gratings spanning a much wider range of spatial frequencies than would be possible by using only local preattentive processing. These results were consistent with the visibility of phase differences being processed by some combination of even- and odd-symmetric simple cells tuned to a wide range of different spatial frequencies.

Contrast sensitivity of wildtype mice wearing diffusers or spectacle lenses, and the effect of atropine.

PubMed

Schmucker, Christine; Schaeffel, Frank

2006-03-01

To find out how spatial vision in mice is affected by wearing of spectacle lenses or diffusers, and by atropine eye drops. This information is necessary to determine which treatments could effectively induce refractive errors in young mice. Whole-body optomotor responses were recorded by automated video analysis in freely ranging mice in a large rotating drum that was covered inside with vertical square-wave gratings with spatial frequencies of 0.03, 0.10 and 0.30 cyc/deg, both at "dim light" (0.10 cd/m(2)), and under photopic conditions (30 cd/m(2)). Contrast thresholds were determined by varying the contrasts of the gratings. Mice wore either no lenses, or binocular plano lenses, or lenses with powers ranging from +25 D to -25 D, or diffusers. In another experiment, contrast thresholds were determined 30 min after binocular installation of one drop of 1% atropine solution which is known to suppress myopia development in other animal models. The range of spatial frequencies, at which the mice still responded to stripes with less than the maximal grating contrast, was rather small. At 0.03 cyc/deg, the mice responded to stripes with low contrast down to 24%. At 0.10 cyc/deg, the minimal contrast was 45%, but at 0.30 cyc/deg, only the maximum contrast elicited a significant response. In dim light, spatial vision was severely impaired and only the lowest spatial frequencies, presented at the highest contrast (91%), were detected. The whole-body optomotor response was largest with spectacle lens powers of plano diopters and +7D lenses. The magnitude of the response decreased symmetrically with increasing lens powers for both signs, providing information on the behavioral depth of field (a second-order fit through the data placed the extreme limits of a response at around +25 D and -25 D lens powers). Finally, atropine improved contrast sensitivity, at least at the lowest spatial frequency tested, a result that was previously obtained also in the chicken and could help to explain the inhibitory effect of atropine on myopia. The study shows that mice have sufficient spatial vision to respond to treatment with powerful spectacle lenses or diffusers. Accordingly, these devices should be effective in inducing refractive errors in this animal model, although primarily under photopic conditions.

Spatiotemporal Characteristics for the Depth from Luminance Contrast

PubMed Central

Matsubara, Kazuya; Matsumiya, Kazumichi; Shioiri, Satoshi; Takahashi, Shuichi; Hyodo, Yasuhide; Ohashi, Isao

2011-01-01

Images with higher luminance contrast tend to be perceived closer in depth. To investigate a spatiotemporal characteristic of this effect, we evaluated subjective depth of a test stimulus with various spatial and temporal frequencies. For the purpose, the depth of a reference stimulus was matched to that of the test stimulus by changing the binocular disparity. The results showed that the test stimulus was perceived closer with higher luminance contrast for all conditions. Contrast efficiency was obtained from the contrast that provided the subjective depth for each spatiotemporal frequency. The shape of the contrast efficiency function was spatially low-pass and temporally band-pass. This characteristic is different from the one measure for a detection task. This suggests that only subset of contrast signals are used for depth from contrast.

The relationship between hue discrimination and contrast sensitivity deficits in patients with diabetes mellitus.

PubMed

Trick, G L; Burde, R M; Gordon, M O; Santiago, J V; Kilo, C

1988-05-01

In an attempt to elucidate more fully the pathophysiologic basis of early visual dysfunction in patients with diabetes mellitus, color vision (hue discrimination) and spatial resolution (contrast sensitivity) were tested in diabetic patients with little or no retinopathy (n = 57) and age-matched visual normals (n = 35). Some evidence of visual dysfunction was observed in 37.8% of the diabetics with no retinopathy and 60.0% of the diabetics with background retinopathy. Although significant hue discrimination and contrast sensitivity deficits were observed in both groups of diabetic patients, contrast sensitivity was abnormal more frequently than hue discrimination. However, only 5.4% of the diabetics with no retinopathy and 10.0% of the diabetics with background retinopathy exhibited both abnormal hue discrimination and abnormal contrast sensitivity. Contrary to previous reports, blue-yellow (B-Y) and red-green (R-G) hue discrimination deficits were observed with approximately equal frequency. In the diabetic group, contrast sensitivity was reduced at all spatial frequencies tested, but for individual diabetic patients, significant deficits were only evident for the mid-range spatial frequencies. Among diabetic patients, the hue discrimination deficits, but not the contrast sensitivity abnormalities, were correlated with the patients' hemoglobin A1 level. A negative correlation between contrast sensitivity at 6.0 cpd and the duration of diabetes also was observed.

Determination of the effect of source intensity profile on speckle contrast using coherent spatial frequency domain imaging

PubMed Central

Rice, Tyler B.; Konecky, Soren D.; Owen, Christopher; Choi, Bernard; Tromberg, Bruce J.

2012-01-01

Laser Speckle Imaging (LSI) is fast, noninvasive technique to image particle dynamics in scattering media such as biological tissue. While LSI measurements are independent of the overall intensity of the laser source, we find that spatial variations in the laser source profile can impact measured flow rates. This occurs due to differences in average photon path length across the profile, and is of significant concern because all lasers have some degree of natural Gaussian profile in addition to artifacts potentially caused by projecting optics. Two in vivo measurement are performed to show that flow rates differ based on location with respect to the beam profile. A quantitative analysis is then done through a speckle contrast forward model generated within a coherent Spatial Frequency Domain Imaging (cSFDI) formalism. The model predicts remitted speckle contrast as a function of spatial frequency, optical properties, and scattering dynamics. Comparison with experimental speckle contrast images were done using liquid phantoms with known optical properties for three common beam shapes. cSFDI is found to accurately predict speckle contrast for all beam shapes to within 5% root mean square error. Suggestions for improving beam homogeneity are given, including a widening of the natural beam Gaussian, proper diffusing glass spreading, and flat top shaping using microlens arrays. PMID:22741080

Brief Report: Developing Spatial Frequency Biases for Face Recognition in Autism and Williams Syndrome

ERIC Educational Resources Information Center

Leonard, Hayley C.; Annaz, Dagmara; Karmiloff-Smith, Annette; Johnson, Mark H.

2011-01-01

The current study investigated whether contrasting face recognition abilities in autism and Williams syndrome could be explained by different spatial frequency biases over developmental time. Typically-developing children and groups with Williams syndrome and autism were asked to recognise faces in which low, middle and high spatial frequency…

Visual evoked potential measurement of contrast sensitivity in a case of retinal laser injury reveals visual function loss despite normal acuity

NASA Astrophysics Data System (ADS)

Glickman, Randolph D.; Harrison, Joseph M.; Zwick, Harry; Longbotham, Harold G.; Ballentine, Charles S.; Pierce, Bennie

1996-04-01

Although visual function following retinal laser injuries has traditionally been assessed by measuring visual acuity, this measure only indicates the highest spatial frequency resolvable under high-contrast viewing conditions. Another visual psychophysical parameter is contrast sensitivity (CS), which measures the minimum contrast required for detection of targets over a range of spatial frequencies, and may evaluate visual mechanisms that do not directly subserve acuity. We used the visual evoked potential (VEP) to measure CS in a population of normal subjects and in patients with ophthalmic conditions affecting retinal function, including one patient with a laser injury in the macula. In this patient, the acuity had recovered from

Chromatic spatial contrast sensitivity estimated by visual evoked cortical potential and psychophysics

PubMed Central

Barboni, M.T.S.; Gomes, B.D.; Souza, G.S.; Rodrigues, A.R.; Ventura, D.F.; Silveira, L.C.L.

2013-01-01

The purpose of the present study was to measure contrast sensitivity to equiluminant gratings using steady-state visual evoked cortical potential (ssVECP) and psychophysics. Six healthy volunteers were evaluated with ssVECPs and psychophysics. The visual stimuli were red-green or blue-yellow horizontal sinusoidal gratings, 5° × 5°, 34.3 cd/m2 mean luminance, presented at 6 Hz. Eight spatial frequencies from 0.2 to 8 cpd were used, each presented at 8 contrast levels. Contrast threshold was obtained by extrapolating second harmonic amplitude values to zero. Psychophysical contrast thresholds were measured using stimuli at 6 Hz and static presentation. Contrast sensitivity was calculated as the inverse function of the pooled cone contrast threshold. ssVECP and both psychophysical contrast sensitivity functions (CSFs) were low-pass functions for red-green gratings. For electrophysiology, the highest contrast sensitivity values were found at 0.4 cpd (1.95 ± 0.15). ssVECP CSF was similar to dynamic psychophysical CSF, while static CSF had higher values ranging from 0.4 to 6 cpd (P < 0.05, ANOVA). Blue-yellow chromatic functions showed no specific tuning shape; however, at high spatial frequencies the evoked potentials showed higher contrast sensitivity than the psychophysical methods (P < 0.05, ANOVA). Evoked potentials can be used reliably to evaluate chromatic red-green CSFs in agreement with psychophysical thresholds, mainly if the same temporal properties are applied to the stimulus. For blue-yellow CSF, correlation between electrophysiology and psychophysics was poor at high spatial frequency, possibly due to a greater effect of chromatic aberration on this kind of stimulus. PMID:23369980

Perceptual learning improves contrast sensitivity, visual acuity, and foveal crowding in amblyopia.

PubMed

Barollo, Michele; Contemori, Giulio; Battaglini, Luca; Pavan, Andrea; Casco, Clara

2017-01-01

Amblyopic observers present abnormal spatial interactions between a low-contrast sinusoidal target and high-contrast collinear flankers. It has been demonstrated that perceptual learning (PL) can modulate these low-level lateral interactions, resulting in improved visual acuity and contrast sensitivity. We measured the extent and duration of generalization effects to various spatial tasks (i.e., visual acuity, Vernier acuity, and foveal crowding) through PL on the target's contrast detection. Amblyopic observers were trained on a contrast-detection task for a central target (i.e., a Gabor patch) flanked above and below by two high-contrast Gabor patches. The pre- and post-learning tasks included lateral interactions at different target-to-flankers separations (i.e., 2, 3, 4, 8λ) and included a range of spatial frequencies and stimulus durations as well as visual acuity, Vernier acuity, contrast-sensitivity function, and foveal crowding. The results showed that perceptual training reduced the target's contrast-detection thresholds more for the longest target-to-flanker separation (i.e., 8λ). We also found generalization of PL to different stimuli and tasks: contrast sensitivity for both trained and untrained spatial frequencies, visual acuity for Sloan letters, and foveal crowding, and partially for Vernier acuity. Follow-ups after 5-7 months showed not only complete maintenance of PL effects on visual acuity and contrast sensitivity function but also further improvement in these tasks. These results suggest that PL improves facilitatory lateral interactions in amblyopic observers, which usually extend over larger separations than in typical foveal vision. The improvement in these basic visual spatial operations leads to a more efficient capability of performing spatial tasks involving high levels of visual processing, possibly due to the refinement of bottom-up and top-down networks of visual areas.

Spatial noise and threshold contrasts in LCD displays

NASA Astrophysics Data System (ADS)

Roehrig, Hans; Krupinski, Elizabeth A.; Chawla, Amarpreet S.; Fan, Jiahua; Gandhi, Kunal

2003-05-01

This paper presents the results of initial physical and psycho-physical evaluations of the noise of high resolution LCDs. 5 LCDs were involved, having 4 different pixel structures. Spatial as well as temporal noise was physically measured with the aid of a high-performance CCD camera. Human contrast sensitivity in the presence of spatial noise was determined psycho-physically using periodic stimuli (square-wave patterns) as well as aperiodic stimuli (squares). For the measurements of the human contrast sensitivity, all LCDs were calibrated to the DICOM 14 Grayscale Standard Display Function (GSDF). The results demonstrate that spatial noise is the dominant noise in all LCDs, while temporal noise is insignificant and plays only a minor part. The magnitude of spatial noise of LCDs is in the range between that of CRTs with a P104 and that of CRTs with a P45. Of particular importance with respect to LCD noise is the contribution of the pixel structure to the Noise Power Spectrum, which shows up as sharp spikes at spatial frequencies beyond the LCDs" Nyquist frequency. The paper does not offer any clues about the importance of these spikes on the human contrast sensitivity.

Steady-state VEP responses to uncomfortable stimuli.

PubMed

O'Hare, Louise

2017-02-01

Periodic stimuli, such as op-art, can evoke a range of aversive sensations included in the term visual discomfort. Illusory motion effects are elicited by fixational eye movements, but the cortex might also contribute to effects of discomfort. To investigate this possibility, steady-state visually evoked responses (SSVEPs) to contrast-matched op-art-based stimuli were measured at the same time as discomfort judgements. On average, discomfort reduced with increasing spatial frequency of the pattern. In contrast, the peak amplitude of the SSVEP response was around the midrange spatial frequencies. Like the discomfort judgements, SSVEP responses to the highest spatial frequencies were lowest amplitude, but the relationship breaks down between discomfort and SSVEP for the lower spatial frequency stimuli. This was not explicable by gross eye movements as measured using the facial electrodes. There was a weak relationship between the peak SSVEP responses and discomfort judgements for some stimuli, suggesting that discomfort can be explained in part by electrophysiological responses measured at the level of the cortex. However, there is a breakdown of this relationship in the case of lower spatial frequency stimuli, which remains unexplained. © 2016 Federation of European Neuroscience Societies and John Wiley & Sons Ltd.

Masking effects of low-frequency sinusoidal gratings on the detection of contrast modulation in high-frequency carriers

NASA Astrophysics Data System (ADS)

Henning, G. Bruce

2004-04-01

A modification and extension of Kortum and Geisler's model [Vision Res. 35, 1595 (1995)] of early visual nonlinearities that incorporates an expansive nonlinearity (consistent with neurophysiological findings [Vision Res. 35, 2725 (1995)], a normalization based on a local average retinal illumination, similar to Mach's proposal [F. Ratliff, Mach Bands: Quantitative Studies on Neural Networks in the Retina (Holden-Day, San Francisco, Calif., 1965)], and a subsequent compression suggested by Henning et al. [J. Opt. Soc. Am A 17, 1147 (2000)] captures a range of hitherto unexplained interactions between a sinusoidal grating of low spatial frequency and a contrast-modulated grating 2 octaves higher in spatial frequency.

Perceptual Learning Improves Contrast Sensitivity of V1 Neurons in Cats

PubMed Central

Hua, Tianmiao; Bao, Pinglei; Huang, Chang-Bing; Wang, Zhenhua; Xu, Jinwang

2010-01-01

Summary Background Perceptual learning has been documented in adult humans over a wide range of tasks. Although the often observed specificity of learning is generally interpreted as evidence for training-induced plasticity in early cortical areas, physiological evidence for training-induced changes in early visual cortical areas is modest, despite reports of learning-induced changes of cortical activities in fMRI studies. To reveal the physiological bases of perceptual learning, we combined psychophysical measurements with extracellular single-unit recording under anesthetized preparations, and examined the effects of training in grating orientation identification on both perceptual and neuronal contrast sensitivity functions of cats. Results We have found that training significantly improved perceptual contrast sensitivity of the cats to gratings with the spatial frequencies near the ‘trained’ spatial frequency, with stronger effects in the trained eye. Consistent with behavioral assessments, the mean contrast sensitivity of neurons recorded from V1 of the trained cats was significantly higher than that of neurons recorded from the untrained cats. Furthermore, in the trained cats, the contrast sensitivity of V1 neurons responding preferentially to stimuli presented via the trained eyes was significantly greater than that of neurons responding preferentially to stimuli presented via the ‘untrained’ eyes. The effect was confined to the trained spatial frequencies. In both trained and untrained cats, the neuronal contrast sensitivity functions derived from the contrast sensitivity of the individual neurons were highly correlated with behaviorally determined perceptual contrast sensitivity functions. Conclusions We suggest that training-induced neuronal contrast-gain in area V1 underlies behaviorally determined perceptual contrast sensitivity improvements. PMID:20451388

Wide-field high spatial frequency domain imaging of tissue microstructure

NASA Astrophysics Data System (ADS)

Lin, Weihao; Zeng, Bixin; Cao, Zili; Zhu, Danfeng; Xu, M.

2018-02-01

Wide-field tissue imaging is usually not capable of resolving tissue microstructure. We present High Spatial Frequency Domain Imaging (HSFDI) - a noncontact imaging modality that spatially maps the tissue microscopic scattering structures over a large field of view. Based on an analytical reflectance model of sub-diffusive light from forward-peaked highly scattering media, HSFDI quantifies the spatially-resolved parameters of the light scattering phase function from the reflectance of structured light modulated at high spatial frequencies. We have demonstrated with ex vivo cancerous tissue to validate the robustness of HSFDI in significant contrast and differentiation of the microstructutral parameters between different types and disease states of tissue.

Visual sensitivity for luminance and chromatic stimuli during the execution of smooth pursuit and saccadic eye movements.

PubMed

Braun, Doris I; Schütz, Alexander C; Gegenfurtner, Karl R

2017-07-01

Visual sensitivity is dynamically modulated by eye movements. During saccadic eye movements, sensitivity is reduced selectively for low-spatial frequency luminance stimuli and largely unaffected for high-spatial frequency luminance and chromatic stimuli (Nature 371 (1994), 511-513). During smooth pursuit eye movements, sensitivity for low-spatial frequency luminance stimuli is moderately reduced while sensitivity for chromatic and high-spatial frequency luminance stimuli is even increased (Nature Neuroscience, 11 (2008), 1211-1216). Since these effects are at least partly of different polarity, we investigated the combined effects of saccades and smooth pursuit on visual sensitivity. For the time course of chromatic sensitivity, we found that detection rates increased slightly around pursuit onset. During saccades to static and moving targets, detection rates dropped briefly before the saccade and reached a minimum at saccade onset. This reduction of chromatic sensitivity was present whenever a saccade was executed and it was not modified by subsequent pursuit. We also measured contrast sensitivity for flashed high- and low-spatial frequency luminance and chromatic stimuli during saccades and pursuit. During saccades, the reduction of contrast sensitivity was strongest for low-spatial frequency luminance stimuli (about 90%). However, a significant reduction was also present for chromatic stimuli (about 58%). Chromatic sensitivity was increased during smooth pursuit (about 12%). These results suggest that the modulation of visual sensitivity during saccades and smooth pursuit is more complex than previously assumed. Copyright © 2017 Elsevier Ltd. All rights reserved.

Clinical skin imaging using color spatial frequency domain imaging (Conference Presentation)

NASA Astrophysics Data System (ADS)

Yang, Bin; Lesicko, John; Moy, Austin J.; Reichenberg, Jason; Tunnell, James W.

2016-02-01

Skin diseases are typically associated with underlying biochemical and structural changes compared with normal tissues, which alter the optical properties of the skin lesions, such as tissue absorption and scattering. Although widely used in dermatology clinics, conventional dermatoscopes don't have the ability to selectively image tissue absorption and scattering, which may limit its diagnostic power. Here we report a novel clinical skin imaging technique called color spatial frequency domain imaging (cSFDI) which enhances contrast by rendering color spatial frequency domain (SFD) image at high spatial frequency. Moreover, by tuning spatial frequency, we can obtain both absorption weighted and scattering weighted images. We developed a handheld imaging system specifically for clinical skin imaging. The flexible configuration of the system allows for better access to skin lesions in hard-to-reach regions. A total of 48 lesions from 31 patients were imaged under 470nm, 530nm and 655nm illumination at a spatial frequency of 0.6mm^(-1). The SFD reflectance images at 470nm, 530nm and 655nm were assigned to blue (B), green (G) and red (R) channels to render a color SFD image. Our results indicated that color SFD images at f=0.6mm-1 revealed properties that were not seen in standard color images. Structural features were enhanced and absorption features were reduced, which helped to identify the sources of the contrast. This imaging technique provides additional insights into skin lesions and may better assist clinical diagnosis.

Orientation tuning of binocular summation: a comparison of colour to achromatic contrast

PubMed Central

Gheiratmand, Mina; Cherniawsky, Avital S.; Mullen, Kathy T.

2016-01-01

A key function of the primary visual cortex is to combine the input from the two eyes into a unified binocular percept. At low, near threshold, contrasts a process of summation occurs if the visual inputs from the two eyes are similar. Here we measure the orientation tuning of binocular summation for chromatic and equivalent achromatic contrast. We derive estimates of orientation tuning by measuring binocular summation as a function of the orientation difference between two sinusoidal gratings presented dichoptically to different eyes. We then use a model to estimate the orientation bandwidth of the neural detectors underlying the binocular combination. We find that orientation bandwidths are similar for chromatic and achromatic stimuli at both low (0.375 c/deg) and mid (1.5 c/deg) spatial frequencies, with an overall average of 29 ± 3 degs (HWHH, s.e.m). This effect occurs despite the overall greater binocular summation found for the low spatial frequency chromatic stimuli. These results suggest that similar, oriented processes underlie both chromatic and achromatic binocular contrast combination. The non-oriented detection process found in colour vision at low spatial frequencies under monocular viewing is not evident at the binocular combination stage. PMID:27168119

Is Suppression Just Normal Dichoptic Masking? Suprathreshold Considerations.

PubMed

Reynaud, Alexandre; Hess, Robert F

2016-10-01

Amblyopic patients have a deficit in visual acuity and contrast sensitivity in their amblyopic eye as well as suppression of the amblyopic eye input under binocular viewing conditions. In this study we wanted to assess the origin of the amblyopic suppression by studying the contrast perception of the amblyopic eye at suprathreshold levels under binocular and monocular viewing. Using a suprathreshold contrast matching task in which the reference and target stimuli were presented to different eyes either simultaneously or successively, we measured interocular contrast matching in 10 controls and 11 amblyopes (mean age 35 ± 15; 5 strabismics; 3 anisometropes; 3 mixed). This was then used as an index of the binocular balance across spatial frequency and compared against the contrast sensitivity ratio measured with the same stimuli. We observed that binocular matching becomes more imbalanced at high spatial frequency for amblyopes, compared with controls; that this imbalance did not depend in either group on whether the stimuli were presented simultaneously or successively; and that for both modes of presentation the matching balance correlates well with the interocular contrast sensitivity ratio (mean correlation coefficient of the slopes R = 0.7125). The results from our amblyopes show comparable losses of contrast perception at and above threshold under these binocular viewing conditions across a wide spatial frequency range, much stronger than that observed for our controls. This occurs under conditions in which there should be no dichoptic masking. Furthermore, the matching contrast could be well predicted by the monocular contrast sensitivity. Altogether, this suggests that amblyopic suppression cannot be explained by normal dichoptic masking but rather an attenuation of the input.

Compact OAM microscope for edge enhancement of biomedical and object samples

NASA Astrophysics Data System (ADS)

Gozali, Richard; Nguyen, Thien-An; Bendau, Ethan; Alfano, Robert R.

2017-09-01

The production of orbital angular momentum (OAM) by using a q-plate, which functions as an electrically tunable spatial frequency filter, provides a simple and efficient method of edge contrast in biological and medical sample imaging for histological evaluation of tissue, smears, and PAP smears. An instrument producing OAM, such as a q-plate, situated at the Fourier plane of a 4f lens system, similar to the use of a high-pass spatial filter, allows the passage of high spatial frequencies and enables the production of an image with highly illuminated edges contrasted against a dark background for both opaque and transparent objects. Compared with ordinary spiral phase plates and spatial light modulators, the q-plate has the added advantage of electric control and tunability.

Importance of phase alignment for interocular suppression.

PubMed

Maehara, Goro; Huang, Pi-Chun; Hess, Robert F

2009-07-01

We measured contrast thresholds for Gabor targets in the presence of maskers which had higher or lower spatial frequencies than the targets. A high-pass fractal masker elevated target contrast thresholds at low and intermediate pedestal contrasts in both monocular and dichoptic modes of presentation, suggesting that the masking occurs after a monocular processing stage. Moreover we found that a high-pass checkerboard masker elevated thresholds at the low and intermediate pedestal contrasts and that most of this threshold elevation disappeared when the phase of the masker's spatial components were scrambled. This masking was effective only in the dichoptic presentation, not in the monocular presentation. These results indicate that phase alignment of the high spatial frequency components plays a crucial role for interocular suppression. We speculate that phase alignments signal the existence of a luminance contour in the monocular image and that this signal suppresses processing of information in the other eye when there is no corresponding signal in that eye.

Contrast affects flicker and speed perception differently

NASA Technical Reports Server (NTRS)

Thompson, P.; Stone, L. S.

1997-01-01

We have previously shown that contrast affects speed perception, with lower-contrast, drifting gratings perceived as moving slower. In a recent study, we examined the implications of this result on models of speed perception that use the amplitude of the response of linear spatio-temporal filters to determine speed. In this study, we investigate whether the contrast dependence of speed can be understood within the context of models in which speed estimation is made using the temporal frequency of the response of linear spatio-temporal filters. We measured the effect of contrast on flicker perception and found that contrast manipulations produce opposite effects on perceived drift rate and perceived flicker rate, i.e., reducing contrast increases the apparent temporal frequency of counterphase modulated gratings. This finding argues that, if a temporal frequency-based algorithm underlies speed perception, either flicker and speed perception must not be based on the output of the same mechanism or contrast effects on perceived spatial frequency reconcile the disparate effects observed for perceived temporal frequency and speed.

Spatial-frequency dependent binocular imbalance in amblyopia

PubMed Central

Kwon, MiYoung; Wiecek, Emily; Dakin, Steven C.; Bex, Peter J.

2015-01-01

While amblyopia involves both binocular imbalance and deficits in processing high spatial frequency information, little is known about the spatial-frequency dependence of binocular imbalance. Here we examined binocular imbalance as a function of spatial frequency in amblyopia using a novel computer-based method. Binocular imbalance at four spatial frequencies was measured with a novel dichoptic letter chart in individuals with amblyopia, or normal vision. Our dichoptic letter chart was composed of band-pass filtered letters arranged in a layout similar to the ETDRS acuity chart. A different chart was presented to each eye of the observer via stereo-shutter glasses. The relative contrast of the corresponding letter in each eye was adjusted by a computer staircase to determine a binocular Balance Point at which the observer reports the letter presented to either eye with equal probability. Amblyopes showed pronounced binocular imbalance across all spatial frequencies, with greater imbalance at high compared to low spatial frequencies (an average increase of 19%, p < 0.01). Good test-retest reliability of the method was demonstrated by the Bland-Altman plot. Our findings suggest that spatial-frequency dependent binocular imbalance may be useful for diagnosing amblyopia and as an outcome measure for recovery of binocular vision following therapy. PMID:26603125

Spatial-frequency dependent binocular imbalance in amblyopia.

PubMed

Kwon, MiYoung; Wiecek, Emily; Dakin, Steven C; Bex, Peter J

2015-11-25

While amblyopia involves both binocular imbalance and deficits in processing high spatial frequency information, little is known about the spatial-frequency dependence of binocular imbalance. Here we examined binocular imbalance as a function of spatial frequency in amblyopia using a novel computer-based method. Binocular imbalance at four spatial frequencies was measured with a novel dichoptic letter chart in individuals with amblyopia, or normal vision. Our dichoptic letter chart was composed of band-pass filtered letters arranged in a layout similar to the ETDRS acuity chart. A different chart was presented to each eye of the observer via stereo-shutter glasses. The relative contrast of the corresponding letter in each eye was adjusted by a computer staircase to determine a binocular Balance Point at which the observer reports the letter presented to either eye with equal probability. Amblyopes showed pronounced binocular imbalance across all spatial frequencies, with greater imbalance at high compared to low spatial frequencies (an average increase of 19%, p < 0.01). Good test-retest reliability of the method was demonstrated by the Bland-Altman plot. Our findings suggest that spatial-frequency dependent binocular imbalance may be useful for diagnosing amblyopia and as an outcome measure for recovery of binocular vision following therapy.

Population receptive field (pRF) measurements of chromatic responses in human visual cortex using fMRI.

PubMed

Welbourne, Lauren E; Morland, Antony B; Wade, Alex R

2018-02-15

The spatial sensitivity of the human visual system depends on stimulus color: achromatic gratings can be resolved at relatively high spatial frequencies while sensitivity to isoluminant color contrast tends to be more low-pass. Models of early spatial vision often assume that the receptive field size of pattern-sensitive neurons is correlated with their spatial frequency sensitivity - larger receptive fields are typically associated with lower optimal spatial frequency. A strong prediction of this model is that neurons coding isoluminant chromatic patterns should have, on average, a larger receptive field size than neurons sensitive to achromatic patterns. Here, we test this assumption using functional magnetic resonance imaging (fMRI). We show that while spatial frequency sensitivity depends on chromaticity in the manner predicted by behavioral measurements, population receptive field (pRF) size measurements show no such dependency. At any given eccentricity, the mean pRF size for neuronal populations driven by luminance, opponent red/green and S-cone isolating contrast, are identical. Changes in pRF size (for example, an increase with eccentricity and visual area hierarchy) are also identical across the three chromatic conditions. These results suggest that fMRI measurements of receptive field size and spatial resolution can be decoupled under some circumstances - potentially reflecting a fundamental dissociation between these parameters at the level of neuronal populations. Copyright © 2017 The Authors. Published by Elsevier Inc. All rights reserved.

Multimodal Medical Image Fusion by Adaptive Manifold Filter.

PubMed

Geng, Peng; Liu, Shuaiqi; Zhuang, Shanna

2015-01-01

Medical image fusion plays an important role in diagnosis and treatment of diseases such as image-guided radiotherapy and surgery. The modified local contrast information is proposed to fuse multimodal medical images. Firstly, the adaptive manifold filter is introduced into filtering source images as the low-frequency part in the modified local contrast. Secondly, the modified spatial frequency of the source images is adopted as the high-frequency part in the modified local contrast. Finally, the pixel with larger modified local contrast is selected into the fused image. The presented scheme outperforms the guided filter method in spatial domain, the dual-tree complex wavelet transform-based method, nonsubsampled contourlet transform-based method, and four classic fusion methods in terms of visual quality. Furthermore, the mutual information values by the presented method are averagely 55%, 41%, and 62% higher than the three methods and those values of edge based similarity measure by the presented method are averagely 13%, 33%, and 14% higher than the three methods for the six pairs of source images.

The role of spatial frequency information for ERP components sensitive to faces and emotional facial expression.

PubMed

Holmes, Amanda; Winston, Joel S; Eimer, Martin

2005-10-01

To investigate the impact of spatial frequency on emotional facial expression analysis, ERPs were recorded in response to low spatial frequency (LSF), high spatial frequency (HSF), and unfiltered broad spatial frequency (BSF) faces with fearful or neutral expressions, houses, and chairs. In line with previous findings, BSF fearful facial expressions elicited a greater frontal positivity than BSF neutral facial expressions, starting at about 150 ms after stimulus onset. In contrast, this emotional expression effect was absent for HSF and LSF faces. Given that some brain regions involved in emotion processing, such as amygdala and connected structures, are selectively tuned to LSF visual inputs, these data suggest that ERP effects of emotional facial expression do not directly reflect activity in these regions. It is argued that higher order neocortical brain systems are involved in the generation of emotion-specific waveform modulations. The face-sensitive N170 component was neither affected by emotional facial expression nor by spatial frequency information.

Binocular combination in abnormal binocular vision

PubMed Central

Ding, Jian; Klein, Stanley A.; Levi, Dennis M.

2013-01-01

We investigated suprathreshold binocular combination in humans with abnormal binocular visual experience early in life. In the first experiment we presented the two eyes with equal but opposite phase shifted sine waves and measured the perceived phase of the cyclopean sine wave. Normal observers have balanced vision between the two eyes when the two eyes' images have equal contrast (i.e., both eyes contribute equally to the perceived image and perceived phase = 0°). However, in observers with strabismus and/or amblyopia, balanced vision requires a higher contrast image in the nondominant eye (NDE) than the dominant eye (DE). This asymmetry between the two eyes is larger than predicted from the contrast sensitivities or monocular perceived contrast of the two eyes and is dependent on contrast and spatial frequency: more asymmetric with higher contrast and/or spatial frequency. Our results also revealed a surprising NDE-to-DE enhancement in some of our abnormal observers. This enhancement is not evident in normal vision because it is normally masked by interocular suppression. However, in these abnormal observers the NDE-to-DE suppression was weak or absent. In the second experiment, we used the identical stimuli to measure the perceived contrast of a cyclopean grating by matching the binocular combined contrast to a standard contrast presented to the DE. These measures provide strong constraints for model fitting. We found asymmetric interocular interactions in binocular contrast perception, which was dependent on both contrast and spatial frequency in the same way as in phase perception. By introducing asymmetric parameters to the modified Ding-Sperling model including interocular contrast gain enhancement, we succeeded in accounting for both binocular combined phase and contrast simultaneously. Adding binocular contrast gain control to the modified Ding-Sperling model enabled us to predict the results of dichoptic and binocular contrast discrimination experiments and provides new insights into the mechanisms of abnormal binocular vision. PMID:23397039

Binocular combination in abnormal binocular vision.

PubMed

Ding, Jian; Klein, Stanley A; Levi, Dennis M

2013-02-08

We investigated suprathreshold binocular combination in humans with abnormal binocular visual experience early in life. In the first experiment we presented the two eyes with equal but opposite phase shifted sine waves and measured the perceived phase of the cyclopean sine wave. Normal observers have balanced vision between the two eyes when the two eyes' images have equal contrast (i.e., both eyes contribute equally to the perceived image and perceived phase = 0°). However, in observers with strabismus and/or amblyopia, balanced vision requires a higher contrast image in the nondominant eye (NDE) than the dominant eye (DE). This asymmetry between the two eyes is larger than predicted from the contrast sensitivities or monocular perceived contrast of the two eyes and is dependent on contrast and spatial frequency: more asymmetric with higher contrast and/or spatial frequency. Our results also revealed a surprising NDE-to-DE enhancement in some of our abnormal observers. This enhancement is not evident in normal vision because it is normally masked by interocular suppression. However, in these abnormal observers the NDE-to-DE suppression was weak or absent. In the second experiment, we used the identical stimuli to measure the perceived contrast of a cyclopean grating by matching the binocular combined contrast to a standard contrast presented to the DE. These measures provide strong constraints for model fitting. We found asymmetric interocular interactions in binocular contrast perception, which was dependent on both contrast and spatial frequency in the same way as in phase perception. By introducing asymmetric parameters to the modified Ding-Sperling model including interocular contrast gain enhancement, we succeeded in accounting for both binocular combined phase and contrast simultaneously. Adding binocular contrast gain control to the modified Ding-Sperling model enabled us to predict the results of dichoptic and binocular contrast discrimination experiments and provides new insights into the mechanisms of abnormal binocular vision.

Nonlinear Y-Like Receptive Fields in the Early Visual Cortex: An Intermediate Stage for Building Cue-Invariant Receptive Fields from Subcortical Y Cells.

PubMed

Gharat, Amol; Baker, Curtis L

2017-01-25

Many of the neurons in early visual cortex are selective for the orientation of boundaries defined by first-order cues (luminance) as well as second-order cues (contrast, texture). The neural circuit mechanism underlying this selectivity is still unclear, but some studies have proposed that it emerges from spatial nonlinearities of subcortical Y cells. To understand how inputs from the Y-cell pathway might be pooled to generate cue-invariant receptive fields, we recorded visual responses from single neurons in cat Area 18 using linear multielectrode arrays. We measured responses to drifting and contrast-reversing luminance gratings as well as contrast modulation gratings. We found that a large fraction of these neurons have nonoriented responses to gratings, similar to those of subcortical Y cells: they respond at the second harmonic (F2) to high-spatial frequency contrast-reversing gratings and at the first harmonic (F1) to low-spatial frequency drifting gratings ("Y-cell signature"). For a given neuron, spatial frequency tuning for linear (F1) and nonlinear (F2) responses is quite distinct, similar to orientation-selective cue-invariant neurons. Also, these neurons respond to contrast modulation gratings with selectivity for the carrier (texture) spatial frequency and, in some cases, orientation. Their receptive field properties suggest that they could serve as building blocks for orientation-selective cue-invariant neurons. We propose a circuit model that combines ON- and OFF-center cortical Y-like cells in an unbalanced push-pull manner to generate orientation-selective, cue-invariant receptive fields. A significant fraction of neurons in early visual cortex have specialized receptive fields that allow them to selectively respond to the orientation of boundaries that are invariant to the cue (luminance, contrast, texture, motion) that defines them. However, the neural mechanism to construct such versatile receptive fields remains unclear. Using multielectrode recording, we found a large fraction of neurons in early visual cortex with receptive fields not selective for orientation that have spatial nonlinearities like those of subcortical Y cells. These are strong candidates for building cue-invariant orientation-selective neurons; we present a neural circuit model that pools such neurons in an imbalanced "push-pull" manner, to generate orientation-selective cue-invariant receptive fields. Copyright © 2017 the authors 0270-6474/17/370998-16$15.00/0.

Distinct spatial frequency sensitivities for processing faces and emotional expressions.

PubMed

Vuilleumier, Patrik; Armony, Jorge L; Driver, Jon; Dolan, Raymond J

2003-06-01

High and low spatial frequency information in visual images is processed by distinct neural channels. Using event-related functional magnetic resonance imaging (fMRI) in humans, we show dissociable roles of such visual channels for processing faces and emotional fearful expressions. Neural responses in fusiform cortex, and effects of repeating the same face identity upon fusiform activity, were greater with intact or high-spatial-frequency face stimuli than with low-frequency faces, regardless of emotional expression. In contrast, amygdala responses to fearful expressions were greater for intact or low-frequency faces than for high-frequency faces. An activation of pulvinar and superior colliculus by fearful expressions occurred specifically with low-frequency faces, suggesting that these subcortical pathways may provide coarse fear-related inputs to the amygdala.

Visible Contrast Energy Metrics for Detection and Discrimination

NASA Technical Reports Server (NTRS)

Ahumada, Albert; Watson, Andrew

2013-01-01

Contrast energy was proposed by Watson, Robson, & Barlow as a useful metric for representing luminance contrast target stimuli because it represents the detectability of the stimulus in photon noise for an ideal observer. Like the eye, the ear is a complex transducer system, but relatively simple sound level meters are used to characterize sounds. These meters provide a range of frequency sensitivity functions and integration times depending on the intended use. We propose here the use of a range of contrast energy measures with different spatial frequency contrast sensitivity weightings, eccentricity sensitivity weightings, and temporal integration times. When detection threshold are plotting using such measures, the results show what the eye sees best when these variables are taken into account in a standard way. The suggested weighting functions revise the Standard Spatial Observer for luminance contrast detection and extend it into the near periphery. Under the assumption that the detection is limited only by internal noise, discrimination performance can be predicted by metrics based on the visible energy of the difference images

A Spatial Frequency Account of the Detriment that Local Processing of Navon Letters Has on Face Recognition

ERIC Educational Resources Information Center

Hills, Peter J.; Lewis, Michael B.

2009-01-01

Five minutes of processing the local features of a Navon letter causes a detriment in subsequent face-recognition performance (Macrae & Lewis, 2002). We hypothesize a perceptual after effect explanation of this effect in which face recognition is less accurate after adapting to high-spatial frequencies at high contrasts. Five experiments were…

Sharpening coarse-to-fine stereo vision by perceptual learning: asymmetric transfer across the spatial frequency spectrum

PubMed Central

Tran, Truyet T.; Craven, Ashley P.; Leung, Tsz-Wing; Chat, Sandy W.; Levi, Dennis M.

2016-01-01

Neurons in the early visual cortex are finely tuned to different low-level visual features, forming a multi-channel system analysing the visual image formed on the retina in a parallel manner. However, little is known about the potential ‘cross-talk’ among these channels. Here, we systematically investigated whether stereoacuity, over a large range of target spatial frequencies, can be enhanced by perceptual learning. Using narrow-band visual stimuli, we found that practice with coarse (low spatial frequency) targets substantially improves performance, and that the improvement spreads from coarse to fine (high spatial frequency) three-dimensional perception, generalizing broadly across untrained spatial frequencies and orientations. Notably, we observed an asymmetric transfer of learning across the spatial frequency spectrum. The bandwidth of transfer was broader when training was at a high spatial frequency than at a low spatial frequency. Stereoacuity training is most beneficial when trained with fine targets. This broad transfer of stereoacuity learning contrasts with the highly specific learning reported for other basic visual functions. We also revealed strategies to boost learning outcomes ‘beyond-the-plateau’. Our investigations contribute to understanding the functional properties of the network subserving stereovision. The ability to generalize may provide a key principle for restoring impaired binocular vision in clinical situations. PMID:26909178

Die Fledermaus: Regarding Optokinetic Contrast Sensitivity and Light-Adaptation, Chicks Are Mice with Wings

PubMed Central

Shi, Qing; Stell, William K.

2013-01-01

Background Through adaptation, animals can function visually under an extremely broad range of light intensities. Light adaptation starts in the retina, through shifts in photoreceptor sensitivity and kinetics plus modulation of visual processing in retinal circuits. Although considerable research has been conducted on retinal adaptation in nocturnal species with rod-dominated retinas, such as the mouse, little is known about how cone-dominated avian retinas adapt to changes in mean light intensity. Methodology/Principal Findings We used the optokinetic response to characterize contrast sensitivity (CS) in the chick retina as a function of spatial frequency and temporal frequency at different mean light intensities. We found that: 1) daytime, cone-driven CS was tuned to spatial frequency; 2) nighttime, presumably rod-driven CS was tuned to temporal frequency and spatial frequency; 3) daytime, presumably cone-driven CS at threshold intensity was invariant with temporal and spatial frequency; and 4) daytime photopic CS was invariant with clock time. Conclusion/Significance Light- and dark-adaptational changes in CS were investigated comprehensively for the first time in the cone-dominated retina of an avian, diurnal species. The chick retina, like the mouse retina, adapts by using a “day/night” or “cone/rod” switch in tuning preference during changes in lighting conditions. The chick optokinetic response is an attractive model for noninvasive, behavioral studies of adaptation in retinal circuitry in health and disease. PMID:24098693

Stereo chromatic contrast sensitivity model to blue-yellow gratings.

PubMed

Yang, Jiachen; Lin, Yancong; Liu, Yun

2016-03-07

As a fundamental metric of human visual system (HVS), contrast sensitivity function (CSF) is typically measured by sinusoidal gratings at the detection of thresholds for psychophysically defined cardinal channels: luminance, red-green, and blue-yellow. Chromatic CSF, which is a quick and valid index to measure human visual performance and various retinal diseases in two-dimensional (2D) space, can not be directly applied into the measurement of human stereo visual performance. And no existing perception model considers the influence of chromatic CSF of inclined planes on depth perception in three-dimensional (3D) space. The main aim of this research is to extend traditional chromatic contrast sensitivity characteristics to 3D space and build a model applicable in 3D space, for example, strengthening stereo quality of 3D images. This research also attempts to build a vision model or method to check human visual characteristics of stereo blindness. In this paper, CRT screen was clockwise and anti-clockwise rotated respectively to form the inclined planes. Four inclined planes were selected to investigate human chromatic vision in 3D space and contrast threshold of each inclined plane was measured with 18 observers. Stimuli were isoluminant blue-yellow sinusoidal gratings. Horizontal spatial frequencies ranged from 0.05 to 5 c/d. Contrast sensitivity was calculated as the inverse function of the pooled cone contrast threshold. According to the relationship between spatial frequency of inclined plane and horizontal spatial frequency, the chromatic contrast sensitivity characteristics in 3D space have been modeled based on the experimental data. The results show that the proposed model can well predicted human chromatic contrast sensitivity characteristics in 3D space.

Spatial covert attention increases contrast sensitivity across the CSF: support for signal enhancement

NASA Technical Reports Server (NTRS)

Carrasco, M.; Penpeci-Talgar, C.; Eckstein, M.

2000-01-01

This study is the first to report the benefits of spatial covert attention on contrast sensitivity in a wide range of spatial frequencies when a target alone was presented in the absence of a local post-mask. We used a peripheral precue (a small circle indicating the target location) to explore the effects of covert spatial attention on contrast sensitivity as assessed by orientation discrimination (Experiments 1-4), detection (Experiments 2 and 3) and localization (Experiment 3) tasks. In all four experiments the target (a Gabor patch ranging in spatial frequency from 0.5 to 10 cpd) was presented alone in one of eight possible locations equidistant from fixation. Contrast sensitivity was consistently higher for peripherally- than for neutrally-cued trials, even though we eliminated variables (distracters, global masks, local masks, and location uncertainty) that are known to contribute to an external noise reduction explanation of attention. When observers were presented with vertical and horizontal Gabor patches an external noise reduction signal detection model accounted for the cueing benefit in a discrimination task (Experiment 1). However, such a model could not account for this benefit when location uncertainty was reduced, either by: (a) Increasing overall performance level (Experiment 2); (b) increasing stimulus contrast to enable fine discriminations of slightly tilted suprathreshold stimuli (Experiment 3); and (c) presenting a local post-mask (Experiment 4). Given that attentional benefits occurred under conditions that exclude all variables predicted by the external noise reduction model, these results support the signal enhancement model of attention.

Pattern masking: the importance of remote spatial frequencies and their phase alignment.

PubMed

Huang, Pi-Chun; Maehara, Goro; May, Keith A; Hess, Robert F

2012-02-16

To assess the effects of spatial frequency and phase alignment of mask components in pattern masking, target threshold vs. mask contrast (TvC) functions for a sine-wave grating (S) target were measured for five types of mask: a sine-wave grating (S), a square-wave grating (Q), a missing fundamental square-wave grating (M), harmonic complexes consisting of phase-scrambled harmonics of a square wave (Qp), and harmonic complexes consisting of phase-scrambled harmonics of a missing fundamental square wave (Mp). Target and masks had the same fundamental frequency (0.46 cpd) and the target was added in phase with the fundamental frequency component of the mask. Under monocular viewing conditions, the strength of masking depends on phase relationships among mask spatial frequencies far removed from that of the target, at least 3 times the target frequency, only when there are common target and mask spatial frequencies. Under dichoptic viewing conditions, S and Q masks produced similar masking to each other and the phase-scrambled masks (Qp and Mp) produced less masking. The results suggest that pattern masking is spatial frequency broadband in nature and sensitive to the phase alignments of spatial components.

Image enhancement filters significantly improve reading performance for low vision observers

NASA Technical Reports Server (NTRS)

Lawton, T. B.

1992-01-01

As people age, so do their photoreceptors; many photoreceptors in central vision stop functioning when a person reaches their late sixties or early seventies. Low vision observers with losses in central vision, those with age-related maculopathies, were studied. Low vision observers no longer see high spatial frequencies, being unable to resolve fine edge detail. We developed image enhancement filters to compensate for the low vision observer's losses in contrast sensitivity to intermediate and high spatial frequencies. The filters work by boosting the amplitude of the less visible intermediate spatial frequencies. The lower spatial frequencies. These image enhancement filters not only reduce the magnification needed for reading by up to 70 percent, but they also increase the observer's reading speed by 2-4 times. A summary of this research is presented.

A complete investigation of monocular and binocular functions in clinically treated amblyopia.

PubMed

Zhao, Wuxiao; Jia, Wu-Li; Chen, Ge; Luo, Yan; Lin, Borong; He, Qing; Lu, Zhong-Lin; Li, Min; Huang, Chang-Bing

2017-09-06

The gold standard of a successful amblyopia treatment is full recovery of visual acuity (VA) in the amblyopic eye, but there has been no systematic study on both monocular and binocular visual functions. In this research, we aimed to quantify visual qualities with a variety of perceptual tasks in subjects with treated amblyopia. We found near stereoacuity and pAE dominance in binocular rivalry in "treated" amblyopia were largely comparable to those of normal subjects. CSF of the pAE remained deficient in high spatial frequencies. The binocular contrast summation ratio is significantly lower than normal standard. The interocular balance point is 34%, indicating that contrast in pAE is much less effective as the same contrast in pFE in binocular phase combination. Although VA, stereoacuity and binocular rivalry at low spatial frequency in treated amblyopes were normal or nearly normal, the pAE remained "lazy" in high frequency domain, binocular contrast summation, and interocular phase combination. Our results suggest that structured monocular and binocular training are necessary to fully recover deficient functions in amblyopia.

Suprathreshold Contrast Sensitivity Vision Test Chart

DTIC Science & Technology

1991-07-14

with data collected on patients having amblyopia , glaucoma and macular degeneration showed that the SCTS may be effectively used as an initial...dramatically in certain cases of abnormal vision, such as amblyopia (Ginsburg, 1978, 1981; Hess, Bradley and Piotrowski, 1983; Loshin and Levi, 1983). The...combination of frequencies. Amblyopia results in marked losses of contrast sensitivity particularly at high spatial frequencies, but may also result in

Cross-media color reproduction using the frequency-based spatial gamut mapping algorithm based on human color vision

NASA Astrophysics Data System (ADS)

Wu, Guangyuan; Niu, Shijun; Li, Xiaozhou; Hu, Guichun

2018-04-01

Due to the increasing globalization of printing industry, remoting proofing will become the inevitable development trend. Cross-media color reproduction will occur in different color gamuts using remote proofing technologies, which usually leads to the problem of incompatible color gamut. In this paper, to achieve equivalent color reproduction between a monitor and a printer, a frequency-based spatial gamut mapping algorithm is proposed for decreasing the loss of visual color information. The design of algorithm is based on the contrast sensitivity functions (CSF), which exploited CSF spatial filter to preserve luminance of the high spatial frequencies and chrominance of the low frequencies. First we show a general framework for how to apply CSF spatial filter in retention of relevant visual information. Then we compare the proposed framework with HPMINDE, CUSP, Bala's algorithm. The psychophysical experimental results indicated the good performance of the proposed algorithm.

Contrast Sensitivity, First-Order Motion and Initial Ocular Following in Demyelinating Optic Neuropathy

PubMed Central

Rucker, Janet C.; Sheliga, Boris M.; FitzGibbon, Edmond J.; Miles, Frederick A.; Leigh, R. John

2008-01-01

The ocular following response (OFR) is a measure of motion vision elicited at ultra-short latencies by sudden movement of a large visual stimulus. We compared the OFR to vertical sinusoidal gratings (spatial frequency 0.153 cycles/° or 0.458 cycles/°) of each eye in a subject with evidence of left optic nerve demyelination due to multiple sclerosis (MS). The subject showed substantial differences in vision measured with stationary low-contrast Sloan letters (20/63 OD and 20/200 OS at 2.5% contrast) and the Lanthony Desaturated 15-hue color test (Color Confusion Index 1.11 OD and 2.14 OS). Compared with controls, all of the subject's OFR to increasing contrast showed a higher threshold. The OFR of each of the subject's eyes were similar for the 0.153 cycles/° stimulus, and psychophysical measurements of his ability to detect these moving gratings were also similar for each eye. However, with the 0.458 cycles/° stimulus, the subject's OFR was asymmetric and the affected eye showed decreased responses (smaller slope constant as estimated by the Naka-Rushton equation). These results suggest that, in this case, optic neuritis caused a selective deficit that affected parvocellular pathways mediating higher spatial frequencies, lower-contrast, and color vision, but spared the field-holding mechanism underlying the OFR to lower spatial frequencies. The OFR may provide a useful method to study motion vision in individuals with disorders affecting anterior visual pathways. PMID:16649097

Contrast gain control in first- and second-order motion perception.

PubMed

Lu, Z L; Sperling, G

1996-12-01

A novel pedestal-plus-test paradigm is used to determine the nonlinear gain-control properties of the first-order (luminance) and the second-order (texture-contrast) motion systems, that is, how these systems' responses to motion stimuli are reduced by pedestals and other masking stimuli. Motion-direction thresholds were measured for test stimuli consisting of drifting luminance and texture-contrast-modulation stimuli superimposed on pedestals of various amplitudes. (A pedestal is a static sine-wave grating of the same type and same spatial frequency as the moving test grating.) It was found that first-order motion-direction thresholds are unaffected by small pedestals, but at pedestal contrasts above 1-2% (5-10 x pedestal threshold), motion thresholds increase proportionally to pedestal amplitude (a Weber law). For first-order stimuli, pedestal masking is specific to the spatial frequency of the test. On the other hand, motion-direction thresholds for texture-contrast stimuli are independent of pedestal amplitude (no gain control whatever) throughout the accessible pedestal amplitude range (from 0 to 40%). However, when baseline carrier contrast increases (with constant pedestal modulation amplitude), motion thresholds increase, showing that gain control in second-order motion is determined not by the modulator (as in first-order motion) but by the carrier. Note that baseline contrast of the carrier is inherently independent of spatial frequency of the modulator. The drastically different gain-control properties of the two motion systems and prior observations of motion masking and motion saturation are all encompassed in a functional theory. The stimulus inputs to both first- and second-order motion process are normalized by feedforward, shunting gain control. The different properties arise because the modulator is used to control the first-order gain and the carrier is used to control the second-order gain.

Enhanced neural function in highly aberrated eyes following perceptual learning with adaptive optics.

PubMed

Sabesan, Ramkumar; Barbot, Antoine; Yoon, Geunyoung

2017-03-01

Highly aberrated keratoconic (KC) eyes do not elicit the expected visual advantage from customized optical corrections. This is attributed to the neural insensitivity arising from chronic visual experience with poor retinal image quality, dominated by low spatial frequencies. The goal of this study was to investigate if targeted perceptual learning with adaptive optics (AO) can stimulate neural plasticity in these highly aberrated eyes. The worse eye of 2 KC subjects was trained in a contrast threshold test under AO correction. Prior to training, tumbling 'E' visual acuity and contrast sensitivity at 4, 8, 12, 16, 20, 24 and 28 c/deg were measured in both the trained and untrained eyes of each subject with their routine prescription and with AO correction for a 6mm pupil. The high spatial frequency requiring 50% contrast for detection with AO correction was picked as the training frequency. Subjects were required to train on a contrast detection test with AO correction for 1h for 5 consecutive days. During each training session, threshold contrast measurement at the training frequency with AO was conducted. Pre-training measures were repeated after the 5 training sessions in both eyes (i.e., post-training). After training, contrast sensitivity under AO correction improved on average across spatial frequency by a factor of 1.91 (range: 1.77-2.04) and 1.75 (1.22-2.34) for the two subjects. This improvement in contrast sensitivity transferred to visual acuity with the two subjects improving by 1.5 and 1.3 lines respectively with AO following training. One of the two subjects denoted an interocular transfer of training and an improvement in performance with their routine prescription post-training. This training-induced visual benefit demonstrates the potential of AO as a tool for neural rehabilitation in patients with abnormal corneas. Moreover, it reveals a sufficient degree of neural plasticity in normally developed adults who have a long history of abnormal visual experience due to optical imperfections. Copyright © 2016 Elsevier Ltd. All rights reserved.

Motion mechanisms with different spatiotemporal characteristics identified by an MAE technique with superimposed gratings.

PubMed

Shioiri, Satoshi; Matsumiya, Kazumichi

2009-05-29

We investigated spatiotemporal characteristics of motion mechanisms using a new type of motion aftereffect (MAE) we found. Our stimulus comprised two superimposed sinusoidal gratings with different spatial frequencies. After exposure to the moving stimulus, observers perceived the MAE in the static test in the direction opposite to that of the high spatial frequency grating even when low spatial frequency motion was perceived during adaptation. In contrast, in the flicker test, the MAE was perceived in the direction opposite to that of the low spatial frequency grating. These MAEs indicate that two different motion systems contribute to motion perception and can be isolated by using different test stimuli. Using a psychophysical technique based on the MAE, we investigated the differences between the two motion mechanisms. The results showed that the static MAE is the aftereffect of the motion system with a high spatial and low temporal frequency tuning (slow motion detector) and the flicker MAE is the aftereffect of the motion system with a low spatial and high temporal frequency tuning (fast motion detector). We also revealed that the two motion detectors differ in orientation tuning, temporal frequency tuning, and sensitivity to relative motion.

Cross-orientation masking in human color vision: application of a two-stage model to assess dichoptic and monocular sources of suppression.

PubMed

Kim, Yeon Jin; Gheiratmand, Mina; Mullen, Kathy T

2013-05-28

Cross-orientation masking (XOM) occurs when the detection of a test grating is masked by a superimposed grating at an orthogonal orientation, and is thought to reveal the suppressive effects mediating contrast normalization. Medina and Mullen (2009) reported that XOM was greater for chromatic than achromatic stimuli at equivalent spatial and temporal frequencies. Here we address whether the greater suppression found in binocular color vision originates from a monocular or interocular site, or both. We measure monocular and dichoptic masking functions for red-green color contrast and achromatic contrast at three different spatial frequencies (0.375, 0.75, and 1.5 cpd, 2 Hz). We fit these functions with a modified two-stage masking model (Meese & Baker, 2009) to extract the monocular and interocular weights of suppression. We find that the weight of monocular suppression is significantly higher for color than achromatic contrast, whereas dichoptic suppression is similar for both. These effects are invariant across spatial frequency. We then apply the model to the binocular masking data using the measured values of the monocular and interocular sources of suppression and show that these are sufficient to account for color binocular masking. We conclude that the greater strength of chromatic XOM has a monocular origin that transfers through to the binocular site.

Training in Contrast Detection Improves Motion Perception of Sinewave Gratings in Amblyopia

PubMed Central

Hou, Fang; Huang, Chang-bing; Tao, Liming; Feng, Lixia; Zhou, Yifeng; Lu, Zhong-Lin

2011-01-01

Purpose. One critical concern about using perceptual learning to treat amblyopia is whether training with one particular stimulus and task generalizes to other stimuli and tasks. In the spatial domain, it has been found that the bandwidth of contrast sensitivity improvement is much broader in amblyopes than in normals. Because previous studies suggested the local motion deficits in amblyopia are explained by the spatial vision deficits, the hypothesis for this study was that training in the spatial domain could benefit motion perception of sinewave gratings. Methods. Nine adult amblyopes (mean age, 22.1 ± 5.6 years) were trained in a contrast detection task in the amblyopic eye for 10 days. Visual acuity, spatial contrast sensitivity functions, and temporal modulation transfer functions (MTF) for sinewave motion detection and discrimination were measured for each eye before and after training. Eight adult amblyopes (mean age, 22.6 ± 6.7 years) served as control subjects. Results. In the amblyopic eye, training improved (1) contrast sensitivity by 6.6 dB (or 113.8%) across spatial frequencies, with a bandwidth of 4.4 octaves; (2) sensitivity of motion detection and discrimination by 3.2 dB (or 44.5%) and 3.7 dB (or 53.1%) across temporal frequencies, with bandwidths of 3.9 and 3.1 octaves, respectively; (3) visual acuity by 3.2 dB (or 44.5%). The fellow eye also showed a small amount of improvement in contrast sensitivities and no significant change in motion perception. Control subjects who received no training demonstrated no obvious improvement in any measure. Conclusions. The results demonstrate substantial plasticity in the amblyopic visual system, and provide additional empirical support for perceptual learning as a potential treatment for amblyopia. PMID:21693615

Spatiotemporal analysis of brightness induction

PubMed Central

McCourt, Mark E.

2011-01-01

Brightness induction refers to a class of visual illusions in which the perceived intensity of a region of space is influenced by the luminance of surrounding regions. These illusions are significant because they provide insight into the neural organization of the visual system. A novel quadrature-phase motion cancelation technique was developed to measure the magnitude of the grating induction brightness illusion across a wide range of spatial frequencies, temporal frequencies and test field heights. Canceling contrast is greatest at low frequencies and declines with increasing frequency in both dimensions, and with increasing test field height. Canceling contrast scales as the product of inducing grating spatial frequency and test field height (the number of inducing grating cycles per test field height). When plotted using a spatial axis which indexes this product, the spatiotemporal induction surfaces for four test field heights can be described as four partially overlapping sections of a single larger surface. These properties of brightness induction are explained in the context of multiscale spatial filtering. The present study is the first to measure the magnitude of grating induction as a function of temporal frequency. Taken in conjunction with several other studies (Blakeslee & McCourt, 2008; Robinson & de Sa, 2008; Magnussen & Glad, 1975) the results of this study illustrate that at least one form of brightness induction is very much faster than that reported by DeValois et al. (1986) and Rossi and Paradiso (1996), and are inconsistent with the proposition that brightness induction results from a slow “filling in” process. PMID:21763339

Syngeneic Schwann cell transplantation preserves vision in RCS rat without immunosuppression.

PubMed

McGill, Trevor J; Lund, Raymond D; Douglas, Robert M; Wang, Shaomei; Lu, Bin; Silver, Byron D; Secretan, Matt R; Arthur, Jennifer N; Prusky, Glen T

2007-04-01

To evaluate the efficacy of immunologically compatible Schwann cells transplanted without immunosuppression in the RCS rat retina to preserve vision. Syngeneic (dystrophic RCS) Schwann cells harvested from sciatic nerves were cultured and transplanted into one eye of dystrophic RCS rats at an early stage of retinal degeneration. Allogeneic (Long-Evans) Schwann cells and unoperated eyes served as controls. Vision through transplanted and unoperated eyes was then quantified using two visual behavior tasks, one measuring the spatial frequency and contrast sensitivity thresholds of the optokinetic response (OKR) and the other measuring grating acuity in a perception task. Spatial frequency thresholds measured through syngeneically transplanted eyes maintained near normal spatial frequency sensitivity for approximately 30 weeks, whereas thresholds through control eyes deteriorated to less than 20% of normal over the same period. Contrast sensitivity was preserved through syngeneically transplanted eyes better than through allogeneic and unoperated eyes, at all spatial frequencies. Grating acuity measured through syngeneically transplanted eyes was maintained at approximately 60% of normal, whereas acuity of allogeneically transplanted eyes was significantly lower at approximately 40% of normal. The ability of immunoprivileged Schwann cell transplants to preserve vision in RCS rats indicates that transplantation of syngeneic Schwann cells holds promise as a preventive treatment for retinal degenerative disease.

The Ohio Contrast Cards: Visual Performance in a Pediatric Low-vision Site

PubMed Central

Hopkins, Gregory R.; Dougherty, Bradley E.; Brown, Angela M.

2017-01-01

SIGNIFICANCE This report describes the first clinical use of the Ohio Contrast Cards, a new test that measures the maximum spatial contrast sensitivity of low-vision patients who cannot recognize and identify optotypes and for whom the spatial frequency of maximum contrast sensitivity is unknown. PURPOSE To compare measurements of the Ohio Contrast Cards to measurements of three other vision tests and a vision-related quality-of-life questionnaire obtained on partially sighted students at Ohio State School for the Blind. METHODS The Ohio Contrast Cards show printed square-wave gratings at very low spatial frequency (0.15 cycle/degree). The patient looks to the left/right side of the card containing the grating. Twenty-five students (13 to 20 years old) provided four measures of visual performance: two grating card tests (the Ohio Contrast Cards and the Teller Acuity Cards) and two letter charts (the Pelli-Robson contrast chart and the Bailey-Lovie acuity chart). Spatial contrast sensitivity functions were modeled using constraints from the grating data. The Impact of Vision Impairment on Children questionnaire measured vision-related quality of life. RESULTS Ohio Contrast Card contrast sensitivity was always less than 0.19 log10 units below the maximum possible contrast sensitivity predicted by the model; average Pelli-Robson letter contrast sensitivity was near the model prediction, but 0.516 log10 units below the maximum. Letter acuity was 0.336 logMAR below the grating acuity results. The model estimated the best testing distance in meters for optimum Pelli-Robson contrast sensitivity from the Bailey-Lovie acuity as distance = 1.5 − logMAR for low-vision patients. Of the four vision tests, only Ohio Contrast Card contrast sensitivity was independently and statistically significantly correlated with students' quality of life. CONCLUSIONS The Ohio Contrast Cards combine a grating stimulus, a looking indicator behavior, and contrast sensitivity measurement. They show promise for the clinical objective of advising the patient and his/her caregivers about the success the patient is likely to enjoy in tasks of everyday life. PMID:28972542

Rebalancing binocular vision in amblyopia.

PubMed

Ding, Jian; Levi, Dennis M

2014-03-01

Humans with amblyopia have an asymmetry in binocular vision: neural signals from the amblyopic eye are suppressed in the cortex by the fellow eye. The purpose of this study was to develop new models and methods for rebalancing this asymmetric binocular vision by manipulating the contrast and luminance in the two eyes. We measured the perceived phase of a cyclopean sinewave by asking normal and amblyopic observers to indicate the apparent location (phase) of the dark trough in the horizontal cyclopean sine wave relative to a black horizontal reference line, and used the same stimuli to measure perceived contrast by matching the binocular combined contrast to a standard contrast presented to one eye. We varied both the relative contrast and luminance of the two eyes' inputs, in order to rebalance the asymmetric binocular vision. Amblyopic binocular vision becomes more and more asymmetric the higher the stimulus contrast or spatial frequency. Reanalysing our previous data, we found that, at a given spatial frequency, the binocular asymmetry could be described by a log-linear formula with two parameters, one for the maximum asymmetry and one for the rate at which the binocular system becomes asymmetric as the contrast increases. Our new data demonstrates that reducing the dominant eye's mean luminance reduces its suppression of the non-dominant eye, and therefore rebalances the asymmetric binocular vision. While the binocular asymmetry in amblyopic vision can be rebalanced by manipulating the relative contrast or luminance of the two eyes at a given spatial frequency and contrast, it is very difficult or even impossible to rebalance the asymmetry for all visual conditions. Nonetheless, wearing a neutral density filter before the dominant eye (or increasing the mean luminance in the non-dominant eye) may be more beneficial than the traditional method of patching the dominant eye for treating amblyopia. © 2014 The Authors Ophthalmic & Physiological Optics © 2014 The College of Optometrists.

Interocular transfer of spatial adaptation is weak at low spatial frequencies.

PubMed

Baker, Daniel H; Meese, Tim S

2012-06-15

Adapting one eye to a high contrast grating reduces sensitivity to similar target gratings shown to the same eye, and also to those shown to the opposite eye. According to the textbook account, interocular transfer (IOT) of adaptation is around 60% of the within-eye effect. However, most previous studies on this were limited to using high spatial frequencies, sustained presentation, and criterion-dependent methods for assessing threshold. Here, we measure IOT across a wide range of spatiotemporal frequencies, using a criterion-free 2AFC method. We find little or no IOT at low spatial frequencies, consistent with other recent observations. At higher spatial frequencies, IOT was present, but weaker than previously reported (around 35%, on average, at 8c/deg). Across all conditions, monocular adaptation raised thresholds by around a factor of 2, and observers showed normal binocular summation, demonstrating that they were not binocularly compromised. These findings prompt a reassessment of our understanding of the binocular architecture implied by interocular adaptation. In particular, the output of monocular channels may be available to perceptual decision making at low spatial frequencies. Copyright © 2012 Elsevier Ltd. All rights reserved.

Spatial contrast sensitivity at twilight: luminance, monocularity, and oxygenation.

PubMed

Connolly, Desmond M

2010-05-01

Visual performance in dim light is compromised by lack of oxygen (hypoxia). The possible influence of altered oxygenation on foveal contrast sensitivity under mesopic (twilight) viewing conditions is relevant to aircrew flying at night, including when using night vision devices, but is poorly documented. Foveal contrast sensitivity was measured binocularly and monocularly in 12 subjects at 7 spatial frequencies, ranging from 0.5 to approximately 16 cycles per degree, using sinusoidal Gabor patch gratings. Hypoxic performance breathing 14.1% oxygen, equivalent to altitude exposure at 3048 m (10,000 ft), was compared with breathing air at sea level (normoxia) at low photopic (28 cd x m(-2)), borderline upper mesopic (approximately 2.1 cd x m(-2)) and midmesopic (approximately 0.26 cd x m(-2)) luminance. Mesopic performance was also assessed breathing 100% oxygen (hyperoxia). Typical 'inverted U' log/log plots of the contrast sensitivity function were obtained, with elevated thresholds (reduced sensitivity) at lower luminance. Binocular viewing enhanced sensitivity by a factor approximating square root of 2 for most conditions, supporting neural summation of the contrast signal, but had greater influence at the lowest light level and highest spatial frequencies (8.26 and 16.51 cpd). Respiratory challenges had no effect. Contrast sensitivity is poorer when viewing monocularly and especially at midmesopic luminance, with relevance to night flying. The foveal contrast sensitivity function is unaffected by respiratory disturbance when twilight conditions favor cone vision, despite known effects on retinal illumination (pupil size). The resilience of the contrast sensitivity function belies the vulnerability of foveal low contrast acuity to mild hypoxia at mesopic luminance.

[Research on Residual Aberrations Correction with Adaptive Optics Technique in Patients Undergoing Orthokeratology].

PubMed

Gong, Rui; Yang, Bi; Liu, Longqian; Dai, Yun; Zhang, Yudong; Zhao, Haoxin

2016-06-01

We conducted this study to explore the influence of the ocular residual aberrations changes on contrast sensitivity(CS)function in eyes undergoing orthokeratology using adaptive optics technique.Nineteen subjects’ nineteen eyes were included in this study.The subjects were between 12 and 20years(14.27±2.23years)of age.An adaptive optics(AO)system was adopted to measure and compensate the residual aberrations through a 4-mm artificial pupil,and at the same time the contrast sensitivities were measured at five spatial frequencies(2,4,8,16,and 32 cycles per degree).The CS measurements with and without AO correction were completed.The sequence of the measurements with and without AO correction was randomly arranged without informing the observers.A two-interval forced-choice procedure was used for the CS measurements.The paired t-test was used to compare the contrast sensitivity with and without AO correction at each spatial frequency.The results revealed that the AO system decreased the mean total root mean square(RMS)from 0.356μm to 0.160μm(t=10.517,P<0.001),and the mean total higher-order RMS from 0.246μm to 0.095μm(t=10.113,P<0.001).The difference in log contrast sensitivity with and without AO correction was significant only at 8cpd(t=-2.51,P=0.02).Thereby we concluded that correcting the ocular residual aberrations using adaptive optics technique could improve the contrast sensitivity function at intermediate spatial frequency in patients undergoing orthokeratology.

Visual Contrast Sensitivity in Early-Stage Parkinson's Disease.

PubMed

Ming, Wendy; Palidis, Dimitrios J; Spering, Miriam; McKeown, Martin J

2016-10-01

Visual impairments are frequent in Parkinson's disease (PD) and impact normal functioning in daily activities. Visual contrast sensitivity is a powerful nonmotor sign for discriminating PD patients from controls. However, it is usually assessed with static visual stimuli. Here we examined the interaction between perception and eye movements in static and dynamic contrast sensitivity tasks in a cohort of mildly impaired, early-stage PD patients. Patients (n = 13) and healthy age-matched controls (n = 12) viewed stimuli of various spatial frequencies (0-8 cyc/deg) and speeds (0°/s, 10°/s, 30°/s) on a computer monitor. Detection thresholds were determined by asking participants to adjust luminance contrast until they could just barely see the stimulus. Eye position was recorded with a video-based eye tracker. Patients' static contrast sensitivity was impaired in the intermediate spatial-frequency range and this impairment correlated with fixational instability. However, dynamic contrast sensitivity and patients' smooth pursuit were relatively normal. An independent component analysis revealed contrast sensitivity profiles differentiating patients and controls. Our study simultaneously assesses perceptual contrast sensitivity and eye movements in PD, revealing a possible link between fixational instability and perceptual deficits. Spatiotemporal contrast sensitivity profiles may represent an easily measurable metric as a component of a broader combined biometric for nonmotor features observed in PD.

Impaired temporal contrast sensitivity in dyslexics is specific to retain-and-compare paradigms.

PubMed

Ben-Yehudah, G; Sackett, E; Malchi-Ginzberg, L; Ahissar, M

2001-07-01

Developmental dyslexia is a specific reading disability that affects 5-10% of the population. Recent studies have suggested that dyslexics may experience a deficit in the visual magnocellular pathway. The most extensively studied prediction deriving from this hypothesis is impaired contrast sensitivity to transient, low-luminance stimuli at low spatial frequencies. However, the findings are inconsistent across studies and even seemingly contradictory. In the present study, we administered several different paradigms for assessing temporal contrast sensitivity, and found both impaired and normal contrast sensitivity within the same group of dyslexic participants. Under sequential presentation, in a temporal forced choice paradigm, dyslexics showed impaired sensitivity to both drifting and flickering gratings. However, under simultaneous presentation, with a spatial forced choice paradigm, dyslexics' sensitivity did not differ from that of the controls. Within each paradigm, dyslexics' sensitivity was poorer at higher temporal frequencies, consistent with the magnocellular hypothesis. These results suggest that a basic perceptual impairment in dyslexics may be their limited ability to retain-and-compare perceptual traces across brief intervals.

Modeling lateral geniculate nucleus response with contrast gain control. Part 2: Analysis

PubMed Central

Cope, Davis; Blakeslee, Barbara; McCourt, Mark E.

2014-01-01

Cope, Blakeslee and McCourt (2013) proposed a class of models for LGN ON-cell behavior consisting of a linear response with divisive normalization by local stimulus contrast. Here we analyze a specific model with the linear response defined by a difference-of-Gaussians filter and a circular Gaussian for the gain pool weighting function. For sinusoidal grating stimuli, the parameter region for band-pass behavior of the linear response is determined, the gain control response is shown to act as a switch (changing from “off” to “on” with increasing spatial frequency), and it is shown that large gain pools stabilize the optimal spatial frequency of the total nonlinear response at a fixed value independent of contrast and stimulus magnitude. Under- and super-saturation as well as contrast saturation occur as typical effects of stimulus magnitude. For circular spot stimuli, it is shown that large gain pools stabilize the spot size that yields the maximum response. PMID:24562034

Input-dependent frequency modulation of cortical gamma oscillations shapes spatial synchronization and enables phase coding.

PubMed

Lowet, Eric; Roberts, Mark; Hadjipapas, Avgis; Peter, Alina; van der Eerden, Jan; De Weerd, Peter

2015-02-01

Fine-scale temporal organization of cortical activity in the gamma range (∼25-80Hz) may play a significant role in information processing, for example by neural grouping ('binding') and phase coding. Recent experimental studies have shown that the precise frequency of gamma oscillations varies with input drive (e.g. visual contrast) and that it can differ among nearby cortical locations. This has challenged theories assuming widespread gamma synchronization at a fixed common frequency. In the present study, we investigated which principles govern gamma synchronization in the presence of input-dependent frequency modulations and whether they are detrimental for meaningful input-dependent gamma-mediated temporal organization. To this aim, we constructed a biophysically realistic excitatory-inhibitory network able to express different oscillation frequencies at nearby spatial locations. Similarly to cortical networks, the model was topographically organized with spatially local connectivity and spatially-varying input drive. We analyzed gamma synchronization with respect to phase-locking, phase-relations and frequency differences, and quantified the stimulus-related information represented by gamma phase and frequency. By stepwise simplification of our models, we found that the gamma-mediated temporal organization could be reduced to basic synchronization principles of weakly coupled oscillators, where input drive determines the intrinsic (natural) frequency of oscillators. The gamma phase-locking, the precise phase relation and the emergent (measurable) frequencies were determined by two principal factors: the detuning (intrinsic frequency difference, i.e. local input difference) and the coupling strength. In addition to frequency coding, gamma phase contained complementary stimulus information. Crucially, the phase code reflected input differences, but not the absolute input level. This property of relative input-to-phase conversion, contrasting with latency codes or slower oscillation phase codes, may resolve conflicting experimental observations on gamma phase coding. Our modeling results offer clear testable experimental predictions. We conclude that input-dependency of gamma frequencies could be essential rather than detrimental for meaningful gamma-mediated temporal organization of cortical activity.

Input-Dependent Frequency Modulation of Cortical Gamma Oscillations Shapes Spatial Synchronization and Enables Phase Coding

PubMed Central

Lowet, Eric; Roberts, Mark; Hadjipapas, Avgis; Peter, Alina; van der Eerden, Jan; De Weerd, Peter

2015-01-01

Fine-scale temporal organization of cortical activity in the gamma range (∼25–80Hz) may play a significant role in information processing, for example by neural grouping (‘binding’) and phase coding. Recent experimental studies have shown that the precise frequency of gamma oscillations varies with input drive (e.g. visual contrast) and that it can differ among nearby cortical locations. This has challenged theories assuming widespread gamma synchronization at a fixed common frequency. In the present study, we investigated which principles govern gamma synchronization in the presence of input-dependent frequency modulations and whether they are detrimental for meaningful input-dependent gamma-mediated temporal organization. To this aim, we constructed a biophysically realistic excitatory-inhibitory network able to express different oscillation frequencies at nearby spatial locations. Similarly to cortical networks, the model was topographically organized with spatially local connectivity and spatially-varying input drive. We analyzed gamma synchronization with respect to phase-locking, phase-relations and frequency differences, and quantified the stimulus-related information represented by gamma phase and frequency. By stepwise simplification of our models, we found that the gamma-mediated temporal organization could be reduced to basic synchronization principles of weakly coupled oscillators, where input drive determines the intrinsic (natural) frequency of oscillators. The gamma phase-locking, the precise phase relation and the emergent (measurable) frequencies were determined by two principal factors: the detuning (intrinsic frequency difference, i.e. local input difference) and the coupling strength. In addition to frequency coding, gamma phase contained complementary stimulus information. Crucially, the phase code reflected input differences, but not the absolute input level. This property of relative input-to-phase conversion, contrasting with latency codes or slower oscillation phase codes, may resolve conflicting experimental observations on gamma phase coding. Our modeling results offer clear testable experimental predictions. We conclude that input-dependency of gamma frequencies could be essential rather than detrimental for meaningful gamma-mediated temporal organization of cortical activity. PMID:25679780

Effects of spatial frequency and location of fearful faces on human amygdala activity.

PubMed

Morawetz, Carmen; Baudewig, Juergen; Treue, Stefan; Dechent, Peter

2011-01-31

Facial emotion perception plays a fundamental role in interpersonal social interactions. Images of faces contain visual information at various spatial frequencies. The amygdala has previously been reported to be preferentially responsive to low-spatial frequency (LSF) rather than to high-spatial frequency (HSF) filtered images of faces presented at the center of the visual field. Furthermore, it has been proposed that the amygdala might be especially sensitive to affective stimuli in the periphery. In the present study we investigated the impact of spatial frequency and stimulus eccentricity on face processing in the human amygdala and fusiform gyrus using functional magnetic resonance imaging (fMRI). The spatial frequencies of pictures of fearful faces were filtered to produce images that retained only LSF or HSF information. Facial images were presented either in the left or right visual field at two different eccentricities. In contrast to previous findings, we found that the amygdala responds to LSF and HSF stimuli in a similar manner regardless of the location of the affective stimuli in the visual field. Furthermore, the fusiform gyrus did not show differential responses to spatial frequency filtered images of faces. Our findings argue against the view that LSF information plays a crucial role in the processing of facial expressions in the amygdala and of a higher sensitivity to affective stimuli in the periphery. Copyright © 2010 Elsevier B.V. All rights reserved.

Anti-aliasing Wiener filtering for wave-front reconstruction in the spatial-frequency domain for high-order astronomical adaptive-optics systems.

PubMed

Correia, Carlos M; Teixeira, Joel

2014-12-01

Computationally efficient wave-front reconstruction techniques for astronomical adaptive-optics (AO) systems have seen great development in the past decade. Algorithms developed in the spatial-frequency (Fourier) domain have gathered much attention, especially for high-contrast imaging systems. In this paper we present the Wiener filter (resulting in the maximization of the Strehl ratio) and further develop formulae for the anti-aliasing (AA) Wiener filter that optimally takes into account high-order wave-front terms folded in-band during the sensing (i.e., discrete sampling) process. We employ a continuous spatial-frequency representation for the forward measurement operators and derive the Wiener filter when aliasing is explicitly taken into account. We further investigate and compare to classical estimates using least-squares filters the reconstructed wave-front, measurement noise, and aliasing propagation coefficients as a function of the system order. Regarding high-contrast systems, we provide achievable performance results as a function of an ensemble of forward models for the Shack-Hartmann wave-front sensor (using sparse and nonsparse representations) and compute point-spread-function raw intensities. We find that for a 32×32 single-conjugated AOs system the aliasing propagation coefficient is roughly 60% of the least-squares filters, whereas the noise propagation is around 80%. Contrast improvements of factors of up to 2 are achievable across the field in the H band. For current and next-generation high-contrast imagers, despite better aliasing mitigation, AA Wiener filtering cannot be used as a standalone method and must therefore be used in combination with optical spatial filters deployed before image formation actually takes place.

Postural stability changes in the elderly with cataract simulation and refractive blur.

PubMed

Anand, Vijay; Buckley, John G; Scally, Andy; Elliott, David B

2003-11-01

To determine the influence of cataractous and refractive blur on postural stability and limb-load asymmetry (LLA) and to establish how postural stability changes with the spatial frequency and contrast of the visual stimulus. Thirteen elderly subjects (mean age, 70.76 +/- 4.14 [SD] years) with no history of falls and normal vision were recruited. Postural stability was determined as the root mean square [RMS] of the center of pressure (COP) signal in the anterior-posterior (A-P) and medial-lateral directions and LLA was determined as the ratio of the average body weight placed on the more-loaded limb to the less-loaded limb, recorded during a 30-second period. Data were collected under normal standing conditions and with somatosensory system input disrupted. Measurements were repeated with four visual targets with high (8 cyc/deg) or low (2 cyc/deg) spatial frequency and high (Weber contrast, approximately 95%) or low (Weber contrast, approximately 25%) contrast. Postural stability was measured under conditions of binocular refractive blur of 0, 1, 2, 4, and 8 D and with cataract simulation. The data were analyzed in a population-averaged linear model. The cataract simulation caused significant increases in postural instability equivalent to that caused by 8-D blur conditions, and its effect was greater when the input from the somatosensory system was disrupted. High spatial frequency targets increased postural instability. Refractive blur, cataract simulation, or eye closure had no effect on LLA. Findings indicate that cataractous and refractive blur increase postural instability, and show why the elderly, many of whom have poor vision along with musculoskeletal and central nervous system degeneration, are at greater risk of falling. Findings also highlight that changes in contrast sensitivity rather than resolution changes are responsible for increasing postural instability. Providing low spatial frequency information in certain environments may be useful in maintaining postural stability. Correcting visual impairment caused by uncorrected refractive error and cataracts could be a useful intervention strategy to help prevent falls and fall-related injuries in the elderly.

Separate channels for the analysis of the shape and the movement of moving visual stimulus.

PubMed

Tolhurst, D J

1973-06-01

1. The effects of temporal modulation on the properties of spatial frequency channels have been investigated using adaptation.2. Adapting to drifting sinusoidal gratings caused threshold elevation that was both spatial frequency and direction specific. Little systematic difference was found between the band widths of the elevation curves for drifting and stationary gratings.3. It was confirmed that adaptation fails to reveal channels at low spatial frequencies when stationary gratings are used. However, channels were revealed at frequencies at least as low as 0.66 c/deg when the test gratings were made to move. These channels are adapted only a little by stationary gratings, confirming their dependence on movement.4. The existence of movement-sensitive channels at low spatial frequencies explains the well known observation that temporal modulation greatly increases the sensitivity of the visual system to low spatial frequencies.5. Temporal modulation was effective at revealing these channels only when the flicker or movement of the test patterns was apparent to the observer; only at low spatial frequencies did patterns, modulated at low rates, actually appear to be temporarily modulated at threshold. At higher spatial frequencies, they were indistinguishable from stationary patterns until the contrast was some way above the detection threshold.6. It is suggested, therefore, that the movement-sensitive channels are responsible for signalling the occurrence of movement; the channels at higher spatial frequencies give no information about temporal changes. These two systems of channels are compared to the Y- and X-cells respectively of the cat.

Contrast computation methods for interferometric measurement of sensor modulation transfer function

NASA Astrophysics Data System (ADS)

Battula, Tharun; Georgiev, Todor; Gille, Jennifer; Goma, Sergio

2018-01-01

Accurate measurement of image-sensor frequency response over a wide range of spatial frequencies is very important for analyzing pixel array characteristics, such as modulation transfer function (MTF), crosstalk, and active pixel shape. Such analysis is especially significant in computational photography for the purposes of deconvolution, multi-image superresolution, and improved light-field capture. We use a lensless interferometric setup that produces high-quality fringes for measuring MTF over a wide range of frequencies (here, 37 to 434 line pairs per mm). We discuss the theoretical framework, involving Michelson and Fourier contrast measurement of the MTF, addressing phase alignment problems using a moiré pattern. We solidify the definition of Fourier contrast mathematically and compare it to Michelson contrast. Our interferometric measurement method shows high detail in the MTF, especially at high frequencies (above Nyquist frequency). We are able to estimate active pixel size and pixel pitch from measurements. We compare both simulation and experimental MTF results to a lens-free slanted-edge implementation using commercial software.

[Study of Image Quality Comparison Based on the MTF Method Between Different Medical Rigid Endoscopes in an In Vitro Model].

PubMed

Wang, Yunlong; Ji, Jun; Jiang, Changsong; Huang, Zengyue

2015-04-01

This study was aimed to use the method of modulation transfer function (MTF) to compare image quality among three different Olympus medical rigid cystoscopes in an in vitro model. During the experimental processes, we firstly used three different types of cystoscopes (i. e. OLYMPUS cystourethroscopy with FOV of 12 degrees, OLYMPUS Germany A22003A and OLYMPUS A2013A) to collect raster images at different brightness with industrial camera and computer from the resolution target which is with different spatial frequency, and then we processed the collected images using MALAB software with the optical transfer function MTF to obtain the values of MTF at different brightness and different spatial frequency. We then did data mathematical statistics and compared imaging quality. The statistical data showed that all three MTF values were smaller than 1. MTF values with the spatial frequency gradually increasing would decrease approaching 0 at the same brightness. When the brightness enhanced in the same process at the same spatial frequency, MTF values showed a slowly increasing trend. The three endoscopes' MTF values were completely different. In some cases the MTF values had a large difference, and the maximum difference could reach 0.7. Conclusion can be derived from analysis of experimental data that three Olympus medical rigid cystoscopes have completely different imaging quality abilities. The No. 3 endoscope OLYMPUS A2013A has low resolution but high contrast. The No. 1 endoscope OLYMPUS cystourethroscopy with FOV of 12 degrees, on the contrary, had high resolution and lower contrast. The No. 2 endoscope OLYMPUS Germany A22003A had high contrast and high resolution, and its image quality was the best.

Spatial Frequency Selectivity Is Impaired in Dopamine D2 Receptor Knockout Mice

PubMed Central

Souza, Bruno Oliveira Ferreira; Abou Rjeili, Mira; Quintana, Clémentine; Beaulieu, Jean M.; Casanova, Christian

2018-01-01

Dopamine is a neurotransmitter implicated in several brain functions, including vision. In the present study, we investigated the impacts of the lack of D2 dopamine receptors on the structure and function of the primary visual cortex (V1) of D2-KO mice using optical imaging of intrinsic signals. Retinotopic maps were generated in order to measure anatomo-functional parameters such as V1 shape, cortical magnification factor, scatter, and ocular dominance. Contrast sensitivity and spatial frequency selectivity (SF) functions were computed from responses to drifting gratings. When compared to control mice, none of the parameters of the retinotopic maps were affected by D2 receptor loss of function. While the contrast sensitivity function of D2-KO mice did not differ from their wild-type counterparts, SF selectivity function was significantly affected as the optimal SF and the high cut-off frequency (p < 0.01) were higher in D2-KO than in WT mice. These findings show that the lack of function of D2 dopamine receptors had no influence on cortical structure whereas it had a significant impact on the spatial frequency selectivity and high cut-off. Taken together, our results suggest that D2 receptors play a specific role on the processing of spatial features in early visual cortex while they do not seem to participate in its development. PMID:29379422

Evaluation of image quality in terahertz pulsed imaging using test objects.

PubMed

Fitzgerald, A J; Berry, E; Miles, R E; Zinovev, N N; Smith, M A; Chamberlain, J M

2002-11-07

As with other imaging modalities, the performance of terahertz (THz) imaging systems is limited by factors of spatial resolution, contrast and noise. The purpose of this paper is to introduce test objects and image analysis methods to evaluate and compare THz image quality in a quantitative and objective way, so that alternative terahertz imaging system configurations and acquisition techniques can be compared, and the range of image parameters can be assessed. Two test objects were designed and manufactured, one to determine the modulation transfer functions (MTF) and the other to derive image signal to noise ratio (SNR) at a range of contrasts. As expected the higher THz frequencies had larger MTFs, and better spatial resolution as determined by the spatial frequency at which the MTF dropped below the 20% threshold. Image SNR was compared for time domain and frequency domain image parameters and time delay based images consistently demonstrated higher SNR than intensity based parameters such as relative transmittance because the latter are more strongly affected by the sources of noise in the THz system such as laser fluctuations and detector shot noise.

Influence of spherical aberration, stimulus spatial frequency, and pupil apodisation on subjective refractions

PubMed Central

Bradley, Arthur; Xu, Renfeng; Thibos, Larry; Marin, Gildas; Hernandez, Martha

2014-01-01

Purpose To test competing hypotheses (Stiles Crawford pupil apodising or superior imaging of high spatial frequencies by the central pupil) for the pupil size independence of subjective refractions in the presence of primary spherical aberration. Methods Subjective refractions were obtained with a variety of test stimuli (high contrast letters, urban cityscape, high and low spatial frequency gratings) while modulating pupil diameter, levels of primary spherical aberration and pupil apodisation. Subjective refractions were also obtained with low-pass and high-pass stimuli and using “darker” and “sharper” subjective criteria. Results Subjective refractions for stimuli containing high spatial frequencies focus a near paraxial region of the pupil and are affected only slightly by level of Seidel spherical aberration, degree of pupil apodisation and pupil diameter, and generally focused a radius of about 1 to 1.5 mm from the pupil centre. Low spatial frequency refractions focus a marginal region of the pupil, and are significantly affected by level of spherical aberration, amount of pupil apodisation, and pupil size. Clinical refractions that employ the “darker” or “sharper” subjective criteria bias the patient to use lower or higher spatial frequencies respectively. Conclusions In the presence of significant levels of spherical aberration, the pupil size independence of subjective refractions occurs with or without Stiles Crawford apodisation for refractions that optimise high spatial frequency content in the image. If low spatial frequencies are optimised by a subjective refraction, spherical refractive error varies with spherical aberration, pupil size, and level of apodisation. As light levels drop from photopic to scotopic, therefore, we expect a shift from pupil size independent to pupil size dependent subjective refractions. Emphasising a “sharper” criterion during subjective refractions will improve image quality for high spatial frequencies and generate pupil size independent refractions. PMID:24397356

Microfabricated high-bandpass foucault aperture for electron microscopy

DOEpatents

Glaeser, Robert; Cambie, Rossana; Jin, Jian

2014-08-26

A variant of the Foucault (knife-edge) aperture is disclosed that is designed to provide single-sideband (SSB) contrast at low spatial frequencies but retain conventional double-sideband (DSB) contrast at high spatial frequencies in transmission electron microscopy. The aperture includes a plate with an inner open area, a support extending from the plate at an edge of the open area, a half-circle feature mounted on the support and located at the center of the aperture open area. The radius of the half-circle portion of reciprocal space that is blocked by the aperture can be varied to suit the needs of electron microscopy investigation. The aperture is fabricated from conductive material which is preferably non-oxidizing, such as gold, for example.

Effects of background stimulation upon eye-movement information.

PubMed

Nakamura, S

1996-04-01

To investigate the effects of background stimulation upon eye-movement information (EMI), the perceived deceleration of the target motion during pursuit eye movement (Aubert-Fleishl paradox) was analyzed. In the experiment, a striped pattern was used as a background stimulus with various brightness contrasts and spatial frequencies for serially manipulating the attributions of the background stimulus. Analysis showed that the retinal-image motion of the background stimulus (optic flow) affected eye-movement information and that the effects of optic flow became stronger when high contrast and low spatial frequency stripes were presented as the background stimulus. In conclusion, optic flow is one source of eye-movement information in determining real object motion, and the effectiveness of optic flow depends on the attributes of the background stimulus.

Proposed New Vision Standards for the 1980’s and Beyond: Contrast Sensitivity

DTIC Science & Technology

1981-09-01

spatial frequency, visual acuity, target aquistion, visual filters, spatial filtering, target detection, recognitio identification, eye charts, workload...visual standards, as well as other performance criteria, are required to be thown relevant to "real-world" performance before acceptance. On the sur- face

Back-illuminated CCD imager adapted for contrast transfer function measurements thereon

NASA Technical Reports Server (NTRS)

Levine, Peter A. (Inventor)

1987-01-01

Stripe patterns of varying spatial frequency, formed in the top-metalization of a back-illuminated solid-state imager, facilitate on-line measurement of contrast transfer function during wafer-probe testing. The imager may be packaged to allow front-illumination during in-the-field testing after its manufacture.

Wavefront-guided versus wavefront-optimized laser in situ keratomileusis: contralateral comparative study.

PubMed

Padmanabhan, Prema; Mrochen, Michael; Basuthkar, Subam; Viswanathan, Deepa; Joseph, Roy

2008-03-01

To compare the outcomes of wavefront-guided and wavefront-optimized treatment in fellow eyes of patients having laser in situ keratomileusis (LASIK) for myopia. Medical and Vision Research Foundation, Tamil Nadu, India. This prospective comparative study comprised 27 patients who had wavefront-guided LASIK in 1 eye and wavefront-optimized LASIK in the fellow eye. The Hansatome (Bausch & Lomb) was used to create a superior-hinged flap and the Allegretto laser (WaveLight Laser Technologie AG), for photoablation. The Allegretto wave analyzer was used to measure ocular wavefront aberrations and the Functional Acuity Contrast Test chart, to measure contrast sensitivity before and 1 month after LASIK. The refractive and visual outcomes and the changes in aberrations and contrast sensitivity were compared between the 2 treatment modalities. One month postoperatively, 92% of eyes in the wavefront-guided group and 85% in the wavefront-optimized group had uncorrected visual acuity of 20/20 or better; 93% and 89%, respectively, had a postoperative spherical equivalent refraction of +/-0.50 diopter. The differences between groups were not statistically significant. Wavefront-guided LASIK induced less change in 18 of 22 higher-order Zernike terms than wavefront-optimized LASIK, with the change in positive spherical aberration the only statistically significant one (P= .01). Contrast sensitivity improved at the low and middle spatial frequencies (not statistically significant) and worsened significantly at high spatial frequencies after wavefront-guided LASIK; there was a statistically significant worsening at all spatial frequencies after wavefront-optimized LASIK. Although both wavefront-guided and wavefront-optimized LASIK gave excellent refractive correction results, the former induced less higher-order aberrations and was associated with better contrast sensitivity.

Visible contrast energy metrics for detection and discrimination

NASA Astrophysics Data System (ADS)

Ahumada, Albert J.; Watson, Andrew B.

2013-03-01

Contrast energy was proposed by Watson, Barlow, and Robson (Science, 1983) as a useful metric for representing luminance contrast target stimuli because it represents the detectability of the stimulus in photon noise for an ideal observer. We propose here the use of visible contrast energy metrics for detection and discrimination among static luminance patterns. The visibility is approximated with spatial frequency sensitivity weighting and eccentricity sensitivity weighting. The suggested weighting functions revise the Standard Spatial Observer (Watson and Ahumada, J. Vision, 2005) for luminance contrast detection , extend it into the near periphery, and provide compensation for duration. Under the assumption that the detection is limited only by internal noise, both detection and discrimination performance can be predicted by metrics based on the visible energy of the difference images.

Attention to local and global levels of hierarchical Navon figures affects rapid scene categorization.

PubMed

Brand, John; Johnson, Aaron P

2014-01-01

In four experiments, we investigated how attention to local and global levels of hierarchical Navon figures affected the selection of diagnostic spatial scale information used in scene categorization. We explored this issue by asking observers to classify hybrid images (i.e., images that contain low spatial frequency (LSF) content of one image, and high spatial frequency (HSF) content from a second image) immediately following global and local Navon tasks. Hybrid images can be classified according to either their LSF, or HSF content; thus, making them ideal for investigating diagnostic spatial scale preference. Although observers were sensitive to both spatial scales (Experiment 1), they overwhelmingly preferred to classify hybrids based on LSF content (Experiment 2). In Experiment 3, we demonstrated that LSF based hybrid categorization was faster following global Navon tasks, suggesting that LSF processing associated with global Navon tasks primed the selection of LSFs in hybrid images. In Experiment 4, replicating Experiment 3 but suppressing the LSF information in Navon letters by contrast balancing the stimuli examined this hypothesis. Similar to Experiment 3, observers preferred to classify hybrids based on LSF content; however and in contrast, LSF based hybrid categorization was slower following global than local Navon tasks.

Attention to local and global levels of hierarchical Navon figures affects rapid scene categorization

PubMed Central

Brand, John; Johnson, Aaron P.

2014-01-01

In four experiments, we investigated how attention to local and global levels of hierarchical Navon figures affected the selection of diagnostic spatial scale information used in scene categorization. We explored this issue by asking observers to classify hybrid images (i.e., images that contain low spatial frequency (LSF) content of one image, and high spatial frequency (HSF) content from a second image) immediately following global and local Navon tasks. Hybrid images can be classified according to either their LSF, or HSF content; thus, making them ideal for investigating diagnostic spatial scale preference. Although observers were sensitive to both spatial scales (Experiment 1), they overwhelmingly preferred to classify hybrids based on LSF content (Experiment 2). In Experiment 3, we demonstrated that LSF based hybrid categorization was faster following global Navon tasks, suggesting that LSF processing associated with global Navon tasks primed the selection of LSFs in hybrid images. In Experiment 4, replicating Experiment 3 but suppressing the LSF information in Navon letters by contrast balancing the stimuli examined this hypothesis. Similar to Experiment 3, observers preferred to classify hybrids based on LSF content; however and in contrast, LSF based hybrid categorization was slower following global than local Navon tasks. PMID:25520675

Flexible Contrast Gain Control in the Right Hemisphere

ERIC Educational Resources Information Center

Okubo, Matia; Nicholls, Michael E. R.

2005-01-01

This study investigates whether the right hemisphere has more flexible contrast gain control settings for the identification of spatial frequency. Right-handed participants identified 1 and 9 cycles per degree sinusoidal gratings presented either to the left visual field-right hemisphere (LVF-RH) or the right visual field-left hemisphere (RVF-LH).…

Experimental characterization of a direct conversion amorphous selenium detector with thicker conversion layer for dual-energy contrast-enhanced breast imaging.

PubMed

Scaduto, David A; Tousignant, Olivier; Zhao, Wei

2017-08-01

Dual-energy contrast-enhanced imaging is being investigated as a tool to identify and localize angiogenesis in the breast, a possible indicator of malignant tumors. This imaging technique requires that x-ray images are acquired at energies above the k-shell binding energy of an appropriate radiocontrast agent. Iodinated contrast agents are commonly used for vascular imaging, and require x-ray energies greater than 33 keV. Conventional direct conversion amorphous selenium (a-Se) flat-panel imagers for digital mammography show suboptimal absorption efficiencies at these higher energies. We use spatial-frequency domain image quality metrics to evaluate the performance of a prototype direct conversion flat-panel imager with a thicker a-Se layer, specifically fabricated for dual-energy contrast-enhanced breast imaging. Imaging performance was evaluated in a prototype digital breast tomosynthesis (DBT) system. The spatial resolution, noise characteristics, detective quantum efficiency, and temporal performance of the detector were evaluated for dual-energy imaging for both conventional full-field digital mammography (FFDM) and DBT. The zero-frequency detective quantum efficiency of the prototype detector is improved by approximately 20% over the conventional detector for higher energy beams required for imaging with iodinated contrast agents. The effect of oblique entry of x-rays on spatial resolution does increase with increasing photoconductor thickness, specifically for the most oblique views of a DBT scan. Degradation of spatial resolution due to focal spot motion was also observed. Temporal performance was found to be comparable to conventional mammographic detectors. Increasing the a-Se thickness in direct conversion flat-panel imagers results in better performance for dual-energy contrast-enhanced breast imaging. The reduction in spatial resolution due to oblique entry of x-rays is appreciable in the most extreme clinically relevant cases, but may not profoundly affect reconstructed images due to the algorithms and filters employed. Degradation to projection domain spatial resolution is thus outweighed by the improvement in detective quantum efficiency for high-energy x-rays. © 2017 American Association of Physicists in Medicine.

Visual signal detection in structured backgrounds. II. Effects of contrast gain control, background variations, and white noise

NASA Technical Reports Server (NTRS)

Eckstein, M. P.; Ahumada, A. J. Jr; Watson, A. B.

1997-01-01

Studies of visual detection of a signal superimposed on one of two identical backgrounds show performance degradation when the background has high contrast and is similar in spatial frequency and/or orientation to the signal. To account for this finding, models include a contrast gain control mechanism that pools activity across spatial frequency, orientation and space to inhibit (divisively) the response of the receptor sensitive to the signal. In tasks in which the observer has to detect a known signal added to one of M different backgrounds grounds due to added visual noise, the main sources of degradation are the stochastic noise in the image and the suboptimal visual processing. We investigate how these two sources of degradation (contrast gain control and variations in the background) interact in a task in which the signal is embedded in one of M locations in a complex spatially varying background (structured background). We use backgrounds extracted from patient digital medical images. To isolate effects of the fixed deterministic background (the contrast gain control) from the effects of the background variations, we conduct detection experiments with three different background conditions: (1) uniform background, (2) a repeated sample of structured background, and (3) different samples of structured background. Results show that human visual detection degrades from the uniform background condition to the repeated background condition and degrades even further in the different backgrounds condition. These results suggest that both the contrast gain control mechanism and the background random variations degrade human performance in detection of a signal in a complex, spatially varying background. A filter model and added white noise are used to generate estimates of sampling efficiencies, an equivalent internal noise, an equivalent contrast-gain-control-induced noise, and an equivalent noise due to the variations in the structured background.

Effects of spatial frequency content on classification of face gender and expression.

PubMed

Aguado, Luis; Serrano-Pedraza, Ignacio; Rodríguez, Sonia; Román, Francisco J

2010-11-01

The role of different spatial frequency bands on face gender and expression categorization was studied in three experiments. Accuracy and reaction time were measured for unfiltered, low-pass (cut-off frequency of 1 cycle/deg) and high-pass (cutoff frequency of 3 cycles/deg) filtered faces. Filtered and unfiltered faces were equated in root-mean-squared contrast. For low-pass filtered faces reaction times were higher than unfiltered and high-pass filtered faces in both categorization tasks. In the expression task, these results were obtained with expressive faces presented in isolation (Experiment 1) and also with neutral-expressive dynamic sequences where each expressive face was preceded by a briefly presented neutral version of the same face (Experiment 2). For high-pass filtered faces different effects were observed on gender and expression categorization. While both speed and accuracy of gender categorization were reduced comparing to unfiltered faces, the efficiency of expression classification remained similar. Finally, we found no differences between expressive and non expressive faces in the effects of spatial frequency filtering on gender categorization (Experiment 3). These results show a common role of information from the high spatial frequency band in the categorization of face gender and expression.

Interactions of spatial and luminance information in the retina of chickens during myopia development.

PubMed

Feldkaemper, M; Diether, S; Kleine, G; Schaeffel, F

1999-01-01

Degrading the retinal image by frosted eye occluders produces elongated eyes and 'deprivation myopia' in a variety of animal models. The postulated retinal 'deprivation detector' is quite sensitive to even small changes in image contrast or spatial frequency composition. Because psychophysical experiments have shown that a decline in luminance shifts the contrast sensitivity function to lower spatial frequencies, it is likely that only a reduced spatial frequency range is available for image analysis to control eye growth. It is even possible that the compression might be sufficient to promote deprivation myopia. We have tested this hypothesis, using the animal model of the chicken. (1) At an ambient illumination of 550 lux (about 76 cd m-2), neutral density (ND) filters placed in front of the eye with 0.0, 0.5 or 1.0 log unit attenuation did not change refractive development. However, monocularly or binocularly attached filters with 2 log units attenuation produced 5-7 D of myopia relative to normal eyes. Black occluders were not more effective. Frosted eye occluders with little effect on image brightness (about 0.5 log units attenuation) produced much more myopia (about 16 D compared with the controls). (2) The effects of the ND filters on refractive development could not be reproduced if the ambient illumination was reduced by 2 log units. Probably, minor effects on image quality were introduced by optical imperfections of the ND filters which were more critical at low retinal image brightness. (3) In an optomotor experiment (spatial frequency 0.2 cyc deg-1, stripe speed 57 deg sec-1), it was shown that the chickens' contrast sensitivity was severely reduced when the eyes were covered by 2.0 ND filters. (4) Since there is evidence that changes in dopamine release from the retina may be one of the factors affecting the development of myopia, we have tested how selective these changes were for spatial information. It was found that dopamine release was controlled by both spatial and luminance information and that the inputs of both could be scarcely separated. (5) Because the experiments show that the eye becomes more sensitive to image degradation at low light, the human eye may also be more prone to develop myopia if the light levels are low during extended periods of near work. Copyright 1999 Academic Press.

Exploring the parahippocampal cortex response to high and low spatial frequency spaces.

PubMed

Zeidman, Peter; Mullally, Sinéad L; Schwarzkopf, Dietrich Samuel; Maguire, Eleanor A

2012-05-30

The posterior parahippocampal cortex (PHC) supports a range of cognitive functions, in particular scene processing. However, it has recently been suggested that PHC engagement during functional MRI simply reflects the representation of three-dimensional local space. If so, PHC should respond to space in the absence of scenes, geometric layout, objects or contextual associations. It has also been reported that PHC activation may be influenced by low-level visual properties of stimuli such as spatial frequency. Here, we tested whether PHC was responsive to the mere sense of space in highly simplified stimuli, and whether this was affected by their spatial frequency distribution. Participants were scanned using functional MRI while viewing depictions of simple three-dimensional space, and matched control stimuli that did not depict a space. Half the stimuli were low-pass filtered to ascertain the impact of spatial frequency. We observed a significant interaction between space and spatial frequency in bilateral PHC. Specifically, stimuli depicting space (more than nonspatial stimuli) engaged the right PHC when they featured high spatial frequencies. In contrast, the interaction in the left PHC did not show a preferential response to space. We conclude that a simple depiction of three-dimensional space that is devoid of objects, scene layouts or contextual associations is sufficient to robustly engage the right PHC, at least when high spatial frequencies are present. We suggest that coding for the presence of space may be a core function of PHC, and could explain its engagement in a range of tasks, including scene processing, where space is always present.

Contralateral Bias of High Spatial Frequency Tuning and Cardinal Direction Selectivity in Mouse Visual Cortex

PubMed Central

Zeitoun, Jack H.; Kim, Hyungtae

2017-01-01

Binocular mechanisms for visual processing are thought to enhance spatial acuity by combining matched input from the two eyes. Studies in the primary visual cortex of carnivores and primates have confirmed that eye-specific neuronal response properties are largely matched. In recent years, the mouse has emerged as a prominent model for binocular visual processing, yet little is known about the spatial frequency tuning of binocular responses in mouse visual cortex. Using calcium imaging in awake mice of both sexes, we show that the spatial frequency preference of cortical responses to the contralateral eye is ∼35% higher than responses to the ipsilateral eye. Furthermore, we find that neurons in binocular visual cortex that respond only to the contralateral eye are tuned to higher spatial frequencies. Binocular neurons that are well matched in spatial frequency preference are also matched in orientation preference. In contrast, we observe that binocularly mismatched cells are more mismatched in orientation tuning. Furthermore, we find that contralateral responses are more direction-selective than ipsilateral responses and are strongly biased to the cardinal directions. The contralateral bias of high spatial frequency tuning was found in both awake and anesthetized recordings. The distinct properties of contralateral cortical responses may reflect the functional segregation of direction-selective, high spatial frequency-preferring neurons in earlier stages of the central visual pathway. Moreover, these results suggest that the development of binocularity and visual acuity may engage distinct circuits in the mouse visual system. SIGNIFICANCE STATEMENT Seeing through two eyes is thought to improve visual acuity by enhancing sensitivity to fine edges. Using calcium imaging of cellular responses in awake mice, we find surprising asymmetries in the spatial processing of eye-specific visual input in binocular primary visual cortex. The contralateral visual pathway is tuned to higher spatial frequencies than the ipsilateral pathway. At the highest spatial frequencies, the contralateral pathway strongly prefers to respond to visual stimuli along the cardinal (horizontal and vertical) axes. These results suggest that monocular, and not binocular, mechanisms set the limit of spatial acuity in mice. Furthermore, they suggest that the development of visual acuity and binocularity in mice involves different circuits. PMID:28924011

Uniform apparent contrast noise: A picture of the noise of the visual contrast detection system

NASA Technical Reports Server (NTRS)

Ahumada, A. J., Jr.; Watson, A. B.

1984-01-01

A picture which is a sample of random contrast noise is generated. The noise amplitude spectrum in each region of the picture is inversely proportional to spatial frequency contrast sensitivity for that region, assuming the observer fixates the center of the picture and is the appropriate distance from it. In this case, the picture appears to have approximately the same contrast everywhere. To the extent that contrast detection thresholds are determined by visual system noise, this picture can be regarded as a picture of the noise of that system. There is evidence that, at different eccentricities, contrast sensitivity functions differ only by a magnification factor. The picture was generated by filtering a sample of white noise with a filter whose frequency response is inversely proportional to foveal contrast sensitivity. It was then stretched by a space-varying magnification function. The picture summmarizes a noise linear model of detection and discrimination of contrast signals by referring the model noise to the input picture domain.

Psychophysical Evaluation of Achromatic and Chromatic Vision of Workers Chronically Exposed to Organic Solvents

PubMed Central

Lacerda, Eliza Maria da Costa Brito; Lima, Monica Gomes; Rodrigues, Anderson Raiol; Teixeira, Cláudio Eduardo Correa; de Lima, Lauro José Barata; Ventura, Dora Fix; Silveira, Luiz Carlos de Lima

2012-01-01

The purpose of this paper was to evaluate achromatic and chromatic vision of workers chronically exposed to organic solvents through psychophysical methods. Thirty-one gas station workers (31.5 ± 8.4 years old) were evaluated. Psychophysical tests were achromatic tests (Snellen chart, spatial and temporal contrast sensitivity, and visual perimetry) and chromatic tests (Ishihara's test, color discrimination ellipses, and Farnsworth-Munsell 100 hue test—FM100). Spatial contrast sensitivities of exposed workers were lower than the control at spatial frequencies of 20 and 30 cpd whilst the temporal contrast sensitivity was preserved. Visual field losses were found in 10–30 degrees of eccentricity in the solvent exposed workers. The exposed workers group had higher error values of FM100 and wider color discrimination ellipses area compared to the controls. Workers occupationally exposed to organic solvents had abnormal visual functions, mainly color vision losses and visual field constriction. PMID:22220188

Spatially encoded phase-contrast MRI-3D MRI movies of 1D and 2D structures at millisecond resolution.

PubMed

Merboldt, Klaus-Dietmar; Uecker, Martin; Voit, Dirk; Frahm, Jens

2011-10-01

This work demonstrates that the principles underlying phase-contrast MRI may be used to encode spatial rather than flow information along a perpendicular dimension, if this dimension contains an MRI-visible object at only one spatial location. In particular, the situation applies to 3D mapping of curved 2D structures which requires only two projection images with different spatial phase-encoding gradients. These phase-contrast gradients define the field of view and mean spin-density positions of the object in the perpendicular dimension by respective phase differences. When combined with highly undersampled radial fast low angle shot (FLASH) and image reconstruction by regularized nonlinear inversion, spatial phase-contrast MRI allows for dynamic 3D mapping of 2D structures in real time. First examples include 3D MRI movies of the acting human hand at a temporal resolution of 50 ms. With an even simpler technique, 3D maps of curved 1D structures may be obtained from only three acquisitions of a frequency-encoded MRI signal with two perpendicular phase encodings. Here, 3D MRI movies of a rapidly rotating banana were obtained at 5 ms resolution or 200 frames per second. In conclusion, spatial phase-contrast 3D MRI of 2D or 1D structures is respective two or four orders of magnitude faster than conventional 3D MRI. Copyright © 2011 Wiley-Liss, Inc.

Objective assessment of chromatic and achromatic pattern adaptation reveals the temporal response properties of different visual pathways.

PubMed

Robson, Anthony G; Kulikowski, Janus J

2012-11-01

The aim was to investigate the temporal response properties of magnocellular, parvocellular, and koniocellular visual pathways using increment/decrement changes in contrast to elicit visual evoked potentials (VEPs). Static achromatic and isoluminant chromatic gratings were generated on a monitor. Chromatic gratings were modulated along red/green (R/G) or subject-specific tritanopic confusion axes, established using a minimum distinct border criterion. Isoluminance was determined using minimum flicker photometry. Achromatic and chromatic VEPs were recorded to contrast increments and decrements of 0.1 or 0.2 superimposed on the static gratings (masking contrast 0-0.6). Achromatic increment/decrement changes in contrast evoked a percept of apparent motion when the spatial frequency was low; VEPs to such stimuli were positive in polarity and largely unaffected by high levels of static contrast, consistent with transient response mechanisms. VEPs to finer achromatic gratings showed marked attenuation as static contrast was increased. Chromatic VEPs to R/G or tritan chromatic contrast increments were of negative polarity and showed progressive attenuation as static contrast was increased, in keeping with increasing desensitization of the sustained responses of the color-opponent visual pathways. Chromatic contrast decrement VEPs were of positive polarity and less sensitive to pattern adaptation. The relative contribution of sustained/transient mechanisms to achromatic processing is spatial frequency dependent. Chromatic contrast increment VEPs reflect the sustained temporal response properties of parvocellular and koniocellular pathways. Cortical VEPs can provide an objective measure of pattern adaptation and can be used to probe the temporal response characteristics of different visual pathways.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kaznatcheev, K.; Bertwistle, D.; Cheng, C.

We have explored the capabilities of synchronous ('lock-in') point detection techniques to enhance the x-ray magnetic circular dichroism (XMCD) contrast in scanning x-ray transmission microscopy (STXM) of magnetic thin-film microstructures. Local absorption contrast, measured synchronously with low-amplitude (<10 Oe) and low-frequency (<200 hz) longitudinal fields perturbing the near-remanent magnetization state, reveal a strong spatial dependence of the response, with a roll-off in frequency response above 200 Hz. In this context, synchronous measurement affords us a basis for imaging the relation between energy loss and the sweeping rate. We speculate that the lock-in approach will be uniquely suited for detailing stochasticmore » and deterministic frequency-dependent events in the process of magnetization reversal.« less

Shortcomings of low-cost imaging systems for viewing computed radiographs.

PubMed

Ricke, J; Hänninen, E L; Zielinski, C; Amthauer, H; Stroszczynski, C; Liebig, T; Wolf, M; Hosten, N

2000-01-01

To assess potential advantages of a new PC-based viewing tool featuring image post-processing for viewing computed radiographs on low-cost hardware (PC) with a common display card and color monitor, and to evaluate the effect of using color versus monochrome monitors. Computed radiographs of a statistical phantom were viewed on a PC, with and without post-processing (spatial frequency and contrast processing), employing a monochrome or a color monitor. Findings were compared with the viewing on a radiological Workstation and evaluated with ROC analysis. Image post-processing improved the perception of low-contrast details significantly irrespective of the monitor used. No significant difference in perception was observed between monochrome and color monitors. The review at the radiological Workstation was superior to the review done using the PC with image processing. Lower quality hardware (graphic card and monitor) used in low cost PCs negatively affects perception of low-contrast details in computed radiographs. In this situation, it is highly recommended to use spatial frequency and contrast processing. No significant quality gain has been observed for the high-end monochrome monitor compared to the color display. However, the color monitor was affected stronger by high ambient illumination.

Testing the quality of images for permanent magnet desktop MRI systems using specially designed phantoms.

PubMed

Qiu, Jianfeng; Wang, Guozhu; Min, Jiao; Wang, Xiaoyan; Wang, Pengcheng

2013-12-21

Our aim was to measure the performance of desktop magnetic resonance imaging (MRI) systems using specially designed phantoms, by testing imaging parameters and analysing the imaging quality. We designed multifunction phantoms with diameters of 18 and 60 mm for desktop MRI scanners in accordance with the American Association of Physicists in Medicine (AAPM) report no. 28. We scanned the phantoms with three permanent magnet 0.5 T desktop MRI systems, measured the MRI image parameters, and analysed imaging quality by comparing the data with the AAPM criteria and Chinese national standards. Image parameters included: resonance frequency, high contrast spatial resolution, low contrast object detectability, slice thickness, geometrical distortion, signal-to-noise ratio (SNR), and image uniformity. The image parameters of three desktop MRI machines could be measured using our specially designed phantoms, and most parameters were in line with MRI quality control criterion, including: resonance frequency, high contrast spatial resolution, low contrast object detectability, slice thickness, geometrical distortion, image uniformity and slice position accuracy. However, SNR was significantly lower than in some references. The imaging test and quality control are necessary for desktop MRI systems, and should be performed with the applicable phantom and corresponding standards.

Gain, noise, and contrast sensitivity of linear visual neurons

NASA Technical Reports Server (NTRS)

Watson, Andrew B.

1990-01-01

Contrast sensitivity is a measure of the ability of an observer to detect contrast signals of particular spatial and temporal frequencies. A formal definition of contrast sensitivity that can be applied to individual linear visual neurons is derived. A neuron is modeled by a contrast transfer function and its modulus, contrast gain, and by a noise power spectrum. The distributions of neural responses to signal and blank presentations are derived, and from these, a definition of contrast sensitivity is obtained. This formal definition may be used to relate the sensitivities of various populations of neurons, and to relate the sensitivities of neurons to that of the behaving animal.

A spherical model for orientation and spatial-frequency tuning in a cortical hypercolumn.

PubMed Central

Bressloff, Paul C; Cowan, Jack D

2003-01-01

A theory is presented of the way in which the hypercolumns in primary visual cortex (V1) are organized to detect important features of visual images, namely local orientation and spatial-frequency. Given the existence in V1 of dual maps for these features, both organized around orientation pinwheels, we constructed a model of a hypercolumn in which orientation and spatial-frequency preferences are represented by the two angular coordinates of a sphere. The two poles of this sphere are taken to correspond, respectively, to high and low spatial-frequency preferences. In Part I of the paper, we use mean-field methods to derive exact solutions for localized activity states on the sphere. We show how cortical amplification through recurrent interactions generates a sharply tuned, contrast-invariant population response to both local orientation and local spatial frequency, even in the case of a weakly biased input from the lateral geniculate nucleus (LGN). A major prediction of our model is that this response is non-separable with respect to the local orientation and spatial frequency of a stimulus. That is, orientation tuning is weaker around the pinwheels, and there is a shift in spatial-frequency tuning towards that of the closest pinwheel at non-optimal orientations. In Part II of the paper, we demonstrate that a simple feed-forward model of spatial-frequency preference, unlike that for orientation preference, does not generate a faithful representation when amplified by recurrent interactions in V1. We then introduce the idea that cortico-geniculate feedback modulates LGN activity to generate a faithful representation, thus providing a new functional interpretation of the role of this feedback pathway. Using linear filter theory, we show that if the feedback from a cortical cell is taken to be approximately equal to the reciprocal of the corresponding feed-forward receptive field (in the two-dimensional Fourier domain), then the mismatch between the feed-forward and cortical frequency representations is eliminated. We therefore predict that cortico-geniculate feedback connections innervate the LGN in a pattern determined by the orientation and spatial-frequency biases of feed-forward receptive fields. Finally, we show how recurrent cortical interactions can generate cross-orientation suppression. PMID:14561324

Improvement of axial excitation confinement in temporal focusing-based multiphoton microscopy via spatially modulated illumination

NASA Astrophysics Data System (ADS)

Chang, Chia-Yuan; Chen, Shean-Jen

2017-02-01

Conventional temporal focusing-based multiphoton excitation microscopy (TFMPEM) can offer widefield optical sectioning with an axial excitation confinement (AEC) of a few microns. Herein, a developed TFMPEM with a digital micromirror device (DMD), acting as the blazed grating for light spatial dispersion and simultaneous patterned illumination, has been extended to implement spatially modulated illumination at structured frequency and orientation. By implementing the spatially modulated illumination, the beam coverage at the back-focal aperture of the objective lens can be increased. As a result, the AEC can be condensed from 3.0 μm to 1.5 μm in full width at half maximum for a 2-fold enhancement. Furthermore, by using HiLo microscopy with two structured illuminations at the same spatial frequency but different orientation, biotissue images according to the structured illumination with condensed AEC is obviously superior in contrast and scattering suppression.

Effects of background motion on eye-movement information.

PubMed

Nakamura, S

1997-02-01

The effect of background stimulus on eye-movement information was investigated by analyzing the underestimation of the target velocity during pursuit eye movement (Aubert-Fleishl paradox). In the experiment, a striped pattern with various brightness contrasts and spatial frequencies was used as a background stimulus, which was moved at various velocities. Analysis showed that the perceived velocity of the pursuit target, which indicated the magnitudes of eye-movement information, decreased when the background stripes moved in the same direction as eye movement at higher velocities and increased when the background moved in the opposite direction. The results suggest that the eye-movement information varied as a linear function of the velocity of the motion of the background retinal image (optic flow). In addition, the effectiveness of optic flow on eye-movement information was determined by the attributes of the background stimulus such as the brightness contrast or the spatial frequency of the striped pattern.

Contrast sensitivity to spatial gratings in moderate and dim light conditions in patients with diabetes in the absence of diabetic retinopathy.

PubMed

Safi, Sare; Rahimi, Anoushiravan; Raeesi, Afsaneh; Safi, Hamid; Aghazadeh Amiri, Mohammad; Malek, Mojtaba; Yaseri, Mehdi; Haeri, Mohammad; Middleton, Frank A; Solessio, Eduardo; Ahmadieh, Hamid

2017-01-01

To evaluate the ability of contrast sensitivity (CS) to discriminate loss of visual function in diabetic subjects with no clinical signs of retinopathy relative to that of normal subjects. In this prospective cross-sectional study, we measured CS in 46 diabetic subjects with a mean age of 48±6 years, a best-corrected visual acuity of 20/20 and no signs of diabetic retinopathy. The CS in these subjects was compared with CS measurements in 46 normal control subjects at four spatial frequencies (3, 6, 12, 18 cycles per degree) under moderate (500 lux) and dim (less than 2 lux) background light conditions. CS was approximately 0.16 log units lower in patients with diabetes relative to controls both in moderate and in dim background light conditions. Logistic regression classification and receiver operating characteristic curve analysis indicated that CS analysis using two light conditions was more accurate (0.78) overall compared with CS analysis using only a single illumination condition (accuracy values were 0.67 and 0.70 in moderate and dim light conditions, respectively). Our results showed that patients with diabetes without clinical signs of retinopathy exhibit a uniform loss in CS at all spatial frequencies tested. Measuring the loss in CS at two spatial frequencies (3 and 6 cycles per degree) and two light conditions (moderate and dim) is sufficiently robust to classify diabetic subjects with no retinopathy versus control subjects.

Enhancing resolution and contrast in second-harmonic generation microscopy using an advanced maximum likelihood estimation restoration method

NASA Astrophysics Data System (ADS)

Sivaguru, Mayandi; Kabir, Mohammad M.; Gartia, Manas Ranjan; Biggs, David S. C.; Sivaguru, Barghav S.; Sivaguru, Vignesh A.; Berent, Zachary T.; Wagoner Johnson, Amy J.; Fried, Glenn A.; Liu, Gang Logan; Sadayappan, Sakthivel; Toussaint, Kimani C.

2017-02-01

Second-harmonic generation (SHG) microscopy is a label-free imaging technique to study collagenous materials in extracellular matrix environment with high resolution and contrast. However, like many other microscopy techniques, the actual spatial resolution achievable by SHG microscopy is reduced by out-of-focus blur and optical aberrations that degrade particularly the amplitude of the detectable higher spatial frequencies. Being a two-photon scattering process, it is challenging to define a point spread function (PSF) for the SHG imaging modality. As a result, in comparison with other two-photon imaging systems like two-photon fluorescence, it is difficult to apply any PSF-engineering techniques to enhance the experimental spatial resolution closer to the diffraction limit. Here, we present a method to improve the spatial resolution in SHG microscopy using an advanced maximum likelihood estimation (AdvMLE) algorithm to recover the otherwise degraded higher spatial frequencies in an SHG image. Through adaptation and iteration, the AdvMLE algorithm calculates an improved PSF for an SHG image and enhances the spatial resolution by decreasing the full-width-at-halfmaximum (FWHM) by 20%. Similar results are consistently observed for biological tissues with varying SHG sources, such as gold nanoparticles and collagen in porcine feet tendons. By obtaining an experimental transverse spatial resolution of 400 nm, we show that the AdvMLE algorithm brings the practical spatial resolution closer to the theoretical diffraction limit. Our approach is suitable for adaptation in micro-nano CT and MRI imaging, which has the potential to impact diagnosis and treatment of human diseases.

Reducing Spatial Uncertainty Through Attentional Cueing Improves Contrast Sensitivity in Regions of the Visual Field With Glaucomatous Defects

PubMed Central

Phu, Jack; Kalloniatis, Michael; Khuu, Sieu K.

2018-01-01

Purpose Current clinical perimetric test paradigms present stimuli randomly to various locations across the visual field (VF), inherently introducing spatial uncertainty, which reduces contrast sensitivity. In the present study, we determined the extent to which spatial uncertainty affects contrast sensitivity in glaucoma patients by minimizing spatial uncertainty through attentional cueing. Methods Six patients with open-angle glaucoma and six healthy subjects underwent laboratory-based psychophysical testing to measure contrast sensitivity at preselected locations at two eccentricities (9.5° and 17.5°) with two stimulus sizes (Goldmann sizes III and V) under different cueing conditions: 1, 2, 4, or 8 points verbally cued. Method of Constant Stimuli and a single-interval forced-choice procedure were used to generate frequency of seeing (FOS) curves at locations with and without VF defects. Results At locations with VF defects, cueing minimizes spatial uncertainty and improves sensitivity under all conditions. The effect of cueing was maximal when one point was cued, and rapidly diminished when more points were cued (no change to baseline with 8 points cued). The slope of the FOS curve steepened with reduced spatial uncertainty. Locations with normal sensitivity in glaucomatous eyes had similar performance to that of healthy subjects. There was a systematic increase in uncertainty with the depth of VF loss. Conclusions Sensitivity measurements across the VF are negatively affected by spatial uncertainty, which increases with greater VF loss. Minimizing uncertainty can improve sensitivity at locations of deficit. Translational Relevance Current perimetric techniques introduce spatial uncertainty and may therefore underestimate sensitivity in regions of VF loss. PMID:29600116

The dependence of binocular contrast sensitivities on binocular single vision in normal and amblyopic human subjects

PubMed Central

Hood, A S; Morrison, J D

2002-01-01

We have measured monocular and binocular contrast sensitivities in response to medium to high spatial frequencies of vertical sinusoidal grating patterns in normal subjects, anisometropic amblyopes, strabismic amblyopes and non-amblyopic esotropes. On binocular viewing, contrast sensitivities were slightly but significantly increased in normal subjects, markedly increased in anisometropes and esotropes with anomalous binocular single vision (BSV) and significantly reduced in esotropes and exotropes without BSV. Application of a prismatic correction to the strabismic eye in order to achieve bifoveal stimulation resulted in a significant reduction in contrast sensitivity in esotropes with and without anomalous BSV, in exotropes and in non-amblyopic esotropes. Control experiments in normal subjects with monocular viewing showed that degradative effects of the prism occurred only with high prism powers and at high spatial frequencies, thus establishing that the reduced contrast sensitivities were the consequence of bifoveal stimulation rather than optical degradation. Displacement of the image of the grating pattern by 2 deg in normal subjects and anisometropes by a dichoptic method to simulate a small angle esotropia had no effect on the contrast sensitivities recorded through the companion eye. By contrast, esotropes showed similar reductions in contrast sensitivity to those obtained with the prism experiments, confirming a fundamental difference between subjects with normal and abnormal ocular alignments. The results have thus established a suppressive action of the fovea of the amblyopic eye acting on the companion, non-amblyopic eye and indicate that correction of ocular misalignments in adult esotropes may be disadvantageous to binocular visual performance. PMID:11956347

Phase-and-amplitude recovery from a single phase-contrast image using partially spatially coherent x-ray radiation

NASA Astrophysics Data System (ADS)

Beltran, Mario A.; Paganin, David M.; Pelliccia, Daniele

2018-05-01

A simple method of phase-and-amplitude extraction is derived that corrects for image blurring induced by partially spatially coherent incident illumination using only a single intensity image as input. The method is based on Fresnel diffraction theory for the case of high Fresnel number, merged with the space-frequency description formalism used to quantify partially coherent fields and assumes the object under study is composed of a single-material. A priori knowledge of the object’s complex refractive index and information obtained by characterizing the spatial coherence of the source is required. The algorithm was applied to propagation-based phase-contrast data measured with a laboratory-based micro-focus x-ray source. The blurring due to the finite spatial extent of the source is embedded within the algorithm as a simple correction term to the so-called Paganin algorithm and is also numerically stable in the presence of noise.

Human speed perception is contrast dependent

NASA Technical Reports Server (NTRS)

Stone, Leland S.; Thompson, Peter

1992-01-01

When two parallel gratings moving at the same speed are presented simultaneously, the lower-contrast grating appears slower. This misperception is evident across a wide range of contrasts (2.5-50 percent) and does not appear to saturate. On average, a 70 percent contrast grating must be slowed by 35 percent to match a 10 percent contrast grating moving at 2 deg/sec (N = 6). Furthermore, the effect is largely independent of the absolute contrast level and is a quasilinear function of log contrast ratio. A preliminary parametric study shows that, although spatial frequency has little effect, relative orientation is important. Finally, the misperception of relative speed appears lessened when the stimuli to be matched are presented sequentially.

Abnormal spatial frequency channels in esotropic cats.

PubMed

Holopigian, K; Blake, R

1984-01-01

A noise masking paradigm was used to measure spatial tuning for the deviating and nondeviating eyes of two esotropic cats and for one eye of a control cat. With increasing noise contrast, masking grew more slowly for both the deviating and the nondeviating eyes of the esotropic cats than for the control cat; apparently, contrast coding is impaired for both eyes of the esotropic cats. Masking with band-reject filtered noise indicated that detection channels for either eye of the esotropic cats were twice as broadly tuned as those for the control cat. In a subsequent experiment, the spatial tuning characteristics of two human esotropes were found to be normal, indicating a fundamental difference between human esotropia and esotropia induced in cats by the section of an eye muscle.

Orientation masking and cross-orientation suppression (XOS): implications for estimates of filter bandwidth.

PubMed

Meese, Tim S; Holmes, David J

2010-10-01

Most contemporary models of spatial vision include a cross-oriented route to suppression (masking from a broadly tuned inhibitory pool), which is most potent at low spatial and high temporal frequencies (T. S. Meese & D. J. Holmes, 2007). The influence of this pathway can elevate orientation-masking functions without exciting the target mechanism, and because early psychophysical estimates of filter bandwidth did not accommodate this, it is likely that they have been overestimated for this corner of stimulus space. Here we show that a transient 40% contrast mask causes substantial binocular threshold elevation for a transient vertical target, and this declines from a mask orientation of 0° to about 40° (indicating tuning), and then more gently to 90°, where it remains at a factor of ∼4. We also confirm that cross-orientation masking is diminished or abolished at high spatial frequencies and for sustained temporal modulation. We fitted a simple model of pedestal masking and cross-orientation suppression (XOS) to our data and those of G. C. Phillips and H. R. Wilson (1984) and found the dependency of orientation bandwidth on spatial frequency to be much less than previously supposed. An extension of our linear spatial pooling model of contrast gain control and dilution masking (T. S. Meese & R. J. Summers, 2007) is also shown to be consistent with our results using filter bandwidths of ±20°. Both models include tightly and broadly tuned components of divisive suppression. More generally, because XOS and/or dilution masking can affect the shape of orientation-masking curves, we caution that variations in bandwidth estimates might reflect variations in processes that have nothing to do with filter bandwidth.

Quantitative spatial frequency fluorescence imaging in the sub-diffusive domain for image-guided glioma resection

PubMed Central

Sibai, Mira; Veilleux, Israel; Elliott, Jonathan T.; Leblond, Frederic; Wilson, Brian C.

2015-01-01

Intraoperative 5- aminolevulinic acid induced-Protoporphyrin IX (PpIX) fluorescence guidance enables maximum safe resection of glioblastomas by providing surgeons with real-time tumor optical contrast. However, visual assessment of PpIX fluorescence is subjective and limited by the distorting effects of light attenuation and tissue autofluorescence. We have previously shown that non-invasive point measurements of absolute PpIX concentration identifies residual tumor that is otherwise non-detectable. Here, we extend this approach to wide-field quantitative fluorescence imaging by implementing spatial frequency domain imaging to recover tissue optical properties across the field-of-view in phantoms and ex vivo tissue. PMID:26713206

Describing contrast across scales

NASA Astrophysics Data System (ADS)

Syed, Sohaib Ali; Iqbal, Muhammad Zafar; Riaz, Muhammad Mohsin

2017-06-01

Due to its sensitive nature against illumination and noise distributions, contrast is not widely used for image description. On the contrary, the human perception of contrast along different spatial frequency bandwidths provides a powerful discriminator function that can be modeled in a robust manner against local illumination. Based upon this observation, a dense local contrast descriptor is proposed and its potential in different applications of computer vision is discussed. Extensive experiments reveal that this simple yet effective description performs well in comparison with state of the art image descriptors. We also show the importance of this description in multiresolution pansharpening framework.

Real-time and quantitative isotropic spatial resolution susceptibility imaging for magnetic nanoparticles

NASA Astrophysics Data System (ADS)

Pi, Shiqiang; Liu, Wenzhong; Jiang, Tao

2018-03-01

The magnetic transparency of biological tissue allows the magnetic nanoparticle (MNP) to be a promising functional sensor and contrast agent. The complex susceptibility of MNPs, strongly influenced by particle concentration, excitation magnetic field and their surrounding microenvironment, provides significant implications for biomedical applications. Therefore, magnetic susceptibility imaging of high spatial resolution will give more detailed information during the process of MNP-aided diagnosis and therapy. In this study, we present a novel spatial magnetic susceptibility extraction method for MNPs under a gradient magnetic field, a low-frequency drive magnetic field, and a weak strength high-frequency magnetic field. Based on this novel method, a magnetic particle susceptibility imaging (MPSI) of millimeter-level spatial resolution (<3 mm) was achieved using our homemade imaging system. Corroborated by the experimental results, the MPSI shows real-time (1 s per frame acquisition) and quantitative abilities, and isotropic high resolution.

Frequency clusters in self-excited dust density waves

NASA Astrophysics Data System (ADS)

Menzel, Kristoffer O.; Arp, Oliver; Piel, Alexander

2010-11-01

Self-excited dust density waves were studied under microgravity conditions. Their non-sinusoidal shape and high degrees of modulation suggests that nonlinear effects play an important role in their spatio-temporal dynamics. The resulting complex wave pattern is analyzed in great detail by means of the Hilbert transform, which provides instantaneous wave attributes, such as the phase and the frequency. Our analysis showed that the spatial frequency distribution of the DDWs is usually not constant over the dust cloud. In contrast, the wave field is divided into regions of different but almost constant frequencies [1]. The boundaries of these so-called frequency clusters coincide with the locations of phase defects in the wave field. It is found that the size of the clusters depends on the strength of spatial gradients in the plasma parameters. We attribute the formation of frequency clusters to synchronization phenomena as a consequence of the nonlinear character of the wave.[1] K. O. Menzel, O. Arp, A.Piel, Phys. Rev. Lett. 104, 235002 (2010)

Spatial patterns in the effects of fire on savanna vegetation three-dimensional structure.

PubMed

Levick, Shaun R; Asner, Gregory P; Smit, Izak P J

2012-12-01

Spatial variability in the effects of fire on savanna vegetation structure is seldom considered in ecology, despite the inherent heterogeneity of savanna landscapes. Much has been learned about the effects of fire on vegetation structure from long-term field experiments, but these are often of limited spatial extent and do not encompass different hillslope catena elements. We mapped vegetation three-dimensional (3-D) structure over 21 000 ha in nine savanna landscapes (six on granite, three on basalt), each with contrasting long-term fire histories (higher and lower fire frequency), as defined from a combination of satellite imagery and 67 years of management records. Higher fire frequency areas contained less woody canopy cover than their lower fire frequency counterparts in all landscapes, and woody cover reduction increased linearly with increasing difference in fire frequency (r2 = 0.58, P = 0.004). Vegetation height displayed a more heterogeneous response to difference in fire frequency, with taller canopies present in the higher fire frequency areas of the wetter sites. Vegetation 3-D structural differences between areas of higher and lower fire frequency differed between geological substrates and varied spatially across hillslopes. Fire had the greatest relative impact on vegetation structure on nutrient-rich basalt substrates, and it imparted different structural responses upon vegetation in upland, midslope, and lowland topographic positions. These results highlight the complexity of fire vegetation relationships in savanna systems, and they suggest that underlying landscape heterogeneity needs more explicit incorporation into fire management policies.

Parameters influencing focalization spot in time reversal of acoustic waves

NASA Astrophysics Data System (ADS)

Zophoniasson, Harald; Bolzmacher, Christian; Hafez, Moustafa

2015-05-01

Time reversal is an approach that can be used to focus acoustic waves in a particular location on a surface, allowing a multitouch tactile feedback interaction. The spatial resolution in this case depends on several parameters, such as geometrical parameters, frequency used and material properties, described by the Lamb wave theory. This paper highlights the impact of frequency, geometrical parameters such as plate thickness and transducer's surface on the focused spot dimensions. In this paper a study of the influence of the plate's thickness and the frequency bandwidth used in the focusing process is presented. It is also shown that the dimension of the piezoelectric diaphragms used has little influence on the spatial resolution. Resonant behavior of the plate and its implication on focus point dimension and focalization contrast were investigated.

Relationship between BOLD amplitude and pattern classification of orientation-selective activity in the human visual cortex.

PubMed

Tong, Frank; Harrison, Stephenie A; Dewey, John A; Kamitani, Yukiyasu

2012-11-15

Orientation-selective responses can be decoded from fMRI activity patterns in the human visual cortex, using multivariate pattern analysis (MVPA). To what extent do these feature-selective activity patterns depend on the strength and quality of the sensory input, and might the reliability of these activity patterns be predicted by the gross amplitude of the stimulus-driven BOLD response? Observers viewed oriented gratings that varied in luminance contrast (4, 20 or 100%) or spatial frequency (0.25, 1.0 or 4.0 cpd). As predicted, activity patterns in early visual areas led to better discrimination of orientations presented at high than low contrast, with greater effects of contrast found in area V1 than in V3. A second experiment revealed generally better decoding of orientations at low or moderate as compared to high spatial frequencies. Interestingly however, V1 exhibited a relative advantage at discriminating high spatial frequency orientations, consistent with the finer scale of representation in the primary visual cortex. In both experiments, the reliability of these orientation-selective activity patterns was well predicted by the average BOLD amplitude in each region of interest, as indicated by correlation analyses, as well as decoding applied to a simple model of voxel responses to simulated orientation columns. Moreover, individual differences in decoding accuracy could be predicted by the signal-to-noise ratio of an individual's BOLD response. Our results indicate that decoding accuracy can be well predicted by incorporating the amplitude of the BOLD response into simple simulation models of cortical selectivity; such models could prove useful in future applications of fMRI pattern classification. Copyright © 2012 Elsevier Inc. All rights reserved.

Relationship between BOLD amplitude and pattern classification of orientation-selective activity in the human visual cortex

PubMed Central

Tong, Frank; Harrison, Stephenie A.; Dewey, John A.; Kamitani, Yukiyasu

2012-01-01

Orientation-selective responses can be decoded from fMRI activity patterns in the human visual cortex, using multivariate pattern analysis (MVPA). To what extent do these feature-selective activity patterns depend on the strength and quality of the sensory input, and might the reliability of these activity patterns be predicted by the gross amplitude of the stimulus-driven BOLD response? Observers viewed oriented gratings that varied in luminance contrast (4, 20 or 100%) or spatial frequency (0.25, 1.0 or 4.0 cpd). As predicted, activity patterns in early visual areas led to better discrimination of orientations presented at high than low contrast, with greater effects of contrast found in area V1 than in V3. A second experiment revealed generally better decoding of orientations at low or moderate as compared to high spatial frequencies. Interestingly however, V1 exhibited a relative advantage at discriminating high spatial frequency orientations, consistent with the finer scale of representation in the primary visual cortex. In both experiments, the reliability of these orientation-selective activity patterns was well predicted by the average BOLD amplitude in each region of interest, as indicated by correlation analyses, as well as decoding applied to a simple model of voxel responses to simulated orientation columns. Moreover, individual differences in decoding accuracy could be predicted by the signal-to-noise ratio of an individual's BOLD response. Our results indicate that decoding accuracy can be well predicted by incorporating the amplitude of the BOLD response into simple simulation models of cortical selectivity; such models could prove useful in future applications of fMRI pattern classification. PMID:22917989

Impact of Primary Spherical Aberration, Spatial Frequency and Stiles Crawford Apodization on Wavefront determined Refractive Error: A Computational Study

PubMed Central

Xu, Renfeng; Bradley, Arthur; Thibos, Larry N.

2013-01-01

Purpose We tested the hypothesis that pupil apodization is the basis for central pupil bias of spherical refractions in eyes with spherical aberration. Methods We employed Fourier computational optics in which we vary spherical aberration levels, pupil size, and pupil apodization (Stiles Crawford Effect) within the pupil function, from which point spread functions and optical transfer functions were computed. Through-focus analysis determined the refractive correction that optimized retinal image quality. Results For a large pupil (7 mm), as spherical aberration levels increase, refractions that optimize the visual Strehl ratio mirror refractions that maximize high spatial frequency modulation in the image and both focus a near paraxial region of the pupil. These refractions are not affected by Stiles Crawford Effect apodization. Refractions that optimize low spatial frequency modulation come close to minimizing wavefront RMS, and vary with level of spherical aberration and Stiles Crawford Effect. In the presence of significant levels of spherical aberration (e.g. C40 = 0.4 µm, 7mm pupil), low spatial frequency refractions can induce −0.7D myopic shift compared to high SF refraction, and refractions that maximize image contrast of a 3 cycle per degree square-wave grating can cause −0.75D myopic drift relative to refractions that maximize image sharpness. Discussion Because of small depth of focus associated with high spatial frequency stimuli, the large change in dioptric power across the pupil caused by spherical aberration limits the effective aperture contributing to the image of high spatial frequencies. Thus, when imaging high spatial frequencies, spherical aberration effectively induces an annular aperture defining that portion of the pupil contributing to a well-focused image. As spherical focus is manipulated during the refraction procedure, the dimensions of the annular aperture change. Image quality is maximized when the inner radius of the induced annulus falls to zero, thus defining a circular near paraxial region of the pupil that determines refraction outcome. PMID:23683093

Role of high-order aberrations in senescent changes in spatial vision

DOE Office of Scientific and Technical Information (OSTI.GOV)

Elliot, S; Choi, S S; Doble, N

2009-01-06

The contributions of optical and neural factors to age-related losses in spatial vision are not fully understood. We used closed-loop adaptive optics to test the visual benefit of correcting monochromatic high-order aberrations (HOAs) on spatial vision for observers ranging in age from 18-81 years. Contrast sensitivity was measured monocularly using a two-alternative forced choice (2AFC) procedure for sinusoidal gratings over 6 mm and 3 mm pupil diameters. Visual acuity was measured using a spatial 4AFC procedure. Over a 6 mm pupil, young observers showed a large benefit of AO at high spatial frequencies, whereas older observers exhibited the greatest benefitmore » at middle spatial frequencies, plus a significantly larger increase in visual acuity. When age-related miosis is controlled, young and old observers exhibited a similar benefit of AO for spatial vision. An increase in HOAs cannot account for the complete senescent decline in spatial vision. These results may indicate a larger role of additional optical factors when the impact of HOAs is removed, but also lend support for the importance of neural factors in age-related changes in spatial vision.« less

Detection of admittivity anomaly on high-contrast heterogeneous backgrounds using frequency difference EIT.

PubMed

Jang, J; Seo, J K

2015-06-01

This paper describes a multiple background subtraction method in frequency difference electrical impedance tomography (fdEIT) to detect an admittivity anomaly from a high-contrast background conductivity distribution. The proposed method expands the use of the conventional weighted frequency difference EIT method, which has been used limitedly to detect admittivity anomalies in a roughly homogeneous background. The proposed method can be viewed as multiple weighted difference imaging in fdEIT. Although the spatial resolutions of the output images by fdEIT are very low due to the inherent ill-posedness, numerical simulations and phantom experiments of the proposed method demonstrate its feasibility to detect anomalies. It has potential application in stroke detection in a head model, which is highly heterogeneous due to the skull.

Contrast sensitivity to spatial gratings in moderate and dim light conditions in patients with diabetes in the absence of diabetic retinopathy

PubMed Central

Safi, Sare; Rahimi, Anoushiravan; Raeesi, Afsaneh; Safi, Hamid; Aghazadeh Amiri, Mohammad; Malek, Mojtaba; Yaseri, Mehdi; Haeri, Mohammad; Middleton, Frank A; Solessio, Eduardo; Ahmadieh, Hamid

2017-01-01

Objective To evaluate the ability of contrast sensitivity (CS) to discriminate loss of visual function in diabetic subjects with no clinical signs of retinopathy relative to that of normal subjects. Research design and methods In this prospective cross-sectional study, we measured CS in 46 diabetic subjects with a mean age of 48±6 years, a best-corrected visual acuity of 20/20 and no signs of diabetic retinopathy. The CS in these subjects was compared with CS measurements in 46 normal control subjects at four spatial frequencies (3, 6, 12, 18 cycles per degree) under moderate (500 lux) and dim (less than 2 lux) background light conditions. Results CS was approximately 0.16 log units lower in patients with diabetes relative to controls both in moderate and in dim background light conditions. Logistic regression classification and receiver operating characteristic curve analysis indicated that CS analysis using two light conditions was more accurate (0.78) overall compared with CS analysis using only a single illumination condition (accuracy values were 0.67 and 0.70 in moderate and dim light conditions, respectively). Conclusions Our results showed that patients with diabetes without clinical signs of retinopathy exhibit a uniform loss in CS at all spatial frequencies tested. Measuring the loss in CS at two spatial frequencies (3 and 6 cycles per degree) and two light conditions (moderate and dim) is sufficiently robust to classify diabetic subjects with no retinopathy versus control subjects. PMID:28878937

Calibration of a Spatial-Temporal Discrimination Model from Forward, Simultaneous, and Backward Masking

NASA Technical Reports Server (NTRS)

Ahumada, Albert J.; Beard, B. L.; Stone, Leland (Technical Monitor)

1997-01-01

We have been developing a simplified spatial-temporal discrimination model similar to our simplified spatial model in that masking is assumed to be a function of the local visible contrast energy. The overall spatial-temporal sensitivity of the model is calibrated to predict the detectability of targets on a uniform background. To calibrate the spatial-temporal integration functions that define local visible contrast energy, spatial-temporal masking data are required. Observer thresholds were measured (2IFC) for the detection of a 12 msec target stimulus in the presence of a 700 msec mask. Targets were 1, 3 or 9 c/deg sine wave gratings. Masks were either one of these gratings or two of them combined. The target was presented in 17 temporal positions with respect to the mask, including positions before, during and after the mask. Peak masking was found near mask onset and offset for 1 and 3 c/deg targets, while masking effects were more nearly uniform during the mask for the 9 c/deg target. As in the purely spatial case, the simplified model can not predict all the details of masking as a function of masking component spatial frequencies, but overall the prediction errors are small.

Highly angular dependent high-contrast grating mirror and its application for transverse-mode control of VCSELs

NASA Astrophysics Data System (ADS)

Inoue, Shunya; Kashino, Junichi; Matsutani, Akihiro; Ohtsuki, Hideo; Miyashita, Takahiro; Koyama, Fumio

2014-09-01

We report on the design and fabrication of a highly angular dependent high contrast grating (HCG) mirror. The modeling and experiment on amorphous-Si/SiO2 HCG clearly show the large angular dependence of reflectivity, which enables single transverse-mode operations of large-area VCSELs. We fabricate 980 nm VCSELs with the angular dependent HCG functioning as a spatial frequency filter. We obtained the single transverse mode operation of the fabricated device in contrast to conventional VCSELs with semiconductor multilayer mirrors.

Detection of briefly flashed sine-gratings in dark-adapted vision.

PubMed

Hofmann, M I; Barnes, C S; Hallett, P E

1990-01-01

Scotopic contrast sensitivity was measured near 20 deg retinal eccentricity for briefly flashed (10 or 20 msec) sine-wave gratings presented in darkness to dark-adapted subjects. For very low spatial frequencies (0.2-0.5 c/deg), curves of contrast sensitivity vs luminous energy show evidence of a low rod plateau and a high scotopic region, with an intervening transition at around -2 to -2.5 log scot td sec. Similar measurements made using long flashed or flickering gratings do not show a plateau. The results suggest that vision in the low rod region is impaired for brief flashes. For the briefly flashed stimuli, curves of contrast sensitivity versus spatial frequency in the low region were best fit by simple Gaussian functions with a variable centre size (sigma c = 0.5----0.25 deg), size decreasing with increasing flash energy. Difference-of-Gaussian functions with constant centre size (sigma c = 0.25 deg) provided the best fit in the high region. Overt input from the cones and grating area artefacts are excluded by appropriate tests. Calculation of photon flux into the receptive field centres suggests that signal compression in P alpha ganglion cells contributes to the low rod plateau.

Feasibility of spatial frequency-domain imaging for monitoring palpable breast lesions

NASA Astrophysics Data System (ADS)

Robbins, Constance M.; Raghavan, Guruprasad; Antaki, James F.; Kainerstorfer, Jana M.

2017-12-01

In breast cancer diagnosis and therapy monitoring, there is a need for frequent, noninvasive disease progression evaluation. Breast tumors differ from healthy tissue in mechanical stiffness as well as optical properties, which allows optical methods to detect and monitor breast lesions noninvasively. Spatial frequency-domain imaging (SFDI) is a reflectance-based diffuse optical method that can yield two-dimensional images of absolute optical properties of tissue with an inexpensive and portable system, although depth penetration is limited. Since the absorption coefficient of breast tissue is relatively low and the tissue is quite flexible, there is an opportunity for compression of tissue to bring stiff, palpable breast lesions within the detection range of SFDI. Sixteen breast tissue-mimicking phantoms were fabricated containing stiffer, more highly absorbing tumor-mimicking inclusions of varying absorption contrast and depth. These phantoms were imaged with an SFDI system at five levels of compression. An increase in absorption contrast was observed with compression, and reliable detection of each inclusion was achieved when compression was sufficient to bring the inclusion center within ˜12 mm of the phantom surface. At highest compression level, contrasts achieved with this system were comparable to those measured with single source-detector near-infrared spectroscopy.

Spatial contrast sensitivity vision loss in children with cortical visual impairment.

PubMed

Good, William V; Hou, Chuan; Norcia, Anthony M

2012-11-19

Although cortical visual impairment (CVI) is the leading cause of bilateral vision impairment in children in Western countries, little is known about the effects of CVI on visual function. The aim of this study was to compare visual evoked potential measures of contrast sensitivity and grating acuity in children with CVI with those of age-matched typically developing controls. The swept parameter visual evoked potential (sVEP) was used to measure contrast sensitivity and grating acuity in 34 children with CVI at 5 months to 5 years of age and in 16 age-matched control children. Contrast thresholds and spatial frequency thresholds (grating acuities) were derived by extrapolating the tuning functions to zero amplitude. These thresholds and maximal suprathreshold response amplitudes were compared between groups. Among 34 children with CVI, 30 had measurable but reduced contrast sensitivity with a median threshold of 10.8% (range 5.0%-30.0% Michelson), and 32 had measurable but reduced grating acuity with median threshold 0.49 logMAR (9.8 c/deg, range 5-14 c/deg). These thresholds were significantly reduced, compared with age-matched control children. In addition, response amplitudes over the entire sweep range for both measures were significantly diminished in children with CVI compared with those of control children. Our results indicate that spatial contrast sensitivity and response amplitudes are strongly affected by CVI. The substantial degree of loss in contrast sensitivity suggests that contrast is a sensitive measure for evaluating vision deficits in patients with CVI.

Perceptual Simulations and Linguistic Representations Have Differential Effects on Speeded Relatedness Judgments and Recognition Memory

PubMed Central

Tse, Chi-Shing; Kurby, Christopher A.; Du, Feng

2010-01-01

We examined the effect of spatial iconicity (a perceptual simulation of canonical locations of objects) and word-order frequency on language processing and episodic memory of orientation. Participants made speeded relatedness judgments to pairs of words presented in locations typical to their real world arrangements (e.g., ceiling on top and floor on bottom). They then engaged in a surprise orientation recognition task for the word pairs. We replicated Louwerse’s finding (2008) that word-order frequency has a stronger effect on semantic relatedness judgments than spatial iconicity. This is consistent with recent suggestions that linguistic representations have a stronger impact on immediate decisions about verbal materials than perceptual simulations. In contrast, spatial iconicity enhanced episodic memory of orientation to a greater extent than word-order frequency did. This new finding indicates that perceptual simulations have an important role in episodic memory. Results are discussed with respect to theories of perceptual representation and linguistic processing. PMID:19742388

Co-registered Frequency-Domain Photoacoustic Radar and Ultrasound System for Subsurface Imaging in Turbid Media

NASA Astrophysics Data System (ADS)

Dovlo, Edem; Lashkari, Bahman; Mandelis, Andreas

2016-03-01

Frequency-domain photoacoustic radar (FD-PAR) imaging of absorbers in turbid media and their comparison and/or validation as well as co-registration with their corresponding ultrasound (US) images are demonstrated in this paper. Also presented are the FD-PAR tomography and the effects of reducing the number of scan lines (or angles) on image quality, resolution, and contrast. The FD-PAR modality uses intensity-modulated (coded) continuous wave laser sources driven by frequency-swept (chirp) waveforms. The spatial cross-correlation function between the PA response and the reference signal used for laser source modulation produces the reconstructed image. Live animal testing is demonstrated, and images of comparable signal-to-noise ratio, contrast, and spatial resolution were obtained. Various image improvement techniques to further reduce absorber spread and artifacts in the images such as normalization, filtering, and amplification were also investigated. The co-registered image produced from the combined US and PA images provides more information than both images independently. The significance of this work lies in the fact that achieving PA imaging functionality on a commercial ultrasound instrument could accelerate its clinical acceptance and use. This work is aimed at functional PA imaging of small animals in vivo.

A novel x-ray detector design with higher DQE and reduced aliasing: Theoretical analysis of x-ray reabsoprtion in detector converter material

NASA Astrophysics Data System (ADS)

Nano, Tomi; Escartin, Terenz; Karim, Karim S.; Cunningham, Ian A.

2016-03-01

The ability to improve visualization of structural information in digital radiography without increasing radiation exposures requires improved image quality across all spatial frequencies, especially at high frequencies. The detective quantum efficiency (DQE) as a function of spatial frequency quantifies image quality given by an x-ray detector. We present a method of increasing DQE at high spatial frequencies by improving the modulation transfer function (MTF) and reducing noise aliasing. The Apodized Aperature Pixel (AAP) design uses a detector with micro-elements to synthesize desired pixels and provide higher DQE than conventional detector designs. A cascaded system analysis (CSA) that incorporates x-ray interactions is used for comparison of the theoretical MTF, noise power spectrum (NPS), and DQE. Signal and noise transfer through the converter material is shown to consist of correlated an uncorrelated terms. The AAP design was shown to improve the DQE of both material types that have predominantly correlated transfer (such as CsI) and predominantly uncorrelated transfer (such as Se). Improvement in the MTF by 50% and the DQE by 100% at the sampling cut-off frequency is obtained when uncorrelated transfer is prevalent through the converter material. Optimizing high-frequency DQE results in improved image contrast and visualization of small structures and fine-detail.

Effects of contrast on smooth pursuit eye movements.

PubMed

Spering, Miriam; Kerzel, Dirk; Braun, Doris I; Hawken, Michael J; Gegenfurtner, Karl R

2005-05-20

It is well known that moving stimuli can appear to move more slowly when contrast is reduced (P. Thompson, 1982). Here we address the question whether changes in stimulus contrast also affect smooth pursuit eye movements. Subjects were asked to smoothly track a moving Gabor patch. Targets varied in velocity (1, 8, and 15 deg/s), spatial frequency (0.1, 1, 4, and 8 c/deg), and contrast, ranging from just below individual thresholds to maximum contrast. Results show that smooth pursuit eye velocity gain rose significantly with increasing contrast. Below a contrast level of two to three times threshold, pursuit gain, acceleration, latency, and positional accuracy were severely impaired. Therefore, the smooth pursuit motor response shows the same kind of slowing at low contrast that was demonstrated in previous studies on perception.

Psychophysical and perceptual performance in a simulated-scotoma model of human eye injury

NASA Astrophysics Data System (ADS)

Brandeis, R.; Egoz, I.; Peri, D.; Sapiens, N.; Turetz, J.

2008-02-01

Macular scotomas, affecting visual functioning, characterize many eye and neurological diseases like AMD, diabetes mellitus, multiple sclerosis, and macular hole. In this work, foveal visual field defects were modeled, and their effects were evaluated on spatial contrast sensitivity and a task of stimulus detection and aiming. The modeled occluding scotomas, of different size, were superimposed on the stimuli presented on the computer display, and were stabilized on the retina using a mono Purkinje Eye-Tracker. Spatial contrast sensitivity was evaluated using square-wave grating stimuli, whose contrast thresholds were measured using the method of constant stimuli with "catch trials". The detection task consisted of a triple conjunctive visual search display of: size (in visual angle), contrast and background (simple, low-level features vs. complex, high-level features). Search/aiming accuracy as well as R.T. measures used for performance evaluation. Artificially generated scotomas suppressed spatial contrast sensitivity in a size dependent manner, similar to previous studies. Deprivation effect was dependent on spatial frequency, consistent with retinal inhomogeneity models. Stimulus detection time was slowed in complex background search situation more than in simple background. Detection speed was dependent on scotoma size and size of stimulus. In contrast, visually guided aiming was more sensitive to scotoma effect in simple background search situation than in complex background. Both stimulus aiming R.T. and accuracy (precision targeting) were impaired, as a function of scotoma size and size of stimulus. The data can be explained by models distinguishing between saliency-based, parallel and serial search processes, guiding visual attention, which are supported by underlying retinal as well as neural mechanisms.

Impaired recognition of facial emotions from low-spatial frequencies in Asperger syndrome.

PubMed

Kätsyri, Jari; Saalasti, Satu; Tiippana, Kaisa; von Wendt, Lennart; Sams, Mikko

2008-01-01

The theory of 'weak central coherence' [Happe, F., & Frith, U. (2006). The weak coherence account: Detail-focused cognitive style in autism spectrum disorders. Journal of Autism and Developmental Disorders, 36(1), 5-25] implies that persons with autism spectrum disorders (ASDs) have a perceptual bias for local but not for global stimulus features. The recognition of emotional facial expressions representing various different levels of detail has not been studied previously in ASDs. We analyzed the recognition of four basic emotional facial expressions (anger, disgust, fear and happiness) from low-spatial frequencies (overall global shapes without local features) in adults with an ASD. A group of 20 participants with Asperger syndrome (AS) was compared to a group of non-autistic age- and sex-matched controls. Emotion recognition was tested from static and dynamic facial expressions whose spatial frequency contents had been manipulated by low-pass filtering at two levels. The two groups recognized emotions similarly from non-filtered faces and from dynamic vs. static facial expressions. In contrast, the participants with AS were less accurate than controls in recognizing facial emotions from very low-spatial frequencies. The results suggest intact recognition of basic facial emotions and dynamic facial information, but impaired visual processing of global features in ASDs.

Experimental power spectral density analysis for mid- to high-spatial frequency surface error control.

PubMed

Hoyo, Javier Del; Choi, Heejoo; Burge, James H; Kim, Geon-Hee; Kim, Dae Wook

2017-06-20

The control of surface errors as a function of spatial frequency is critical during the fabrication of modern optical systems. A large-scale surface figure error is controlled by a guided removal process, such as computer-controlled optical surfacing. Smaller-scale surface errors are controlled by polishing process parameters. Surface errors of only a few millimeters may degrade the performance of an optical system, causing background noise from scattered light and reducing imaging contrast for large optical systems. Conventionally, the microsurface roughness is often given by the root mean square at a high spatial frequency range, with errors within a 0.5×0.5  mm local surface map with 500×500 pixels. This surface specification is not adequate to fully describe the characteristics for advanced optical systems. The process for controlling and minimizing mid- to high-spatial frequency surface errors with periods of up to ∼2-3  mm was investigated for many optical fabrication conditions using the measured surface power spectral density (PSD) of a finished Zerodur optical surface. Then, the surface PSD was systematically related to various fabrication process parameters, such as the grinding methods, polishing interface materials, and polishing compounds. The retraceable experimental polishing conditions and processes used to produce an optimal optical surface PSD are presented.

Directional antenna array (DAA) for communications, control, and data link protection

NASA Astrophysics Data System (ADS)

Molchanov, Pavlo A.; Contarino, Vincent M.

2013-06-01

A next generation of Smart antennas with point-to-point communication and jam, spoof protection capability by verification of spatial position is offered. A directional antenna array (DAA) with narrow irradiation beam provides counter terrorism protection for communications, data link, control and GPS. Communications are "invisible" to guided missiles because of 20 dB smaller irradiation outside the beam and spatial separation. This solution can be implemented with current technology. Directional antennas have higher gain and can be multi-frequency or have wide frequency band in contrast to phase antenna arrays. This multi-directional antenna array provides a multi-functional communication network and simultaneously can be used for command control, data link and GPS.

Genetic influence on contrast sensitivity in middle-aged male twins.

PubMed

Cronin-Golomb, Alice; Panizzon, Matthew S; Lyons, Michael J; Franz, Carol E; Grant, Michael D; Jacobson, Kristen C; Eisen, Seth A; Laudate, Thomas M; Kremen, William S

2007-07-01

Contrast sensitivity is strongly associated with daily functioning among older adults, but the genetic and environmental contributions to this ability are unknown. Using the classical twin method, we addressed this issue by examining contrast sensitivity at five spatial frequencies (1.5-18 cycles per degree) in 718 middle-aged male twins from the Vietnam Era Twin Study of Aging (VETSA). Heritability estimates were modest (14-38%), whereas individual-specific environmental influences accounted for 62-86% of the variance. Identifying the types of individual-specific events that impact contrast sensitivity may suggest interventions to modulate this ability and thereby improve overall quality of life as adults age.

Passive microlesion detection and mapping for treatment of hypertrophic cardiomyopathy

NASA Astrophysics Data System (ADS)

Zhu, Yiying I.; Miller, Douglas L.; Dou, Chunyan; Kripfgans, Oliver D.

2017-03-01

Intermittent high intensity ultrasound pulses with circulating contrast agent microbubbles can induce scattered microlesions of potential value for myocardial reduction therapy. This paper presents an in vitro setup imitating the treatment for monitoring development. A preclinical imaging system with a single element transducer, synchronization and receive-only imaging transducer array has been implemented on a research platform. Contrast agent microbubbles pumped in a dialysis tubing setup were exposed to high intensity focused ultrasound at 1.0/3.5 MHz center frequencies. Polystyrene spheres were employed as linear scatterers compared to contrast agents for system transfer function equalization. A cavitation mapping technique was employed to spatially locate and depict microbubble activity during treatment. For high acoustic pressure amplitudes a 5 dB difference between contrast agent and solid spheres was observed and spatially mapped. The in-plane resolution was 4.5 mm for axial and 1.5 mm laterally. In the future, this cavitation detection scheme will be applied to monitor in vivo microlesioning in real-time.

Spatial-frequency requirements for reading revisited

PubMed Central

Kwon, MiYoung; Legge, Gordon E.

2012-01-01

Blur is one of many visual factors that can limit reading in both normal and low vision. Legge et al. [Legge, G. E., Pelli, D. G., Rubin, G. S., & Schleske, M. M. (1985). Psychophysics of reading. I. Normal vision. Vision Research, 25, 239–252.] measured reading speed for text that was low-pass filtered with a range of cutoff spatial frequencies. Above 2 cycles per letter (CPL) reading speed was constant at its maximum level, but decreased rapidly for lower cutoff frequencies. It remains unknown why the critical cutoff for reading speed is near 2 CPL. The goal of the current study was to ask whether the spatial-frequency requirement for rapid reading is related to the effects of cutoff frequency on letter recognition and the size of the visual span. Visual span profiles were measured by asking subjects to recognize letters in trigrams (random strings of three letters) flashed for 150 ms at varying letter positions left and right of the fixation point. Reading speed was measured with Rapid Serial Visual Presentation (RSVP). The size of the visual span and reading speed were measured for low-pass filtered stimuli with cutoff frequencies from 0.8 to 8 CPL. Low-pass letter recognition data, obtained under similar testing conditions, were available from our previous study (Kwon & Legge, 2011). We found that the spatial-frequency requirement for reading is very similar to the spatial-frequency requirements for the size of the visual span and single letter recognition. The critical cutoff frequencies for reading speed, the size of the visual span and a contrast-invariant measure of letter recognition were all near 1.4 CPL, which is lower than the previous estimate of 2 CPL for reading speed. Although correlational in nature, these results are consistent with the hypothesis that the size of the visual span is closely linked to reading speed. PMID:22521659

Human speed perception is contrast dependent

NASA Technical Reports Server (NTRS)

Stone, Leland S.; Thompson, Peter

1992-01-01

When two parallel gratings moving at the same speed are presented simultaneously, the lower-contrast grating appears slower. This misperception is evident across a wide range of contrasts (2.5-50 percent) and does not appear to saturate (e.g. a 50 percent contrast grating appears slower than a 70 percent contrast grating moving at the same speed). On average, a 70 percent contrast grating must be slowed by 35 percent to match a 10 percent contrast grating moving at 2 deg/sec (N = 6). Furthermore, the effect is largely independent of the absolute contrast level and is a quasi-linear function of log contrast ratio. A preliminary parametric study shows that, although spatial frequency has little effect, relative orientation is important. Finally, the misperception of relative speed appears lessened when the stimuli to be matched are presented sequentially.

Color, contrast sensitivity, and the cone mosaic.

PubMed Central

Williams, D; Sekiguchi, N; Brainard, D

1993-01-01

This paper evaluates the role of various stages in the human visual system in the detection of spatial patterns. Contrast sensitivity measurements were made for interference fringe stimuli in three directions in color space with a psychophysical technique that avoided blurring by the eye's optics including chromatic aberration. These measurements were compared with the performance of an ideal observer that incorporated optical factors, such as photon catch in the cone mosaic, that influence the detection of interference fringes. The comparison of human and ideal observer performance showed that neural factors influence the shape as well as the height of the foveal contrast sensitivity function for all color directions, including those that involve luminance modulation. Furthermore, when optical factors are taken into account, the neural visual system has the same contrast sensitivity for isoluminant stimuli seen by the middle-wavelength-sensitive (M) and long-wavelength-sensitive (L) cones and isoluminant stimuli seen by the short-wavelength-sensitive (S) cones. Though the cone submosaics that feed these chromatic mechanisms have very different spatial properties, the later neural stages apparently have similar spatial properties. Finally, we review the evidence that cone sampling can produce aliasing distortion for gratings with spatial frequencies exceeding the resolution limit. Aliasing can be observed with gratings modulated in any of the three directions in color space we used. We discuss mechanisms that prevent aliasing in most ordinary viewing conditions. Images Fig. 1 Fig. 8 PMID:8234313

Digital image enhancement techniques used in some ERTS application problems. [geology, geomorphology, and oceanography

NASA Technical Reports Server (NTRS)

Goetz, A. F. H.; Billingsley, F. C.

1974-01-01

Enhancements discussed include contrast stretching, multiratio color displays, Fourier plane operations to remove striping and boosting MTF response to enhance high spatial frequency content. The use of each technique in a specific application in the fields of geology, geomorphology and oceanography is demonstrated.

Contrast Sensitivity in Army Aviator Candidates: Cycloplegia Effects and Population Norms

DTIC Science & Technology

1988-09-01

administration; this effect is highly significant statistically (F=169.41, df=l,104, p<.0001). The interaction of cycloplegia and spatial frequency is not...and universal acceptance. Deficits observed in visual acuity immediately alert the practitioner to the presence of an ametropia or other

Quantitative, depth-resolved determination of particle motion using multi-exposure, spatial frequency domain laser speckle imaging.

PubMed

Rice, Tyler B; Kwan, Elliott; Hayakawa, Carole K; Durkin, Anthony J; Choi, Bernard; Tromberg, Bruce J

2013-01-01

Laser Speckle Imaging (LSI) is a simple, noninvasive technique for rapid imaging of particle motion in scattering media such as biological tissue. LSI is generally used to derive a qualitative index of relative blood flow due to unknown impact from several variables that affect speckle contrast. These variables may include optical absorption and scattering coefficients, multi-layer dynamics including static, non-ergodic regions, and systematic effects such as laser coherence length. In order to account for these effects and move toward quantitative, depth-resolved LSI, we have developed a method that combines Monte Carlo modeling, multi-exposure speckle imaging (MESI), spatial frequency domain imaging (SFDI), and careful instrument calibration. Monte Carlo models were used to generate total and layer-specific fractional momentum transfer distributions. This information was used to predict speckle contrast as a function of exposure time, spatial frequency, layer thickness, and layer dynamics. To verify with experimental data, controlled phantom experiments with characteristic tissue optical properties were performed using a structured light speckle imaging system. Three main geometries were explored: 1) diffusive dynamic layer beneath a static layer, 2) static layer beneath a diffuse dynamic layer, and 3) directed flow (tube) submerged in a dynamic scattering layer. Data fits were performed using the Monte Carlo model, which accurately reconstructed the type of particle flow (diffusive or directed) in each layer, the layer thickness, and absolute flow speeds to within 15% or better.

Cue combination in a combined feature contrast detection and figure identification task.

PubMed

Meinhardt, Günter; Persike, Malte; Mesenholl, Björn; Hagemann, Cordula

2006-11-01

Target figures defined by feature contrast in spatial frequency, orientation or both cues had to be detected in Gabor random fields and their shape had to be identified in a dual task paradigm. Performance improved with increasing feature contrast and was strongly correlated among both tasks. Subjects performed significantly better with combined cues than with single cues. The improvement due to cue summation was stronger than predicted by the assumption of independent feature specific mechanisms, and increased with the performance level achieved with single cues until it was limited by ceiling effects. Further, cue summation was also strongly correlated among tasks: when there was benefit due to the additional cue in feature contrast detection, there was also benefit in figure identification. For the same performance level achieved with single cues, cue summation was generally larger in figure identification than in feature contrast detection, indicating more benefit when processes of shape and surface formation are involved. Our results suggest that cue combination improves spatial form completion and figure-ground segregation in noisy environments, and therefore leads to more stable object vision.

Local and Global Spatial Organization of Interaural Level Difference and Frequency Preferences in Auditory Cortex

PubMed Central

Panniello, Mariangela; King, Andrew J; Dahmen, Johannes C; Walker, Kerry M M

2018-01-01

Abstract Despite decades of microelectrode recordings, fundamental questions remain about how auditory cortex represents sound-source location. Here, we used in vivo 2-photon calcium imaging to measure the sensitivity of layer II/III neurons in mouse primary auditory cortex (A1) to interaural level differences (ILDs), the principal spatial cue in this species. Although most ILD-sensitive neurons preferred ILDs favoring the contralateral ear, neurons with either midline or ipsilateral preferences were also present. An opponent-channel decoder accurately classified ILDs using the difference in responses between populations of neurons that preferred contralateral-ear-greater and ipsilateral-ear-greater stimuli. We also examined the spatial organization of binaural tuning properties across the imaged neurons with unprecedented resolution. Neurons driven exclusively by contralateral ear stimuli or by binaural stimulation occasionally formed local clusters, but their binaural categories and ILD preferences were not spatially organized on a more global scale. In contrast, the sound frequency preferences of most neurons within local cortical regions fell within a restricted frequency range, and a tonotopic gradient was observed across the cortical surface of individual mice. These results indicate that the representation of ILDs in mouse A1 is comparable to that of most other mammalian species, and appears to lack systematic or consistent spatial order. PMID:29136122

Detection of fresh bruises in apples by structured-illumination reflectance imaging

NASA Astrophysics Data System (ADS)

Lu, Yuzhen; Li, Richard; Lu, Renfu

2016-05-01

Detection of fresh bruises in apples remains a challenging task due to the absence of visual symptoms and significant chemical alterations of fruit tissues during the initial stage after the fruit have been bruised. This paper reports on a new structured-illumination reflectance imaging (SIRI) technique for enhanced detection of fresh bruises in apples. Using a digital light projector engine, sinusoidally-modulated illumination at the spatial frequencies of 50, 100, 150 and 200 cycles/m was generated. A digital camera was then used to capture the reflectance images from `Gala' and `Jonagold' apples, immediately after they had been subjected to two levels of bruising by impact tests. A conventional three-phase demodulation (TPD) scheme was applied to the acquired images for obtaining the planar (direct component or DC) and amplitude (alternating component or AC) images. Bruises were identified in the amplitude images with varying image contrasts, depending on spatial frequency. The bruise visibility was further enhanced through post-processing of the amplitude images. Furthermore, three spiral phase transform (SPT)-based demodulation methods, using single and two images and two phase-shifted images, were proposed for obtaining AC images. Results showed that the demodulation methods greatly enhanced the contrast and spatial resolution of the AC images, making it feasible to detect the fresh bruises that, otherwise, could not be achieved by conventional imaging technique with planar or uniform illumination. The effectiveness of image enhancement, however, varied with spatial frequency. Both 2-image and 2-phase SPT methods achieved the performance similar to that by conventional TPD. SIRI technique has demonstrated the capability of detecting fresh bruises in apples, and it has the potential as a new imaging modality for enhancing food quality and safety detection.

Contextual Interactions in Grating Plaid Configurations Are Explained by Natural Image Statistics and Neural Modeling

PubMed Central

Ernst, Udo A.; Schiffer, Alina; Persike, Malte; Meinhardt, Günter

2016-01-01

Processing natural scenes requires the visual system to integrate local features into global object descriptions. To achieve coherent representations, the human brain uses statistical dependencies to guide weighting of local feature conjunctions. Pairwise interactions among feature detectors in early visual areas may form the early substrate of these local feature bindings. To investigate local interaction structures in visual cortex, we combined psychophysical experiments with computational modeling and natural scene analysis. We first measured contrast thresholds for 2 × 2 grating patch arrangements (plaids), which differed in spatial frequency composition (low, high, or mixed), number of grating patch co-alignments (0, 1, or 2), and inter-patch distances (1° and 2° of visual angle). Contrast thresholds for the different configurations were compared to the prediction of probability summation (PS) among detector families tuned to the four retinal positions. For 1° distance the thresholds for all configurations were larger than predicted by PS, indicating inhibitory interactions. For 2° distance, thresholds were significantly lower compared to PS when the plaids were homogeneous in spatial frequency and orientation, but not when spatial frequencies were mixed or there was at least one misalignment. Next, we constructed a neural population model with horizontal laminar structure, which reproduced the detection thresholds after adaptation of connection weights. Consistent with prior work, contextual interactions were medium-range inhibition and long-range, orientation-specific excitation. However, inclusion of orientation-specific, inhibitory interactions between populations with different spatial frequency preferences were crucial for explaining detection thresholds. Finally, for all plaid configurations we computed their likelihood of occurrence in natural images. The likelihoods turned out to be inversely related to the detection thresholds obtained at larger inter-patch distances. However, likelihoods were almost independent of inter-patch distance, implying that natural image statistics could not explain the crowding-like results at short distances. This failure of natural image statistics to resolve the patch distance modulation of plaid visibility remains a challenge to the approach. PMID:27757076

The edge of awareness: Mask spatial density, but not color, determines optimal temporal frequency for continuous flash suppression.

PubMed

Drewes, Jan; Zhu, Weina; Melcher, David

2018-01-01

The study of how visual processing functions in the absence of visual awareness has become a major research interest in the vision-science community. One of the main sources of evidence that stimuli that do not reach conscious awareness-and are thus "invisible"-are still processed to some degree by the visual system comes from studies using continuous flash suppression (CFS). Why and how CFS works may provide more general insight into how stimuli access awareness. As spatial and temporal properties of stimuli are major determinants of visual perception, we hypothesized that these properties of the CFS masks would be of significant importance to the achieved suppression depth. In previous studies however, the spatial and temporal properties of the masks themselves have received little study, and masking parameters vary widely across studies, making a metacomparison difficult. To investigate the factors that determine the effectiveness of CFS, we varied both the temporal frequency and the spatial density of Mondrian-style masks. We consistently found the longest suppression duration for a mask temporal frequency of around 6 Hz. In trials using masks with reduced spatial density, suppression was weaker and frequency tuning was less precise. In contrast, removing color reduced mask effectiveness but did not change the pattern of suppression strength as a function of frequency. Overall, this pattern of results stresses the importance of CFS mask parameters and is consistent with the idea that CFS works by disrupting the spatiotemporal mechanisms that underlie conscious access to visual input.

Hadamard-Encoded Multipulses for Contrast-Enhanced Ultrasound Imaging.

PubMed

Gong, Ping; Song, Pengfei; Chen, Shigao

2017-11-01

The development of contrast-enhanced ultrasound (CEUS) imaging offers great opportunities for new ultrasound clinical applications such as myocardial perfusion imaging and abdominal lesion characterization. In CEUS imaging, the contrast agents (i.e., microbubbles) are utilized to improve the contrast between blood and tissue based on their high nonlinearity under low ultrasound pressure. In this paper, we propose a new CEUS pulse sequence by combining Hadamard-encoded multipulses (HEM) with fundamental frequency bandpass filter (i.e., filter centered on transmit frequency). HEM consecutively emits multipulses encoded by a second-order Hadamard matrix in each of the two transmission events (i.e., pulse-echo events), as opposed to conventional CEUS methods which emit individual pulses in two separate transmission events (i.e., pulse inversion (PI), amplitude modulation (AM), and PIAM). In HEM imaging, the microbubble responses can be improved by the longer transmit pulse, and the tissue harmonics can be suppressed by the fundamental frequency filter, leading to significantly improved contrast-to-tissue ratio (CTR) and signal-to-noise ratio (SNR). In addition, the fast polarity change between consecutive coded pulse emissions excites strong nonlinear microbubble echoes, further enhancing the CEUS image quality. The spatial resolution of HEM image is compromised as compared to other microbubble imaging methods due to the longer transmit pulses and the lower imaging frequency (i.e., fundamental frequency). However, the resolution loss was shown to be negligible and could be offset by the significantly enhanced CTR, SNR, and penetration depth. These properties of HEM can potentially facilitate robust CEUS imaging for many clinical applications, especially for deep abdominal organs and heart.

Effects of ocular aberrations on contrast detection in noise.

PubMed

Liang, Bo; Liu, Rong; Dai, Yun; Zhou, Jiawei; Zhou, Yifeng; Zhang, Yudong

2012-08-06

We use adaptive optics (AO) techniques to manipulate the ocular aberrations and elucidate the effects of these ocular aberrations on contrast detection in a noisy background. The detectability of sine wave gratings at frequencies of 4, 8, and 16 circles per degree (cpd) was measured in a standard two-interval force-choice staircase procedure against backgrounds of various levels of white noise. The observer's ocular aberrations were either corrected with AO or left uncorrected. In low levels of external noise, contrast detection thresholds are always lowered by AO correction, whereas in high levels of external noise, they are generally elevated by AO correction. Higher levels of external noise are required to make this threshold elevation observable when signal spatial frequencies increase from 4 to 16 cpd. The linear-amplifier-model fit shows that mostly sampling efficiency and equivalent noise both decrease with AO correction. Our findings indicate that ocular aberrations could be beneficial for contrast detection in high-level noises. The implications of these findings are discussed.

Concept of contrast transfer function for edge illumination x-ray phase-contrast imaging and its comparison with the free-space propagation technique.

PubMed

Diemoz, Paul C; Vittoria, Fabio A; Olivo, Alessandro

2016-05-16

Previous studies on edge illumination (EI) X-ray phase-contrast imaging (XPCi) have investigated the nature and amplitude of the signal provided by this technique. However, the response of the imaging system to different object spatial frequencies was never explicitly considered and studied. This is required in order to predict the performance of a given EI setup for different classes of objects. To this scope, in the present work we derive analytical expressions for the contrast transfer function of an EI imaging system, using the approximation of near-field regime, and study its dependence upon the main experimental parameters. We then exploit these results to compare the frequency response of an EI system with respect of that of a free-space propagation XPCi one. The results achieved in this work can be useful for predicting the signals obtainable for different types of objects and also as a basis for new retrieval methods.

Temporal focusing-based widefield multiphoton microscopy with spatially modulated illumination for biotissue imaging.

PubMed

Chang, Chia-Yuan; Lin, Cheng-Han; Lin, Chun-Yu; Sie, Yong-Da; Hu, Yvonne Yuling; Tsai, Sheng-Feng; Chen, Shean-Jen

2018-01-01

A developed temporal focusing-based multiphoton excitation microscope (TFMPEM) has a digital micromirror device (DMD) which is adopted not only as a blazed grating for light spatial dispersion but also for patterned illumination simultaneously. Herein, the TFMPEM has been extended to implement spatially modulated illumination at structured frequency and orientation to increase the beam coverage at the back-focal aperture of the objective lens. The axial excitation confinement (AEC) of TFMPEM can be condensed from 3.0 μm to 1.5 μm for a 50 % improvement. By using the TFMPEM with HiLo technique as two structured illuminations at the same spatial frequency but different orientation, reconstructed biotissue images according to the condensed AEC structured illumination are shown obviously superior in contrast and better scattering suppression. Picture: TPEF images of the eosin-stained mouse cerebellar cortex by conventional TFMPEM (left), and the TFMPEM with HiLo technique as 1.09 μm -1 spatially modulated illumination at 90° (center) and 0° (right) orientations. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Intravascular photoacoustic imaging at 35 and 80 MHz

PubMed Central

Li, Xiang; Wei, Wei; Shung, K. Kirk

2012-01-01

Abstract. The catheter-based intravascular photoacoustic (IVPA) imaging for diagnosing atherosclerosis, which can provide optical absorption contrast of the arterial wall besides acoustic scattering contrast from the conventional intravascular ultrasound (IVUS) imaging, has been intensively researched recently. The resolution of IVPA is determined by the frequency bandwidth of an ultrasonic transducer. Higher resolution can be achieved by increasing the transducer’s working frequency and bandwidth. We introduce IVPA imaging at 35 and 80 MHz by using newly designed integrated IVUS/IVPA probes. This is the first time IVPA has been achieved as high as 80 MHz. Six-micrometer tungsten wires were imaged to evaluate the probes’ spatial resolutions and beam patterns. Healthy rabbit aorta was imaged in vitro. Imaging results show that IVPA has superior contrast over IVUS in identifying the arterial wall, and IVPA at 80 MHz demonstrates extraordinary resolution (35 μm) compared to 35 MHz. PMID:23224004

Contrast in Terahertz Images of Archival Documents—Part II: Influence of Topographic Features

NASA Astrophysics Data System (ADS)

Bardon, Tiphaine; May, Robert K.; Taday, Philip F.; Strlič, Matija

2017-04-01

We investigate the potential of terahertz time-domain imaging in reflection mode to reveal archival information in documents in a non-invasive way. In particular, this study explores the parameters and signal processing tools that can be used to produce well-contrasted terahertz images of topographic features commonly found in archival documents, such as indentations left by a writing tool, as well as sieve lines. While the amplitude of the waveforms at a specific time delay can provide the most contrasted and legible images of topographic features on flat paper or parchment sheets, this parameter may not be suitable for documents that have a highly irregular surface, such as water- or fire-damaged documents. For analysis of such documents, cross-correlation of the time-domain signals can instead yield images with good contrast. Analysis of the frequency-domain representation of terahertz waveforms can also provide well-contrasted images of topographic features, with improved spatial resolution when utilising high-frequency content. Finally, we point out some of the limitations of these means of analysis for extracting information relating to topographic features of interest from documents.

Genetic Influence on Contrast Sensitivity in Middle-Aged Male Twins

PubMed Central

Cronin-Golomb, Alice; Panizzon, Matthew S.; Lyons, Michael J.; Franz, Carol E.; Grant, Michael D.; Jacobson, Kristen C.; Eisen, Seth A.; Laudate, Thomas M.; Kremen, William S.

2007-01-01

Contrast sensitivity is strongly associated with daily functioning among older adults, but the genetic and environmental contributions to this ability are unknown. Using the classical twin method, we addressed this issue by examining contrast sensitivity at five spatial frequencies (1.5–18 cycles per degree) in 718 middle-aged male twins from the Vietnam Era Twin Study of Aging (VETSA). Heritability estimates were modest (14%–38%), whereas individual-specific environmental influences accounted for 62%–86% of the variance. Identifying the types of individual-specific events that impact contrast sensitivity may suggest interventions to modulate this ability and thereby improve overall quality of life as adults age. PMID:17604073

Spatial resolution, contrast sensitivity, and sensitivity to defocus of chicken retinal ganglion cells in vitro.

PubMed

Diedrich, Erich; Schaeffel, Frank

2009-11-01

The chicken has been extensively studied as an animal model for myopia because its eye growth is tightly controlled by visual experience. It has been found that the retina controls the axial eye growth rates depending on the amount and the sign of defocus imposed in the projected image. Glucagonergic amacrine cells were discovered that appear to encode for the sign of imposed defocus. It is not clear whether the downstream neurons, the retinal ganglion cells, still have access to this information-and whether it ultimately reaches the brain. We have analyzed the spike rates of chicken retinal ganglion cells in vitro using a microelectrode array. For this purpose, we initially defined spatial resolution and contrast sensitivity in vitro. Two classes of chicken retinal ganglions were found, depending on the linearity of their responses with increasing contrast. Responses generally declined with increasing defocus of the visual stimulus. These responses were well predicted by the modulation transfer function for a diffraction-limited defocused optical system, the first Bessel function. Thus, the studied retinal ganglion cells did not distinguish between a loss of contrast at a given spatial frequency due to reduced contrast of the stimulus pattern or because the pattern was presented out of focus. Furthermore, there was no indication that the retinal ganglion cells responded differently to defocus of either sign, at least for the cells that were recorded in this study.

Spatial frequency domain tomography of protoporphyrin IX fluorescence in preclinical glioma models

PubMed Central

Konecky, Soren D.; Owen, Chris M.; Rice, Tyler; Valdés, Pablo A.; Kolste, Kolbein; Wilson, Brian C.; Leblond, Frederic; Roberts, David W.; Paulsen, Keith D.

2012-01-01

Abstract. Multifrequency (0 to 0.3  mm−1), multiwavelength (633, 680, 720, 800, and 820 nm) spatial frequency domain imaging (SFDI) of 5-aminolevulinic acid-induced protoporphyrin IX (PpIX) was used to recover absorption, scattering, and fluorescence properties of glioblastoma multiforme spheroids in tissue-simulating phantoms and in vivo in a mouse model. Three-dimensional tomographic reconstructions of the frequency-dependent remitted light localized the depths of the spheroids within 500 μm, and the total amount of PpIX in the reconstructed images was constant to within 30% when spheroid depth was varied. In vivo tumor-to-normal contrast was greater than ∼1.5 in reduced scattering coefficient for all wavelengths and was ∼1.3 for the tissue concentration of deoxyhemoglobin (ctHb). The study demonstrates the feasibility of SFDI for providing enhanced image guidance during surgical resection of brain tumors. PMID:22612131

Sensitivity to spatial frequency content is not specific to face perception

PubMed Central

Williams, N. Rankin; Willenbockel, Verena; Gauthier, Isabel

2010-01-01

Prior work using a matching task between images that were complementary in spatial frequency and orientation information suggested that the representation of faces, but not objects, retains low-level spatial frequency (SF) information (Biederman & Kalocsai. 1997). In two experiments, we reexamine the claim that faces are uniquely sensitive to changes in SF. In contrast to prior work, we used a design allowing the computation of sensitivity and response criterion for each category, and in one experiment, equalized low-level image properties across object categories. In both experiments, we find that observers are sensitive to SF changes for upright and inverted faces and nonface objects. Differential response biases across categories contributed to a larger sensitivity for faces, but even sensitivity showed a larger effect for faces, especially when faces were upright and in a front-facing view. However, when objects were inverted, or upright but shown in a three-quarter view, the matching of objects and faces was equally sensitive to SF changes. Accordingly, face perception does not appear to be uniquely affected by changes in SF content. PMID:19576237

Preoperative Mapping of Nonmelanoma Skin Cancer Using Spatial Frequency Domain and Ultrasound Imaging

PubMed Central

Rohrbach, Daniel J.; Muffoletto, Daniel; Huihui, Jonathan; Saager, Rolf; Keymel, Kenneth; Paquette, Anne; Morgan, Janet; Zeitouni, Nathalie; Sunar, Ulas

2014-01-01

Rationale and Objectives The treatment of nonmelanoma skin cancer (NMSC) is usually by surgical excision or Mohs micrographic surgery and alternatively may include photodynamic therapy (PDT). To guide surgery and to optimize PDT, information about the tumor structure, optical parameters, and vasculature is desired. Materials and Methods Spatial frequency domain imaging (SFDI) can map optical absorption, scattering, and fluorescence parameters that can enhance tumor contrast and quantify light and photosensitizer dose. High frequency ultrasound (HFUS) imaging can provide high-resolution tumor structure and depth, which is useful for both surgery and PDT planning. Results Here, we present preliminary results from our recently developed clinical instrument for patients with NMSC. We quantified optical absorption and scattering, blood oxygen saturation (StO2), and total hemoglobin concentration (THC) with SFDI and lesion thickness with ultrasound. These results were compared to histological thickness of excised tumor sections. Conclusions SFDI quantified optical parameters with high precision, and multiwavelength analysis enabled 2D mappings of tissue StO2 and THC. HFUS quantified tumor thickness that correlated well with histology. The results demonstrate the feasibility of the instrument for noninvasive mapping of optical, physiological, and ultrasound contrasts in human skin tumors for surgery guidance and therapy planning. PMID:24439339

An Attempt to Simulate Letter-by-Letter Dyslexia in Normal Readers

ERIC Educational Resources Information Center

Fiset, Stephanie; Arguin, Martin; Fiset, Daniel

2006-01-01

We attempted to simulate the main features of letter-by-letter (LBL) dyslexia in normal readers through stimulus degradation (i.e. contrast reduction and removal of high spatial frequencies). The results showed the word length and the letter confusability effects characteristic of LBL dyslexia. However, the interaction of letter confusability and…

Effect of glare on reaction time for peripheral vision at mesopic adaptation.

PubMed

Aguirre, Rolando C; Colombo, Elisa M; Barraza, José F

2011-10-01

When a bright light is present in the field of view, visibility is dramatically reduced. Many studies have investigated the effect of glare on visibility considering foveal vision. However, the effects on peripheral vision have received little attention. In a previous work [J. Opt. Soc. Am. A 25, 1790 (2008)], we showed that the effect of glare on reaction time (RT) for foveal vision at mesopic adaptation depends on the stimulus spatial frequency. In this work, we extend this study to peripheral vision. We measured the RT of achromatic sinusoidal gratings as a function of contrast for a range of spatial frequency, and eccentricity, and for two glare levels, in addition to the no-glare condition. Data were fitted with Piéron's law, following a linear relationship. We found that glare increases the slope of these lines for all conditions. These slopes seem to depend critically on eccentricity for 4 cycles/degree (c/deg), but not for 1 and 2 c/deg. We explain our results in terms of the contrast sensitivity (gain) of the underlying detection mechanisms.

Image enhancement by spatial frequency post-processing of images obtained with pupil filters

NASA Astrophysics Data System (ADS)

Estévez, Irene; Escalera, Juan C.; Stefano, Quimey Pears; Iemmi, Claudio; Ledesma, Silvia; Yzuel, María J.; Campos, Juan

2016-12-01

The use of apodizing or superresolving filters improves the performance of an optical system in different frequency bands. This improvement can be seen as an increase in the OTF value compared to the OTF for the clear aperture. In this paper we propose a method to enhance the contrast of an image in both its low and its high frequencies. The method is based on the generation of a synthetic Optical Transfer Function, by multiplexing the OTFs given by the use of different non-uniform transmission filters on the pupil. We propose to capture three images, one obtained with a clear pupil, one obtained with an apodizing filter that enhances the low frequencies and another one taken with a superresolving filter that improves the high frequencies. In the Fourier domain the three spectra are combined by using smoothed passband filters, and then the inverse transform is performed. We show that we can create an enhanced image better than the image obtained with the clear aperture. To evaluate the performance of the method, bar tests (sinusoidal tests) with different frequency content are used. The results show that a contrast improvement in the high and low frequencies is obtained.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Foxley, Sean, E-mail: sean.foxley@ndcn.ox.ac.uk; Karczmar, Gregory S.; Domowicz, Miriam

Purpose: Widely used MRI methods show brain morphology both in vivo and ex vivo at very high resolution. Many of these methods (e.g., T{sub 2}{sup *}-weighted imaging, phase-sensitive imaging, or susceptibility-weighted imaging) are sensitive to local magnetic susceptibility gradients produced by subtle variations in tissue composition. However, the spectral resolution of commonly used methods is limited to maintain reasonable run-time combined with very high spatial resolution. Here, the authors report on data acquisition at increased spectral resolution, with 3-dimensional high spectral and spatial resolution MRI, in order to analyze subtle variations in water proton resonance frequency and lineshape that reflectmore » local anatomy. The resulting information compliments previous studies based on T{sub 2}{sup *} and resonance frequency. Methods: The proton free induction decay was sampled at high resolution and Fourier transformed to produce a high-resolution water spectrum for each image voxel in a 3D volume. Data were acquired using a multigradient echo pulse sequence (i.e., echo-planar spectroscopic imaging) with a spatial resolution of 50 × 50 × 70 μm{sup 3} and spectral resolution of 3.5 Hz. Data were analyzed in the spectral domain, and images were produced from the various Fourier components of the water resonance. This allowed precise measurement of local variations in water resonance frequency and lineshape, at the expense of significantly increased run time (16–24 h). Results: High contrast T{sub 2}{sup *}-weighted images were produced from the peak of the water resonance (peak height image), revealing a high degree of anatomical detail, specifically in the hippocampus and cerebellum. In images produced from Fourier components of the water resonance at −7.0 Hz from the peak, the contrast between deep white matter tracts and the surrounding tissue is the reverse of the contrast in water peak height images. This indicates the presence of a shoulder in the water resonance that is not present at +7.0 Hz and may be specific to white matter anatomy. Moreover, a frequency shift of 6.76 ± 0.55 Hz was measured between the molecular and granular layers of the cerebellum. This shift is demonstrated in corresponding spectra; water peaks from voxels in the molecular and granular layers are consistently 2 bins apart (7.0 Hz, as dictated by the spectral resolution) from one another. Conclusions: High spectral and spatial resolution MR imaging has the potential to accurately measure the changes in the water resonance in small voxels. This information can guide optimization and interpretation of more commonly used, more rapid imaging methods that depend on image contrast produced by local susceptibility gradients. In addition, with improved sampling methods, high spectral and spatial resolution data could be acquired in reasonable run times, and used for in vivo scans to increase sensitivity to variations in local susceptibility.« less

A gaze-contingent display to study contrast sensitivity under natural viewing conditions

NASA Astrophysics Data System (ADS)

Dorr, Michael; Bex, Peter J.

2011-03-01

Contrast sensitivity has been extensively studied over the last decades and there are well-established models of early vision that were derived by presenting the visual system with synthetic stimuli such as sine-wave gratings near threshold contrasts. Natural scenes, however, contain a much wider distribution of orientations, spatial frequencies, and both luminance and contrast values. Furthermore, humans typically move their eyes two to three times per second under natural viewing conditions, but most laboratory experiments require subjects to maintain central fixation. We here describe a gaze-contingent display capable of performing real-time contrast modulations of video in retinal coordinates, thus allowing us to study contrast sensitivity when dynamically viewing dynamic scenes. Our system is based on a Laplacian pyramid for each frame that efficiently represents individual frequency bands. Each output pixel is then computed as a locally weighted sum of pyramid levels to introduce local contrast changes as a function of gaze. Our GPU implementation achieves real-time performance with more than 100 fps on high-resolution video (1920 by 1080 pixels) and a synthesis latency of only 1.5ms. Psychophysical data show that contrast sensitivity is greatly decreased in natural videos and under dynamic viewing conditions. Synthetic stimuli therefore only poorly characterize natural vision.

Time course of dichoptic masking in normals and suppression in amblyopes.

PubMed

Zhou, Jiawei; McNeal, Suzanne; Babu, Raiju J; Baker, Daniel H; Bobier, William R; Hess, Robert F

2014-04-17

To better understand the relationship between dichoptic masking in normal vision and suppression in amblyopia we address three questions: First, what is the time course of dichoptic masking in normals and amblyopes? Second, is interocular suppression low-pass or band-pass in its spatial dependence? And third, in the above two regards, is dichoptic masking in normals different from amblyopic suppression? We measured the dependence of dichoptic masking in normal controls and amblyopes on the temporal duration of presentation under three conditions; monocular (the nontested eye-i.e., dominant eye of normals or nonamblyopic eye of amblyopes, being patched), dichoptic-luminance (the nontested eye seeing a mean luminance-i.e., a DC component) and dichoptic-contrast (the nontested eye seeing high-contrast visual noise). The subject had to detect a letter in the other eye, the contrast of which was varied. We found that threshold elevation relative to the patched condition occurred in both normals and amblyopes when the nontested eye saw either 1/f or band-pass filtered noise, but not just mean luminance (i.e., there was no masking from the DC component that corresponds to a channel responsive to a spatial frequency of 0 cyc/deg); longer presentation of the target (corresponding to lower temporal frequencies) produced greater threshold elevation. Dichoptic masking exhibits similar properties in both subject groups, being low-pass temporally and band-pass spatially, so that masking was greatest at the longest presentation durations and was not greatly affected by mean luminance in the nontested eye. Copyright 2014 The Association for Research in Vision and Ophthalmology, Inc.

Using Anatomic Magnetic Resonance Image Information to Enhance Visualization and Interpretation of Functional Images: A Comparison of Methods Applied to Clinical Arterial Spin Labeling Images

PubMed Central

Dai, Weiying; Soman, Salil; Hackney, David B.; Wong, Eric T.; Robson, Philip M.; Alsop, David C.

2017-01-01

Functional imaging provides hemodynamic and metabolic information and is increasingly being incorporated into clinical diagnostic and research studies. Typically functional images have reduced signal-to-noise ratio and spatial resolution compared to other non-functional cross sectional images obtained as part of a routine clinical protocol. We hypothesized that enhancing visualization and interpretation of functional images with anatomic information could provide preferable quality and superior diagnostic value. In this work, we implemented five methods (frequency addition, frequency multiplication, wavelet transform, non-subsampled contourlet transform and intensity-hue-saturation) and a newly proposed ShArpening by Local Similarity with Anatomic images (SALSA) method to enhance the visualization of functional images, while preserving the original functional contrast and quantitative signal intensity characteristics over larger spatial scales. Arterial spin labeling blood flow MR images of the brain were visualization enhanced using anatomic images with multiple contrasts. The algorithms were validated on a numerical phantom and their performance on images of brain tumor patients were assessed by quantitative metrics and neuroradiologist subjective ratings. The frequency multiplication method had the lowest residual error for preserving the original functional image contrast at larger spatial scales (55%–98% of the other methods with simulated data and 64%–86% with experimental data). It was also significantly more highly graded by the radiologists (p<0.005 for clear brain anatomy around the tumor). Compared to other methods, the SALSA provided 11%–133% higher similarity with ground truth images in the simulation and showed just slightly lower neuroradiologist grading score. Most of these monochrome methods do not require any prior knowledge about the functional and anatomic image characteristics, except the acquired resolution. Hence, automatic implementation on clinical images should be readily feasible. PMID:27723582

Zeroth-order phase-contrast technique.

PubMed

Pizolato, José Carlos; Cirino, Giuseppe Antonio; Gonçalves, Cristhiane; Neto, Luiz Gonçalves

2007-11-01

What we believe to be a new phase-contrast technique is proposed to recover intensity distributions from phase distributions modulated by spatial light modulators (SLMs) and binary diffractive optical elements (DOEs). The phase distribution is directly transformed into intensity distributions using a 4f optical correlator and an iris centered in the frequency plane as a spatial filter. No phase-changing plates or phase dielectric dots are used as a filter. This method allows the use of twisted nematic liquid-crystal televisions (LCTVs) operating in the real-time phase-mostly regime mode between 0 and p to generate high-intensity multiple beams for optical trap applications. It is also possible to use these LCTVs as input SLMs for optical correlators to obtain high-intensity Fourier transform distributions of input amplitude objects.

Emergence of increased frequency and severity of multiple infections by viruses due to spatial clustering of hosts

NASA Astrophysics Data System (ADS)

Taylor, Bradford P.; Penington, Catherine J.; Weitz, Joshua S.

2016-12-01

Multiple virus particles can infect a target host cell. Such multiple infections (MIs) have significant and varied ecological and evolutionary consequences for both virus and host populations. Yet, the in situ rates and drivers of MIs in virus-microbe systems remain largely unknown. Here, we develop an individual-based model (IBM) of virus-microbe dynamics to probe how spatial interactions drive the frequency and nature of MIs. In our IBMs, we identify increasingly spatially correlated clusters of viruses given sufficient decreases in viral movement. We also identify increasingly spatially correlated clusters of viruses and clusters of hosts given sufficient increases in viral infectivity. The emergence of clusters is associated with an increase in multiply infected hosts as compared to expectations from an analogous mean field model. We also observe long-tails in the distribution of the multiplicity of infection in contrast to mean field expectations that such events are exponentially rare. We show that increases in both the frequency and severity of MIs occur when viruses invade a cluster of uninfected microbes. We contend that population-scale enhancement of MI arises from an aggregate of invasion dynamics over a distribution of microbe cluster sizes. Our work highlights the need to consider spatially explicit interactions as a potentially key driver underlying the ecology and evolution of virus-microbe communities.

Colour vision and contrast sensitivity losses of mercury intoxicated industry workers in Brazil.

PubMed

Ventura, D F; Simões, A L; Tomaz, S; Costa, M F; Lago, M; Costa, M T V; Canto-Pereira, L H M; de Souza, J M; Faria, M A M; Silveira, L C L

2005-05-01

We evaluated vision loss in workers from fluorescent lamp industries (n=39) who had retired due to intoxication with mercury vapour and had been away from the work situation for several years (mean=6.32 years). An age-matched control group was submitted to the same tests for comparison. The luminance contrast sensitivity (CSF) was measured psychophysically and with the sweep visual evoked potential (sVEP) method. Chromatic red-green and blue-yellow CSFs were measured psychophysically. Colour discrimination was assessed with the Farnsworth-Munsell 100-hue test, Lanthony D-15d test and Cambridge Colour Vision Test. Patient data showed significantly lower scores in all colour tests compared to controls (p<.001). The behavioural luminance CSF of the patients was lower than that of controls (p<.001 at all frequencies tested). This result was confirmed by the electrophysiologically measured sweep VEP luminance CSF except at the highest frequencies-a difference that might be related to stimulus differences in the two situations. Chromatic CSFs were also statistically significantly lower for the patients than for the controls, for both chromatic equiluminant stimuli: red-green (p<.005) and blue-yellow (p<.04 for all frequencies, except 2 cycles per degree (cpd), the highest spatial frequency tested) spatial gratings. We conclude that exposure to elemental mercury vapour is associated with profound and lasting losses in achromatic and chromatic visual functions, affecting the magno-, parvo- and koniocellular visual pathways.

Stimulus specificity of a steady-state visual-evoked potential-based brain-computer interface.

PubMed

Ng, Kian B; Bradley, Andrew P; Cunnington, Ross

2012-06-01

The mechanisms of neural excitation and inhibition when given a visual stimulus are well studied. It has been established that changing stimulus specificity such as luminance contrast or spatial frequency can alter the neuronal activity and thus modulate the visual-evoked response. In this paper, we study the effect that stimulus specificity has on the classification performance of a steady-state visual-evoked potential-based brain-computer interface (SSVEP-BCI). For example, we investigate how closely two visual stimuli can be placed before they compete for neural representation in the cortex and thus influence BCI classification accuracy. We characterize stimulus specificity using the four stimulus parameters commonly encountered in SSVEP-BCI design: temporal frequency, spatial size, number of simultaneously displayed stimuli and their spatial proximity. By varying these quantities and measuring the SSVEP-BCI classification accuracy, we are able to determine the parameters that provide optimal performance. Our results show that superior SSVEP-BCI accuracy is attained when stimuli are placed spatially more than 5° apart, with size that subtends at least 2° of visual angle, when using a tagging frequency of between high alpha and beta band. These findings may assist in deciding the stimulus parameters for optimal SSVEP-BCI design.

Stimulus specificity of a steady-state visual-evoked potential-based brain-computer interface

NASA Astrophysics Data System (ADS)

Ng, Kian B.; Bradley, Andrew P.; Cunnington, Ross

2012-06-01

The mechanisms of neural excitation and inhibition when given a visual stimulus are well studied. It has been established that changing stimulus specificity such as luminance contrast or spatial frequency can alter the neuronal activity and thus modulate the visual-evoked response. In this paper, we study the effect that stimulus specificity has on the classification performance of a steady-state visual-evoked potential-based brain-computer interface (SSVEP-BCI). For example, we investigate how closely two visual stimuli can be placed before they compete for neural representation in the cortex and thus influence BCI classification accuracy. We characterize stimulus specificity using the four stimulus parameters commonly encountered in SSVEP-BCI design: temporal frequency, spatial size, number of simultaneously displayed stimuli and their spatial proximity. By varying these quantities and measuring the SSVEP-BCI classification accuracy, we are able to determine the parameters that provide optimal performance. Our results show that superior SSVEP-BCI accuracy is attained when stimuli are placed spatially more than 5° apart, with size that subtends at least 2° of visual angle, when using a tagging frequency of between high alpha and beta band. These findings may assist in deciding the stimulus parameters for optimal SSVEP-BCI design.

Active Optics: stress polishing of toric mirrors for the VLT SPHERE adaptive optics system.

PubMed

Hugot, Emmanuel; Ferrari, Marc; El Hadi, Kacem; Vola, Pascal; Gimenez, Jean Luc; Lemaitre, Gérard R; Rabou, Patrick; Dohlen, Kjetil; Puget, Pascal; Beuzit, Jean Luc; Hubin, Norbert

2009-05-20

The manufacturing of toric mirrors for the Very Large Telescope-Spectro-Polarimetric High-Contrast Exoplanet Research instrument (SPHERE) is based on Active Optics and stress polishing. This figuring technique allows minimizing mid and high spatial frequency errors on an aspherical surface by using spherical polishing with full size tools. In order to reach the tight precision required, the manufacturing error budget is described to optimize each parameter. Analytical calculations based on elasticity theory and finite element analysis lead to the mechanical design of the Zerodur blank to be warped during the stress polishing phase. Results on the larger (366 mm diameter) toric mirror are evaluated by interferometry. We obtain, as expected, a toric surface within specification at low, middle, and high spatial frequencies ranges.

Spatial-impulse-response-dependent back-projection using the non-stationary convolution in optoacoustic mesoscopy

NASA Astrophysics Data System (ADS)

Lu, Tong; Wang, Yihan; Gao, Feng; Zhao, Huijuan; Ntziachristos, Vasilis; Li, Jiao

2018-02-01

Photoacoustic mesoscopy (PAMe), offering high-resolution (sub-100-μm) and high optical contrast imaging at the depth of 1-10 mm, generally obtains massive collection data using a high-frequency focused ultrasonic transducer. The spatial impulse response (SIR) of this focused transducer causes the distortion of measured signals in both duration and amplitude. Thus, the reconstruction method considering the SIR needs to be investigated in the computation-economic way for PAMe. Here, we present a modified back-projection algorithm, by introducing a SIR-dependent calibration process using a non-satationary convolution method. The proposed method is performed on numerical simulations and phantom experiments of microspheres with diameter of both 50 μm and 100 μm, and the improvement of image fidelity of this method is proved to be evident by methodology parameters. The results demonstrate that, the images reconstructed when the SIR of transducer is accounted for have higher contrast-to-noise ratio and more reasonable spatial resolution, compared to the common back-projection algorithm.

Vibrational dephasing in matter-wave interferometers

NASA Astrophysics Data System (ADS)

Rembold, A.; Schütz, G.; Röpke, R.; Chang, W. T.; Hwang, I. S.; Günther, A.; Stibor, A.

2017-03-01

Matter-wave interferometry is a highly sensitive tool to measure small perturbations in a quantum system. This property allows the creation of precision sensors for dephasing mechanisms such as mechanical vibrations. They are a challenge for phase measurements under perturbing conditions that cannot be perfectly decoupled from the interferometer, e.g. for mobile interferometric devices or vibrations with a broad frequency range. Here, we demonstrate a method based on second-order correlation theory in combination with Fourier analysis, to use an electron interferometer as a sensor that precisely characterizes the mechanical vibration spectrum of the interferometer. Using the high spatial and temporal single-particle resolution of a delay line detector, the data allows to reveal the original contrast and spatial periodicity of the interference pattern from ‘washed-out’ matter-wave interferograms that have been vibrationally disturbed in the frequency region between 100 and 1000 Hz. Other than with electromagnetic dephasing, due to excitations of higher harmonics and additional frequencies induced from the environment, the parts in the setup oscillate with frequencies that can be different to the applied ones. The developed numerical search algorithm is capable to determine those unknown oscillations and corresponding amplitudes. The technique can identify vibrational dephasing and decrease damping and shielding requirements in electron, ion, neutron, atom and molecule interferometers that generate a spatial fringe pattern on the detector plane.

The significance of retinal image contrast and spatial frequency composition for eye growth modulation in young chicks

PubMed Central

Tran, Nina; Chiu, Sara; Tian, Yibin; Wildsoet, Christine F.

2009-01-01

Purpose This study sought further insight into the stimulus dependence of form deprivation myopia, a common response to retinal image degradation in young animals. Methods Each of 4 Bangerter diffusing filters (0.6, 0.1, <0.1, and LP (light perception only)) combined with clear plano lenses, as well as plano lenses alone, were fitted monocularly to 4-day-old chicks. Axial ocular dimensions and refractive errors were monitored over a 14-day treatment period, using high frequency A-scan ultrasonography and an autorefractor, respectively. Results Only the <0.1 and LP filters induced significant form deprivation myopia; these filters induced similarly large myopic shifts in refractive error (mean interocular differences ±SEM: -9.92 ±1.99, -7.26 ± 1.60 D respectively), coupled to significant increases in both vitreous chamber depths and optical axial lengths (p<0.001). The other 3 groups showed comparable, small changes in their ocular dimensions (p>0.05), and only small myopic shifts in refraction (<3.00 D). The myopia-inducing filters eliminated mid-and-high spatial frequency information. Conclusions Our results are consistent with emmetropization being tuned to mid-spatial frequencies. They also imply that form deprivation is not a graded phenomenon. PMID:18533221

Modulated-alignment dual-axis (MAD) confocal microscopy for deep optical sectioning in tissues

PubMed Central

Leigh, Steven Y.; Chen, Ye; Liu, Jonathan T.C.

2014-01-01

A strategy is presented to enable optical-sectioning microscopy with improved contrast and imaging depth using low-power (0.5 - 1 mW) diode laser illumination. This technology combines the inherent strengths of focal-modulation microscopy and dual-axis confocal (DAC) microscopy for rejecting out-of-focus and multiply scattered background light in tissues. The DAC architecture is unique in that it utilizes an intersecting pair of illumination and collection beams to improve the spatial-filtering and optical-sectioning performance of confocal microscopy while focal modulation selectively ‘labels’ in-focus signals via amplitude modulation. Simulations indicate that modulating the spatial alignment of dual-axis beams at a frequency f generates signals from the focal volume of the microscope that are modulated at 2f with minimal modulation of background signals, thus providing nearly an order-of-magnitude improvement in optical-sectioning contrast compared to DAC microscopy alone. Experiments show that 2f lock-in detection enhances contrast and imaging depth within scattering phantoms and fresh tissues. PMID:24940534

Quantitative Susceptibility Mapping: Contrast Mechanisms and Clinical Applications

PubMed Central

Liu, Chunlei; Wei, Hongjiang; Gong, Nan-Jie; Cronin, Matthew; Dibb, Russel; Decker, Kyle

2016-01-01

Quantitative susceptibility mapping (QSM) is a recently developed MRI technique for quantifying the spatial distribution of magnetic susceptibility within biological tissues. It first uses the frequency shift in the MRI signal to map the magnetic field profile within the tissue. The resulting field map is then used to determine the spatial distribution of the underlying magnetic susceptibility by solving an inverse problem. The solution is achieved by deconvolving the field map with a dipole field, under the assumption that the magnetic field is a result of the superposition of the dipole fields generated by all voxels and that each voxel has its unique magnetic susceptibility. QSM provides improved contrast to noise ratio for certain tissues and structures compared to its magnitude counterpart. More importantly, magnetic susceptibility is a direct reflection of the molecular composition and cellular architecture of the tissue. Consequently, by quantifying magnetic susceptibility, QSM is becoming a quantitative imaging approach for characterizing normal and pathological tissue properties. This article reviews the mechanism generating susceptibility contrast within tissues and some associated applications. PMID:26844301

Symmetry-protected zero-mode laser with a tunable spatial profile

NASA Astrophysics Data System (ADS)

Ge, Li

Majorana zero modes in condense matter systems have attracted considerable interest in topological quantum computation. In contrast, while robust zero modes have been observed in various photonic lattices, it remains an open question whether they can be used for the same purpose. To advance significantly the state-of-the-art in zero-mode photonics, new inspirations are needed for a better design and control of photonic systems. Using the zero modes protected by non-Hermitian particle-hole symmetry in a photonic lattice and the spatial degrees of freedom they offer, we propose a single-mode, fixed-frequency, and spatially tunable zero-mode laser. The system does not need to have zero modes before a localized pump is applied; they are created by the spontaneous restoration of particle-hole symmetry. By modifying this process using different pump configurations, we present a versatile way to tune the spatial profile of our zero-mode laser, with its lasing frequency pinned at the zero energy. Such a zero-mode laser may find applications in telecommunication, where spatial encoding is held by some to be last frontier of signal processing. This project is supported by the NSF under Grant No. DMR-1506987.

Quality of image of grating target placed in vitreous of isolated pig eyes photographed through different implanted multifocal intraocular lenses.

PubMed

Inoue, Makoto; Noda, Toru; Ohnuma, Kazuhiko; Bissen-Miyajima, Hiroko; Hirakata, Akito

2011-11-01

To determine the quality of the image of a grating target placed in the vitreous of isolated pig eyes and photographed through implanted refractive and diffractive multifocal intraocular lenses (IOL). Refractive multifocal (NXG1, PY60MV), diffractive multifocal (ZM900, SA60D3) and monofocal (SA60AT, ZA9003) IOL were implanted in the capsular bag of isolated pig eyes. A grating target was placed in the vitreous and photographed through a flat or a wide-field viewing contact lens. The contrast of the grating targets of different spatial frequencies was measured. With the flat corneal contact lens, the gratings appeared clear and not distorted when viewed through the optics of the NXG1 and PY60MV for far vision but were distorted with reduced contrast when viewed through the optical zone for near vision. The images through the diffractive zone of the ZM900 and SA60D3 were more defocused than with the monofocal IOL (p < 0.005). Ghost images oriented centrifugally of the original image were seen with the ZM900 resulting in lower contrast at higher spatial frequencies than with the SA60D3 with less defocused images only in the central area. With the wide-field viewing contact lens, the images were less defocused and the contrast was comparable to both refractive and diffractive multifocal IOL. Both refractive and diffractive multifocal IOL reduced the contrast of the retinal image when viewed through a flat corneal contact lens but less defocused when viewed through a wide-field viewing contact lens. © 2011 The Authors. Acta Ophthalmologica © 2011 Acta Ophthalmologica Scandinavica Foundation.

Three filters for visualization of phase objects with large variations of phase gradients

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sagan, Arkadiusz; Antosiewicz, Tomasz J.; Szoplik, Tomasz

2009-02-20

We propose three amplitude filters for visualization of phase objects. They interact with the spectra of pure-phase objects in the frequency plane and are based on tangent and error functions as well as antisymmetric combination of square roots. The error function is a normalized form of the Gaussian function. The antisymmetric square-root filter is composed of two square-root filters to widen its spatial frequency spectral range. Their advantage over other known amplitude frequency-domain filters, such as linear or square-root graded ones, is that they allow high-contrast visualization of objects with large variations of phase gradients.

Study of Tissue Phantoms, Tissues, and Contrast Agent with the Biophotoacoustic Radar and Comparison to Ultrasound Imaging for Deep Subsurface Imaging

NASA Astrophysics Data System (ADS)

Alwi, R.; Telenkov, S.; Mandelis, A.; Gu, F.

2012-11-01

In this study, the imaging capability of our wide-spectrum frequency-domain photoacoustic (FD-PA) imaging alias "photoacoustic radar" methodology for imaging of soft tissues is explored. A practical application of the mathematical correlation processing method with relatively long (1 ms) frequency-modulated optical excitation is demonstrated for reconstruction of the spatial location of the PA sources. Image comparison with ultrasound (US) modality was investigated to see the complementarity between the two techniques. The obtained results with a phased array probe on tissue phantoms and their comparison to US images demonstrated that the FD-PA technique has strong potential for deep subsurface imaging with excellent contrast and high signal-to-noise ratio. FD-PA images of blood vessels in a human wrist and an in vivo subcutaneous tumor in a rat model are presented. As in other imaging modalities, the employment of contrast agents is desirable to improve the capability of medical diagnostics. Therefore, this study also evaluated and characterized the use of Food and Drug Administration (FDA)-approved superparamagnetic iron oxide nanoparticles (SPION) as PA contrast agents.

Frequency of gamma oscillations in humans is modulated by velocity of visual motion

PubMed Central

Butorina, Anna V.; Sysoeva, Olga V.; Prokofyev, Andrey O.; Nikolaeva, Anastasia Yu.; Stroganova, Tatiana A.

2015-01-01

Gamma oscillations are generated in networks of inhibitory fast-spiking (FS) parvalbumin-positive (PV) interneurons and pyramidal cells. In animals, gamma frequency is modulated by the velocity of visual motion; the effect of velocity has not been evaluated in humans. In this work, we have studied velocity-related modulations of gamma frequency in children using MEG/EEG. We also investigated whether such modulations predict the prominence of the “spatial suppression” effect (Tadin D, Lappin JS, Gilroy LA, Blake R. Nature 424: 312-315, 2003) that is thought to depend on cortical center-surround inhibitory mechanisms. MEG/EEG was recorded in 27 normal boys aged 8–15 yr while they watched high-contrast black-and-white annular gratings drifting with velocities of 1.2, 3.6, and 6.0°/s and performed a simple detection task. The spatial suppression effect was assessed in a separate psychophysical experiment. MEG gamma oscillation frequency increased while power decreased with increasing velocity of visual motion. In EEG, the effects were less reliable. The frequencies of the velocity-specific gamma peaks were 64.9, 74.8, and 87.1 Hz for the slow, medium, and fast motions, respectively. The frequency of the gamma response elicited during slow and medium velocity of visual motion decreased with subject age, whereas the range of gamma frequency modulation by velocity increased with age. The frequency modulation range predicted spatial suppression even after controlling for the effect of age. We suggest that the modulation of the MEG gamma frequency by velocity of visual motion reflects excitability of cortical inhibitory circuits and can be used to investigate their normal and pathological development in the human brain. PMID:25925324

Co-registration of ultrasound and frequency-domain photoacoustic radar images and image improvement for tumor detection

NASA Astrophysics Data System (ADS)

Dovlo, Edem; Lashkari, Bahman; Choi, Sung soo Sean; Mandelis, Andreas

2015-03-01

This paper demonstrates the co-registration of ultrasound (US) and frequency domain photoacoustic radar (FD-PAR) images with significant image improvement from applying image normalization, filtering and amplification techniques. Achieving PA imaging functionality on a commercial Ultrasound instrument could accelerate clinical acceptance and use. Experimental results presented demonstrate live animal testing and show enhancements in signal-to-noise ratio (SNR), contrast and spatial resolution. The co-registered image produced from the US and phase PA images, provides more information than both images independently.

Developmental dyslexia and vision

PubMed Central

Quercia, Patrick; Feiss, Léonard; Michel, Carine

2013-01-01

Developmental dyslexia affects almost 10% of school-aged children and represents a significant public health problem. Its etiology is unknown. The consistent presence of phonological difficulties combined with an inability to manipulate language sounds and the grapheme–phoneme conversion is widely acknowledged. Numerous scientific studies have also documented the presence of eye movement anomalies and deficits of perception of low contrast, low spatial frequency, and high frequency temporal visual information in dyslexics. Anomalies of visual attention with short visual attention spans have also been demonstrated in a large number of cases. Spatial orientation is also affected in dyslexics who manifest a preference for spatial attention to the right. This asymmetry may be so pronounced that it leads to a veritable neglect of space on the left side. The evaluation of treatments proposed to dyslexics whether speech or oriented towards the visual anomalies remains fragmentary. The advent of new explanatory theories, notably cerebellar, magnocellular, or proprioceptive, is an incentive for ophthalmologists to enter the world of multimodal cognition given the importance of the eye’s visual input. PMID:23690677

Spatial cell firing during virtual navigation of open arenas by head-restrained mice.

PubMed

Chen, Guifen; King, John Andrew; Lu, Yi; Cacucci, Francesca; Burgess, Neil

2018-06-18

We present a mouse virtual reality (VR) system which restrains head-movements to horizontal rotations, compatible with multi-photon imaging. This system allows expression of the spatial navigation and neuronal firing patterns characteristic of real open arenas (R). Comparing VR to R: place and grid, but not head-direction, cell firing had broader spatial tuning; place, but not grid, cell firing was more directional; theta frequency increased less with running speed; whereas increases in firing rates with running speed and place and grid cells' theta phase precession were similar. These results suggest that the omni-directional place cell firing in R may require local-cues unavailable in VR, and that the scale of grid and place cell firing patterns, and theta frequency, reflect translational motion inferred from both virtual (visual and proprioceptive) and real (vestibular translation and extra-maze) cues. By contrast, firing rates and theta phase precession appear to reflect visual and proprioceptive cues alone. © 2018, Chen et al.

Spatially variable stage-driven groundwater-surface water interaction inferred from time-frequency analysis of distributed temperature sensing data

USGS Publications Warehouse

Mwakanyamale, Kisa; Slater, Lee; Day-Lewis, Frederick D.; Elwaseif, Mehrez; Johnson, Carole D.

2012-01-01

Characterization of groundwater-surface water exchange is essential for improving understanding of contaminant transport between aquifers and rivers. Fiber-optic distributed temperature sensing (FODTS) provides rich spatiotemporal datasets for quantitative and qualitative analysis of groundwater-surface water exchange. We demonstrate how time-frequency analysis of FODTS and synchronous river stage time series from the Columbia River adjacent to the Hanford 300-Area, Richland, Washington, provides spatial information on the strength of stage-driven exchange of uranium contaminated groundwater in response to subsurface heterogeneity. Although used in previous studies, the stage-temperature correlation coefficient proved an unreliable indicator of the stage-driven forcing on groundwater discharge in the presence of other factors influencing river water temperature. In contrast, S-transform analysis of the stage and FODTS data definitively identifies the spatial distribution of discharge zones and provided information on the dominant forcing periods (≥2 d) of the complex dam operations driving stage fluctuations and hence groundwater-surface water exchange at the 300-Area.

Ring-like spatial distribution of laser accelerated protons in the ultra-high-contrast TNSA-regime

NASA Astrophysics Data System (ADS)

Becker, G. A.; Tietze, S.; Keppler, S.; Reislöhner, J.; Bin, J. H.; Bock, L.; Brack, F.-E.; Hein, J.; Hellwing, M.; Hilz, P.; Hornung, M.; Kessler, A.; Kraft, S. D.; Kuschel, S.; Liebetrau, H.; Ma, W.; Polz, J.; Schlenvoigt, H.-P.; Schorcht, F.; Schwab, M. B.; Seidel, A.; Zeil, K.; Schramm, U.; Zepf, M.; Schreiber, J.; Rykovanov, S.; Kaluza, M. C.

2018-05-01

The spatial distribution of protons accelerated from submicron-thick plastic foil targets using multi-terawatt, frequency-doubled laser pulses with ultra-high temporal contrast has been investigated experimentally. A very stable, ring-like beam profile of the accelerated protons, oriented around the target’s normal direction has been observed. The ring’s opening angle has been found to decrease with increasing foil thicknesses. Two-dimensional particle-in-cell simulations reproduce our results indicating that the ring is formed during the expansion of the proton density distribution into the vacuum as described by the mechanism of target-normal sheath acceleration. Here—in addition to the longitudinal electric fields responsible for the forward acceleration of the protons—a lateral charge separation leads to transverse field components accelerating the protons in the lateral direction.

A look at motion in the frequency domain

NASA Technical Reports Server (NTRS)

Watson, A. B.; Ahumada, A. J., Jr.

1983-01-01

A moving image can be specified by a contrast distribution, c(x,y,t), over the dimensions of space x,y, and time t. Alternatively, it can be specified by the distribution C(u,v,w) over spatial frequency u,v and temporal frequency w. The frequency representation of a moving image is shown to have a characteristic form. This permits two useful observations. The first is that the apparent smoothness of time-sampled moving images (apparent motion) can be explained by the filtering action of the human visual system. This leads to the following formula for the required update rate for time-sampled displays. W(c)=W(l)+ru(l) where w(c) is the required update rate in Hz, W(l) is the limit of human temporal resolution in Hz, r is the velocity of the moving image in degrees/sec, and u(l) is the limit of human spatial resolution in cycles/deg. The second observation is that it is possible to construct a linear sensor that responds to images moving in a particular direction. The sensor is derived and its properties are discussed.

Modulation of hippocampal NCAM polysialylation and spatial memory consolidation by fear conditioning.

PubMed

Sandi, Carmen; Merino, José J; Cordero, M Isabel; Kruyt, Nyika D; Murphy, Keith J; Regan, Ciaran M

2003-09-15

Cell adhesion molecule function is involved in hippocampal synaptic plasticity and associated with memory consolidation. At the infragranular zone of the dentate gyrus, neurons expressing the polysialylated form of the neural cell adhesion molecule (NCAM PSA) transiently increase their frequency 12 hours after training in different tasks. Using immunohistochemical procedures, we investigated NCAM polysialylation following training in a contextual fear conditioning paradigm that employed increasing shock intensities to separately model stressful and traumatic experiences in adult male Wistar rats. Fear conditioning with a stressful.4-mA stimulus resulted in an increased frequency of dentate polysialylated neurons, the magnitude of which was indistinguishable from that observed following water maze training. By contrast, training with a traumatic 1-mA stimulus resulted in a significant decrease in the frequency of polysialylated neurons at the 12 hours posttraining time. Whereas sequential training in the water maze paradigm followed by fear conditioning resulted in potentiated consolidation of spatial information when conditioning involved a.4-mA stimulus, amnesia for spatial learning occurred when conditioning was performed with a 1-mA stimulus. These results suggest traumatic fear conditioning suppresses NCAM-PSA-mediated plasticity and the concomitant inability to store the trace of recently acquired information.

Wave propagation in axially moving periodic strings

NASA Astrophysics Data System (ADS)

Sorokin, Vladislav S.; Thomsen, Jon Juel

2017-04-01

The paper deals with analytically studying transverse waves propagation in an axially moving string with periodically modulated cross section. The structure effectively models various relevant technological systems, e.g. belts, thread lines, band saws, etc., and, in particular, roller chain drives for diesel engines by capturing both their spatial periodicity and axial motion. The Method of Varying Amplitudes is employed in the analysis. It is shown that the compound wave traveling in the axially moving periodic string comprises many components with different frequencies and wavenumbers. This is in contrast to non-moving periodic structures, for which all components of the corresponding compound wave feature the same frequency. Due to this "multi-frequency" character of the wave motion, the conventional notion of frequency band-gaps appears to be not applicable for the moving periodic strings. Thus, for such structures, by frequency band-gaps it is proposed to understand frequency ranges in which the primary component of the compound wave attenuates. Such frequency band-gaps can be present for a moving periodic string, but only if its axial velocity is lower than the transverse wave speed, and, the higher the axial velocity, the narrower the frequency band-gaps. The revealed effects could be of potential importance for applications, e.g. they indicate that due to spatial inhomogeneity, oscillations of axially moving periodic chains always involve a multitude of frequencies.

Brain State Effects on Layer 4 of the Awake Visual Cortex

PubMed Central

Zhuang, Jun; Bereshpolova, Yulia; Stoelzel, Carl R.; Huff, Joseph M.; Hei, Xiaojuan; Alonso, Jose-Manuel

2014-01-01

Awake mammals can switch between alert and nonalert brain states hundreds of times per day. Here, we study the effects of alertness on two cell classes in layer 4 of primary visual cortex of awake rabbits: presumptive excitatory “simple” cells and presumptive fast-spike inhibitory neurons (suspected inhibitory interneurons). We show that in both cell classes, alertness increases the strength and greatly enhances the reliability of visual responses. In simple cells, alertness also increases the temporal frequency bandwidth, but preserves contrast sensitivity, orientation tuning, and selectivity for direction and spatial frequency. Finally, alertness selectively suppresses the simple cell responses to high-contrast stimuli and stimuli moving orthogonal to the preferred direction, effectively enhancing mid-contrast borders. Using a population coding model, we show that these effects of alertness in simple cells—enhanced reliability, higher gain, and increased suppression in orthogonal orientation—could play a major role at increasing the speed of cortical feature detection. PMID:24623767

Contrast sensitivity test and conventional and high frequency audiometry: information beyond that required to prescribe lenses and headsets

NASA Astrophysics Data System (ADS)

Comastri, S. A.; Martin, G.; Simon, J. M.; Angarano, C.; Dominguez, S.; Luzzi, F.; Lanusse, M.; Ranieri, M. V.; Boccio, C. M.

2008-04-01

In Optometry and in Audiology, the routine tests to prescribe correction lenses and headsets are respectively the visual acuity test (the first chart with letters was developed by Snellen in 1862) and conventional pure tone audiometry (the first audiometer with electrical current was devised by Hartmann in 1878). At present there are psychophysical non invasive tests that, besides evaluating visual and auditory performance globally and even in cases catalogued as normal according to routine tests, supply early information regarding diseases such as diabetes, hypertension, renal failure, cardiovascular problems, etc. Concerning Optometry, one of these tests is the achromatic luminance contrast sensitivity test (introduced by Schade in 1956). Concerning Audiology, one of these tests is high frequency pure tone audiometry (introduced a few decades ago) which yields information relative to pathologies affecting the basal cochlea and complements data resulting from conventional audiometry. These utilities of the contrast sensitivity test and of pure tone audiometry derive from the facts that Fourier components constitute the basis to synthesize stimuli present at the entrance of the visual and auditory systems; that these systems responses depend on frequencies and that the patient's psychophysical state affects frequency processing. The frequency of interest in the former test is the effective spatial frequency (inverse of the angle subtended at the eye by a cycle of a sinusoidal grating and measured in cycles/degree) and, in the latter, the temporal frequency (measured in cycles/sec). Both tests have similar duration and consist in determining the patient's threshold (corresponding to the inverse multiplicative of the contrast or to the inverse additive of the sound intensity level) for each harmonic stimulus present at the system entrance (sinusoidal grating or pure tone sound). In this article the frequencies, standard normality curves and abnormal threshold shifts inherent to the contrast sensitivity test (which for simplicity could be termed "visionmetry") and to pure tone audiometry (also termed auditory sensitivity test) are analyzed with the purpose of contributing to divulge their ability to supply early information associated to pathologies not solely related to the visual and auditory systems respectively.

α-Lipoic Acid Treatment Improves Vision-Related Quality of Life in Patients with Dry Age-Related Macular Degeneration.

PubMed

Tao, Yuan; Jiang, Pengfei; Wei, Yuhua; Wang, Ping; Sun, Xiaoling; Wang, Hong

2016-11-01

Dry form of age-related macular degeneration (AMD) constitutes 90% of AMD cases, and it is characterized by the formation of drusen under the retina and the slow breakdown of the light-sensing cells in the macula, which causes a gradual loss of central vision. Since oxidative stress is involved in the pathogenesis of dry AMD, α-lipoic acid (LA) with antioxidant properties was selected, and its effect on anti-oxidative markers and visual quality in patients with dry AMD was assessed. A total of 100 dry AMD patients (60-83 years old) were randomly assigned to LA treatment group (n = 50) and placebo control group (n = 50). We measured the serum superoxide dismutase (SOD) activity, an important marker of antioxidant defense, best-corrected visual acuity (BCVA), contrast sensitivity, and Chinese-Version Low Vision Quality of Life (CLVQOL) before and after LA or placebo intervention. Pearson correlation coefficients were calculated to explore the relationship between contrast sensitivity values and CLVQOL scores. There was a statistically significant increase in serum SOD activity after LA intervention. The CLVQOL score was improved significantly after LA treatment. The contrast sensitivity measured at middle and low spatial frequency was significantly higher after LA treatment. CLVQOL scores were positively correlated with contrast sensitivity at low spatial frequency (3 cyc/degree) in LA-treated group. These results indicate that LA treatment improves vision-related quality of life in patients with dry AMD probably by increasing antioxidant activity. Thus, LA can be regarded as a promising agent for the treatment of AMD.

Reference frames for spatial frequency in face representation differ in the temporal visual cortex and amygdala.

PubMed

Inagaki, Mikio; Fujita, Ichiro

2011-07-13

Social communication in nonhuman primates and humans is strongly affected by facial information from other individuals. Many cortical and subcortical brain areas are known to be involved in processing facial information. However, how the neural representation of faces differs across different brain areas remains unclear. Here, we demonstrate that the reference frame for spatial frequency (SF) tuning of face-responsive neurons differs in the temporal visual cortex and amygdala in monkeys. Consistent with psychophysical properties for face recognition, temporal cortex neurons were tuned to image-based SFs (cycles/image) and showed viewing distance-invariant representation of face patterns. On the other hand, many amygdala neurons were influenced by retina-based SFs (cycles/degree), a characteristic that is useful for social distance computation. The two brain areas also differed in the luminance contrast sensitivity of face-responsive neurons; amygdala neurons sharply reduced their responses to low luminance contrast images, while temporal cortex neurons maintained the level of their responses. From these results, we conclude that different types of visual processing in the temporal visual cortex and the amygdala contribute to the construction of the neural representations of faces.

Perceptual and cognitive effects of antipsychotics in first-episode schizophrenia: the potential impact of GABA concentration in the visual cortex.

PubMed

Kelemen, Oguz; Kiss, Imre; Benedek, György; Kéri, Szabolcs

2013-12-02

Schizophrenia is characterized by anomalous perceptual experiences (e.g., sensory irritation, inundation, and flooding) and specific alterations in visual perception. We aimed to investigate the effects of short-term antipsychotic medication on these perceptual alterations. We assessed 28 drug-naïve first episode patients with schizophrenia and 20 matched healthy controls at baseline and follow-up 8 weeks later. Contrast sensitivity was measured with steady- and pulsed-pedestal tests. Participants also received a motion coherence task, the Structured Interview for Assessing Perceptual Anomalies (SIAPA), and the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS). Proton magnetic resonance spectroscopy was used to measure gamma-aminobutyric acid (GABA) levels in the occipital cortex (GABA/total creatine [Cr] ratio). Results revealed that, comparing baseline and follow-up values, patients with schizophrenia exhibited a marked sensitivity reduction on the steady-pedestal test at low spatial frequency. Anomalous perceptual experiences were also significantly ameliorated. Antipsychotic medications had no effect on motion perception. RBANS scores showed mild improvements. At baseline, but not at follow-up, patients with schizophrenia outperformed controls on the steady-pedestal test at low spatial frequency. The dysfunction of motion perception (higher coherence threshold in patients relative to controls) was similar at both assessments. There were reduced GABA levels in schizophrenia at both assessments, which were not related to perceptual functions. These results suggest that antipsychotics dominantly affect visual contrast sensitivity and anomalous perceptual experiences. The prominent dampening effect on low spatial frequency in the steady-pedestal test might indicate the normalization of putatively overactive magnocellular retino-geniculo-cortical pathways. © 2013.

Organic solvent exposure and contrast sensitivity: comparing men and women

PubMed Central

Oliveira, A.R.; Campos, A.A.; de Andrade, M.J.O.; de Medeiros, P.C.B.; dos Santos, N.A.

2018-01-01

The goal of this study was to compare the visual contrast sensitivity (CS) of men and women exposed and not exposed to organic solvents. Forty-six volunteers of both genders aged between 18 and 41 years (mean±SD=27.72±6.28) participated. Gas station attendants were exposed to gas containing 46.30 ppm of solvents at a temperature of 304±274.39 K, humidity of 62.25±7.59% and ventilation of 0.69±0.46 m/s (a passive gas chromatography-based sampling method was used considering the microclimate variables). Visual CS was measured via the psychophysical method of two-alternative forced choice using vertical sinusoidal gratings with spatial frequencies of 0.2, 0.5, 1.0, 2.0, 5.0, 10.0, and 16.0 cpd (cycles per degree) and an average luminance of 34.4 cd/m2. The results showed that visual CS was significantly lower (P<0.05) in the following groups: i) exposed men compared to unexposed men at frequencies of 0.2, 0.5, 1.0, and 2.0 cpd; ii) exposed women compared to unexposed women at a frequency of 5.0 cpd; and iii) exposed women compared to exposed men at a frequency of 0.5 cpd, even at exposures below the tolerance limit (300 ppm). These results suggest that the visual CS of exposed men was impaired over a wider range of spatial frequencies than that of exposed women. This difference may have been due to the higher body fat content of women compared to that of men, suggesting that body fat in women can serve as a protective factor against neurotoxic effects. PMID:29340521

Noise frame duration, masking potency and whiteness of temporal noise.

PubMed

Kukkonen, Heljä; Rovamo, Jyrki; Donner, Kristian; Tammikallio, Marja; Raninen, Antti

2002-09-01

Because of the limited contrast range, increasing the duration of the noise frame is often the only option for increasing the masking potency of external, white temporal noise. This, however, reduces the high-frequency cutoff beyond which noise is no longer white. This study was conducted to determine the longest noise frame duration that produces the strongest masking effect and still mimics white noise on the detection of sinusoidal flicker. Contrast energy thresholds (E(th)) were measured for flicker at 1.25 to 20 Hz in strong, purely temporal (spatially uniform), additive, external noise. The masking power of white external noise, characterized by its spectral density at zero frequency N0, increases with the duration of the noise frame. For short noise frame durations, E(th) increased in direct proportion to N0, keeping the nominal signal-to-noise ratio [SNR = (E(th)/N0)(0.5)] constant at threshold. The masking effect thus increased with the duration of the noise frame and the noise mimicked white noise. When noise frame duration and N0 increased further, the nominal SNR at threshold started to decrease, indicating that noise no longer mimicked white noise. The minimum number of noise frames per flicker cycle needed to mimic white noise decreased with increasing flicker frequency from 8.3 at 1.25 Hz to 1.6 at 20 Hz. The critical high-frequency cutoff of detection-limiting temporal noise in terms of noise frames per signal cycle depends on the temporal frequency of the signal. This is opposite to the situation in the spatial domain and must be taken into consideration when temporal signals are masked with temporal noise.

A Third-Generation Adaptive Statistical Iterative Reconstruction Technique: Phantom Study of Image Noise, Spatial Resolution, Lesion Detectability, and Dose Reduction Potential.

PubMed

Euler, André; Solomon, Justin; Marin, Daniele; Nelson, Rendon C; Samei, Ehsan

2018-06-01

The purpose of this study was to assess image noise, spatial resolution, lesion detectability, and the dose reduction potential of a proprietary third-generation adaptive statistical iterative reconstruction (ASIR-V) technique. A phantom representing five different body sizes (12-37 cm) and a contrast-detail phantom containing lesions of five low-contrast levels (5-20 HU) and three sizes (2-6 mm) were deployed. Both phantoms were scanned on a 256-MDCT scanner at six different radiation doses (1.25-10 mGy). Images were reconstructed with filtered back projection (FBP), ASIR-V with 50% blending with FBP (ASIR-V 50%), and ASIR-V without blending (ASIR-V 100%). In the first phantom, noise properties were assessed by noise power spectrum analysis. Spatial resolution properties were measured by use of task transfer functions for objects of different contrasts. Noise magnitude, noise texture, and resolution were compared between the three groups. In the second phantom, low-contrast detectability was assessed by nine human readers independently for each condition. The dose reduction potential of ASIR-V was estimated on the basis of a generalized linear statistical regression model. On average, image noise was reduced 37.3% with ASIR-V 50% and 71.5% with ASIR-V 100% compared with FBP. ASIR-V shifted the noise power spectrum toward lower frequencies compared with FBP. The spatial resolution of ASIR-V was equivalent or slightly superior to that of FBP, except for the low-contrast object, which had lower resolution. Lesion detection significantly increased with both ASIR-V levels (p = 0.001), with an estimated radiation dose reduction potential of 15% ± 5% (SD) for ASIR-V 50% and 31% ± 9% for ASIR-V 100%. ASIR-V reduced image noise and improved lesion detection compared with FBP and had potential for radiation dose reduction while preserving low-contrast detectability.

Casimir-Lifshitz interaction between dielectrics of arbitrary geometry: A dielectric contrast perturbation theory

DOE Office of Scientific and Technical Information (OSTI.GOV)

Golestanian, Ramin

2009-07-15

The general theory of electromagnetic-fluctuation-induced interactions in dielectric bodies as formulated by Dzyaloshinskii, Lifshitz, and Pitaevskii is rewritten as a perturbation theory in terms of the spatial contrast in (imaginary) frequency dependent dielectric function. The formulation can be used to calculate the Casimir-Lifshitz forces for dielectric objects of arbitrary geometry, as a perturbative expansion in the dielectric contrast, and could thus complement the existing theories that use perturbation in geometrical features. We find that expansion in dielectric contrast recasts the resulting Lifshitz energy into a sum of the different many-body contributions. The limit of validity and convergence properties of themore » perturbation theory is discussed using the example of parallel semi-infinite objects for which the exact result is known.« less

Magnetic Resonance Imaging of Ventilation and Perfusion in the Lung

NASA Technical Reports Server (NTRS)

Prisk, Gordon Kim (Inventor); Hopkins, Susan Roberta (Inventor); Pereira De Sa, Rui Carlos (Inventor); Theilmann, Rebecca Jean (Inventor); Buxton, Richard Bruce (Inventor); Cronin, Matthew Vincent (Inventor)

2017-01-01

Methods, devices, and systems are disclosed for implementing a fully quantitative non-injectable contrast proton MRI technique to measure spatial ventilation-perfusion (VA/Q) matching and spatial distribution of ventilation and perfusion. In one aspect, a method using MRI to characterize ventilation and perfusion in a lung includes acquiring an MR image of the lung having MR data in a voxel and obtaining a breathing frequency parameter, determining a water density value, a specific ventilation value, and a perfusion value in at least one voxel of the MR image based on the MR data and using the water density value to determine an air content value, and determining a ventilation-perfusion ratio value that is the product of the specific ventilation value, the air content value, the inverse of the perfusion value, and the breathing frequency.

Evaluating visual function in cataract.

PubMed

Elliott, D B

1993-11-01

This paper reviews recent research on the evaluation of visual function in cataract. Visual impairment in cataract is principally caused by increased intraocular forward light scatter. It is assumed that visual acuity (VA) measurements assess the impact of narrow angle light scatter. This also makes the measurement of high spatial frequency contrast sensitivity (CS) unnecessary. However, VA measurements alone are an inadequate assessment of visual impairment in some patients with cataract. In addition, it is suggested that a measurement of wide-angle light scatter is required. This can be evaluated directly using the van den Berg Straylightmeter, or indirectly using low spatial frequency CS or disability glare (DG) tests. The following are discussed: (1) the relative usefulness of these tests; (2) how they can be incorporated into the decision as to when to extract a cataract; and (3) the importance of considering binocular visual function.

Carotid dual-energy CT angiography: Evaluation of low keV calculated monoenergetic datasets by means of a frequency-split approach for noise reduction at low keV levels.

PubMed

Riffel, Philipp; Haubenreisser, Holger; Meyer, Mathias; Sudarski, Sonja; Morelli, John N; Schmidt, Bernhard; Schoenberg, Stefan O; Henzler, Thomas

2016-04-01

Calculated monoenergetic ultra-low keV datasets did not lead to improved contrast-to-noise ratio (CNR) due to the dramatic increase in image noise. The aim of the present study was to evaluate the objective image quality of ultra-low keV monoenergetic images (MEIs) calculated from carotid DECT angiography data with a new monoenergetic imaging algorithm using a frequency-split technique. 20 patients (12 male; mean age 53±17 years) were retrospectively analyzed. MEIs from 40 to 120 keV were reconstructed using the monoenergetic split frequency approach (MFSA). Additionally MEIs were reconstructed for 40 and 50 keV using a conventional monoenergetic (CM) software application. Signal intensity, noise, signal-to-noise ratio (SNR) and CNR were assessed in the basilar, common, internal carotid arteries. Ultra-low keV MEIs at 40 keV and 50 keV demonstrated highest vessel attenuation, significantly greater than those of the polyenergetic images (PEI) (all p-values <0.05). The highest SNR level and CNR level was found at 40 keV and 50 keV (all p-values <0.05). MEIs with MFSA showed significantly lower noise levels than those processed with CM (all p-values <0.05) and no significant differences in vessel attenuation (p>0.05). Thus MEIs with MFSA showed significantly higher SNR and CNR compared to MEIs with CM. Combining the lower spatial frequency stack for contrast at low keV levels with the high spatial frequency stack for noise at high keV levels (frequency-split technique) leads to improved image quality of ultra-low keV monoenergetic DECT datasets when compared to previous monoenergetic reconstruction techniques without the frequency-split technique. Copyright © 2016 Elsevier Ireland Ltd. All rights reserved.

Adaptive windowing in contrast-enhanced intravascular ultrasound imaging

PubMed Central

Lindsey, Brooks D.; Martin, K. Heath; Jiang, Xiaoning; Dayton, Paul A.

2016-01-01

Intravascular ultrasound (IVUS) is one of the most commonly-used interventional imaging techniques and has seen recent innovations which attempt to characterize the risk posed by atherosclerotic plaques. One such development is the use of microbubble contrast agents to image vasa vasorum, fine vessels which supply oxygen and nutrients to the walls of coronary arteries and typically have diameters less than 200 µm. The degree of vasa vasorum neovascularization within plaques is positively correlated with plaque vulnerability. Having recently presented a prototype dual-frequency transducer for contrast agent-specific intravascular imaging, here we describe signal processing approaches based on minimum variance (MV) beamforming and the phase coherence factor (PCF) for improving the spatial resolution and contrast-to-tissue ratio (CTR) in IVUS imaging. These approaches are examined through simulations, phantom studies, ex vivo studies in porcine arteries, and in vivo studies in chicken embryos. In phantom studies, PCF processing improved CTR by a mean of 4.2 dB, while combined MV and PCF processing improved spatial resolution by 41.7%. Improvements of 2.2 dB in CTR and 37.2% in resolution were observed in vivo. Applying these processing strategies can enhance image quality in conventional B-mode IVUS or in contrast-enhanced IVUS, where signal-to-noise ratio is relatively low and resolution is at a premium. PMID:27161022

Deconstructing continuous flash suppression

PubMed Central

Yang, Eunice; Blake, Randolph

2012-01-01

In this paper, we asked to what extent the depth of interocular suppression engendered by continuous flash suppression (CFS) varies depending on spatiotemporal properties of the suppressed stimulus and CFS suppressor. An answer to this question could have implications for interpreting the results in which CFS influences the processing of different categories of stimuli to different extents. In a series of experiments, we measured the selectivity and depth of suppression (i.e., elevation in contrast detection thresholds) as a function of the visual features of the stimulus being suppressed and the stimulus evoking suppression, namely, the popular “Mondrian” CFS stimulus (N. Tsuchiya & C. Koch, 2005). First, we found that CFS differentially suppresses the spatial components of the suppressed stimulus: Observers' sensitivity for stimuli of relatively low spatial frequency or cardinally oriented features was more strongly impaired in comparison to high spatial frequency or obliquely oriented stimuli. Second, we discovered that this feature-selective bias primarily arises from the spatiotemporal structure of the CFS stimulus, particularly within information residing in the low spatial frequency range and within the smooth rather than abrupt luminance changes over time. These results imply that this CFS stimulus operates by selectively attenuating certain classes of low-level signals while leaving others to be potentially encoded during suppression. These findings underscore the importance of considering the contribution of low-level features in stimulus-driven effects that are reported under CFS. PMID:22408039

Deconstructing continuous flash suppression.

PubMed

Yang, Eunice; Blake, Randolph

2012-03-08

In this paper, we asked to what extent the depth of interocular suppression engendered by continuous flash suppression (CFS) varies depending on spatiotemporal properties of the suppressed stimulus and CFS suppressor. An answer to this question could have implications for interpreting the results in which CFS influences the processing of different categories of stimuli to different extents. In a series of experiments, we measured the selectivity and depth of suppression (i.e., elevation in contrast detection thresholds) as a function of the visual features of the stimulus being suppressed and the stimulus evoking suppression, namely, the popular "Mondrian" CFS stimulus (N. Tsuchiya & C. Koch, 2005). First, we found that CFS differentially suppresses the spatial components of the suppressed stimulus: Observers' sensitivity for stimuli of relatively low spatial frequency or cardinally oriented features was more strongly impaired in comparison to high spatial frequency or obliquely oriented stimuli. Second, we discovered that this feature-selective bias primarily arises from the spatiotemporal structure of the CFS stimulus, particularly within information residing in the low spatial frequency range and within the smooth rather than abrupt luminance changes over time. These results imply that this CFS stimulus operates by selectively attenuating certain classes of low-level signals while leaving others to be potentially encoded during suppression. These findings underscore the importance of considering the contribution of low-level features in stimulus-driven effects that are reported under CFS.

1st- and 2nd-order motion and texture resolution in central and peripheral vision

NASA Technical Reports Server (NTRS)

Solomon, J. A.; Sperling, G.

1995-01-01

STIMULI. The 1st-order stimuli are moving sine gratings. The 2nd-order stimuli are fields of static visual texture, whose contrasts are modulated by moving sine gratings. Neither the spatial slant (orientation) nor the direction of motion of these 2nd-order (microbalanced) stimuli can be detected by a Fourier analysis; they are invisible to Reichardt and motion-energy detectors. METHOD. For these dynamic stimuli, when presented both centrally and in an annular window extending from 8 to 10 deg in eccentricity, we measured the highest spatial frequency for which discrimination between +/- 45 deg texture slants and discrimination between opposite directions of motion were each possible. RESULTS. For sufficiently low spatial frequencies, slant and direction can be discriminated in both central and peripheral vision, for both 1st- and for 2nd-order stimuli. For both 1st- and 2nd-order stimuli, at both retinal locations, slant discrimination is possible at higher spatial frequencies than direction discrimination. For both 1st- and 2nd-order stimuli, motion resolution decreases 2-3 times more rapidly with eccentricity than does texture resolution. CONCLUSIONS. (1) 1st- and 2nd-order motion scale similarly with eccentricity. (2) 1st- and 2nd-order texture scale similarly with eccentricity. (3) The central/peripheral resolution fall-off is 2-3 times greater for motion than for texture.

Amblyopia in Astigmatic Children: Patterns of Deficits

PubMed Central

Harvey, Erin M.; Dobson, Velma; Miller, Joseph M.; Clifford-Donaldson, Candice E.

2007-01-01

Neural changes that result from disruption of normal visual experience during development are termed amblyopia. To characterize visual deficits specific to astigmatism-related amblyopia, we compared best-corrected visual performance in 330 astigmatic and 475 non-astigmatic kindergarten through 6th grade children. Astigmatism was associated with deficits in letter, grating and vernier acuity, high and middle spatial frequency contrast sensitivity, and stereoacuity. Although grating acuity, vernier acuity, and contrast sensitivity were reduced across stimulus orientation, astigmats demonstrated orientation-dependent deficits (meridional amblyopia) only for grating acuity. Astigmatic children are at risk for deficits across a range of visual functions. PMID:17184807

Biomedical terahertz imaging with a quantum cascade laser

NASA Astrophysics Data System (ADS)

Kim, Seongsin M.; Hatami, Fariba; Harris, James S.; Kurian, Allison W.; Ford, James; King, Douglas; Scalari, Giacomo; Giovannini, Marcella; Hoyler, Nicolas; Faist, Jerome; Harris, Geoff

2006-04-01

We present biomedical imaging using a single frequency terahertz imaging system based on a low threshold quantum cascade laser emitting at 3.7THz (λ=81μm). With a peak output power of 4mW, coherent terahertz radiation and detection provide a relatively large dynamic range and high spatial resolution. We study image contrast based on water/fat content ratios in different tissues. Terahertz transmission imaging demonstrates a distinct anatomy in a rat brain slice. We also demonstrate malignant tissue contrast in an image of a mouse liver with developed tumors, indicating potential use of terahertz imaging for probing cancerous tissues.

Nonlinear vibrational microscopy

DOEpatents

Holtom, Gary R.; Xie, Xiaoliang Sunney; Zumbusch, Andreas

2000-01-01

The present invention is a method and apparatus for microscopic vibrational imaging using coherent Anti-Stokes Raman Scattering or Sum Frequency Generation. Microscopic imaging with a vibrational spectroscopic contrast is achieved by generating signals in a nonlinear optical process and spatially resolved detection of the signals. The spatial resolution is attained by minimizing the spot size of the optical interrogation beams on the sample. Minimizing the spot size relies upon a. directing at least two substantially co-axial laser beams (interrogation beams) through a microscope objective providing a focal spot on the sample; b. collecting a signal beam together with a residual beam from the at least two co-axial laser beams after passing through the sample; c. removing the residual beam; and d. detecting the signal beam thereby creating said pixel. The method has significantly higher spatial resolution then IR microscopy and higher sensitivity than spontaneous Raman microscopy with much lower average excitation powers. CARS and SFG microscopy does not rely on the presence of fluorophores, but retains the resolution and three-dimensional sectioning capability of confocal and two-photon fluorescence microscopy. Complementary to these techniques, CARS and SFG microscopy provides a contrast mechanism based on vibrational spectroscopy. This vibrational contrast mechanism, combined with an unprecedented high sensitivity at a tolerable laser power level, provides a new approach for microscopic investigations of chemical and biological samples.

Spatial layout of letters in nonwords affects visual short-term memory load: evidence from human electrophysiology.

PubMed

Prime, David; Dell'acqua, Roberto; Arguin, Martin; Gosselin, Frédéric; Jolicœur, Pierre

2011-03-01

The sustained posterior contralateral negativity (SPCN) was used to investigate the effect of spatial layout on the maintenance of letters in VSTM. SPCN amplitude was measured for words, nonwords, and scrambled nonwords. We reexamined the effects of spatial layout of letters on SPCN amplitude in a design that equated the mean frequency of use of each position. Scrambled letters that did not form words elicited a larger SPCN than either words or nonwords, indicating lower VSTM load for nonwords presented in a typical horizontal array than the load observed for the same letters presented in spatially scrambled locations. In contrast, prior research has shown that the spatial extent of arrays of simple stimuli did not influence the amplitude of the SPCN. Thus, the present results indicate the existence of encoding and VSTM maintenance mechanisms specific to letter and word processing. Copyright © 2010 Society for Psychophysiological Research.

Grating test of contrast sensitivity in patients with Minamata disease.

PubMed Central

Mukuno, K; Ishikawa, S; Okamura, R

1981-01-01

Thirty cases of Minamata disease caused by methyl mercury poisoning with the lesion mainly at the occipital cortex were selected and their spatial contrast sensitivity of vision was examined by the Arden grating chart. At the same time their visual acuity, visual field, and visual evoked cortical potential (VECP) were also investigated. In all cases the results of the Arden test indicated abnormality. Poor results were obtained at higher frequencies of the gratings. VECP elicited by grating pattern reversal stimulus was undertaken in 12 cases out of the 30. The results revealed abnormality almost equal to that shown by the Arden test. Seven out of the 12 cases showed no VECP response. The other 5, giving a response, showed abnormality: when the size of the grating became smaller at higher frequencies, the VECP paused or was not recorded, whereas at low frequencies it was recorded. This finding was in good agreement or was not recorded, whereas at low frequencies it was recorded. This finding was in good agreement with the results of the Arden test. Visual acuity and visual field tests were less sensitive in detecting abnormality. The Arden chart is a sensitive clinical tool for patients with lesions at the cerebral cortex. Furthermore, the test can be used for screening patients who may have come in contrast with organic mercury. PMID:7236573

Performance evaluation of spatial compounding in the presence of aberration and adaptive imaging

NASA Astrophysics Data System (ADS)

Dahl, Jeremy J.; Guenther, Drake; Trahey, Gregg E.

2003-05-01

Spatial compounding has been used for years to reduce speckle in ultrasonic images and to resolve anatomical features hidden behind the grainy appearance of speckle. Adaptive imaging restores image contrast and resolution by compensating for beamforming errors caused by tissue-induced phase errors. Spatial compounding represents a form of incoherent imaging, whereas adaptive imaging attempts to maintain a coherent, diffraction-limited aperture in the presence of aberration. Using a Siemens Antares scanner, we acquired single channel RF data on a commercially available 1-D probe. Individual channel RF data was acquired on a cyst phantom in the presence of a near field electronic phase screen. Simulated data was also acquired for both a 1-D and a custom built 8x96, 1.75-D probe (Tetrad Corp.). The data was compounded using a receive spatial compounding algorithm; a widely used algorithm because it takes advantage of parallel beamforming to avoid reductions in frame rate. Phase correction was also performed by using a least mean squares algorithm to estimate the arrival time errors. We present simulation and experimental data comparing the performance of spatial compounding to phase correction in contrast and resolution tasks. We evaluate spatial compounding and phase correction, and combinations of the two methods, under varying aperture sizes, aperture overlaps, and aberrator strength to examine the optimum configuration and conditions in which spatial compounding will provide a similar or better result than adaptive imaging. We find that, in general, phase correction is hindered at high aberration strengths and spatial frequencies, whereas spatial compounding is helped by these aberrators.

Assessing spatial and temporal properties of perimetric stimuli for resistance to clinical variations in retinal illumination.

PubMed

Swanson, William H; Dul, Mitchell W; Horner, Douglas G; Liu, Tiffany; Tran, Irene

2014-01-20

To develop perimetric stimuli for which sensitivities are more resistant to reduced retinal illumination than current clinical perimeters. Fifty-four people free of eye disease were dilated and tested monocularly. For each test, retinal illumination was attenuated with neutral density (ND) filters, and a standard adaptation model was fit to derive mean and SEM for the adaptation parameter (NDhalf). For different stimuli, t-tests on NDhalf were used to assess significance of differences in consistency with Weber's law. Three experiments used custom Gaussian-windowed contrast sensitivity perimetry (CSP). Experiment 1 used CSP-1, with a Gaussian temporal pulse, a spatial frequency of 0.375 cyc/deg (cpd), and SD of 1.5°. Experiment 1 also used the Humphrey Matrix perimeter, with the N-30 test using 0.25 cpd and 25 Hz flicker. Experiment 2 used a rectangular temporal pulse, SDs of 0.25° and 0.5°, and spatial frequencies of 0.0 and 1.0 cpd. Experiment 3 used CSP-2, with 5-Hz flicker, SDs from 0.5° to 1.8°, and spatial frequencies from 0.14 to 0.50 cpd. In Experiment 1, CSP-1 was more consistent with Weber's law (NDhalf ± SEM = 1.86 ± 0.08 log unit) than N-30 (NDhalf = 1.03 ± 0.03 log unit; t > 9, P < 0.0001). All stimuli used in Experiments 2 and 3 had comparable consistency with Weber's law (NDhalf = 1.49-1.69 log unit; t < 2). Perimetric sensitivities were consistent with Weber's law when higher temporal frequencies were avoided.

New technologies lead to a new frontier: cognitive multiple data representation

NASA Astrophysics Data System (ADS)

Buffat, S.; Liege, F.; Plantier, J.; Roumes, C.

2005-05-01

The increasing number and complexity of operational sensors (radar, infrared, hyperspectral...) and availability of huge amount of data, lead to more and more sophisticated information presentations. But one key element of the IMINT line cannot be improved beyond initial system specification: the operator.... In order to overcome this issue, we have to better understand human visual object representation. Object recognition theories in human vision balance between matching 2D templates representation with viewpoint-dependant information, and a viewpoint-invariant system based on structural description. Spatial frequency content is relevant due to early vision filtering. Orientation in depth is an important variable to challenge object constancy. Three objects, seen from three different points of view in a natural environment made the original images in this study. Test images were a combination of spatial frequency filtered original images and an additive contrast level of white noise. In the first experiment, the observer's task was a same versus different forced choice with spatial alternative. Test images had the same noise level in a presentation row. Discrimination threshold was determined by modifying the white noise contrast level by means of an adaptative method. In the second experiment, a repetition blindness paradigm was used to further investigate the viewpoint effect on object recognition. The results shed some light on the human visual system processing of objects displayed under different physical descriptions. This is an important achievement because targets which not always match physical properties of usual visual stimuli can increase operational workload.

Pitch perception deficits in nonverbal learning disability.

PubMed

Fernández-Prieto, I; Caprile, C; Tinoco-González, D; Ristol-Orriols, B; López-Sala, A; Póo-Argüelles, P; Pons, F; Navarra, J

2016-12-01

The nonverbal learning disability (NLD) is a neurological dysfunction that affects cognitive functions predominantly related to the right hemisphere such as spatial and abstract reasoning. Previous evidence in healthy adults suggests that acoustic pitch (i.e., the relative difference in frequency between sounds) is, under certain conditions, encoded in specific areas of the right hemisphere that also encode the spatial elevation of external objects (e.g., high vs. low position). Taking this evidence into account, we explored the perception of pitch in preadolescents and adolescents with NLD and in a group of healthy participants matched by age, gender, musical knowledge and handedness. Participants performed four speeded tests: a stimulus detection test and three perceptual categorization tests based on colour, spatial position and pitch. Results revealed that both groups were equally fast at detecting visual targets and categorizing visual stimuli according to their colour. In contrast, the NLD group showed slower responses than the control group when categorizing space (direction of a visual object) and pitch (direction of a change in sound frequency). This pattern of results suggests the presence of a subtle deficit at judging pitch in NLD along with the traditionally-described difficulties in spatial processing. Copyright © 2016. Published by Elsevier Ltd.

Modeling astronomical adaptive optics performance with temporally filtered Wiener reconstruction of slope data

NASA Astrophysics Data System (ADS)

Correia, Carlos M.; Bond, Charlotte Z.; Sauvage, Jean-François; Fusco, Thierry; Conan, Rodolphe; Wizinowich, Peter L.

2017-10-01

We build on a long-standing tradition in astronomical adaptive optics (AO) of specifying performance metrics and error budgets using linear systems modeling in the spatial-frequency domain. Our goal is to provide a comprehensive tool for the calculation of error budgets in terms of residual temporally filtered phase power spectral densities and variances. In addition, the fast simulation of AO-corrected point spread functions (PSFs) provided by this method can be used as inputs for simulations of science observations with next-generation instruments and telescopes, in particular to predict post-coronagraphic contrast improvements for planet finder systems. We extend the previous results and propose the synthesis of a distributed Kalman filter to mitigate both aniso-servo-lag and aliasing errors whilst minimizing the overall residual variance. We discuss applications to (i) analytic AO-corrected PSF modeling in the spatial-frequency domain, (ii) post-coronagraphic contrast enhancement, (iii) filter optimization for real-time wavefront reconstruction, and (iv) PSF reconstruction from system telemetry. Under perfect knowledge of wind velocities, we show that $\\sim$60 nm rms error reduction can be achieved with the distributed Kalman filter embodying anti- aliasing reconstructors on 10 m class high-order AO systems, leading to contrast improvement factors of up to three orders of magnitude at few ${\\lambda}/D$ separations ($\\sim1-5{\\lambda}/D$) for a 0 magnitude star and reaching close to one order of magnitude for a 12 magnitude star.

Contrast sensitivity measured by two different test methods in healthy, young adults with normal visual acuity.

PubMed

Koefoed, Vilhelm F; Baste, Valborg; Roumes, Corinne; Høvding, Gunnar

2015-03-01

This study reports contrast sensitivity (CS) reference values obtained by two different test methods in a strictly selected population of healthy, young adults with normal uncorrected visual acuity. Based on these results, the index of contrast sensitivity (ICS) is calculated, aiming to establish ICS reference values for this population and to evaluate the possible usefulness of ICS as a tool to compare the degree of agreement between different CS test methods. Military recruits with best eye uncorrected visual acuity 0.00 LogMAR or better, normal colour vision and age 18-25 years were included in a study to record contrast sensitivity using Optec 6500 (FACT) at spatial frequencies of 1.5, 3, 6, 12 and 18 cpd in photopic and mesopic light and CSV-1000E at spatial frequencies of 3, 6, 12 and 18 cpd in photopic light. Index of contrast sensitivity was calculated based on data from the three tests, and the Bland-Altman technique was used to analyse the agreement between ICS obtained by the different test methods. A total of 180 recruits were included. Contrast sensitivity frequency data for all tests were highly skewed with a marked ceiling effect for the photopic tests. The median ICS for Optec 6500 at 85 cd/m2 was -0.15 (95% percentile 0.45), compared with -0.00 (95% percentile 1.62) for Optec at 3 cd/m2 and 0.30 (95% percentile 1.20) FOR CSV-1000E. The mean difference between ICSFACT 85 and ICSCSV was -0.43 (95% CI -0.56 to -0.30, p<0.00) with limits of agreement (LoA) within -2.10 and 1.22. The regression line on the difference of average was near to zero (R2=0.03). The results provide reference CS and ICS values in a young, adult population with normal visual acuity. The agreement between the photopic tests indicated that they may be used interchangeably. There was little agreement between the mesopic and photopic tests. The mesopic test seemed best suited to differentiate between candidates and may therefore possibly be useful for medical selection purposes. © 2014 Acta Ophthalmologica Scandinavica Foundation. Published by John Wiley & Sons Ltd.

Digital holographic microscope with low-frequency attenuation filter for position measurement of a nanoparticle.

PubMed

Pham, Quang Duc; Kusumi, Yuichi; Hasegawa, Satoshi; Hayasaki, Yoshio

2012-10-01

We propose a new method for three-dimensional (3D) position measurement of nanoparticles using an in-line digital holographic microscope. The method improves the signal-to-noise ratio of the amplitude of the interference fringes to achieve higher accuracy in the position measurement by increasing weak scattered light from a nanoparticle relative to the reference light by using a low spatial frequency attenuation filter. We demonstrated the improvements of signal-to-noise ratio of the optical system and contrast of the interference fringes, allowing the 3D positions of nanoparticles to be determined more precisely.

Singular value decomposition metrics show limitations of detector design in diffuse fluorescence tomography

PubMed Central

Leblond, Frederic; Tichauer, Kenneth M.; Pogue, Brian W.

2010-01-01

The spatial resolution and recovered contrast of images reconstructed from diffuse fluorescence tomography data are limited by the high scattering properties of light propagation in biological tissue. As a result, the image reconstruction process can be exceedingly vulnerable to inaccurate prior knowledge of tissue optical properties and stochastic noise. In light of these limitations, the optimal source-detector geometry for a fluorescence tomography system is non-trivial, requiring analytical methods to guide design. Analysis of the singular value decomposition of the matrix to be inverted for image reconstruction is one potential approach, providing key quantitative metrics, such as singular image mode spatial resolution and singular data mode frequency as a function of singular mode. In the present study, these metrics are used to analyze the effects of different sources of noise and model errors as related to image quality in the form of spatial resolution and contrast recovery. The image quality is demonstrated to be inherently noise-limited even when detection geometries were increased in complexity to allow maximal tissue sampling, suggesting that detection noise characteristics outweigh detection geometry for achieving optimal reconstructions. PMID:21258566

Cholinergic enhancement of visual attention and neural oscillations in the human brain.

PubMed

Bauer, Markus; Kluge, Christian; Bach, Dominik; Bradbury, David; Heinze, Hans Jochen; Dolan, Raymond J; Driver, Jon

2012-03-06

Cognitive processes such as visual perception and selective attention induce specific patterns of brain oscillations. The neurochemical bases of these spectral changes in neural activity are largely unknown, but neuromodulators are thought to regulate processing. The cholinergic system is linked to attentional function in vivo, whereas separate in vitro studies show that cholinergic agonists induce high-frequency oscillations in slice preparations. This has led to theoretical proposals that cholinergic enhancement of visual attention might operate via gamma oscillations in visual cortex, although low-frequency alpha/beta modulation may also play a key role. Here we used MEG to record cortical oscillations in the context of administration of a cholinergic agonist (physostigmine) during a spatial visual attention task in humans. This cholinergic agonist enhanced spatial attention effects on low-frequency alpha/beta oscillations in visual cortex, an effect correlating with a drug-induced speeding of performance. By contrast, the cholinergic agonist did not alter high-frequency gamma oscillations in visual cortex. Thus, our findings show that cholinergic neuromodulation enhances attentional selection via an impact on oscillatory synchrony in visual cortex, for low rather than high frequencies. We discuss this dissociation between high- and low-frequency oscillations in relation to proposals that lower-frequency oscillations are generated by feedback pathways within visual cortex. Copyright © 2012 Elsevier Ltd. All rights reserved.

Y-cell receptive field and collicular projection of parasol ganglion cells in macaque monkey retina

PubMed Central

Crook, Joanna D.; Peterson, Beth B.; Packer, Orin S.; Robinson, Farrel R.; Troy, John B.; Dacey, Dennis M.

2009-01-01

The distinctive parasol ganglion cell of the primate retina transmits a transient, spectrally non-opponent signal to the magnocellular layers of the lateral geniculate nucleus (LGN). Parasol cells show well-recognized parallels with the alpha-Y cell of other mammals, yet two key alpha-Y cell properties, a collateral projection to the superior colliculus and nonlinear spatial summation, have not been clearly established for parasol cells. Here we show by retrograde photodynamic staining that parasol cells project to the superior colliculus. Photostained dendritic trees formed characteristic spatial mosaics and afforded unequivocal identification of the parasol cells among diverse collicular-projecting cell types. Loose-patch recordings were used to demonstrate for all parasol cells a distinct Y-cell receptive field ‘signature’ marked by a non-linear mechanism that responded to contrast-reversing gratings at twice the stimulus temporal frequency (second Fourier harmonic, F2) independent of stimulus spatial phase. The F2 component showed high contrast gain and temporal sensitivity and appeared to originate from a region coextensive with that of the linear receptive field center. The F2 spatial frequency response peaked well beyond the resolution limit of the linear receptive field center, showing a Gaussian center radius of ~15 μm. Blocking inner retinal inhibition elevated the F2 response, suggesting that amacrine circuitry does not generate this non-linearity. Our data are consistent with a pooled-subunit model of the parasol-Y cell receptive field in which summation from an array of transient, partially rectifying cone bipolar cells accounts for both linear and non-linear components of the receptive field. PMID:18971470

The highs and lows of object impossibility: effects of spatial frequency on holistic processing of impossible objects.

PubMed

Freud, Erez; Avidan, Galia; Ganel, Tzvi

2015-02-01

Holistic processing, the decoding of a stimulus as a unified whole, is a basic characteristic of object perception. Recent research using Garner's speeded classification task has shown that this processing style is utilized even for impossible objects that contain an inherent spatial ambiguity. In particular, similar Garner interference effects were found for possible and impossible objects, indicating similar holistic processing styles for the two object categories. In the present study, we further investigated the perceptual mechanisms that mediate such holistic representation of impossible objects. We relied on the notion that, whereas information embedded in the high-spatial-frequency (HSF) content supports fine-detailed processing of object features, the information conveyed by low spatial frequencies (LSF) is more crucial for the emergence of a holistic shape representation. To test the effects of image frequency on the holistic processing of impossible objects, participants performed the Garner speeded classification task on images of possible and impossible cubes filtered for their LSF and HSF information. For images containing only LSF, similar interference effects were observed for possible and impossible objects, indicating that the two object categories were processed in a holistic manner. In contrast, for the HSF images, Garner interference was obtained only for possible, but not for impossible objects. Importantly, we provided evidence to show that this effect could not be attributed to a lack of sensitivity to object possibility in the LSF images. Particularly, even for full-spectrum images, Garner interference was still observed for both possible and impossible objects. Additionally, performance in an object classification task revealed high sensitivity to object possibility, even for LSF images. Taken together, these findings suggest that the visual system can tolerate the spatial ambiguity typical to impossible objects by relying on information embedded in LSF, whereas HSF information may underlie the visual system's susceptibility to distortions in objects' spatial layouts.

Complex-valued time-series correlation increases sensitivity in FMRI analysis.

PubMed

Kociuba, Mary C; Rowe, Daniel B

2016-07-01

To develop a linear matrix representation of correlation between complex-valued (CV) time-series in the temporal Fourier frequency domain, and demonstrate its increased sensitivity over correlation between magnitude-only (MO) time-series in functional MRI (fMRI) analysis. The standard in fMRI is to discard the phase before the statistical analysis of the data, despite evidence of task related change in the phase time-series. With a real-valued isomorphism representation of Fourier reconstruction, correlation is computed in the temporal frequency domain with CV time-series data, rather than with the standard of MO data. A MATLAB simulation compares the Fisher-z transform of MO and CV correlations for varying degrees of task related magnitude and phase amplitude change in the time-series. The increased sensitivity of the complex-valued Fourier representation of correlation is also demonstrated with experimental human data. Since the correlation description in the temporal frequency domain is represented as a summation of second order temporal frequencies, the correlation is easily divided into experimentally relevant frequency bands for each voxel's temporal frequency spectrum. The MO and CV correlations for the experimental human data are analyzed for four voxels of interest (VOIs) to show the framework with high and low contrast-to-noise ratios in the motor cortex and the supplementary motor cortex. The simulation demonstrates the increased strength of CV correlations over MO correlations for low magnitude contrast-to-noise time-series. In the experimental human data, the MO correlation maps are noisier than the CV maps, and it is more difficult to distinguish the motor cortex in the MO correlation maps after spatial processing. Including both magnitude and phase in the spatial correlation computations more accurately defines the correlated left and right motor cortices. Sensitivity in correlation analysis is important to preserve the signal of interest in fMRI data sets with high noise variance, and avoid excessive processing induced correlation. Copyright © 2016 Elsevier Inc. All rights reserved.

Transcranial magnetic stimulation changes response selectivity of neurons in the visual cortex

PubMed Central

Kim, Taekjun; Allen, Elena A.; Pasley, Brian N.; Freeman, Ralph D.

2015-01-01

Background Transcranial magnetic stimulation (TMS) is used to selectively alter neuronal activity of specific regions in the cerebral cortex. TMS is reported to induce either transient disruption or enhancement of different neural functions. However, its effects on tuning properties of sensory neurons have not been studied quantitatively. Objective/Hypothesis Here, we use specific TMS application parameters to determine how they may alter tuning characteristics (orientation, spatial frequency, and contrast sensitivity) of single neurons in the cat’s visual cortex. Methods Single unit spikes were recorded with tungsten microelectrodes from the visual cortex of anesthetized and paralyzed cats (12 males). Repetitive TMS (4Hz, 4sec) was delivered with a 70mm figure-8 coil. We quantified basic tuning parameters of individual neurons for each pre- and post-TMS condition. The statistical significance of changes for each tuning parameter between the two conditions was evaluated with a Wilcoxon signed-rank test. Results We generally find long-lasting suppression which persists well beyond the stimulation period. Pre- and post-TMS orientation tuning curves show constant peak values. However, strong suppression at non-preferred orientations tends to narrow the widths of tuning curves. Spatial frequency tuning exhibits an asymmetric change in overall shape, which results in an emphasis on higher frequencies. Contrast tuning curves show nonlinear changes consistent with a gain control mechanism. Conclusions These findings suggest that TMS causes extended interruption of the balance between sub-cortical and intra-cortical inputs. PMID:25862599

Maximum bandwidth snapshot channeled imaging polarimeter with polarization gratings

NASA Astrophysics Data System (ADS)

LaCasse, Charles F.; Redman, Brian J.; Kudenov, Michael W.; Craven, Julia M.

2016-05-01

Compact snapshot imaging polarimeters have been demonstrated in literature to provide Stokes parameter estimations for spatially varying scenes using polarization gratings. However, the demonstrated system does not employ aggressive modulation frequencies to take full advantage of the bandwidth available to the focal plane array. A snapshot imaging Stokes polarimeter is described and demonstrated through results. The simulation studies the challenges of using a maximum bandwidth configuration for a snapshot polarization grating based polarimeter, such as the fringe contrast attenuation that results from higher modulation frequencies. Similar simulation results are generated and compared for a microgrid polarimeter. Microgrid polarimeters are instruments where pixelated polarizers are superimposed onto a focal plan array, and this is another type of spatially modulated polarimeter, and the most common design uses a 2x2 super pixel of polarizers which maximally uses the available bandwidth of the focal plane array.

A resolution-enhancing image reconstruction method for few-view differential phase-contrast tomography

NASA Astrophysics Data System (ADS)

Guan, Huifeng; Anastasio, Mark A.

2017-03-01

It is well-known that properly designed image reconstruction methods can facilitate reductions in imaging doses and data-acquisition times in tomographic imaging. The ability to do so is particularly important for emerging modalities such as differential X-ray phase-contrast tomography (D-XPCT), which are currently limited by these factors. An important application of D-XPCT is high-resolution imaging of biomedical samples. However, reconstructing high-resolution images from few-view tomographic measurements remains a challenging task. In this work, a two-step sub-space reconstruction strategy is proposed and investigated for use in few-view D-XPCT image reconstruction. It is demonstrated that the resulting iterative algorithm can mitigate the high-frequency information loss caused by data incompleteness and produce images that have better preserved high spatial frequency content than those produced by use of a conventional penalized least squares (PLS) estimator.

Demographic stochasticity in small remnant populations of the declining distylous plant Primula veris

USGS Publications Warehouse

Kery, M.; Matthies, D.; Schmid, B.

2003-01-01

We studied ecological consequences of distyly for the declining perennial plant Primula veris in the Swiss Jura. Distyly favours cross-fertilization and avoids inbreeding, but may lead to pollen limitation and reduced reproduction if morph frequencies deviate from 50 %. Disassortative mating is promoted by the reciprocal position of stigmas and anthers in the two morphs (pin and thrum) and by intramorph incompatibility and should result in equal frequencies of morphs at equilibrium. However, deviations could arise because of demographic stochasticity, the lower intra-morph incompatibility of the pin morph, and niche differentiation between morphs. Demographic stochasticity should result in symmetric deviations from an even morph frequency among populations and in increased deviations with decreasing population size. If crosses between pins occurred, these would only generate pins, and this could result in a pin-bias of morph frequencies in general and in small populations in particular. If the morphs have different niches, morph frequencies should be related to environmental factors, morphs might be spatially segregated, and morphological differences between morphs would be expected. We tested these hypotheses in the declining distylous P. veris. We studied morph frequencies in relation to environmental conditions and population size, spatial segregation in field populations, morphological differences between morphs, and growth responses to nutrient addition. Morph frequencies in 76 populations with 1 - 80000 flowering plants fluctuated symmetrically about 50 %. Deviations from 50 % were much larger in small populations, and sixof the smallest populations had lost one morph altogether. In contrast, morph frequencies were neither related to population size nor to 17 measures of environmental conditions. We found no spatial segregation or morphological differences in the field or in the common garden. The results suggest that demographic stochasticity caused deviations of the morph ratiofrom unity in small populations. Demographic stochasticity was probably caused by the random elimination of plants during the fragmentation of formerly large continuous populations. Biased morph frequencies may be one of the reasons for the strongly reduced reproduction in small populations of P. veris.

Greater magnocellular saccadic suppression in high versus low autistic tendency suggests a causal path to local perceptual style.

PubMed

Crewther, David P; Crewther, Daniel; Bevan, Stephanie; Goodale, Melvyn A; Crewther, Sheila G

2015-12-01

Saccadic suppression-the reduction of visual sensitivity during rapid eye movements-has previously been proposed to reflect a specific suppression of the magnocellular visual system, with the initial neural site of that suppression at or prior to afferent visual information reaching striate cortex. Dysfunction in the magnocellular visual pathway has also been associated with perceptual and physiological anomalies in individuals with autism spectrum disorder or high autistic tendency, leading us to question whether saccadic suppression is altered in the broader autism phenotype. Here we show that individuals with high autistic tendency show greater saccadic suppression of low versus high spatial frequency gratings while those with low autistic tendency do not. In addition, those with high but not low autism spectrum quotient (AQ) demonstrated pre-cortical (35-45 ms) evoked potential differences (saccade versus fixation) to a large, low contrast, pseudo-randomly flashing bar. Both AQ groups showed similar differential visual evoked potential effects in later epochs (80-160 ms) at high contrast. Thus, the magnocellular theory of saccadic suppression appears untenable as a general description for the typically developing population. Our results also suggest that the bias towards local perceptual style reported in autism may be due to selective suppression of low spatial frequency information accompanying every saccadic eye movement.

Spatially-Resolved Hydraulic Conductivity Estimation Via Poroelastic Magnetic Resonance Elastography

PubMed Central

McGarry, Matthew; Weaver, John B.; Paulsen, Keith D.

2015-01-01

Poroelastic magnetic resonance elastography is an imaging technique that could recover mechanical and hydrodynamical material properties of in vivo tissue. To date, mechanical properties have been estimated while hydrodynamical parameters have been assumed homogeneous with literature-based values. Estimating spatially-varying hydraulic conductivity would likely improve model accuracy and provide new image information related to a tissue’s interstitial fluid compartment. A poroelastic model was reformulated to recover hydraulic conductivity with more appropriate fluid-flow boundary conditions. Simulated and physical experiments were conducted to evaluate the accuracy and stability of the inversion algorithm. Simulations were accurate (property errors were < 2%) even in the presence of Gaussian measurement noise up to 3%. The reformulated model significantly decreased variation in the shear modulus estimate (p≪0.001) and eliminated the homogeneity assumption and the need to assign hydraulic conductivity values from literature. Material property contrast was recovered experimentally in three different tofu phantoms and the accuracy was improved through soft-prior regularization. A frequency-dependence in hydraulic conductivity contrast was observed suggesting that fluid-solid interactions may be more prominent at low frequency. In vivo recovery of both structural and hydrodynamical characteristics of tissue could improve detection and diagnosis of neurological disorders such as hydrocephalus and brain tumors. PMID:24771571

Modeling of Nonlinear Beat Signals of TAE's

NASA Astrophysics Data System (ADS)

Zhang, Bo; Berk, Herbert; Breizman, Boris; Zheng, Linjin

2012-03-01

Experiments on Alcator C-Mod reveal Toroidal Alfven Eigenmodes (TAE) together with signals at various beat frequencies, including those at twice the mode frequency. The beat frequencies are sidebands driven by quadratic nonlinear terms in the MHD equations. These nonlinear sidebands have not yet been quantified by any existing codes. We extend the AEGIS code to capture nonlinear effects by treating the nonlinear terms as a driving source in the linear MHD solver. Our goal is to compute the spatial structure of the sidebands for realistic geometry and q-profile, which can be directly compared with experiment in order to interpret the phase contrast imaging diagnostic measurements and to enable the quantitative determination of the Alfven wave amplitude in the plasma core

Chromatic summation and receptive field properties of blue-on and blue-off cells in marmoset lateral geniculate nucleus.

PubMed

Eiber, C D; Pietersen, A N J; Zeater, N; Solomon, S G; Martin, P R

2017-11-22

The "blue-on" and "blue-off" receptive fields in retina and dorsal lateral geniculate nucleus (LGN) of diurnal primates combine signals from short-wavelength sensitive (S) cone photoreceptors with signals from medium/long wavelength sensitive (ML) photoreceptors. Three questions about this combination remain unresolved. Firstly, is the combination of S and ML signals in these cells linear or non-linear? Secondly, how does the timing of S and ML inputs to these cells influence their responses? Thirdly, is there spatial antagonism within S and ML subunits of the receptive field of these cells? We measured contrast sensitivity and spatial frequency tuning for four types of drifting sine gratings: S cone isolating, ML cone isolating, achromatic (S + ML), and counterphase chromatic (S - ML), in extracellular recordings from LGN of marmoset monkeys. We found that responses to stimuli which modulate both S and ML cones are well predicted by a linear sum of S and ML signals, followed by a saturating contrast-response relation. Differences in sensitivity and timing (i.e. vector combination) between S and ML inputs are needed to explain the amplitude and phase of responses to achromatic (S + ML) and counterphase chromatic (S - ML) stimuli. Best-fit spatial receptive fields for S and/or ML subunits in most cells (>80%) required antagonistic surrounds, usually in the S subunit. The surrounds were however generally weak and had little influence on spatial tuning. The sensitivity and size of S and ML subunits were correlated on a cell-by-cell basis, adding to evidence that blue-on and blue-off receptive fields are specialised to signal chromatic but not spatial contrast. Copyright © 2017 Elsevier Ltd. All rights reserved.

Toxic risks and nutritional benefits of traditional diet on near visual contrast sensitivity and color vision in the Brazilian Amazon.

PubMed

Fillion, Myriam; Lemire, Mélanie; Philibert, Aline; Frenette, Benoît; Weiler, Hope Alberta; Deguire, Jason Robert; Guimarães, Jean Remy Davée; Larribe, Fabrice; Barbosa, Fernando; Mergler, Donna

2013-07-01

Visual functions are known to be sensitive to toxins such as mercury (Hg) and lead (Pb), while omega-3 fatty acids (FA) and selenium (Se) may be protective. In the Tapajós region of the Brazilian Amazon, all of these elements are present in the local diet. Examine how near visual contrast sensitivity and acquired color vision loss vary with biomarkers of toxic exposures (Hg and Pb) and the nutrients Se and omega-3 FA in riverside communities of the Tapajós. Complete visuo-ocular examinations were performed. Near visual contrast sensitivity and color vision were assessed in 228 participants (≥15 years) without diagnosed age-related cataracts or ocular pathologies and with near visual acuity refracted to at least 20/40. Biomarkers of Hg (hair), Pb (blood), Se (plasma), and the omega-3 FAs eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in plasma phospholipids were measured. Multiple linear regressions were used to examine the relations between visual outcomes and biomarkers, taking into account age, sex, drinking and smoking. Reduced contrast sensitivity at all spatial frequencies was associated with hair Hg, while %EPA, and to a lesser extent %EPA+DHA, were associated with better visual function. The intermediate spatial frequency of contrast sensitivity (12 cycles/degree) was negatively related to blood Pb and positively associated with plasma Se. Acquired color vision loss increased with hair Hg and decreased with plasma Se and %EPA. These findings suggest that the local diet of riverside communities of the Amazon contain toxic substances that can have deleterious effects on vision as well as nutrients that are beneficial for visual function. Since remediation at the source is a long process, a better knowledge of the nutrient content and health effects of traditional foods would be useful to minimize harmful effects of Hg and Pb exposure. Copyright © 2013 Elsevier Inc. All rights reserved.

Every photon counts: improving low, mid, and high-spatial frequency errors on astronomical optics and materials with MRF

NASA Astrophysics Data System (ADS)

Maloney, Chris; Lormeau, Jean Pierre; Dumas, Paul

2016-07-01

Many astronomical sensing applications operate in low-light conditions; for these applications every photon counts. Controlling mid-spatial frequencies and surface roughness on astronomical optics are critical for mitigating scattering effects such as flare and energy loss. By improving these two frequency regimes higher contrast images can be collected with improved efficiency. Classically, Magnetorheological Finishing (MRF) has offered an optical fabrication technique to correct low order errors as well has quilting/print-through errors left over in light-weighted optics from conventional polishing techniques. MRF is a deterministic, sub-aperture polishing process that has been used to improve figure on an ever expanding assortment of optical geometries, such as planos, spheres, on and off axis aspheres, primary mirrors and freeform optics. Precision optics are routinely manufactured by this technology with sizes ranging from 5-2,000mm in diameter. MRF can be used for form corrections; turning a sphere into an asphere or free form, but more commonly for figure corrections achieving figure errors as low as 1nm RMS while using careful metrology setups. Recent advancements in MRF technology have improved the polishing performance expected for astronomical optics in low, mid and high spatial frequency regimes. Deterministic figure correction with MRF is compatible with most materials, including some recent examples on Silicon Carbide and RSA905 Aluminum. MRF also has the ability to produce `perfectly-bad' compensating surfaces, which may be used to compensate for measured or modeled optical deformation from sources such as gravity or mounting. In addition, recent advances in MRF technology allow for corrections of mid-spatial wavelengths as small as 1mm simultaneously with form error correction. Efficient midspatial frequency corrections make use of optimized process conditions including raster polishing in combination with a small tool size. Furthermore, a novel MRF fluid, called C30, has been developed to finish surfaces to ultra-low roughness (ULR) and has been used as the low removal rate fluid required for fine figure correction of mid-spatial frequency errors. This novel MRF fluid is able to achieve <4Å RMS on Nickel-plated Aluminum and even <1.5Å RMS roughness on Silicon, Fused Silica and other materials. C30 fluid is best utilized within a fine figure correction process to target mid-spatial frequency errors as well as smooth surface roughness 'for free' all in one step. In this paper we will discuss recent advancements in MRF technology and the ability to meet requirements for precision optics in low, mid and high spatial frequency regimes and how improved MRF performance addresses the need for achieving tight specifications required for astronomical optics.

WE-FG-207B-09: Experimental Assessment of Noise and Spatial Resolution in Virtual Non-Contrast Dual-Energy CT Images Across Multiple Patient Sizes and CT Systems

DOE Office of Scientific and Technical Information (OSTI.GOV)

Montoya, J; Ferrero, A; Yu, L

Purpose: To investigate the noise and spatial resolution properties of virtual non-contrast (VNC) dual-energy CT images compared to true non-contrast (TNC) images across multiple patient sizes and CT systems. Methods: Torso-shaped water phantoms with lateral widths of 25, 30, 35, 40 and 45 cm and a high resolution bar pattern phantom (Catphan CTP528) were scanned using 2nd and 3rd generation dual-source CT systems (Scanner A: Somatom Definition Flash, Scanner B: Somatom Force, Siemens Healthcare) in dual-energy scan mode with the same radiation dose for a given phantom size. Tube potentials of 80/Sn140 and 100/Sn140 on Scanner A and 80/Sn150, 90/Sn150more » and 100/Sn150 on Scanner B were evaluated to examine the impact of spectral separation. Images were reconstructed using a medium sharp quantitative kernel (Qr40), 1.0-mm thickness, 1.0-mm interval and 20 cm field of view. Mixed images served as TNC images. VNC images were created using commercial software (Virtual Unenhanced, Syngo VIA Version VA30, Siemens Healthcare). The noise power spectrum (NPS), area under the NPS, peak frequency of the NPS and image noise were measured for every phantom size and tube potential combination in TNC and VNC images. Results were compared within and between CT systems. Results: Minimal shift in NPS peak frequencies was observed in VNC images compared to TNC for NPS having pronounced peaks. Image noise and area under the NPS were higher in VNC images compared to TNC images across all tube potentials and for scanner A compared to scanner B. Limiting spatial resolution was deemed to be identical between VNC and TNC images. Conclusion: Quantitative assessment of image quality in VNC images demonstrated higher noise but equivalent spatial resolution compared to TNC images. Decreased noise was observed in the 3rd generation dual-source CT system for tube potential pairs having greater spectral separation. Dr. McCollough receives research support from Siemens Healthcare.« less

Sound pressure distribution and power flow within the gerbil ear canal from 100 Hz to 80 kHz

PubMed Central

Ravicz, Michael E.; Olson, Elizabeth S.; Rosowski, John J.

2008-01-01

Sound pressure was mapped in the bony ear canal of gerbils during closed-field sound stimulation at frequencies from 0.1 to 80 kHz. A 1.27-mm-diam probe-tube microphone or a 0.17-mm-diam fiber-optic miniature microphone was positioned along approximately longitudinal trajectories within the 2.3-mm-diam ear canal. Substantial spatial variations in sound pressure, sharp minima in magnitude, and half-cycle phase changes occurred at frequencies >30 kHz. The sound frequencies of these transitions increased with decreasing distance from the tympanic membrane (TM). Sound pressure measured orthogonally across the surface of the TM showed only small variations at frequencies below 60 kHz. Hence, the ear canal sound field can be described fairly well as a one-dimensional standing wave pattern. Ear-canal power reflectance estimated from longitudinal spatial variations was roughly constant at 0.2–0.5 at frequencies between 30 and 45 kHz. In contrast, reflectance increased at higher frequencies to at least 0.8 above 60 kHz. Sound pressure was also mapped in a microphone-terminated uniform tube—an “artificial ear.” Comparison with ear canal sound fields suggests that an artificial ear or “artificial cavity calibration” technique may underestimate the in situ sound pressure by 5–15 dB between 40 and 60 kHz. PMID:17902852

Adaptive windowing in contrast-enhanced intravascular ultrasound imaging.

PubMed

Lindsey, Brooks D; Martin, K Heath; Jiang, Xiaoning; Dayton, Paul A

2016-08-01

Intravascular ultrasound (IVUS) is one of the most commonly-used interventional imaging techniques and has seen recent innovations which attempt to characterize the risk posed by atherosclerotic plaques. One such development is the use of microbubble contrast agents to image vasa vasorum, fine vessels which supply oxygen and nutrients to the walls of coronary arteries and typically have diameters less than 200μm. The degree of vasa vasorum neovascularization within plaques is positively correlated with plaque vulnerability. Having recently presented a prototype dual-frequency transducer for contrast agent-specific intravascular imaging, here we describe signal processing approaches based on minimum variance (MV) beamforming and the phase coherence factor (PCF) for improving the spatial resolution and contrast-to-tissue ratio (CTR) in IVUS imaging. These approaches are examined through simulations, phantom studies, ex vivo studies in porcine arteries, and in vivo studies in chicken embryos. In phantom studies, PCF processing improved CTR by a mean of 4.2dB, while combined MV and PCF processing improved spatial resolution by 41.7%. Improvements of 2.2dB in CTR and 37.2% in resolution were observed in vivo. Applying these processing strategies can enhance image quality in conventional B-mode IVUS or in contrast-enhanced IVUS, where signal-to-noise ratio is relatively low and resolution is at a premium. Copyright © 2016 Elsevier B.V. All rights reserved.

Differential verbal, visual, and spatial working memory in written language production.

PubMed

Raulerson, Bascom A; Donovan, Michael J; Whiteford, Alison P; Kellogg, Ronald T

2010-02-01

The contributions of verbal, visual, and spatial working memory to written language production were investigated. Participants composed definitions for nouns while concurrently performing a task which required updating, storing, and retrieving information coded either verbally, visually, or spatially. The present study extended past findings by showing the linguistic encoding of planned conceptual content makes its largest demand on verbal working memory for both low and high frequency nouns. Kellogg, Olive, and Piolat in 2007 found that concrete nouns place substantial demands on visual working memory when imaging the nouns' referents during planning, whereas abstract nouns make no demand. The current study further showed that this pattern was not an artifact of visual working memory being sensitive to manipulation of just any lexical property of the noun prompts. In contrast to past results, writing made a small but detectible demand on spatial working memory.

Visual detection following retinal damage: predictions of an inhomogeneous retino-cortical model

NASA Astrophysics Data System (ADS)

Arnow, Thomas L.; Geisler, Wilson S.

1996-04-01

A model of human visual detection performance has been developed, based on available anatomical and physiological data for the primate visual system. The inhomogeneous retino- cortical (IRC) model computes detection thresholds by comparing simulated neural responses to target patterns with responses to a uniform background of the same luminance. The model incorporates human ganglion cell sampling distributions; macaque monkey ganglion cell receptive field properties; macaque cortical cell contrast nonlinearities; and a optical decision rule based on ideal observer theory. Spatial receptive field properties of cortical neurons were not included. Two parameters were allowed to vary while minimizing the squared error between predicted and observed thresholds. One parameter was decision efficiency, the other was the relative strength of the ganglion-cell center and surround. The latter was only allowed to vary within a small range consistent with known physiology. Contrast sensitivity was measured for sinewave gratings as a function of spatial frequency, target size and eccentricity. Contrast sensitivity was also measured for an airplane target as a function of target size, with and without artificial scotomas. The results of these experiments, as well as contrast sensitivity data from the literature were compared to predictions of the IRC model. Predictions were reasonably good for grating and airplane targets.

Assessing Spatial and Temporal Properties of Perimetric Stimuli for Resistance to Clinical Variations in Retinal Illumination

PubMed Central

Swanson, William H.; Dul, Mitchell W.; Horner, Douglas G.; Liu, Tiffany; Tran, Irene

2014-01-01

Purpose. To develop perimetric stimuli for which sensitivities are more resistant to reduced retinal illumination than current clinical perimeters. Methods. Fifty-four people free of eye disease were dilated and tested monocularly. For each test, retinal illumination was attenuated with neutral density (ND) filters, and a standard adaptation model was fit to derive mean and SEM for the adaptation parameter (NDhalf). For different stimuli, t-tests on NDhalf were used to assess significance of differences in consistency with Weber's law. Three experiments used custom Gaussian-windowed contrast sensitivity perimetry (CSP). Experiment 1 used CSP-1, with a Gaussian temporal pulse, a spatial frequency of 0.375 cyc/deg (cpd), and SD of 1.5°. Experiment 1 also used the Humphrey Matrix perimeter, with the N-30 test using 0.25 cpd and 25 Hz flicker. Experiment 2 used a rectangular temporal pulse, SDs of 0.25° and 0.5°, and spatial frequencies of 0.0 and 1.0 cpd. Experiment 3 used CSP-2, with 5-Hz flicker, SDs from 0.5° to 1.8°, and spatial frequencies from 0.14 to 0.50 cpd. Results. In Experiment 1, CSP-1 was more consistent with Weber's law (NDhalf ± SEM = 1.86 ± 0.08 log unit) than N-30 (NDhalf = 1.03 ± 0.03 log unit; t > 9, P < 0.0001). All stimuli used in Experiments 2 and 3 had comparable consistency with Weber's law (NDhalf = 1.49–1.69 log unit; t < 2). Conclusions. Perimetric sensitivities were consistent with Weber's law when higher temporal frequencies were avoided. PMID:24370832

Basic abnormalities in visual processing affect face processing at an early age in autism spectrum disorder.

PubMed

Vlamings, Petra Hendrika Johanna Maria; Jonkman, Lisa Marthe; van Daalen, Emma; van der Gaag, Rutger Jan; Kemner, Chantal

2010-12-15

A detailed visual processing style has been noted in autism spectrum disorder (ASD); this contributes to problems in face processing and has been directly related to abnormal processing of spatial frequencies (SFs). Little is known about the early development of face processing in ASD and the relation with abnormal SF processing. We investigated whether young ASD children show abnormalities in low spatial frequency (LSF, global) and high spatial frequency (HSF, detailed) processing and explored whether these are crucially involved in the early development of face processing. Three- to 4-year-old children with ASD (n = 22) were compared with developmentally delayed children without ASD (n = 17). Spatial frequency processing was studied by recording visual evoked potentials from visual brain areas while children passively viewed gratings (HSF/LSF). In addition, children watched face stimuli with different expressions, filtered to include only HSF or LSF. Enhanced activity in visual brain areas was found in response to HSF versus LSF information in children with ASD, in contrast to control subjects. Furthermore, facial-expression processing was also primarily driven by detail in ASD. Enhanced visual processing of detailed (HSF) information is present early in ASD and occurs for neutral (gratings), as well as for socially relevant stimuli (facial expressions). These data indicate that there is a general abnormality in visual SF processing in early ASD and are in agreement with suggestions that a fast LSF subcortical face processing route might be affected in ASD. This could suggest that abnormal visual processing is causative in the development of social problems in ASD. Copyright © 2010 Society of Biological Psychiatry. Published by Elsevier Inc. All rights reserved.

Higher locus coeruleus MRI contrast is associated with lower parasympathetic influence over heart rate variability

PubMed Central

Mather, Mara; Yoo, Hyun Joo; Clewett, David V.; Lee, Tae-Ho; Greening, Steven G.; Ponzio, Allison; Min, Jungwon; Thayer, Julian F.

2017-01-01

The locus coeruleus (LC) is a key node of the sympathetic nervous system and suppresses parasympathetic activity that would otherwise increase heart rate variability. In the current study, we examined whether LC-MRI contrast reflecting neuromelanin accumulation in the LC was associated with high-frequency heart rate variability (HF-HRV), a measure reflecting parasympathetic influences on the heart. Recent evidence indicates that neuromelanin, a byproduct of catecholamine metabolism, accumulates in the LC through young and mid adulthood, suggesting that LC-MRI contrast may be a useful biomarker of individual differences in habitual LC activation. We found that, across younger and older adults, greater LC-MRI contrast was negatively associated with HF-HRV during fear conditioning and spatial detection tasks. This correlation was not accounted for by individual differences in age or anxiety. These findings indicate that individual differences in LC structure relate to key cardiovascular parameters. PMID:28215623

Subsurface imaging of metal lines embedded in a dielectric with a scanning microwave microscope

NASA Astrophysics Data System (ADS)

You, Lin; Ahn, Jung-Joon; Obeng, Yaw S.; Kopanski, Joseph J.

2016-02-01

We demonstrate the ability of the scanning microwave microscope (SMM) to detect subsurface metal lines embedded in a dielectric film with sub-micrometer resolution. The SMM was used to image 1.2 μm-wide Al-Si-Cu metal lines encapsulated with either 800 nm or 2300 nm of plasma deposited silicon dioxide. Both the reflected microwave (S 11) amplitude and phase shifted near resonance frequency while the tip scanned across these buried lines. The shallower line edge could be resolved within 900 nm  ±  70 nm, while the deeper line was resolved within 1200 nm  ±  260 nm. The spatial resolution obtained in this work is substantially better that the 50 μm previously reported in the literature. Our observations agree very well with the calculated change in peak frequency and phase using a simple lumped element model for an SMM with a resonant transmission line. By conducting experiments at various eigenmodes, different contrast levels and signal-to-noise ratios have been compared. With detailed sensitivity studies, centered around 9.3 GHz, it has been revealed that the highest amplitude contrast is obtained when the probe microwave frequency matches the exact resonance frequency of the experimental setup. By RLC equivalent circuit modeling of the tip-sample system, two competing effects have been identified to account for the positive and negative S 11 amplitude and phase contrasts, which can be leveraged to further improve the contrast and resolution. Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States.

Electrophysiological correlates of figure-ground segregation directly reflect perceptual saliency.

PubMed

Straube, Sirko; Grimsen, Cathleen; Fahle, Manfred

2010-03-05

In a figure identification task, we investigated the influence of different visual cue configurations (spatial frequency, orientation or a combination of both) on the human EEG. Combining psychophysics with ERP and time-frequency analysis, we show that the neural response at about 200ms reflects perceptual saliency rather than physical cue contrast. Increasing saliency caused (i) a negative shift of the posterior P2 coinciding with a power decrease in the posterior theta-band and (ii) an amplitude and latency increase of the posterior P3. We demonstrate that visual cues interact for a percept that is non-linearly related to the physical figure-ground properties.

Optical functional performance of the osteo-odonto-keratoprosthesis.

PubMed

Lee, Richard M H; Ong, Gek L; Lam, Fook Chang; White, Joy; Crook, David; Liu, Christopher S C; Hull, Chris C

2014-10-01

The aim of this study was to evaluate optical and visual functional performance of the osteo-odonto-keratoprosthesis (OOKP). Optical design and analysis was performed with customized optical design software. Nine patients with implanted OOKP devices and 9 age-matched control patients were assessed. Contrast sensitivity was assessed and glare effect was measured with a brightness acuity test. All OOKP patients underwent kinetic Goldmann perimetry and wavefront aberrometry and completed the National Eye Institute Visual Function Questionnaire-25 (NEI VFQ-25). Optical analysis showed that the optical cylinder is near diffraction-limited. A reduction in median visual acuity (VA) with increasing glare settings was observed from 0.04 logMAR (without glare) to 0.20 logMAR (with glare at "high" setting) and significantly reduced statistically when compared with the control group at all levels of glare (P < 0.05). Contrast sensitivity was significantly reduced when compared with age-matched controls at medium and high spatial frequencies (P < 0.05). Median Goldmann perimetry was 65 degrees (interquartile range, 64-74 degrees; V-4e isopters) and 69 degrees excluding 2 glaucomatous subjects. Several vision-related NEI VFQ-25 subscales correlated significantly with VA at various brightness acuity test levels and contrast sensitivity at medium spatial frequencies, including dependency, general vision, near activities and distance activities. The OOKP optical cylinder provides patients with a good level of VA that is significantly reduced by glare. We have shown in vivo that updates to the optical cylinder design have improved the patient's field of view. Reduction of glare and refinement of cylinder alignment methods may further improve visual function and patient satisfaction.

Retinal Layer Abnormalities as Biomarkers of Schizophrenia.

PubMed

Samani, Niraj N; Proudlock, Frank A; Siram, Vasantha; Suraweera, Chathurie; Hutchinson, Claire; Nelson, Christopher P; Al-Uzri, Mohammed; Gottlob, Irene

2018-06-06

Schizophrenia is associated with several brain deficits, as well as visual processing deficits, but clinically useful biomarkers are elusive. We hypothesized that retinal layer changes, noninvasively visualized using spectral-domain optical coherence tomography (SD-OCT), may represent a possible "window" to these abnormalities. A Leica EnvisuTM SD-OCT device was used to obtain high-resolution central foveal B-scans in both eyes of 35 patients with schizophrenia and 50 demographically matched controls. Manual retinal layer segmentation was performed to acquire individual and combined layer thickness measurements in 3 macular regions. Contrast sensitivity was measured at 3 spatial frequencies in a subgroup of each cohort. Differences were compared using adjusted linear models and significantly different layer measures in patients underwent Spearman Rank correlations with contrast sensitivity, quantified symptoms severity, disease duration, and antipsychotic medication dose. Total retinal and photoreceptor complex thickness was reduced in all regions in patients (P < .0001). Segmentation revealed consistent thinning of the outer nuclear layer (P < .001) and inner segment layer (P < .05), as well as a pattern of parafoveal ganglion cell changes. Low spatial frequency contrast sensitivity was reduced in patients (P = .002) and correlated with temporal parafoveal ganglion cell complex thinning (R = .48, P = .01). Negative symptom severity was inversely correlated with foveal photoreceptor complex thickness (R = -.54, P = .001) and outer nuclear layer thickness (R = -.47, P = .005). Our novel findings demonstrate considerable retinal layer abnormalities in schizophrenia that are related to clinical features and visual function. With time, SD-OCT could provide easily-measurable biomarkers to facilitate clinical assessment and further our understanding of the disease.

Streaming flow from ultrasound contrast agents by acoustic waves in a blood vessel model.

PubMed

Cho, Eunjin; Chung, Sang Kug; Rhee, Kyehan

2015-09-01

To elucidate the effects of streaming flow on ultrasound contrast agent (UCA)-assisted drug delivery, streaming velocity fields from sonicated UCA microbubbles were measured using particle image velocimetry (PIV) in a blood vessel model. At the beginning of ultrasound sonication, the UCA bubbles formed clusters and translated in the direction of the ultrasound field. Bubble cluster formation and translation were faster with 2.25MHz sonication, a frequency close to the resonance frequency of the UCA. Translation of bubble clusters induced streaming jet flow that impinged on the vessel wall, forming symmetric vortices. The maximum streaming velocity was about 60mm/s at 2.25MHz and decreased to 15mm/s at 1.0MHz for the same acoustic pressure amplitude. The effect of the ultrasound frequency on wall shear stress was more noticeable. Maximum wall shear stress decreased from 0.84 to 0.1Pa as the ultrasound frequency decreased from 2.25 to 1.0MHz. The maximum spatial gradient of the wall shear stress also decreased from 1.0 to 0.1Pa/mm. This study showed that streaming flow was induced by bubble cluster formation and translation and was stronger upon sonication by an acoustic wave with a frequency near the UCA resonance frequency. Therefore, the secondary radiant force, which is much stronger at the resonance frequency, should play an important role in UCA-assisted drug delivery. Copyright © 2015 Elsevier B.V. All rights reserved.

Assessment of Human Visual Performance with a Swept Evoked Potential Technique

DTIC Science & Technology

1984-07-01

obtained in naive patients. Retinitis pigmentosa patients with < 20/50 vision have shown contrast sensitivity losses at the higher spatial frequencies...X and Y visual subsystems The new visual duplicity. Th« observation that cat retinal ganglion cells can be divided into those which sum luminous...bias in retinal ganglion cells (cat: Levick & Thibos, 1980; monkey: DeMonasterio, 1978). The bias is weak. In cat, when the stimulus orientation was

Quantitative characterization of edge enhancement in phase contrast x-ray imaging.

PubMed

Monnin, P; Bulling, S; Hoszowska, J; Valley, J F; Meuli, R; Verdun, F R

2004-06-01

The aim of this study was to model the edge enhancement effect in in-line holography phase contrast imaging. A simple analytical approach was used to quantify refraction and interference contrasts in terms of beam energy and imaging geometry. The model was applied to predict the peak intensity and frequency of the edge enhancement for images of cylindrical fibers. The calculations were compared with measurements, and the relationship between the spatial resolution of the detector and the amplitude of the phase contrast signal was investigated. Calculations using the analytical model were in good agreement with experimental results for nylon, aluminum and copper wires of 50 to 240 microm diameter, and with numerical simulations based on Fresnel-Kirchhoff theory. A relationship between the defocusing distance and the pixel size of the image detector was established. This analytical model is a useful tool for optimizing imaging parameters in phase contrast in-line holography, including defocusing distance, detector resolution and beam energy.

Facilitation of contrast detection in near-peripheral vision.

PubMed

Giorgi, Robert G; Soong, Grace P; Woods, Russell L; Peli, Eli

2004-12-01

Foveal detection of a Gabor patch (target) is facilitated by collinear, displaced high-contrast flankers. Polat and Sagi reported that the same phenomenon occurred in the periphery, but no data were presented [Proc. Natl. Acad. Sci. 91 (1994) 1206]. Others have found no facilitation in a limited number of conditions tested. To resolve this apparent conflict, we measured lateral facilitation in the near-periphery using a range of stimulus parameters. We found facilitation for a range of target-flanker distances for peripheral eccentricities up to 6 degrees , but the magnitude of the effect was less than found in central vision. Facilitation varied across subjects and with spatial frequency. Flanker contrast had no effect over the range evaluated (10-80%). Equal facilitation was found for two global arrangements of the stimulus pattern. Facilitation was found using a temporal, but not a spatial two-alternative forced-choice paradigm, accounting for the different results among previous studies. This finding supports previous indications of the role of attention in altering such facilitation. The value of facilitation from lateral interactions for persons with central vision impairment, who have to shift their attention to a peripheral locus constantly, needs to be examined.

Exploring the Role of Spatial Frequency Information during Neural Emotion Processing in Human Infants.

PubMed

Jessen, Sarah; Grossmann, Tobias

2017-01-01

Enhanced attention to fear expressions in adults is primarily driven by information from low as opposed to high spatial frequencies contained in faces. However, little is known about the role of spatial frequency information in emotion processing during infancy. In the present study, we examined the role of low compared to high spatial frequencies in the processing of happy and fearful facial expressions by using filtered face stimuli and measuring event-related brain potentials (ERPs) in 7-month-old infants ( N = 26). Our results revealed that infants' brains discriminated between emotional facial expressions containing high but not between expressions containing low spatial frequencies. Specifically, happy faces containing high spatial frequencies elicited a smaller Nc amplitude than fearful faces containing high spatial frequencies and happy and fearful faces containing low spatial frequencies. Our results demonstrate that already in infancy spatial frequency content influences the processing of facial emotions. Furthermore, we observed that fearful facial expressions elicited a comparable Nc response for high and low spatial frequencies, suggesting a robust detection of fearful faces irrespective of spatial frequency content, whereas the detection of happy facial expressions was contingent upon frequency content. In summary, these data provide new insights into the neural processing of facial emotions in early development by highlighting the differential role played by spatial frequencies in the detection of fear and happiness.

Phase calibration target for quantitative phase imaging with ptychography.

PubMed

Godden, T M; Muñiz-Piniella, A; Claverley, J D; Yacoot, A; Humphry, M J

2016-04-04

Quantitative phase imaging (QPI) utilizes refractive index and thickness variations that lead to optical phase shifts. This gives contrast to images of transparent objects. In quantitative biology, phase images are used to accurately segment cells and calculate properties such as dry mass, volume and proliferation rate. The fidelity of the measured phase shifts is of critical importance in this field. However to date, there has been no standardized method for characterizing the performance of phase imaging systems. Consequently, there is an increasing need for protocols to test the performance of phase imaging systems using well-defined phase calibration and resolution targets. In this work, we present a candidate for a standardized phase resolution target, and measurement protocol for the determination of the transfer of spatial frequencies, and sensitivity of a phase imaging system. The target has been carefully designed to contain well-defined depth variations over a broadband range of spatial frequencies. In order to demonstrate the utility of the target, we measure quantitative phase images on a ptychographic microscope, and compare the measured optical phase shifts with Atomic Force Microscopy (AFM) topography maps and surface profile measurements from coherence scanning interferometry. The results show that ptychography has fully quantitative nanometer sensitivity in optical path differences over a broadband range of spatial frequencies for feature sizes ranging from micrometers to hundreds of micrometers.

Comparison of fundamental, second harmonic, and superharmonic imaging: a simulation study.

PubMed

van Neer, Paul L M J; Danilouchkine, Mikhail G; Verweij, Martin D; Demi, Libertario; Voormolen, Marco M; van der Steen, Anton F W; de Jong, Nico

2011-11-01

In medical ultrasound, fundamental imaging (FI) uses the reflected echoes from the same spectral band as that of the emitted pulse. The transmission frequency determines the trade-off between penetration depth and spatial resolution. Tissue harmonic imaging (THI) employs the second harmonic of the emitted frequency band to construct images. Recently, superharmonic imaging (SHI) has been introduced, which uses the third to the fifth (super) harmonics. The harmonic level is determined by two competing phenomena: nonlinear propagation and frequency dependent attenuation. Thus, the transmission frequency yielding the optimal trade-off between the spatial resolution and the penetration depth differs for THI and SHI. This paper quantitatively compares the concepts of fundamental, second harmonic, and superharmonic echocardiography at their optimal transmission frequencies. Forward propagation is modeled using a 3D-KZK implementation and the iterative nonlinear contrast source (INCS) method. Backpropagation is assumed to be linear. Results show that the fundamental lateral beamwidth is the narrowest at focus, while the superharmonic one is narrower outside the focus. The lateral superharmonic roll-off exceeds the fundamental and second harmonic roll-off. Also, the axial resolution of SHI exceeds that of FI and THI. The far-field pulse-echo superharmonic pressure is lower than that of the fundamental and second harmonic. SHI appears suited for echocardiography and is expected to improve its image quality at the cost of a slight reduction in depth-of-field.

Visual contrast sensitivity deficits in Bohemian children.

PubMed

Hudnell, H K; Skalik, I; Otto, D; House, D; Subrt, P; Sram, R

1996-01-01

Visual contrast sensitivity (VCS) tests have been used successfully in medical diagnosis and subclinical neurotoxicity detection. This paper reports VCS measurements in three studies of children in the Czech Republic. Study 1 compared children in standard schools and schools for the learning disabled. Studies 2 and 3 compared children in Teplice, an area in which soft-brown coal combustion produced high levels of pollutants (e.g. Hg, As, SO2, NOx, and aromatic hydrocarbons), with children in areas of low air pollution, Znojmo and/or Prachatice. It was hypothesized that in utero exposure to the combustion products disrupted neurological development (Sram, 1991). The VCS test (Stereo Optical Co.) consisted of circular fields containing sinusoidal gratings at 5 spatial frequencies (1.5-18 cycles/degree) and various levels of contrast. Subjects indicated orientation of the gratings by pointing left, up, or right. Visual acuity and VCS were measured in each eye of 74 children in Study 1,327 second-grade children in Study 2, and 426 fourth-grade children in Study 3. Hair samples were collected in Studies 2 and 3 analyzed for Hg and As content. Children attending schools for the learning disabled scored significantly lower than controls on VCS, whereas visual acuity was normal. The deficit was greatest at mid- to high spatial frequency. In Study 2, significant VCS deficits were seen in exposed second-grade children at low to mid-spatial frequency, even though visual acuity was slightly above control level. Regression analyses showed that VCS had no relationship to As, but a significant negative correlation with hair Hg was observed in the exposed district. However, current Hg levels were higher in Prachatice. VCS deficits were not observed in the fourth-grade students of Teplice in Study 3. The results of Study 1 indicated that behavioral VCS testing in field studies is practical in young, non-English speaking children, and suggested that vision may be compromised in learning-disabled children. Studies 2 and 3 indicated that at these levels, current Hg body-burdens are poor predictors of VCS. If the VCS deficits seen in Study 2 were related to prenatal exposures, the results of Study 3 suggest that they represent a developmental delay. A longitudinal-study design is needed to address this issue.

Dual-high-frequency ultrasound excitation on microbubble destruction volume.

PubMed

Shen, Che-Chou; Su, Shin-Yuan; Cheng, Chih-Hao; Yeh, Chih-Kuang

2010-06-01

The goal of this work was to test experimentally that exposing air bubbles or ultrasound contrast agents in water to amplitude modulated wave allows control of inertial cavitation affected volume and hence could limit the undesirable bioeffects. Focused transducer operating at the center frequency of 10 MHz and having about 65% fractional bandwidth was excited by 3 micros 8.5 and 11.5 MHz tone-bursts to produce 3 MHz envelope signal. The 3 MHz frequency was selected because it corresponds to the resonance frequency of the microbubbles used in the experiment. Another 5 MHz transducer was used as a receiver to produce B-mode image. Peak negative acoustic pressure was adjusted in the range from 0.5 to 3.5 MPa. The spectrum amplitudes obtained from the imaging of SonoVue contrast agent when using the envelope and a separate 3 MHz transducer were compared to determine their cross-section at the -6 dB level. The conventional 3 MHz tone-burst excitation resulted in the region of interest (ROI) cross-section of 2.47 mm while amplitude modulated, dual-frequency excitation with difference frequency of 3 MHz produced cross-section equal to 1.2mm. These results corroborate our hypothesis that, in addition to the considerably higher penetration depth of dual-frequency excitation due to the lower attenuation at 3 MHz than that at 8.5 and 11.5 MHz, the sample volume of dual-frequency excitation is also smaller than that of linear 3-MHz method for more spatially confined destruction of microbubbles. 2010 Elsevier B.V. All rights reserved.

Hurricane frequency and landfall distribution for coastal wetlands of the Gulf coast, USA

USGS Publications Warehouse

Doyle, T.W.

2009-01-01

The regularity and severity of tropical storms are major determinants controlling ecosystem structure and succession for coastal ecosystems. Hurricane landfall rates vary greatly with high and low frequency for given coastal stretches of the southeastern United States. Site-specific meteorological data of hurricane wind speeds and direction, however, are only available for select populated cities of relatively sparse distribution and inland from the coast. A spatial simulation model of hurricane circulation, HURASIM, was applied to reconstruct chronologies of hurricane wind speeds and vectors for northern Gulf coast locations derived from historical tracking data of North Atlantic tropical storms dating back to 1851. Contrasts of storm frequencies showed that tropical storm incidence is nearly double for Florida coastal ecosystems than the westernmost stretches of Texas coastline. Finer-scale spatial simulations for the north-central Gulf coast exhibited sub-regional differences in storm strength and frequency with coastal position and latitude. The overall pattern of storm incidence in the Gulf basin indicates that the disturbance regime of coastal areas varies greatly along the coast, inland from the coast, and temporally over the period of record. Field and modeling studies of coastal ecosystems will benefit from this retrospective analysis of hurricane incidence and intensity both on a local or regional basis. ?? 2009 The Society of Wetland Scientists.

Spatial Congruity Effects Reveal Metaphorical Thinking, not Polarity Correspondence.

PubMed

Dolscheid, Sarah; Casasanto, Daniel

2015-01-01

Spatial congruity effects have often been interpreted as evidence for metaphorical thinking, but an alternative account based on polarity correspondence (a.k.a. markedness) has challenged this view. Here we compared metaphor- and polarity-correspondence-based explanations for spatial congruity effects, using musical pitch as a testbed. In one experiment, English speakers classified high- and low-frequency pitches as "high" and "low," or as "front" and "back," to determine whether space-pitch congruity effects could be elicited by any marked spatial continuum. Although both pairs of terms describe bipolar spatial continuums, we found congruity effects only for high/low judgments, indicating that markedness is not sufficient to produce space-pitch congruity effects. A second experiment confirmed that there were no space-pitch congruity effects for another pair of terms that have clear markedness (big/small), but which do not denote spatial height. By contrast, this experiment showed congruity effects for words that cued an appropriate vertical spatial schema (tall/short), even though these words are not used conventionally in English to describe pitches, ruling out explanations for the observed pattern of results based on verbal polysemy. Together, results suggest that space-pitch congruity effects reveal metaphorical uses of spatial schemas, not polarity correspondence effects.

Image jitter enhances visual performance when spatial resolution is impaired.

PubMed

Watson, Lynne M; Strang, Niall C; Scobie, Fraser; Love, Gordon D; Seidel, Dirk; Manahilov, Velitchko

2012-09-06

Visibility of low-spatial frequency stimuli improves when their contrast is modulated at 5 to 10 Hz compared with stationary stimuli. Therefore, temporal modulations of visual objects could enhance the performance of low vision patients who primarily perceive images of low-spatial frequency content. We investigated the effect of retinal-image jitter on word recognition speed and facial emotion recognition in subjects with central visual impairment. Word recognition speed and accuracy of facial emotion discrimination were measured in volunteers with AMD under stationary and jittering conditions. Computer-driven and optoelectronic approaches were used to induce retinal-image jitter with duration of 100 or 166 ms and amplitude within the range of 0.5 to 2.6° visual angle. Word recognition speed was also measured for participants with simulated (Bangerter filters) visual impairment. Text jittering markedly enhanced word recognition speed for people with severe visual loss (101 ± 25%), while for those with moderate visual impairment, this effect was weaker (19 ± 9%). The ability of low vision patients to discriminate the facial emotions of jittering images improved by a factor of 2. A prototype of optoelectronic jitter goggles produced similar improvement in facial emotion discrimination. Word recognition speed in participants with simulated visual impairment was enhanced for interjitter intervals over 100 ms and reduced for shorter intervals. Results suggest that retinal-image jitter with optimal frequency and amplitude is an effective strategy for enhancing visual information processing in the absence of spatial detail. These findings will enable the development of novel tools to improve the quality of life of low vision patients.

Spatial-frequency cutoff requirements for pattern recognition in central and peripheral vision

PubMed Central

Kwon, MiYoung; Legge, Gordon E.

2011-01-01

It is well known that object recognition requires spatial frequencies exceeding some critical cutoff value. People with central scotomas who rely on peripheral vision have substantial difficulty with reading and face recognition. Deficiencies of pattern recognition in peripheral vision, might result in higher cutoff requirements, and may contribute to the functional problems of people with central-field loss. Here we asked about differences in spatial-cutoff requirements in central and peripheral vision for letter and face recognition. The stimuli were the 26 letters of the English alphabet and 26 celebrity faces. Each image was blurred using a low-pass filter in the spatial frequency domain. Critical cutoffs (defined as the minimum low-pass filter cutoff yielding 80% accuracy) were obtained by measuring recognition accuracy as a function of cutoff (in cycles per object). Our data showed that critical cutoffs increased from central to peripheral vision by 20% for letter recognition and by 50% for face recognition. We asked whether these differences could be accounted for by central/peripheral differences in the contrast sensitivity function (CSF). We addressed this question by implementing an ideal-observer model which incorporates empirical CSF measurements and tested the model on letter and face recognition. The success of the model indicates that central/peripheral differences in the cutoff requirements for letter and face recognition can be accounted for by the information content of the stimulus limited by the shape of the human CSF, combined with a source of internal noise and followed by an optimal decision rule. PMID:21854800

Contrast Gain Control Model Fits Masking Data

NASA Technical Reports Server (NTRS)

Watson, Andrew B.; Solomon, Joshua A.; Null, Cynthia H. (Technical Monitor)

1994-01-01

We studied the fit of a contrast gain control model to data of Foley (JOSA 1994), consisting of thresholds for a Gabor patch masked by gratings of various orientations, or by compounds of two orientations. Our general model includes models of Foley and Teo & Heeger (IEEE 1994). Our specific model used a bank of Gabor filters with octave bandwidths at 8 orientations. Excitatory and inhibitory nonlinearities were power functions with exponents of 2.4 and 2. Inhibitory pooling was broad in orientation, but narrow in spatial frequency and space. Minkowski pooling used an exponent of 4. All of the data for observer KMF were well fit by the model. We have developed a contrast gain control model that fits masking data. Unlike Foley's, our model accepts images as inputs. Unlike Teo & Heeger's, our model did not require multiple channels for different dynamic ranges.

Stereoscopic depth constancy

PubMed Central

Guan, Phillip

2016-01-01

Depth constancy is the ability to perceive a fixed depth interval in the world as constant despite changes in viewing distance and the spatial scale of depth variation. It is well known that the spatial frequency of depth variation has a large effect on threshold. In the first experiment, we determined that the visual system compensates for this differential sensitivity when the change in disparity is suprathreshold, thereby attaining constancy similar to contrast constancy in the luminance domain. In a second experiment, we examined the ability to perceive constant depth when the spatial frequency and viewing distance both changed. To attain constancy in this situation, the visual system has to estimate distance. We investigated this ability when vergence, accommodation and vertical disparity are all presented accurately and therefore provided veridical information about viewing distance. We found that constancy is nearly complete across changes in viewing distance. Depth constancy is most complete when the scale of the depth relief is constant in the world rather than when it is constant in angular units at the retina. These results bear on the efficacy of algorithms for creating stereo content. This article is part of the themed issue ‘Vision in our three-dimensional world’. PMID:27269596

Color structured light imaging of skin

NASA Astrophysics Data System (ADS)

Yang, Bin; Lesicko, John; Moy, Austin; Reichenberg, Jason; Sacks, Michael; Tunnell, James W.

2016-05-01

We illustrate wide-field imaging of skin using a structured light (SL) approach that highlights the contrast from superficial tissue scattering. Setting the spatial frequency of the SL in a regime that limits the penetration depth effectively gates the image for photons that originate from the skin surface. Further, rendering the SL images in a color format provides an intuitive format for viewing skin pathologies. We demonstrate this approach in skin pathologies using a custom-built handheld SL imaging system.

Greater magnocellular saccadic suppression in high versus low autistic tendency suggests a causal path to local perceptual style

PubMed Central

Crewther, David P.; Crewther, Daniel; Bevan, Stephanie; Goodale, Melvyn A.; Crewther, Sheila G.

2015-01-01

Saccadic suppression—the reduction of visual sensitivity during rapid eye movements—has previously been proposed to reflect a specific suppression of the magnocellular visual system, with the initial neural site of that suppression at or prior to afferent visual information reaching striate cortex. Dysfunction in the magnocellular visual pathway has also been associated with perceptual and physiological anomalies in individuals with autism spectrum disorder or high autistic tendency, leading us to question whether saccadic suppression is altered in the broader autism phenotype. Here we show that individuals with high autistic tendency show greater saccadic suppression of low versus high spatial frequency gratings while those with low autistic tendency do not. In addition, those with high but not low autism spectrum quotient (AQ) demonstrated pre-cortical (35–45 ms) evoked potential differences (saccade versus fixation) to a large, low contrast, pseudo-randomly flashing bar. Both AQ groups showed similar differential visual evoked potential effects in later epochs (80–160 ms) at high contrast. Thus, the magnocellular theory of saccadic suppression appears untenable as a general description for the typically developing population. Our results also suggest that the bias towards local perceptual style reported in autism may be due to selective suppression of low spatial frequency information accompanying every saccadic eye movement. PMID:27019719

Accounting for the phase, spatial frequency and orientation demands of the task improves metrics based on the visual Strehl ratio.

PubMed

Young, Laura K; Love, Gordon D; Smithson, Hannah E

2013-09-20

Advances in ophthalmic instrumentation have allowed high order aberrations to be measured in vivo. These measurements describe the distortions to a plane wavefront entering the eye, but not the effect they have on visual performance. One metric for predicting visual performance from a wavefront measurement uses the visual Strehl ratio, calculated in the optical transfer function (OTF) domain (VSOTF) (Thibos et al., 2004). We considered how well such a metric captures empirical measurements of the effects of defocus, coma and secondary astigmatism on letter identification and on reading. We show that predictions using the visual Strehl ratio can be significantly improved by weighting the OTF by the spatial frequency band that mediates letter identification and further improved by considering the orientation of phase and contrast changes imposed by the aberration. We additionally showed that these altered metrics compare well to a cross-correlation-based metric. We suggest a version of the visual Strehl ratio, VScombined, that incorporates primarily those phase disruptions and contrast changes that have been shown independently to affect object recognition processes. This metric compared well to VSOTF for letter identification and was the best predictor of reading performance, having a higher correlation with the data than either the VSOTF or cross-correlation-based metric. Copyright © 2013 The Authors. Published by Elsevier Ltd.. All rights reserved.

Film characteristics pertinent to coherent optical data processing systems.

PubMed

Thomas, C E

1972-08-01

Photographic film is studied quantitatively as the input mechanism for coherent optical data recording and processing systems. The two important film characteristics are the amplitude transmission vs exposure (T(A) - E) curve and the film noise power spectral density. Both functions are measured as a function of the type of film, the type of developer, developer time and temperature, and the exposing and readout light wavelengths. A detailed analysis of a coherent optical spatial frequency analyzer reveals that the optimum do bias point for 649-F film is an amplitude transmission of about 70%. This operating point yields minimum harmonic and intermodulation distortion, whereas the 50% amplitude transmission bias point recommended by holographers yields maximum diffraction efficiency. It is also shown that the effective ac gain or contrast of the film is nearly independent of the development conditions for a given film. Finally, the linear dynamic range of one particular coherent optical spatial frequency analyzer is shown to be about 40-50 dB.

MTF evaluation of in-line phase contrast imaging system

NASA Astrophysics Data System (ADS)

Sun, Xiaoran; Gao, Feng; Zhao, Huijuan; Zhang, Limin; Li, Jiao; Zhou, Zhongxing

2017-02-01

X-ray phase contrast imaging (XPCI) is a novel method that exploits the phase shift for the incident X-ray to form an image. Various XPCI methods have been proposed, among which, in-line phase contrast imaging (IL-PCI) is regarded as one of the most promising clinical methods. The contrast of the interface is enhanced due to the introduction of the boundary fringes in XPCI, thus it is generally used to evaluate the image quality of XPCI. But the contrast is a comprehensive index and it does not reflect the information of image quality in the frequency range. The modulation transfer function (MTF), which is the Fourier transform of the system point spread function, is recognized as the metric to characterize the spatial response of conventional X-ray imaging system. In this work, MTF is introduced into the image quality evaluation of the IL-PCI system. Numerous simulations based on Fresnel - Kirchhoff diffraction theory are performed with varying system settings and the corresponding MTFs were calculated for comparison. The results show that MTF can provide more comprehensive information of image quality comparing to contrast in IL-PCI.

Application of Optical Coherence Tomography and Contrast Sensitivity Test for Observing Fundus Changes of Patients With Pregnancy-Induced Hypertension Syndrome.

PubMed

Wang, Zhixue; Zou, Yuanyuan; Li, Wenying; Wang, Xueyan; Zhang, Min; Wang, Wenying

2015-11-01

This study was aimed to investigate the fundus changes of patients with pregnancy-induced hypertension syndrome (PIHS) using optical coherence tomography (OCT) technology and contrast sensitivity (CS) tests.Ninety-eight patients with PIHS underwent routine eye examinations including vision correction, fundus examination, OCT, and CS tests. The CS test was performed at low, medium, and high frequency, respectively. Moreover, the difference in CS tests between 2 groups was analyzed by independent-samples T test. The Kruskal-Wallis rank sum test and linear regression model were used to detect the correlation of OCT with CS, respectively. Meanwhile Satterthwaite approximate T test was adopted for pairwise comparisons after nonparametric analysis of variance.The OCT test revealed that 56.76% of the examined eyes showed shallow retinal detachment in the macula lutea and around the optic disk. The differences in CS at each spatial frequency between the case and control group were statistically significant (P < 0.01). Besides, OCT manifestations were associated with CS at each spacial frequency including 1.5, 3, 6, 12, and 18 frequency (P < 0.01). And patients with abnormal manifestations of OCT showed lower CS at each spacial frequency than those without abnormal OCT manifestations. What's more the OCT manifestation 1 showed the greatest impact on CS at each spacial frequency.The results showed that abnormal OCT manifestations were correlated with CS in PIHS. OCT and CS tests might be valuable methods in observing fundus changes for PIHS patients.

Modulation transfer function estimation of optical lens system by adaptive neuro-fuzzy methodology

NASA Astrophysics Data System (ADS)

Petković, Dalibor; Shamshirband, Shahaboddin; Pavlović, Nenad T.; Anuar, Nor Badrul; Kiah, Miss Laiha Mat

2014-07-01

The quantitative assessment of image quality is an important consideration in any type of imaging system. The modulation transfer function (MTF) is a graphical description of the sharpness and contrast of an imaging system or of its individual components. The MTF is also known and spatial frequency response. The MTF curve has different meanings according to the corresponding frequency. The MTF of an optical system specifies the contrast transmitted by the system as a function of image size, and is determined by the inherent optical properties of the system. In this study, the adaptive neuro-fuzzy (ANFIS) estimator is designed and adapted to estimate MTF value of the actual optical system. Neural network in ANFIS adjusts parameters of membership function in the fuzzy logic of the fuzzy inference system. The back propagation learning algorithm is used for training this network. This intelligent estimator is implemented using Matlab/Simulink and the performances are investigated. The simulation results presented in this paper show the effectiveness of the developed method.

Investigating the performance of reconstruction methods used in structured illumination microscopy as a function of the illumination pattern's modulation frequency

NASA Astrophysics Data System (ADS)

Shabani, H.; Sánchez-Ortiga, E.; Preza, C.

2016-03-01

Surpassing the resolution of optical microscopy defined by the Abbe diffraction limit, while simultaneously achieving optical sectioning, is a challenging problem particularly for live cell imaging of thick samples. Among a few developing techniques, structured illumination microscopy (SIM) addresses this challenge by imposing higher frequency information into the observable frequency band confined by the optical transfer function (OTF) of a conventional microscope either doubling the spatial resolution or filling the missing cone based on the spatial frequency of the pattern when the patterned illumination is two-dimensional. Standard reconstruction methods for SIM decompose the low and high frequency components from the recorded low-resolution images and then combine them to reach a high-resolution image. In contrast, model-based approaches rely on iterative optimization approaches to minimize the error between estimated and forward images. In this paper, we study the performance of both groups of methods by simulating fluorescence microscopy images from different type of objects (ranging from simulated two-point sources to extended objects). These simulations are used to investigate the methods' effectiveness on restoring objects with various types of power spectrum when modulation frequency of the patterned illumination is changing from zero to the incoherent cut-off frequency of the imaging system. Our results show that increasing the amount of imposed information by using a higher modulation frequency of the illumination pattern does not always yield a better restoration performance, which was found to be depended on the underlying object. Results from model-based restoration show performance improvement, quantified by an up to 62% drop in the mean square error compared to standard reconstruction, with increasing modulation frequency. However, we found cases for which results obtained with standard reconstruction methods do not always follow the same trend.

Lower bound for LCD image quality

NASA Astrophysics Data System (ADS)

Olson, William P.; Balram, Nikhil

1996-03-01

The paper presents an objective lower bound for the discrimination of patterns and fine detail in images on a monochrome LCD. In applications such as medical imaging and military avionics the information of interest is often at the highest frequencies in the image. Since LCDs are sampled data systems, their output modulation is dependent on the phase between the input signal and the sampling points. This phase dependence becomes particularly significant at high spatial frequencies. In order to use an LCD for applications such as those mentioned above it is essential to have a lower (worst case) bound on the performance of the display. We address this problem by providing a mathematical model for the worst case output modulation of an LCD in response to a sine wave input. This function can be interpreted as a worst case modulation transfer function (MTF). The intersection of the worst case MTF with the contrast threshold function (CTF) of the human visual system defines the highest spatial frequency that will always be detectable. In addition to providing the worst case limiting resolution, this MTF is combined with the CTF to produce objective worst case image quality values using the modulation transfer function area (MTFA) metric.

Attentional selection of relative SF mediates global versus local processing: evidence from EEG.

PubMed

Flevaris, Anastasia V; Bentin, Shlomo; Robertson, Lynn C

2011-06-13

Previous research on functional hemispheric differences in visual processing has associated global perception with low spatial frequency (LSF) processing biases of the right hemisphere (RH) and local perception with high spatial frequency (HSF) processing biases of the left hemisphere (LH). The Double Filtering by Frequency (DFF) theory expanded this hypothesis by proposing that visual attention selects and is directed to relatively LSFs by the RH and relatively HSFs by the LH, suggesting a direct causal relationship between SF selection and global versus local perception. We tested this idea in the current experiment by comparing activity in the EEG recorded at posterior right and posterior left hemisphere sites while participants' attention was directed to global or local levels of processing after selection of relatively LSFs versus HSFs in a previous stimulus. Hemispheric asymmetry in the alpha band (8-12 Hz) during preparation for global versus local processing was modulated by the selected SF. In contrast, preparatory activity associated with selection of SF was not modulated by the previously attended level (global/local). These results support the DFF theory that top-down attentional selection of SF mediates global and local processing.

Undersampled digital holographic interferometry

NASA Astrophysics Data System (ADS)

Halaq, H.; Demoli, N.; Sović, I.; Šariri, K.; Torzynski, M.; Vukičević, D.

2008-04-01

In digital holography, primary holographic fringes are recorded using a matricial CCD sensor. Because of the low spatial resolution of currently available CCD arrays, the angle between the reference and object beams must be limited to a few degrees. Namely, due to the digitization involved, the Shannon's criterion imposes that the Nyquist sampling frequency be at least twice the highest signal frequency. This means that, in the case of the recording of an interference fringe pattern by a CCD sensor, the inter-fringe distance must be larger than twice the pixel period. This in turn limits the angle between the object and the reference beams. If this angle, in a practical holographic interferometry measuring setup, cannot be limited to the required value, aliasing will occur in the reconstructed image. In this work, we demonstrate that the low spatial frequency metrology data could nevertheless be efficiently extracted by careful choice of twofold, and even threefold, undersampling of the object field. By combining the time-averaged recording with subtraction digital holography method, we present results for a loudspeaker membrane interferometric study obtained under strong aliasing conditions. High-contrast fringes, as a consequence of the vibration modes of the membrane, are obtained.

A bandpass filter for the enhancement of an X-ray reconstruction of the tissue in the spinal canal

NASA Technical Reports Server (NTRS)

Reed, I. S.; Glenn, W. V.; Kwoh, Y. S.; Truong, T. K.

1980-01-01

In this communication, a new bandpass reconstruction filter is developed to partially remove the low spatial frequencies of the bone and the soft tissue in an X-ray reconstruction of a lumbar spine. This partial removal of the low frequencies suppresses the bony vertebral body and the soft tissue components within the projections of actual clinical data. It also has the effect of enhancing the sharp edges of the fatty tissue surrounding the spinal cord region. The intent of this effort is to directly visualize the spinal cord without the need for water-soluble contrast (e.g., metrizamide) to be installed through lumbar punctures.

An optimized method to calculate error correction capability of tool influence function in frequency domain

NASA Astrophysics Data System (ADS)

Wang, Jia; Hou, Xi; Wan, Yongjian; Shi, Chunyan

2017-10-01

An optimized method to calculate error correction capability of tool influence function (TIF) in certain polishing conditions will be proposed based on smoothing spectral function. The basic mathematical model for this method will be established in theory. A set of polishing experimental data with rigid conformal tool is used to validate the optimized method. The calculated results can quantitatively indicate error correction capability of TIF for different spatial frequency errors in certain polishing conditions. The comparative analysis with previous method shows that the optimized method is simpler in form and can get the same accuracy results with less calculating time in contrast to previous method.

A polyphonic acoustic vortex and its complementary chords

NASA Astrophysics Data System (ADS)

Wilson, C.; Padgett, M. J.

2010-02-01

Using an annular phased array of eight loudspeakers, we generate sound beams that simultaneously contain phase singularities at a number of different frequencies. These frequencies correspond to different musical notes and the singularities can be set to overlap along the beam axis, creating a polyphonic acoustic vortex. Perturbing the drive amplitudes of the speakers means that the singularities no longer overlap, each note being nulled at a slightly different lateral position, where the volume of the other notes is now nonzero. The remaining notes form a tri-note chord. We contrast this acoustic phenomenon to the optical case where the perturbation of a white light vortex leads to a spectral spatial distribution.

Testing the generality of the zoom-lens model: Evidence for visual-pathway specific effects of attended-region size on perception.

PubMed

Goodhew, Stephanie C; Lawrence, Rebecca K; Edwards, Mark

2017-05-01

There are volumes of information available to process in visual scenes. Visual spatial attention is a critically important selection mechanism that prevents these volumes from overwhelming our visual system's limited-capacity processing resources. We were interested in understanding the effect of the size of the attended area on visual perception. The prevailing model of attended-region size across cognition, perception, and neuroscience is the zoom-lens model. This model stipulates that the magnitude of perceptual processing enhancement is inversely related to the size of the attended region, such that a narrow attended-region facilitates greater perceptual enhancement than a wider region. Yet visual processing is subserved by two major visual pathways (magnocellular and parvocellular) that operate with a degree of independence in early visual processing and encode contrasting visual information. Historically, testing of the zoom-lens has used measures of spatial acuity ideally suited to parvocellular processing. This, therefore, raises questions about the generality of the zoom-lens model to different aspects of visual perception. We found that while a narrow attended-region facilitated spatial acuity and the perception of high spatial frequency targets, it had no impact on either temporal acuity or the perception of low spatial frequency targets. This pattern also held up when targets were not presented centrally. This supports the notion that visual attended-region size has dissociable effects on magnocellular versus parvocellular mediated visual processing.

The effect of Bangerter filters on binocular function in observers with amblyopia.

PubMed

Chen, Zidong; Li, Jinrong; Thompson, Benjamin; Deng, Daming; Yuan, Junpeng; Chan, Lily; Hess, Robert F; Yu, Minbin

2014-10-28

We assessed whether partial occlusion of the nonamblyopic eye with Bangerter filters can immediately reduce suppression and promote binocular summation of contrast in observers with amblyopia. In Experiment 1, suppression was measured for 22 observers (mean age, 20 years; range, 14-32 years; 10 females) with strabismic or anisometropic amblyopia and 10 controls using our previously established "balance point" protocol. Measurements were made at baseline and with 0.6-, 0.4-, and 0.2-strength Bangerter filters placed over the nonamblyopic/dominant eye. In Experiment 2, psychophysical measurements of contrast sensitivity were made under binocular and monocular viewing conditions for 25 observers with anisometropic amblyopia (mean age, 17 years; range, 11-28 years; 14 females) and 22 controls (mean age, 24 years; range, 22-27; 12 female). Measurements were made at baseline, and with 0.4- and 0.2-strength Bangerter filters placed over the nonamblyopic/dominant eye. Binocular summation ratios (BSRs) were calculated at baseline and with Bangerter filters in place. Experiment 1: Bangerter filters reduced suppression in observers with amblyopia and induced suppression in controls (P = 0.025). The 0.2-strength filter eliminated suppression in observers with amblyopia and this was not a visual acuity effect. Experiment 2: Bangerter filters were able to induce normal levels of binocular contrast summation in the group of observers with anisometropic amblyopia for a stimulus with a spatial frequency of 3 cycles per degree (cpd, P = 0.006). The filters reduced binocular summation in controls. Bangerter filters can immediately reduce suppression and promote binocular summation for mid/low spatial frequencies in observers with amblyopia. Copyright 2014 The Association for Research in Vision and Ophthalmology, Inc.

Evaluation of possible factors affecting contrast sensitivity function in patients with primary Sjögren's syndrome.

PubMed

Arikan, Sedat; Gokmen, Ferhat; Comez, Arzu Taskiran; Gencer, Baran; Kara, Selcuk; Akbal, Ayla

2015-01-01

The contrast sensitivity (CS) function in patients with primary Sjögren's syndrome (pSS) may be impaired either frequently as a result of dry eye diseases or rarely as a result of optic neuropathy. In this study, we aimed to evaluate the CS function in pSS patients as well as to assess corneal aberrations and thickness of the peripapillary retinal nerve fiber layer (pRNFL). Fourteen eyes of 14 pSS patients (pSS group) and 14 eyes of 14 healthy participants (control group) were subjected to assessment of CS at the spatial frequencies of 1.5, 3.0, 6.0, 12, and 18 cycles/degree (cpd) using a functional visual acuity contrast test (FACT); measurement of corneal high-order aberrations (HOAs) in terms of coma-like, spherical-like, and total HOAs using Scheimpflug corneal topography; and measurement of the thickness of both the macular ganglion cell-inner plexiform layer (mGCIPL) and pRNFL in all quadrants using optical coherence tomography. None of the participants were under treatment with artificial tears. The results of the CS test did not differ between the 2 groups at all spatial frequencies (p>0.05). In addition, there were no statistically significant differences between the 2 groups in terms of corneal HOAs (p>0.05) and thickness of mGCIPL (p>0.05). However, among all quadrants, only the inferior quadrant of pRNFL in pSS patients was statistically significantly thinner than that in the healthy participants (p=0.04). The CS function in pSS patients can be maintained with normal thickness of both pRNFL and mGCIPL and with lack of increased corneal HOAs, which may be present even in the absence of artificial tear usage.

A Spatial and Temporal Frequency Based Figure-Ground Processor

NASA Astrophysics Data System (ADS)

Weisstein, Namoi; Wong, Eva

1990-03-01

Recent findings in visual psychophysics have shown that figure-ground perception can be specified by the spatial and temporal response characteristics of the visual system. Higher spatial frequency regions of the visual field are perceived as figure and lower spatial frequency regions are perceived as background/ (Klymenko and Weisstein, 1986, Wong and Weisstein, 1989). Higher temporal frequency regions are seen as background and lower temporal frequency regions are seen as figure (Wong and Weisstein, 1987, Klymenko, Weisstein, Topolski, and Hsieh, 1988). Thus, high spatial and low temporal frequencies appear to be associated with figure and low spatial and high temporal frequencies appear to be associated with background.

[Possibilities of magnetotherapy in stabilization of visual function in patients with glaucoma].

PubMed

Bisvas Shutanto Kumar; Listopadova, N A

1996-01-01

Courses of magnetotherapy (MT) using ATOS device with 33 mT magnetic field induction were administered to 31 patients (43 eyes) with primary open-angle glaucoma with compensated intraocular pressure. The operation mode was intermittent, with 1.0 to 1.5 Hz field rotation frequency by 6 radii. The procedure is administered to a patient in a sitting posture with magnetic inductor held before the eye. The duration of a session is 10 min, a course consists of 10 sessions. Untreated eyes (n = 15) of the same patients were examined for control. The patients were examined before and 4 to 5 months after MT course. Vision acuity improved by 0.16 diopters, on an average, in 29 eyes (96.7%) out of 30 with vision acuity below 1.0 before treatment. Visocontrastometry was carried out using Visokontrastometer-DT device with spatial frequency range from 0.4 to 19 cycle/degree (12 frequencies) and 125 x 125 monitor. The orientation of lattices was horizontal and vertical. The contrasts ranged from 0.03 to 0.9 (12 levels). MT brought about an improvement of spatial contrast sensitivity by at least 7 values of 12 levels in 22 (84.6%) out of 26 eyes and was unchanged in 4 eyes. Visual field was examined using Humphry automated analyzer. A 120-point threshold test was used. After a course of MT, visual field deficit decreased by at least 10% in 31 (72%) out of 43 eyes, increased in 3, and was unchanged in 9 eyes; on an average, visual field deficit decreased by 22.4% vs. the initial value. After 4 to 5 months the changes in the vision acuity and visual field deficit were negligible. In controls these parameters did not appreciably change over the entire follow-up period.

Opportunistic Beamforming with Wireless Powered 1-bit Feedback Through Rectenna Array

NASA Astrophysics Data System (ADS)

Krikidis, Ioannis

2015-11-01

This letter deals with the opportunistic beamforming (OBF) scheme for multi-antenna downlink with spatial randomness. In contrast to conventional OBF, the terminals return only 1-bit feedback, which is powered by wireless power transfer through a rectenna array. We study two fundamental topologies for the combination of the rectenna elements; the direct-current combiner and the radio-frequency combiner. The beam outage probability is derived in closed form for both combination schemes, by using high order statistics and stochastic geometry.

Spatial-temporal filter effect in a computer model study of ventricular fibrillation.

PubMed

Nowak, Claudia N; Fischer, Gerald; Wieser, Leonhard; Tilg, Bernhard; Neurauter, Andreas; Strohmenger, Hans U

2008-08-01

Prediction of countershock success from ventricular fibrillation (VF) ECG is a major challenge in critical care medicine. Recent findings indicate that stable, high frequency mother rotors are one possible mechanism maintaining VF. A computer model study was performed to investigate how epicardiac sources are reflected in the ECG. In the cardiac tissues of two computer models - a model with cubic geometry and a simplified torso model with a left ventricle - a mother rotor was induced by increasing the potassium rectifier current. On the epicardium, the dominant frequency (DF) map revealed a constant DF of 23 Hz (cubic model) and 24.4 Hz (torso model) in the region of the mother rotor, respectively. A sharp drop of frequency (3-18 Hz in the cubic model and 12.4-18 Hz in the torso model) occurred in the surrounding epicardial tissue of chaotic fibrillatory conduction. While no organized pattern was observable on the body surface of the cubic model, the mother rotor frequency can be identified in the anterior surface of the torso model because of the chosen position of the mother rotor in the ventricle (shortest distance to the body surface). Nevertheless, the DFs were damped on the body surfaces of both models (4.6-8.5 Hz in the cubic model and 14.4-16.4 Hz in the torso model). Thus, it was shown in this computer model study that wave propagation transforms the spatial low pass filtering of the thorax into a temporal low pass. In contrast to the resistive-capacitive low pass filter formed by the tissue, this spatial-temporal low pass filter becomes effective at low frequencies (tens of Hertz). This effect damps the high frequency components arising from the heart and it hampers a direct observation of rapid, organized sources of VF in the ECGs, when in an emergency case an artifact-free recording is not possible.

Hollow Cone Electron Imaging for Single Particle 3D Reconstruction of Proteins

PubMed Central

Tsai, Chun-Ying; Chang, Yuan-Chih; Lobato, Ivan; Van Dyck, Dirk; Chen, Fu-Rong

2016-01-01

The main bottlenecks for high-resolution biological imaging in electron microscopy are radiation sensitivity and low contrast. The phase contrast at low spatial frequencies can be enhanced by using a large defocus but this strongly reduces the resolution. Recently, phase plates have been developed to enhance the contrast at small defocus but electrical charging remains a problem. Single particle cryo-electron microscopy is mostly used to minimize the radiation damage and to enhance the resolution of the 3D reconstructions but it requires averaging images of a massive number of individual particles. Here we present a new route to achieve the same goals by hollow cone dark field imaging using thermal diffuse scattered electrons giving about a 4 times contrast increase as compared to bright field imaging. We demonstrate the 3D reconstruction of a stained GroEL particle can yield about 13.5 Å resolution but using a strongly reduced number of images. PMID:27292544

Higher locus coeruleus MRI contrast is associated with lower parasympathetic influence over heart rate variability.

PubMed

Mather, Mara; Joo Yoo, Hyun; Clewett, David V; Lee, Tae-Ho; Greening, Steven G; Ponzio, Allison; Min, Jungwon; Thayer, Julian F

2017-04-15

The locus coeruleus (LC) is a key node of the sympathetic nervous system and suppresses parasympathetic activity that would otherwise increase heart rate variability. In the current study, we examined whether LC-MRI contrast reflecting neuromelanin accumulation in the LC was associated with high-frequency heart rate variability (HF-HRV), a measure reflecting parasympathetic influences on the heart. Recent evidence indicates that neuromelanin, a byproduct of catecholamine metabolism, accumulates in the LC through young and mid adulthood, suggesting that LC-MRI contrast may be a useful biomarker of individual differences in habitual LC activation. We found that, across younger and older adults, greater LC-MRI contrast was negatively associated with HF-HRV during fear conditioning and spatial detection tasks. This correlation was not accounted for by individual differences in age or anxiety. These findings indicate that individual differences in LC structure relate to key cardiovascular parameters. Copyright © 2017 Elsevier Inc. All rights reserved.

Lateralized hippocampal oscillations underlie distinct aspects of human spatial memory and navigation.

PubMed

Miller, Jonathan; Watrous, Andrew J; Tsitsiklis, Melina; Lee, Sang Ah; Sheth, Sameer A; Schevon, Catherine A; Smith, Elliot H; Sperling, Michael R; Sharan, Ashwini; Asadi-Pooya, Ali Akbar; Worrell, Gregory A; Meisenhelter, Stephen; Inman, Cory S; Davis, Kathryn A; Lega, Bradley; Wanda, Paul A; Das, Sandhitsu R; Stein, Joel M; Gorniak, Richard; Jacobs, Joshua

2018-06-21

The hippocampus plays a vital role in various aspects of cognition including both memory and spatial navigation. To understand electrophysiologically how the hippocampus supports these processes, we recorded intracranial electroencephalographic activity from 46 neurosurgical patients as they performed a spatial memory task. We measure signals from multiple brain regions, including both left and right hippocampi, and we use spectral analysis to identify oscillatory patterns related to memory encoding and navigation. We show that in the left but not right hippocampus, the amplitude of oscillations in the 1-3-Hz "low theta" band increases when viewing subsequently remembered object-location pairs. In contrast, in the right but not left hippocampus, low-theta activity increases during periods of navigation. The frequencies of these hippocampal signals are slower than task-related signals in the neocortex. These results suggest that the human brain includes multiple lateralized oscillatory networks that support different aspects of cognition.

Perception of global image contrast involves transparent spatial filtering and the integration and suppression of local contrasts (not RMS contrast)

PubMed Central

2017-01-01

When adjusting the contrast setting on a television set, we experience a perceptual change in the global image contrast. But how is that statistic computed? We addressed this using a contrast-matching task for checkerboard configurations of micro-patterns in which the contrasts and spatial spreads of two interdigitated components were controlled independently. When the patterns differed greatly in contrast, the higher contrast determined the perceived global contrast. Crucially, however, low contrast additions of one pattern to intermediate contrasts of the other caused a paradoxical reduction in the perceived global contrast. None of the following metrics/models predicted this: max, linear sum, average, energy, root mean squared (RMS), Legge and Foley. However, a nonlinear gain control model, derived from contrast detection and discrimination experiments, incorporating wide-field summation and suppression, did predict the results with no free parameters, but only when spatial filtering was removed. We conclude that our model describes fundamental processes in human contrast vision (the pattern of results was the same for expert and naive observers), but that above threshold—when contrast pedestals are clearly visible—vision's spatial filtering characteristics become transparent, tending towards those of a delta function prior to spatial summation. The global contrast statistic from our model is as easily derived as the RMS contrast of an image, and since it more closely relates to human perception, we suggest it be used as an image contrast metric in practical applications. PMID:28989735

Sex-biased dispersal creates spatial genetic structure in a parthenogenetic ant with a dependent-lineage reproductive system.

PubMed

Kuhn, A; Bauman, D; Darras, H; Aron, S

2017-10-01

Reproduction and dispersal are key aspects of species life history that influence spatial genetic structure in populations. Several ant species in the genus Cataglyphis have evolved a unique breeding system in which new reproductives (that is, queens and males) are produced asexually by parthenogenesis; in contrast, non-reproductives (that is, workers) are produced via sexual reproduction by mates from distinct genetic lineages. We investigated how these two coexisting reproductive methods affect population-level spatial genetic structure using the ant Cataglyphis mauritanica as a model. We obtained genotypes for queens and their male mates from 338 colonies, and we found that the two lineages present in the study population occurred with equal frequency. Furthermore, analysis of spatial genetic structure revealed strong sex-biased dispersal. Because queens were produced by parthenogenesis and because they dispersed over short distances, there was an extreme level of spatial structuring: a mosaic of patches composed of clonal queens was formed. Males, on the other hand, dispersed over several hundred metres and, thus, across patches, ensuring successful interlineage mating.

Towards an Analytical Age-Dependent Model of Contrast Sensitivity Functions for an Ageing Society

PubMed Central

Joulan, Karine; Brémond, Roland

2015-01-01

The Contrast Sensitivity Function (CSF) describes how the visibility of a grating depends on the stimulus spatial frequency. Many published CSF data have demonstrated that contrast sensitivity declines with age. However, an age-dependent analytical model of the CSF is not available to date. In this paper, we propose such an analytical CSF model based on visual mechanisms, taking into account the age factor. To this end, we have extended an existing model from Barten (1999), taking into account the dependencies of this model's optical and physiological parameters on age. Age-dependent models of the cones and ganglion cells densities, the optical and neural MTF, and optical and neural noise are proposed, based on published data. The proposed age-dependent CSF is finally tested against available experimental data, with fair results. Such an age-dependent model may be beneficial when designing real-time age-dependent image coding and display applications. PMID:26078994

Experimental study of microwave-induced thermoacoustic imaging

NASA Astrophysics Data System (ADS)

Jacobs, Ryan T.

Microwave-Induced Thermoacoustic Imaging (TAI) is a noninvasive hybrid modality which improves contrast by using thermoelastic wave generation induced by microwave absorption. Ultrasonography is widely used in medical practice as a low-cost alternative and supplement to magnetic resonance imaging (MRI). Although ultrasonography has relatively high image resolution (depending on the ultrasonic wavelength at diagnostic frequencies), it suffers from low image contrast of soft tissues. In this work samples are irradiated with sub-microsecond electromagnetic pulses inducing acoustic waves in the sample that are then detected with an unfocused transducer. The advantage of this hybrid modality is the ability to take advantage of the microwave absorption coefficients which provide high contrast in tissue samples. This in combination with the superior spatial resolution of ultrasound waves is important to providing a low-cost alternative to MRI and early breast cancer detection methods. This work describes the implementation of a thermoacoustic experiment using a 5 kW peak power microwave source.

Biomolecular Imaging with Coherent Nonlinear Vibrational Microscopy

PubMed Central

Chung, Chao-Yu; Boik, John; Potma, Eric O.

2014-01-01

Optical imaging with spectroscopic vibrational contrast is a label-free solution for visualizing, identifying, and quantifying a wide range of biomolecular compounds in biological materials. Both linear and nonlinear vibrational microscopy techniques derive their imaging contrast from infrared active or Raman allowed molecular transitions, which provide a rich palette for interrogating chemical and structural details of the sample. Yet nonlinear optical methods, which include both second-order sum-frequency generation (SFG) and third-order coherent Raman scattering (CRS) techniques, offer several improved imaging capabilities over their linear precursors. Nonlinear vibrational microscopy features unprecedented vibrational imaging speeds, provides strategies for higher spatial resolution, and gives access to additional molecular parameters. These advances have turned vibrational microscopy into a premier tool for chemically dissecting live cells and tissues. This review discusses the molecular contrast of SFG and CRS microscopy and highlights several of the advanced imaging capabilities that have impacted biological and biomedical research. PMID:23245525

Large-Scale Multiantenna Multisine Wireless Power Transfer

NASA Astrophysics Data System (ADS)

Huang, Yang; Clerckx, Bruno

2017-11-01

Wireless Power Transfer (WPT) is expected to be a technology reshaping the landscape of low-power applications such as the Internet of Things, Radio Frequency identification (RFID) networks, etc. Although there has been some progress towards multi-antenna multi-sine WPT design, the large-scale design of WPT, reminiscent of massive MIMO in communications, remains an open challenge. In this paper, we derive efficient multiuser algorithms based on a generalizable optimization framework, in order to design transmit sinewaves that maximize the weighted-sum/minimum rectenna output DC voltage. The study highlights the significant effect of the nonlinearity introduced by the rectification process on the design of waveforms in multiuser systems. Interestingly, in the single-user case, the optimal spatial domain beamforming, obtained prior to the frequency domain power allocation optimization, turns out to be Maximum Ratio Transmission (MRT). In contrast, in the general weighted sum criterion maximization problem, the spatial domain beamforming optimization and the frequency domain power allocation optimization are coupled. Assuming channel hardening, low-complexity algorithms are proposed based on asymptotic analysis, to maximize the two criteria. The structure of the asymptotically optimal spatial domain precoder can be found prior to the optimization. The performance of the proposed algorithms is evaluated. Numerical results confirm the inefficiency of the linear model-based design for the single and multi-user scenarios. It is also shown that as nonlinear model-based designs, the proposed algorithms can benefit from an increasing number of sinewaves.

Optimization of neural retinal visual motor strategies in recovery of visual acuity following acute laser-induced macula injury

NASA Astrophysics Data System (ADS)

Zwick, Harry; Ness, James W.; Loveday, J.; Molchany, Jerome W.; Stuck, Bruce E.

1997-05-01

Laser induced damage to the retina may produce immediate and serious loss in visual acuity as well as subsequent recovery of visual acuity over a 1 to 6 month post exposure period. While acuity may recover, full utilization of the foveal region may not return. In one patient, a superior/temporal preferred retinal location (PRL) was apparent, while a second patient demonstrated significant foveal involvement and contrast sensitivity more reflective of foveal than parafoveal involvement. These conditions of injury wee simulated by using an artificial scotoma technique which optically stabilized a 5 degree opacity in the center of the visual field. The transmission of spatially degraded target information in the scotoma was 0 percent, 5 percent and 95 percent. Contrast sensitivity for the 0 percent and 5 percent transmission scotoma showed broad spatial frequency suppression as opposed to a bipartite contrast sensitivity function with a narrow sensitivity loss at 3 cycles/degree for the 95 percent transmission scotoma. A PRL shift to superior temporal retina with a concomitant change in accommodation was noted as target resolution became more demanding. These findings suggest that restoration of visual acuity in human laser accidents may depend upon the functionality of complex retinal and cortical adaptive mechanisms.

Close similarity between spatiotemporal frequency tunings of human cortical responses and involuntary manual following responses to visual motion.

PubMed

Amano, Kaoru; Kimura, Toshitaka; Nishida, Shin'ya; Takeda, Tsunehiro; Gomi, Hiroaki

2009-02-01

Human brain uses visual motion inputs not only for generating subjective sensation of motion but also for directly guiding involuntary actions. For instance, during arm reaching, a large-field visual motion is quickly and involuntarily transformed into a manual response in the direction of visual motion (manual following response, MFR). Previous attempts to correlate motion-evoked cortical activities, revealed by brain imaging techniques, with conscious motion perception have resulted only in partial success. In contrast, here we show a surprising degree of similarity between the MFR and the population neural activity measured by magnetoencephalography (MEG). We measured the MFR and MEG induced by the same motion onset of a large-field sinusoidal drifting grating with changing the spatiotemporal frequency of the grating. The initial transient phase of these two responses had very similar spatiotemporal tunings. Specifically, both the MEG and MFR amplitudes increased as the spatial frequency was decreased to, at most, 0.05 c/deg, or as the temporal frequency was increased to, at least, 10 Hz. We also found in peak latency a quantitative agreement (approximately 100-150 ms) and correlated changes against spatiotemporal frequency changes between MEG and MFR. In comparison with these two responses, conscious visual motion detection is known to be most sensitive (i.e., have the lowest detection threshold) at higher spatial frequencies and have longer and more variable response latencies. Our results suggest a close relationship between the properties of involuntary motor responses and motion-evoked cortical activity as reflected by the MEG.

Myopic astigmatism correction: comparison of a Toric Implantable Collamer Lens and a bioptics technique by an adaptive optics visual simulator.

PubMed

Pérez-Vives, Cari; Domínguez-Vicent, Alberto; Madrid-Costa, David; Ferrer-Blasco, Teresa; Montés-Micó, Robert

2013-03-01

To compare the optical and visual quality of a simulated Toric Implantable Collamer Lens (TICL) and a bioptics technique to treat high myopic astigmatism. An adaptive optics visual simulator was used to simulate the vision after TICL implantation and a bioptics procedure from the wavefront aberration pattern for moderate and high-myopic astigmatism. Visual acuity (VA) at different contrasts and contrast sensitivity (CS) at 10, 20 and 25 cycles degree(-1) were measured for 3 and 5-mm pupils. Modulation Transfer Function (MTF) and Point Spread Function (PSF) were calculated for a 5-mm pupil. At a 3-mm pupil we only found statistically significant differences in VA between the two simulated surgeries at low-contrast for moderate- and high-myopic astigmatism (p < 0.05). Statistically significant differences were found in CS at 3-mm pupil between both procedures at the highest spatial frequency for moderate-myopic astigmatism and at all frequencies for high-myopic astigmatism (p < 0.05). At a 5-mm pupil we found statistically significant differences in VA and CS between both simulated surgeries at all contrasts and frequencies evaluated for both groups (p < 0.05). In all cases VA and CS were better with the TICL than with the bioptics technique. MTFs for the bioptics technique were worse than those computed for the TICL. The TICL showed less spread out of the PSF than the bioptics procedure. Simulated TICL and bioptics procedures provided good optical and visual quality, although TICL implantation provided slightly better outcomes than the bioptics procedure, especially when the pupil diameter was increased. Ophthalmic & Physiological Optics © 2013 The College of Optometrists.

Natural images dominate in binocular rivalry

PubMed Central

Baker, Daniel H.; Graf, Erich W.

2009-01-01

Ecological approaches to perception have demonstrated that information encoding by the visual system is informed by the natural environment, both in terms of simple image attributes like luminance and contrast, and more complex relationships corresponding to Gestalt principles of perceptual organization. Here, we ask if this optimization biases perception of visual inputs that are perceptually bistable. Using the binocular rivalry paradigm, we designed stimuli that varied in either their spatiotemporal amplitude spectra or their phase spectra. We found that noise stimuli with “natural” amplitude spectra (i.e., amplitude content proportional to 1/f, where f is spatial or temporal frequency) dominate over those with any other systematic spectral slope, along both spatial and temporal dimensions. This could not be explained by perceived contrast measurements, and occurred even though all stimuli had equal energy. Calculating the effective contrast following attenuation by a model contrast sensitivity function suggested that the strong contrast dependency of rivalry provides the mechanism by which binocular vision is optimized for viewing natural images. We also compared rivalry between natural and phase-scrambled images and found a strong preference for natural phase spectra that could not be accounted for by observer biases in a control task. We propose that this phase specificity relates to contour information, and arises either from the activity of V1 complex cells, or from later visual areas, consistent with recent neuroimaging and single-cell work. Our findings demonstrate that human vision integrates information across space, time, and phase to select the input most likely to hold behavioral relevance. PMID:19289828

Application of Optical Coherence Tomography and Contrast Sensitivity Test for Observing Fundus Changes of Patients With Pregnancy-Induced Hypertension Syndrome

PubMed Central

Wang, Zhixue; Zou, Yuanyuan; Li, Wenying; Wang, Xueyan; Zhang, Min; Wang, Wenying

2015-01-01

Abstract This study was aimed to investigate the fundus changes of patients with pregnancy-induced hypertension syndrome (PIHS) using optical coherence tomography (OCT) technology and contrast sensitivity (CS) tests. Ninety-eight patients with PIHS underwent routine eye examinations including vision correction, fundus examination, OCT, and CS tests. The CS test was performed at low, medium, and high frequency, respectively. Moreover, the difference in CS tests between 2 groups was analyzed by independent-samples T test. The Kruskal–Wallis rank sum test and linear regression model were used to detect the correlation of OCT with CS, respectively. Meanwhile Satterthwaite approximate T test was adopted for pairwise comparisons after nonparametric analysis of variance. The OCT test revealed that 56.76% of the examined eyes showed shallow retinal detachment in the macula lutea and around the optic disk. The differences in CS at each spatial frequency between the case and control group were statistically significant (P < 0.01). Besides, OCT manifestations were associated with CS at each spacial frequency including 1.5, 3, 6, 12, and 18 frequency (P < 0.01). And patients with abnormal manifestations of OCT showed lower CS at each spacial frequency than those without abnormal OCT manifestations. What's more the OCT manifestation 1 showed the greatest impact on CS at each spacial frequency. The results showed that abnormal OCT manifestations were correlated with CS in PIHS. OCT and CS tests might be valuable methods in observing fundus changes for PIHS patients. PMID:26554764

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ogura, Toshihiko, E-mail: t-ogura@aist.go.jp

Scanning electron microscopy (SEM) has been widely used to examine biological specimens of bacteria, viruses and proteins. Until now, atmospheric and/or wet biological specimens have been examined using various atmospheric holders or special equipment involving SEM. Unfortunately, they undergo heavy radiation damage by the direct electron beam. In addition, images of unstained biological samples in water yield poor contrast. We recently developed a new analytical technology involving a frequency transmission electric-field (FTE) method based on thermionic SEM. This method is suitable for high-contrast imaging of unstained biological specimens. Our aim was to optimise the method. Here we describe a high-resolutionmore » FTE system based on field-emission SEM; it allows for imaging and nanoscale examination of various biological specimens in water without radiation damage. The spatial resolution is 8 nm, which is higher than 41 nm of the existing FTE system. Our new method can be easily utilised for examination of unstained biological specimens including bacteria, viruses and protein complexes. Furthermore, our high-resolution FTE system can be used for diverse liquid samples across a broad range of scientific fields, e.g. nanoparticles, nanotubes and organic and catalytic materials. - Highlights: • We developed a high-resolution frequency transmission electric-field (FTE) system. • High-resolution FTE system is introduced in the field-emission SEM. • The spatial resolution of high-resolution FTE method is 8 nm. • High-resolution FTE system enables observation of the intact IgM particles in water.« less

B0 concomitant field compensation for MRI systems employing asymmetric transverse gradient coils.

PubMed

Weavers, Paul T; Tao, Shengzhen; Trzasko, Joshua D; Frigo, Louis M; Shu, Yunhong; Frick, Matthew A; Lee, Seung-Kyun; Foo, Thomas K-F; Bernstein, Matt A

2018-03-01

Imaging gradients result in the generation of concomitant fields, or Maxwell fields, which are of increasing importance at higher gradient amplitudes. These time-varying fields cause additional phase accumulation, which must be compensated for to avoid image artifacts. In the case of gradient systems employing symmetric design, the concomitant fields are well described with second-order spatial variation. Gradient systems employing asymmetric design additionally generate concomitant fields with global (zeroth-order or B 0 ) and linear (first-order) spatial dependence. This work demonstrates a general solution to eliminate the zeroth-order concomitant field by applying the correct B 0 frequency shift in real time to counteract the concomitant fields. Results are demonstrated for phase contrast, spiral, echo-planar imaging (EPI), and fast spin-echo imaging. A global phase offset is reduced in the phase-contrast exam, and blurring is virtually eliminated in spiral images. The bulk image shift in the phase-encode direction is compensated for in EPI, whereas signal loss, ghosting, and blurring are corrected in the fast-spin echo images. A user-transparent method to compensate the zeroth-order concomitant field term by center frequency shifting is proposed and implemented. This solution allows all the existing pulse sequences-both product and research-to be retained without any modifications. Magn Reson Med 79:1538-1544, 2018. © 2017 International Society for Magnetic Resonance in Medicine. © 2017 International Society for Magnetic Resonance in Medicine.

Visual defects in a mouse model of fetal alcohol spectrum disorder.

PubMed

Lantz, Crystal L; Pulimood, Nisha S; Rodrigues-Junior, Wandilson S; Chen, Ching-Kang; Manhaes, Alex C; Kalatsky, Valery A; Medina, Alexandre Esteves

2014-01-01

Alcohol consumption during pregnancy can lead to a multitude of neurological problems in offspring, varying from subtle behavioral changes to severe mental retardation. These alterations are collectively referred to as Fetal Alcohol Spectrum Disorders (FASD). Early alcohol exposure can strongly affect the visual system and children with FASD can exhibit an amblyopia-like pattern of visual acuity deficits even in the absence of optical and oculomotor disruption. Here, we test whether early alcohol exposure can lead to a disruption in visual acuity, using a model of FASD to mimic alcohol consumption in the last months of human gestation. To accomplish this, mice were exposed to ethanol (5 g/kg i.p.) or saline on postnatal days (P) 5, 7, and 9. Two to three weeks later we recorded visually evoked potentials to assess spatial frequency detection and contrast sensitivity, conducted electroretinography (ERG) to further assess visual function and imaged retinotopy using optical imaging of intrinsic signals. We observed that animals exposed to ethanol displayed spatial frequency acuity curves similar to controls. However, ethanol-treated animals showed a significant deficit in contrast sensitivity. Moreover, ERGs revealed a market decrease in both a- and b-waves amplitudes, and optical imaging suggest that both elevation and azimuth maps in ethanol-treated animals have a 10-20° greater map tilt compared to saline-treated controls. Overall, our findings suggest that binge alcohol drinking restricted to the last months of gestation in humans can lead to marked deficits in visual function.

Spatially-Heterodyned Holography

DOEpatents

Thomas, Clarence E [Knoxville, TN; Hanson, Gregory R [Clinton, TN

2006-02-21

A method of recording a spatially low-frequency heterodyne hologram, including spatially heterodyne fringes for Fourier analysis, includes: splitting a laser beam into a reference beam and an object beam; interacting the object beam with an object; focusing the reference beam and the object beam at a focal plane of a digital recorder to form a spatially low-frequency heterodyne hologram including spatially heterodyne fringes for Fourier analysis; digital recording the spatially low-frequency heterodyne hologram; Fourier transforming axes of the recorded spatially low-frequency heterodyne hologram including spatially heterodyne fringes in Fourier space to sit on top of a heterodyne carrier frequency defined by an angle between the reference beam and the object beam; cutting off signals around an origin; and performing an inverse Fourier transform.

The Thaayorre think of Time Like They Talk of Space.

PubMed

Gaby, Alice

2012-01-01

Around the world, it is common to both talk and think about time in terms of space. But does our conceptualization of time simply reflect the space/time metaphors of the language we speak? Evidence from the Australian language Kuuk Thaayorre suggests not. Kuuk Thaayorre speakers do not employ active spatial metaphors in describing time. But this is not to say that spatial language is irrelevant to temporal construals: non-linguistic representations of time are shown here to covary with the linguistic system of describing space. This article contrasts two populations of ethnic Thaayorre from Pormpuraaw - one comprising Kuuk Thaayorre/English bilinguals and the other English-monolinguals - in order to distinguish the effects of language from environmental and other factors. Despite their common physical, social, and cultural context, the two groups differ in their representations of time in ways that are congruent with the language of space in Kuuk Thaayorre and English, respectively. Kuuk Thaayorre/English bilinguals represent time along an absolute east-to-west axis, in alignment with the high frequency of absolute frame of reference terms in Kuuk Thaayorre spatial description. The English-monolinguals, in contrast, represent time from left-to-right, aligning with the dominant relative frame of reference in English spatial description. This occurs in the absence of any east-to-west metaphors in Kuuk Thaayorre, or left-to-right metaphors in English. Thus the way these two groups think about time appears to reflect the language of space and not the language of time.

Paleohydrologic techniques used to define the spatial occurrence of floods

USGS Publications Warehouse

Jarrett, R.D.

1990-01-01

Defining the cause and spatial characteristics of floods may be difficult because of limited streamflow and precipitation data. New paleohydrologic techniques that incorporate information from geomorphic, sedimentologic, and botanic studies provide important supplemental information to define homogeneous hydrologic regions. These techniques also help to define the spatial structure of rainstorms and floods and improve regional flood-frequency estimates. The occurrence and the non-occurrence of paleohydrologic evidence of floods, such as flood bars, alluvial fans, and tree scars, provide valuable hydrologic information. The paleohydrologic research to define the spatial characteristics of floods improves the understanding of flood hydrometeorology. This research was used to define the areal extent and contributing drainage area of flash floods in Colorado. Also, paleohydrologic evidence was used to define the spatial boundaries for the Colorado foothills region in terms of the meteorologic cause of flooding and elevation. In general, above 2300 m, peak flows are caused by snowmelt. Below 2300 m, peak flows primarily are caused by rainfall. The foothills region has an upper elevation limit of about 2300 m and a lower elevation limit of about 1500 m. Regional flood-frequency estimates that incorporate the paleohydrologic information indicate that the Big Thompson River flash flood of 1976 had a recurrence interval of approximately 10,000 years. This contrasts markedly with 100 to 300 years determined by using conventional hydrologic analyses. Flood-discharge estimates based on rainfall-runoff methods in the foothills of Colorado result in larger values than those estimated with regional flood-frequency relations, which are based on long-term streamflow data. Preliminary hydrologic and paleohydrologic research indicates that intense rainfall does not occur at higher elevations in other Rocky Mountain states and that the highest elevations for rainfall-producing floods vary by latitude. The study results have implications for floodplain management and design of hydraulic structures in the mountains of Colorado and other Rocky Mountain States. ?? 1990.

Projected Response of Low-Level Convergence and Associated Precipitation to Greenhouse Warming

NASA Astrophysics Data System (ADS)

Weller, Evan; Jakob, Christian; Reeder, Michael J.

2017-10-01

The parameterization of convection in climate models is a large source of uncertainty in projecting future precipitation changes. Here an objective method to identify organized low-level convergence lines has been used to better understand how atmospheric convection is organized and projected to change, as low-level convergence plays an important role in the processes leading to precipitation. The frequency and strength of convergence lines over both ocean and land in current climate simulations is too low compared to reanalysis data. Projections show a further reduction in the frequency and strength of convergence lines over the midlatitudes. In the tropics, the largest changes in frequency are generally associated with shifts in major low-latitude convergence zones, consistent with changes in the precipitation. Further, examining convergence lines when in the presence or absence of precipitation results in large spatial contrasts, providing a better understanding of regional changes in terms of thermodynamic and dynamic effects.

Mapping cardiogenic oscillations using synchrotron-based phase contrast CT imaging

NASA Astrophysics Data System (ADS)

Thurgood, Jordan; Dubsky, Stephen; Siu, Karen K. W.; Wallace, Megan; Siew, Melissa; Hooper, Stuart; Fouras, Andreas

2012-10-01

In many animals, including humans, the lungs encase the majority of the heart thus the motion of each organ affects the other. The effects of the motion of the heart on the lungs potentially provides information with regards to both lung and heart health. We present a novel technique that is capable of measuring the effect of the heart on the surrounding lung tissue through the use of advanced synchrotron imaging techniques and recently developed X-ray velocimetry methods. This technique generates 2D frequency response maps of the lung tissue motion at multiple projection angles from projection X-ray images. These frequency response maps are subsequently used to generate 3D reconstructions of the lung tissue exhibiting motion at the frequency of ventilation and the lung tissue exhibiting motion at the frequency of the heart. This technique has a combined spatial and temporal resolution sufficient to observe the dynamic and complex 3D nature of lung-heart interactions.

Automatic digital image analysis for identification of mitotic cells in synchronous mammalian cell cultures.

PubMed

Eccles, B A; Klevecz, R R

1986-06-01

Mitotic frequency in a synchronous culture of mammalian cells was determined fully automatically and in real time using low-intensity phase-contrast microscopy and a newvicon video camera connected to an EyeCom III image processor. Image samples, at a frequency of one per minute for 50 hours, were analyzed by first extracting the high-frequency picture components, then thresholding and probing for annular objects indicative of putative mitotic cells. Both the extraction of high-frequency components and the recognition of rings of varying radii and discontinuities employed novel algorithms. Spatial and temporal relationships between annuli were examined to discern the occurrences of mitoses, and such events were recorded in a computer data file. At present, the automatic analysis is suited for random cell proliferation rate measurements or cell cycle studies. The automatic identification of mitotic cells as described here provides a measure of the average proliferative activity of the cell population as a whole and eliminates more than eight hours of manual review per time-lapse video recording.

Positive contrast of SPIO-labeled cells by off-resonant reconstruction of 3D radial half-echo bSSFP.

PubMed

Diwoky, Clemens; Liebmann, Daniel; Neumayer, Bernhard; Reinisch, Andreas; Knoll, Florian; Strunk, Dirk; Stollberger, Rudolf

2015-01-01

This article describes a new acquisition and reconstruction concept for positive contrast imaging of cells labeled with superparamagnetic iron oxides (SPIOs). Overcoming the limitations of a negative contrast representation as gained with gradient echo and fully balanced steady state (bSSFP), the proposed method delivers a spatially localized contrast with high cellular sensitivity not accomplished by other positive contrast methods. Employing a 3D radial bSSFP pulse sequence with half-echo sampling, positive cellular contrast is gained by adding artificial global frequency offsets to each half-echo before image reconstruction. The new contrast regime is highlighted with numerical intravoxel simulations including the point-spread function for 3D half-echo acquisitions. Furthermore, the new method is validated on the basis of in vitro cell phantom measurements on a clinical MRI platform, where the measured contrast-to-noise ratio (CNR) of the new approach exceeds even the negative contrast of bSSFP. Finally, an in vivo proof of principle study based on a mouse model with a clear depiction of labeled cells within a subcutaneous cell islet containing a cell density as low as 7 cells/mm(3) is presented. The resultant isotropic images show robustness to motion and a high CNR, in addition to an enhanced specificity due to the positive contrast of SPIO-labeled cells. Copyright © 2014 John Wiley & Sons, Ltd.

A Model for an Angular Velocity-Tuned Motion Detector Accounting for Deviations in the Corridor-Centering Response of the Bee.

PubMed

Cope, Alex J; Sabo, Chelsea; Gurney, Kevin; Vasilaki, Eleni; Marshall, James A R

2016-05-01

We present a novel neurally based model for estimating angular velocity (AV) in the bee brain, capable of quantitatively reproducing experimental observations of visual odometry and corridor-centering in free-flying honeybees, including previously unaccounted for manipulations of behaviour. The model is fitted using electrophysiological data, and tested using behavioural data. Based on our model we suggest that the AV response can be considered as an evolutionary extension to the optomotor response. The detector is tested behaviourally in silico with the corridor-centering paradigm, where bees navigate down a corridor with gratings (square wave or sinusoidal) on the walls. When combined with an existing flight control algorithm the detector reproduces the invariance of the average flight path to the spatial frequency and contrast of the gratings, including deviations from perfect centering behaviour as found in the real bee's behaviour. In addition, the summed response of the detector to a unit distance movement along the corridor is constant for a large range of grating spatial frequencies, demonstrating that the detector can be used as a visual odometer.

A Model for an Angular Velocity-Tuned Motion Detector Accounting for Deviations in the Corridor-Centering Response of the Bee

PubMed Central

Sabo, Chelsea; Gurney, Kevin; Vasilaki, Eleni; Marshall, James A. R.

2016-01-01

We present a novel neurally based model for estimating angular velocity (AV) in the bee brain, capable of quantitatively reproducing experimental observations of visual odometry and corridor-centering in free-flying honeybees, including previously unaccounted for manipulations of behaviour. The model is fitted using electrophysiological data, and tested using behavioural data. Based on our model we suggest that the AV response can be considered as an evolutionary extension to the optomotor response. The detector is tested behaviourally in silico with the corridor-centering paradigm, where bees navigate down a corridor with gratings (square wave or sinusoidal) on the walls. When combined with an existing flight control algorithm the detector reproduces the invariance of the average flight path to the spatial frequency and contrast of the gratings, including deviations from perfect centering behaviour as found in the real bee’s behaviour. In addition, the summed response of the detector to a unit distance movement along the corridor is constant for a large range of grating spatial frequencies, demonstrating that the detector can be used as a visual odometer. PMID:27148968

Pattern Formation and Strong Nonlinear Interactions in Exciton-Polariton Condensates

NASA Astrophysics Data System (ADS)

Ge, Li; Nersisyan, Ani; Oztop, Baris; Tureci, Hakan

2014-03-01

Exciton-polaritons generated by light-induced potentials can spontaneously condense into macroscopic quantum states that display nontrivial spatial and temporal density modulation. While these patterns and their dynamics can be reproduced through the solution of the generalized Gross-Pitaevskii equation, a predictive theory of their thresholds, oscillation frequencies, and multi-pattern interactions has so far been lacking. Here we represent such an approach based on current-carrying quasi-modes of the non-Hermitian potential induced by the pump. The presented theory allows us to capture the patterns formed in the steady-state directly and account for nonlinearities exactly. We find a simple but powerful expression for thresholds of condensation and the associated frequencies of oscillations, quantifying the contribution of particle formation, leakage, and interactions. We also show that the evolution of the condensate with increasing pump strength is strongly geometry dependent and can display contrasting features such as enhancement or reduction of the spatial localization of the condensate. We acknowledge support by DARPA under Grant No. N66001-11-1-4162 and NSF under CAREER Grant No. DMR-1151810.

Intraoperative spatial frequency domain diffuse optical tomography with indo-cyanine green (ICG) fluorescence contrast (Conference Presentation)

NASA Astrophysics Data System (ADS)

Chong, Sang Hoon; Parthasarathy, Ashwin B.; Kavuri, Venkaiah C.; Moscatelli, Frank A.; Singhal, Sunil; Yodh, Arjun G.

2017-02-01

Surgical resection is the most effective treatment strategy for solid tumors, but complete removal of the tumor is critical for post-surgical recovery/long-term survival and is dependent on correct identification of the tumor margin and accurate excision of microscopic residual tumor in the surgical field. Fluorescence image guided surgery is an emerging technique that has shown promise for intraoperative location of tumors and tumor margins. Current versions of such intraoperative fluorescence imaging, however, are generally limited to 2D near-surface images, i.e., without information about tumor depth. Here we present an intraoperative fluorescence imaging system for 3D volumetric imaging of tumors; the system uses spatial frequency domain diffuse optical tomography with an analytic inversion reconstruction method. The new instrument can derive depth-sensitive 3D tumor images at depths up to 1 cm, and it employs compact epi-imaging and illumination suitable for the operating room, with quasi-real-time image reconstruction for surgical visualization. We present experimental results with FDA-approved Indocynanine Green using an extensive array of tissue phantoms and in a pilot in-vivo study.

On the role of grain boundary character distribution in grain growth of Al-Mg alloys

DOE Office of Scientific and Technical Information (OSTI.GOV)

Matsumoto, K.; Shibayanagi, T.; Umakoshi, Y.

1997-02-01

Grain growth behavior of recrystallized Al-Mg alloys containing 0.3 and 2.7 mass% Mg was investigated, focusing on the interconnection between development of the texture and grain boundary character distribution. An Al-0.3 mass% Mg alloy showed two stages in the change of microstructure during grain growth: the frequency of cube oriented grains and the {Sigma}1 boundary significantly increased at an early stage and then decreased. In the second stage a small amount of isolated large grains with the non-cube component grew and consumed the surrounding cube grains. In contrast, the frequency of cube oriented grains and the grain boundary character distributionmore » showed no significant change during grain growth of Al-2.7 mass% Mg. Small clusters composed of several cube grains containing {Sigma}1 boundaries were formed and their spatial distribution played an important role in the change of microstructure during grain growth. The effect of the spatial distribution on the grain growth behavior was discussed considering the energy balance at triple junctions of grain boundaries.« less

Broadband spatial optical filtering with a volume Bragg grating and a blazed grating pair

NASA Astrophysics Data System (ADS)

Chen, Guanjin; Sun, Xiaojie; Yuan, Xiao; Zhang, Guiju

2017-10-01

A broadband spatial optical filtering system is presented in this paper, which is composed of a Volume Bragg Grating (VBG) and a blazed grating pair. The diffraction efficiency and filtering properties are calculated and simulated by using Fourier diffraction analysis and Coupled Wave Theory. A blazed grating pair and VBG structures are designed and optimized in our simulation. The diffraction efficiency of filtering system shows more than 77.2% during the wavelength period from 953nm to 1153nm, especially 84.1% at the center wavelength. The beam quality is described with near-field modulation (M) and contrast ratio (C). The M of filtering beam are 1.44, 1.49 and 1.55, respectively and the C of filtering beam are 10.1%, 10.2% and 10.5% , respectively and the beam intensity distribution is great improved. The cut-off frequencies of three filtering systems are 1.57mm-1 , 2.06 mm-1 and 2.38 mm-1 , respectively from power spectral density (PSD) curve. It's clear that the cut-off frequency of filtering system is closely related to the angular selectivity of VBG, and the value of cut-off frequency is decided by VBG's Half Width at First Zero (HWFZ) and center wavelength.

Coarse-to-Fine Encoding of Spatial Frequency Information into Visual Short-Term Memory for Faces but Impartial Decay

ERIC Educational Resources Information Center

Gao, Zaifeng; Bentin, Shlomo

2011-01-01

Face perception studies investigated how spatial frequencies (SF) are extracted from retinal display while forming a perceptual representation, or their selective use during task-imposed categorization. Here we focused on the order of encoding low-spatial frequencies (LSF) and high-spatial frequencies (HSF) from perceptual representations into…

Color matrix display simulation based upon luminance and chromatic contrast sensitivity of early vision

NASA Technical Reports Server (NTRS)

Martin, Russel A.; Ahumada, Albert J., Jr.; Larimer, James O.

1992-01-01

This paper describes the design and operation of a new simulation model for color matrix display development. It models the physical structure, the signal processing, and the visual perception of static displays, to allow optimization of display design parameters through image quality measures. The model is simple, implemented in the Mathematica computer language, and highly modular. Signal processing modules operate on the original image. The hardware modules describe backlights and filters, the pixel shape, and the tiling of the pixels over the display. Small regions of the displayed image can be visualized on a CRT. Visual perception modules assume static foveal images. The image is converted into cone catches and then into luminance, red-green, and blue-yellow images. A Haar transform pyramid separates the three images into spatial frequency and direction-specific channels. The channels are scaled by weights taken from human contrast sensitivity measurements of chromatic and luminance mechanisms at similar frequencies and orientations. Each channel provides a detectability measure. These measures allow the comparison of images displayed on prospective devices and, by that, the optimization of display designs.

WE-DE-206-03: MRI Image Formation - Slice Selection, Phase Encoding, Frequency Encoding, K-Space, SNR

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lin, C.

Magnetic resonance imaging (MRI) has become an essential part of clinical imaging due to its ability to render high soft tissue contrast. Instead of ionizing radiation, MRI use strong magnetic field, radio frequency waves and field gradients to create diagnostic useful images. It can be used to image the anatomy and also functional and physiological activities within the human body. Knowledge of the basic physical principles underlying MRI acquisition is vitally important to successful image production and proper image interpretation. This lecture will give an overview of the spin physics, imaging principle of MRI, the hardware of the MRI scanner,more » and various pulse sequences and their applications. It aims to provide a conceptual foundation to understand the image formation process of a clinical MRI scanner. Learning Objectives: Understand the origin of the MR signal and contrast from the spin physics level. Understand the main hardware components of a MRI scanner and their purposes Understand steps for MR image formation including spatial encoding and image reconstruction Understand the main kinds of MR pulse sequences and their characteristics.« less

Evaluation of modulation transfer function of optical lens system by support vector regression methodologies - A comparative study

NASA Astrophysics Data System (ADS)

Petković, Dalibor; Shamshirband, Shahaboddin; Saboohi, Hadi; Ang, Tan Fong; Anuar, Nor Badrul; Rahman, Zulkanain Abdul; Pavlović, Nenad T.

2014-07-01

The quantitative assessment of image quality is an important consideration in any type of imaging system. The modulation transfer function (MTF) is a graphical description of the sharpness and contrast of an imaging system or of its individual components. The MTF is also known and spatial frequency response. The MTF curve has different meanings according to the corresponding frequency. The MTF of an optical system specifies the contrast transmitted by the system as a function of image size, and is determined by the inherent optical properties of the system. In this study, the polynomial and radial basis function (RBF) are applied as the kernel function of Support Vector Regression (SVR) to estimate and predict estimate MTF value of the actual optical system according to experimental tests. Instead of minimizing the observed training error, SVR_poly and SVR_rbf attempt to minimize the generalization error bound so as to achieve generalized performance. The experimental results show that an improvement in predictive accuracy and capability of generalization can be achieved by the SVR_rbf approach in compare to SVR_poly soft computing methodology.

[Comparative clinical study of wavefront-guided laser in situ keratomileusis with versus without iris recognition for myopia or myopic astigmatism].

PubMed

Wang, Wei-qun; Zhang, Jin-song; Zhao, Xiao-jin

2011-10-01

To explore the postoperative visual acuity results of wavefront-guided LASIK with iris recognition for myopia or myopic astigmatism and the changes of higher-order aberrations and contrast sensitivity function (CSF). Series of prospective case studies, 158 eyes (85 cases) of myopia or myopic astigmatism were divided into two groups: one group underwent wavefront-guided LASIK with iris recognition (iris recognition group); another group underwent wavefront-guided LASIK treatment without iris recognition through the limbus maring point (non-iris recognition group). To comparative analyze the postoperative visual acuity, residual refraction, the RMS of higher-order aberrations and CSF of two groups. There was no statistical significance difference between two groups of the average uncorrected visual acuity (t = 0.039, 0.058, 0.898; P = 0.844, 0.810, 0.343), best corrected visual acuity (t = 0.320, 0.440, 1.515; P = 0.572, 0.507, 0.218), and residual refraction [spherical equivalent (t = 0.027, 0.215, 0.238; P = 0.869, 0.643, 0.626), spherical (t = 0.145, 0.117, 0.038; P = 0.704, 0.732, 0.845) and cylinder (t = 1.676, 1.936, 0.334; P = 0.195, 0.164, 0.563)] at postoperative 10 days, 1 month and 3 month. The security index of iris recognition group at postoperative 3 month was 1.06 and non-iris recognition group was 1.03; the efficacy index of iris recognition group is 1.01 and non-iris recognition group was 1.00. Postoperative 3 month iris recognition group 93.83% eyes and non-iris recognition group of 90.91% eyes spherical equivalent within ± 0.50 D (χ(2) = 0.479, P = 0.489), iris recognition group of 98.77% eyes and non-iris recognition group of 97.40% eyes spherical equivalent within ± 1.00 D (Fisher test, P = 0.613). There was no significance difference between the two groups of security, efficacy and predictability. Non-iris recognition group postoperative 1 month and postoperative 3 months 3-order order aberrations root mean square value (RMS) higher than the iris recognition group increased (t = 3.414, -2.870; P = 0.027, 0.045), in particular of coma; the general higher-order aberrations (t = 0.386, 1.132; P = 0.719, 0.321), 4-order aberrations (t = 0.808, 2.720; P = 0.464, 0.063), and 5-order aberrations (t = 0.148, -1.717; P = 0.890, 0.161) show no statistically significant difference. Three months after surgery, two groups have recovered at all spatial frequencies of CSF, iris recognition group at 3.0 c/d (t = 3.209, P = 0.002) and 6.0 c/d (t = 2.997, P = 0.004) spatial frequencies of CSF under mesopic condition was better than non-iris recognition group, glare contrast sensitivity function (GCSF) for 3.0 c/d (t = 3.423, P = 0.001) and 6.0 c/d (t = 6.986, P = 0.000) spatial frequencies under mesopic condition and 1.5 c/d (t = 9.839, P = 0.000) and 3.0 c/d (t = 7.367, P = 0.000) spatial frequencies under photopic condition in iris recognition group were better than non-iris recognition group, there were no significant difference between two groups at the other spatial frequencies. Wavefront-guided LASIK with or without iris recognition both acquired better postoperative visual acuity, but in comparison with without iris recognition, wavefront-guided LASIK with iris recognition is efficient to reduce coma and enhance contrast sensitivity of postoperative.

Sub-nanosecond time-resolved near-field scanning magneto-optical microscope.

PubMed

Rudge, J; Xu, H; Kolthammer, J; Hong, Y K; Choi, B C

2015-02-01

We report on the development of a new magnetic microscope, time-resolved near-field scanning magneto-optical microscope, which combines a near-field scanning optical microscope and magneto-optical contrast. By taking advantage of the high temporal resolution of time-resolved Kerr microscope and the sub-wavelength spatial resolution of a near-field microscope, we achieved a temporal resolution of ∼50 ps and a spatial resolution of <100 nm. In order to demonstrate the spatiotemporal magnetic imaging capability of this microscope, the magnetic field pulse induced gyrotropic vortex dynamics occurring in 1 μm diameter, 20 nm thick CoFeB circular disks has been investigated. The microscope provides sub-wavelength resolution magnetic images of the gyrotropic motion of the vortex core at a resonance frequency of ∼240 MHz.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ogura, Toshihiko, E-mail: t-ogura@aist.go.jp

Highlights: • We developed a high-sensitive frequency transmission electric-field (FTE) system. • The output signal was highly enhanced by applying voltage to a metal layer on SiN. • The spatial resolution of new FTE method is 41 nm. • New FTE system enables observation of the intact bacteria and virus in water. - Abstract: The high-resolution structural analysis of biological specimens by scanning electron microscopy (SEM) presents several advantages. Until now, wet bacterial specimens have been examined using atmospheric sample holders. However, images of unstained specimens in water using these holders exhibit very poor contrast and heavy radiation damage. Recently,more » we developed the frequency transmission electric-field (FTE) method, which facilitates the SEM observation of biological specimens in water without radiation damage. However, the signal detection system presents low sensitivity. Therefore, a high EB current is required to generate clear images, and thus reducing spatial resolution and inducing thermal damage to the samples. Here a high-sensitivity detection system is developed for the FTE method, which enhances the output signal amplitude by hundredfold. The detection signal was highly enhanced when voltage was applied to the metal layer on silicon nitride thin film. This enhancement reduced the EB current and improved the spatial resolution as well as the signal-to-noise ratio. The spatial resolution of a high-sensitive FTE system is 41 nm, which is considerably higher than previous FTE system. New FTE system can easily be utilised to examine various unstained biological specimens in water, such as living bacteria and viruses.« less

Influence of spatial frequency and emotion expression on face processing in patients with panic disorder.

PubMed

Shim, Miseon; Kim, Do-Won; Yoon, Sunkyung; Park, Gewnhi; Im, Chang-Hwan; Lee, Seung-Hwan

2016-06-01

Deficits in facial emotion processing is a major characteristic of patients with panic disorder. It is known that visual stimuli with different spatial frequencies take distinct neural pathways. This study investigated facial emotion processing involving stimuli presented at broad, high, and low spatial frequencies in patients with panic disorder. Eighteen patients with panic disorder and 19 healthy controls were recruited. Seven event-related potential (ERP) components: (P100, N170, early posterior negativity (EPN); vertex positive potential (VPP), N250, P300; and late positive potential (LPP)) were evaluated while the participants looked at fearful and neutral facial stimuli presented at three spatial frequencies. When a fearful face was presented, panic disorder patients showed a significantly increased P100 amplitude in response to low spatial frequency compared to high spatial frequency; whereas healthy controls demonstrated significant broad spatial frequency dependent processing in P100 amplitude. Vertex positive potential amplitude was significantly increased in high and broad spatial frequency, compared to low spatial frequency in panic disorder. Early posterior negativity amplitude was significantly different between HSF and BSF, and between LSF and BSF processing in both groups, regardless of facial expression. The possibly confounding effects of medication could not be controlled. During early visual processing, patients with panic disorder prefer global to detailed information. However, in later processing, panic disorder patients overuse detailed information for the perception of facial expressions. These findings suggest that unique spatial frequency-dependent facial processing could shed light on the neural pathology associated with panic disorder. Copyright © 2016 Elsevier B.V. All rights reserved.

The neural bases of spatial frequency processing during scene perception

PubMed Central

Kauffmann, Louise; Ramanoël, Stephen; Peyrin, Carole

2014-01-01

Theories on visual perception agree that scenes are processed in terms of spatial frequencies. Low spatial frequencies (LSF) carry coarse information whereas high spatial frequencies (HSF) carry fine details of the scene. However, how and where spatial frequencies are processed within the brain remain unresolved questions. The present review addresses these issues and aims to identify the cerebral regions differentially involved in low and high spatial frequency processing, and to clarify their attributes during scene perception. Results from a number of behavioral and neuroimaging studies suggest that spatial frequency processing is lateralized in both hemispheres, with the right and left hemispheres predominantly involved in the categorization of LSF and HSF scenes, respectively. There is also evidence that spatial frequency processing is retinotopically mapped in the visual cortex. HSF scenes (as opposed to LSF) activate occipital areas in relation to foveal representations, while categorization of LSF scenes (as opposed to HSF) activates occipital areas in relation to more peripheral representations. Concomitantly, a number of studies have demonstrated that LSF information may reach high-order areas rapidly, allowing an initial coarse parsing of the visual scene, which could then be sent back through feedback into the occipito-temporal cortex to guide finer HSF-based analysis. Finally, the review addresses spatial frequency processing within scene-selective regions areas of the occipito-temporal cortex. PMID:24847226

Simultaneous monitoring of multiple contrast agents using a hybrid MR-DOT system

NASA Astrophysics Data System (ADS)

Gulsen, Gultekin; Unlu, Mehmet Burcin; Birgul, Ozlem; Nalcioglu, Orhan

2007-02-01

Frequency domain diffuse optical tomography (DOT) is a recently emerging technique that uses arrays of sources and detectors to obtain spatially dependent optical parameters of tissue. Here, we describe the design of a hybrid MR-DOT system for dynamic imaging cancer. The combined system acquires both MR and optical data simultaneously. The performance of the system is tested with phantom and in-vivo studies. Gd-DTPA and ICG was used for this purpose and the enhancement kinetics of both agents are recorded using the hybrid system.

Acoustic metamaterials with broadband and wide-angle impedance matching

NASA Astrophysics Data System (ADS)

Liu, Chenkai; Luo, Jie; Lai, Yun

2018-04-01

We propose a general approach to design broadband and wide-angle impedance-matched acoustic metamaterials. Such an unusual acoustic impedance matching characteristic can be well explained by using a spatially dispersive effective medium theory. For demonstrations, we used silicone rubber, which has a huge impedance contrast with water, to design one- and two-dimensional acoustic structures which are almost perfectly impedance matched to water for a wide range of incident angles and in a broad frequency band. Our work opens up an approach to realize extraordinary acoustic impedance matching properties via metamaterial-design techniques.

High-Frequency Subband Compressed Sensing MRI Using Quadruplet Sampling

PubMed Central

Sung, Kyunghyun; Hargreaves, Brian A

2013-01-01

Purpose To presents and validates a new method that formalizes a direct link between k-space and wavelet domains to apply separate undersampling and reconstruction for high- and low-spatial-frequency k-space data. Theory and Methods High- and low-spatial-frequency regions are defined in k-space based on the separation of wavelet subbands, and the conventional compressed sensing (CS) problem is transformed into one of localized k-space estimation. To better exploit wavelet-domain sparsity, CS can be used for high-spatial-frequency regions while parallel imaging can be used for low-spatial-frequency regions. Fourier undersampling is also customized to better accommodate each reconstruction method: random undersampling for CS and regular undersampling for parallel imaging. Results Examples using the proposed method demonstrate successful reconstruction of both low-spatial-frequency content and fine structures in high-resolution 3D breast imaging with a net acceleration of 11 to 12. Conclusion The proposed method improves the reconstruction accuracy of high-spatial-frequency signal content and avoids incoherent artifacts in low-spatial-frequency regions. This new formulation also reduces the reconstruction time due to the smaller problem size. PMID:23280540

Diversity in spatial scope of contrast adaptation among mouse retinal ganglion cells.

PubMed

Khani, Mohammad Hossein; Gollisch, Tim

2017-12-01

Retinal ganglion cells adapt to changes in visual contrast by adjusting their response kinetics and sensitivity. While much work has focused on the time scales of these adaptation processes, less is known about the spatial scale of contrast adaptation. For example, do small, localized contrast changes affect a cell's signal processing across its entire receptive field? Previous investigations have provided conflicting evidence, suggesting that contrast adaptation occurs either locally within subregions of a ganglion cell's receptive field or globally over the receptive field in its entirety. Here, we investigated the spatial extent of contrast adaptation in ganglion cells of the isolated mouse retina through multielectrode-array recordings. We applied visual stimuli so that ganglion cell receptive fields contained regions where the average contrast level changed periodically as well as regions with constant average contrast level. This allowed us to analyze temporal stimulus integration and sensitivity separately for stimulus regions with and without contrast changes. We found that the spatial scope of contrast adaptation depends strongly on cell identity, with some ganglion cells displaying clear local adaptation, whereas others, in particular large transient ganglion cells, adapted globally to contrast changes. Thus, the spatial scope of contrast adaptation in mouse retinal ganglion cells appears to be cell-type specific. This could reflect differences in mechanisms of contrast adaptation and may contribute to the functional diversity of different ganglion cell types. NEW & NOTEWORTHY Understanding whether adaptation of a neuron in a sensory system can occur locally inside the receptive field or whether it always globally affects the entire receptive field is important for understanding how the neuron processes complex sensory stimuli. For mouse retinal ganglion cells, we here show that both local and global contrast adaptation exist and that this diversity in spatial scope can contribute to the functional diversity of retinal ganglion cell types. Copyright © 2017 the American Physiological Society.

Diversity in spatial scope of contrast adaptation among mouse retinal ganglion cells

PubMed Central

Khani, Mohammad Hossein

2017-01-01

Retinal ganglion cells adapt to changes in visual contrast by adjusting their response kinetics and sensitivity. While much work has focused on the time scales of these adaptation processes, less is known about the spatial scale of contrast adaptation. For example, do small, localized contrast changes affect a cell’s signal processing across its entire receptive field? Previous investigations have provided conflicting evidence, suggesting that contrast adaptation occurs either locally within subregions of a ganglion cell’s receptive field or globally over the receptive field in its entirety. Here, we investigated the spatial extent of contrast adaptation in ganglion cells of the isolated mouse retina through multielectrode-array recordings. We applied visual stimuli so that ganglion cell receptive fields contained regions where the average contrast level changed periodically as well as regions with constant average contrast level. This allowed us to analyze temporal stimulus integration and sensitivity separately for stimulus regions with and without contrast changes. We found that the spatial scope of contrast adaptation depends strongly on cell identity, with some ganglion cells displaying clear local adaptation, whereas others, in particular large transient ganglion cells, adapted globally to contrast changes. Thus, the spatial scope of contrast adaptation in mouse retinal ganglion cells appears to be cell-type specific. This could reflect differences in mechanisms of contrast adaptation and may contribute to the functional diversity of different ganglion cell types. NEW & NOTEWORTHY Understanding whether adaptation of a neuron in a sensory system can occur locally inside the receptive field or whether it always globally affects the entire receptive field is important for understanding how the neuron processes complex sensory stimuli. For mouse retinal ganglion cells, we here show that both local and global contrast adaptation exist and that this diversity in spatial scope can contribute to the functional diversity of retinal ganglion cell types. PMID:28904106

Effect of DM Actuator Errors on the WFIRST/AFTA Coronagraph Contrast Performance

NASA Technical Reports Server (NTRS)

Sidick, Erkin; Shi, Fang

2015-01-01

The WFIRST/AFTA 2.4 m space telescope currently under study includes a stellar coronagraph for the imaging and the spectral characterization of extrasolar planets. The coronagraph employs two sequential deformable mirrors (DMs) to compensate for phase and amplitude errors in creating dark holes. DMs are critical elements in high contrast coronagraphs, requiring precision and stability measured in picometers to enable detection of Earth-like exoplanets. Working with a low-order wavefront-sensor the DM that is conjugate to a pupil can also be used to correct low-order wavefront drift during a scientific observation. However, not all actuators in a DM have the same gain. When using such a DM in low-order wavefront sensing and control subsystem, the actuator gain errors introduce high-spatial frequency errors to the DM surface and thus worsen the contrast performance of the coronagraph. We have investigated the effects of actuator gain errors and the actuator command digitization errors on the contrast performance of the coronagraph through modeling and simulations, and will present our results in this paper.

SU-E-I-33: Initial Evaluation of Model-Based Iterative CT Reconstruction Using Standard Image Quality Phantoms

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gingold, E; Dave, J

2014-06-01

Purpose: The purpose of this study was to compare a new model-based iterative reconstruction with existing reconstruction methods (filtered backprojection and basic iterative reconstruction) using quantitative analysis of standard image quality phantom images. Methods: An ACR accreditation phantom (Gammex 464) and a CATPHAN600 phantom were scanned using 3 routine clinical acquisition protocols (adult axial brain, adult abdomen, and pediatric abdomen) on a Philips iCT system. Each scan was acquired using default conditions and 75%, 50% and 25% dose levels. Images were reconstructed using standard filtered backprojection (FBP), conventional iterative reconstruction (iDose4) and a prototype model-based iterative reconstruction (IMR). Phantom measurementsmore » included CT number accuracy, contrast to noise ratio (CNR), modulation transfer function (MTF), low contrast detectability (LCD), and noise power spectrum (NPS). Results: The choice of reconstruction method had no effect on CT number accuracy, or MTF (p<0.01). The CNR of a 6 HU contrast target was improved by 1–67% with iDose4 relative to FBP, while IMR improved CNR by 145–367% across all protocols and dose levels. Within each scan protocol, the CNR improvement from IMR vs FBP showed a general trend of greater improvement at lower dose levels. NPS magnitude was greatest for FBP and lowest for IMR. The NPS of the IMR reconstruction showed a pronounced decrease with increasing spatial frequency, consistent with the unusual noise texture seen in IMR images. Conclusion: Iterative Model Reconstruction reduces noise and improves contrast-to-noise ratio without sacrificing spatial resolution in CT phantom images. This offers the possibility of radiation dose reduction and improved low contrast detectability compared with filtered backprojection or conventional iterative reconstruction.« less

Pollinator visitation in populations of tristylous Eichhornia paniculata in northeastern Brazil.

PubMed

Husband, Brian C; Barrett, Spencer C H

1992-03-01

The frequencies of floral morphs in populations of tristylous Eichhornia paniculata often deviate from the theoretical expectation of equality. This variation is associated with the breakdown of tristyly and the evolution of self-fertilization. Differences in morph frequencies could result from selection pressures due to variable levels of insect visitation to populations and contrasting foraging behavior among the floral morphs. We estimated pollinator densities in 16 populations and quantified visitation sequences to morphs in five populations of E. paniculata in northeastern Brazil. Foraging behavior among floral morphs was measured as the frequency of visits to morphs relative to their frequency in the population (preference) and number of flights between inflorescences of the same versus different morphs (constancy). Pollinator density (number/m 2 /minute) was not correlated with population size, plant density or morph diversity. Pollinator densities varied most among populations of less than 200 plants. Whether pollinators discriminated among the morphs, depended on whether they primarily collected nectar or pollen. In four populations, nectar-feeding bees (Ancyloscelis and Florilegus spp.) and butterflies showed no consistent preference or constancy among the morphs. In contrast, pollen-collecting bees (Trigona sp.) visited a lower proportion of longstyled inflorescences than expected and tended to visit more mid-and short-styled inflorescences in succession, once they were encountered. Pollinator constancy for morphs did not result from differences in inflorescence production or spatial patchiness among the morphs. Although non-random pollinator visitation to morphs in heterostylous populations could potentially affect mating and hence morph frequencies, the observed visitation patterns in this study do not provide evidence that pollinators play a major role in influencing floral morph frequencies.

On-axis programmable microscope using liquid crystal spatial light modulator

NASA Astrophysics Data System (ADS)

García-Martínez, Pascuala; Martínez, José Luís.; Moreno, Ignacio

2017-06-01

Spatial light modulators (SLM) are currently used in many applications in optical microscopy and imaging. One of the most promising methods is the use of liquid crystal displays (LCD) as programmable phase diffractive optical elements (DOE) placed in the Fourier plane giving access to the spatial frequencies which can be phased shifted individually, allowing to emulate a wealth of contrast enhancing methods for both amplitude and phase samples. We use phase and polarization modulation of LCD to implement an on-axis microscope optical system. The LCD used are Hamamatsu liquid crystal on silicon (LCOS) SLM free of flicker, thus showing a full profit of the SLM space bandwidth, as opposed to optical systems in the literature forced to work off-axis due to the strong zero-order component. Taking benefits of the phase modulation of the LCOS we have implemented different microscopic imaging operations, such as high-pass and low-pass filtering in parallel using programmable blazed gratings. Moreover, we are able to control polarization modulation to display two orthogonal linear state of polarization images than can be subtracted or added by changing the period of the blazed grating. In that sense, Differential Interference Contrast (DIC) microscopy can be easily done by generating two images exploiting the polarization splitting properties when a blazed grating is displayed in the SLM. Biological microscopy samples are also used.

Investigating local network interactions underlying first- and second-order processing.

PubMed

Ellemberg, Dave; Allen, Harriet A; Hess, Robert F

2004-01-01

We compared the spatial lateral interactions for first-order cues to those for second-order cues, and investigated spatial interactions between these two types of cues. We measured the apparent modulation depth of a target Gabor at fixation, in the presence and the absence of horizontally flanking Gabors. The Gabors' gratings were either added to (first-order) or multiplied with (second-order) binary 2-D noise. Apparent "contrast" or modulation depth (i.e., the perceived difference between the high and low luminance regions for the first-order stimulus, or between the high and low contrast regions for the second-order stimulus) was measured with a modulation depth-matching paradigm. For each observer, the first- and second-order Gabors were equated for apparent modulation depth without the flankers. Our results indicate that at the smallest inter-element spacing, the perceived reduction in modulation depth is significantly smaller for the second-order than for the first-order stimuli. Further, lateral interactions operate over shorter distances and the spatial frequency and orientation tuning of the suppression effect are broader for second- than first-order stimuli. Finally, first- and second-order information interact in an asymmetrical fashion; second-order flankers do not reduce the apparent modulation depth of the first-order target, whilst first-order flankers reduce the apparent modulation depth of the second-order target.

A microarray analysis of retinal transcripts that are controlled by image contrast in mice.

PubMed

Brand, Christine; Schaeffel, Frank; Feldkaemper, Marita Pauline

2007-06-18

The development of myopia is controlled by still largely unknown retinal signals. The aim of this study was to investigate the changes in retinal mRNA expression after different periods of visual deprivation in mice, while controlling for retinal illuminance. Each group consisted of three male C57BL/6 mice. Treatment periods were 30 min, 4 h, and 6+6 h. High spatial frequencies were filtered from the retinal image by frosted diffusers over one eye while the fellow eyes were covered by clear neutral density (ND) filters that exhibited similar light attenuating properties (0.1 log units) as the diffusers. For the final 30 min of the respective treatment period mice were individually placed in a clear Perspex cylinder that was positioned in the center of a rotating (60 degrees) large drum. The inside of the drum was covered with a 0.1 cyc/degree vertical square wave grating. This visual environment was chosen to standardize illuminances and contrasts seen by the mice. Labeled cRNA was prepared and hybridized to Affymetrix GeneChip Mouse Genome 430 2.0 arrays. Alterations in mRNA expression levels of candidate genes with potential biological relevance were confirmed by semi-quantitative real-time reverse transcription polymerase chain reaction (RT-PCR). In all groups, Egr-1 mRNA expression was reduced in diffuser-treated eyes. Furthermore, the degradation of the spatial frequency spectrum also changed the cFos mRNA level, with reduced expression after 4 h of diffuser treatment. Other interesting candidates were Akt2, which was up-regulated after 30 min of deprivation and Mapk8ip3, a neuron specific JNK binding and scaffolding protein that was temporally regulated in the diffuser-treated eyes only. The microarray analysis demonstrated a pattern of differential transcriptional changes, even though differences in the retinal images were restricted to spatial features. The candidate genes may provide further insight into the biochemical short-term changes following retinal image degradation in mice. Because deprivation of spatial vision leads to increased eye growth and myopia in both animals and humans, it is believed some of the identified genes play a role in myopia development.

Quantification of resolution in multiplanar reconstructions for digital breast tomosynthesis

NASA Astrophysics Data System (ADS)

Vent, Trevor L.; Acciavatti, Raymond J.; Kwon, Young Joon; Maidment, Andrew D. A.

2016-03-01

Multiplanar reconstruction (MPR) in digital breast tomosynthesis (DBT) allows tomographic images to be portrayed in various orientations. We have conducted research to determine the resolution of tomosynthesis MPR. We built a phantom that houses a star test pattern to measure resolution. This phantom provides three rotational degrees of freedom. The design consists of two hemispheres with longitudinal and latitudinal grooves that reference angular increments. When joined together, the hemispheres form a dome that sits inside a cylindrical encasement. The cylindrical encasement contains reference notches to match the longitudinal and latitudinal grooves that guide the phantom's rotations. With this design, any orientation of the star-pattern can be analyzed. Images of the star-pattern were acquired using a DBT mammography system at the Hospital of the University of Pennsylvania. Images taken were reconstructed and analyzed by two different methods. First, the maximum visible frequency (in line pairs per millimeter) of the star test pattern was measured. Then, the contrast was calculated at a fixed spatial frequency. These analyses confirm that resolution decreases with tilt relative to the breast support. They also confirm that resolution in tomosynthesis MPR is dependent on object orientation. Current results verify that the existence of super-resolution depends on the orientation of the frequency; the direction parallel to x-ray tube motion shows super-resolution. In conclusion, this study demonstrates that the direction of the spatial frequency relative to the motion of the x-ray tube is a determinant of resolution in MPR for DBT.

Human hippocampal theta power indicates movement onset and distance travelled

PubMed Central

Bird, Chris M.; Gollwitzer, Stephanie; Rodionov, Roman; Diehl, Beate; McEvoy, Andrew W.; Walker, Matthew C.; Burgess, Neil

2017-01-01

Theta frequency oscillations in the 6- to 10-Hz range dominate the rodent hippocampal local field potential during translational movement, suggesting that theta encodes self-motion. Increases in theta power have also been identified in the human hippocampus during both real and virtual movement but appear as transient bursts in distinct high- and low-frequency bands, and it is not yet clear how these bursts relate to the sustained oscillation observed in rodents. Here, we examine depth electrode recordings from the temporal lobe of 13 presurgical epilepsy patients performing a self-paced spatial memory task in a virtual environment. In contrast to previous studies, we focus on movement-onset periods that incorporate both initial acceleration and an immediately preceding stationary interval associated with prominent theta oscillations in the rodent hippocampal formation. We demonstrate that movement-onset periods are associated with a significant increase in both low (2–5 Hz)- and high (6–9 Hz)-frequency theta power in the human hippocampus. Similar increases in low- and high-frequency theta power are seen across lateral temporal lobe recording sites and persist throughout the remainder of movement in both regions. In addition, we show that movement-related theta power is greater both before and during longer paths, directly implicating human hippocampal theta in the encoding of translational movement. These findings strengthen the connection between studies of theta-band activity in rodents and humans and offer additional insight into the neural mechanisms of spatial navigation. PMID:29078334

Lesion contrast and detection using sonoelastographic shear velocity imaging: preliminary results

NASA Astrophysics Data System (ADS)

Hoyt, Kenneth; Parker, Kevin J.

2007-03-01

This paper assesses lesion contrast and detection using sonoelastographic shear velocity imaging. Shear wave interference patterns, termed crawling waves, for a two phase medium were simulated assuming plane wave conditions. Shear velocity estimates were computed using a spatial autocorrelation algorithm that operates in the direction of shear wave propagation for a given kernel size. Contrast was determined by analyzing shear velocity estimate transition between mediums. Experimental results were obtained using heterogeneous phantoms with spherical inclusions (5 or 10 mm in diameter) characterized by elevated shear velocities. Two vibration sources were applied to opposing phantom edges and scanned (orthogonal to shear wave propagation) with an ultrasound scanner equipped for sonoelastography. Demodulated data was saved and transferred to an external computer for processing shear velocity images. Simulation results demonstrate shear velocity transition between contrasting mediums is governed by both estimator kernel size and source vibration frequency. Experimental results from phantoms further indicates that decreasing estimator kernel size produces corresponding decrease in shear velocity estimate transition between background and inclusion material albeit with an increase in estimator noise. Overall, results demonstrate the ability to generate high contrast shear velocity images using sonoelastographic techniques and detect millimeter-sized lesions.

Multiscale energy reallocation during low-frequency steady-state brain response.

PubMed

Wang, Yifeng; Chen, Wang; Ye, Liangkai; Biswal, Bharat B; Yang, Xuezhi; Zou, Qijun; Yang, Pu; Yang, Qi; Wang, Xinqi; Cui, Qian; Duan, Xujun; Liao, Wei; Chen, Huafu

2018-05-01

Traditional task-evoked brain activations are based on detection and estimation of signal change from the mean signal. By contrast, the low-frequency steady-state brain response (lfSSBR) reflects frequency-tagging activity at the fundamental frequency of the task presentation and its harmonics. Compared to the activity at these resonant frequencies, brain responses at nonresonant frequencies are largely unknown. Additionally, because the lfSSBR is defined by power change, we hypothesize using Parseval's theorem that the power change reflects brain signal variability rather than the change of mean signal. Using a face recognition task, we observed power increase at the fundamental frequency (0.05 Hz) and two harmonics (0.1 and 0.15 Hz) and power decrease within the infra-slow frequency band (<0.1 Hz), suggesting a multifrequency energy reallocation. The consistency of power and variability was demonstrated by the high correlation (r > .955) of their spatial distribution and brain-behavior relationship at all frequency bands. Additionally, the reallocation of finite energy was observed across various brain regions and frequency bands, forming a particular spatiotemporal pattern. Overall, results from this study strongly suggest that frequency-specific power and variability may measure the same underlying brain activity and that these results may shed light on different mechanisms between lfSSBR and brain activation, and spatiotemporal characteristics of energy reallocation induced by cognitive tasks. © 2018 Wiley Periodicals, Inc.

Cortical feedback signals generalise across different spatial frequencies of feedforward inputs.

PubMed

Revina, Yulia; Petro, Lucy S; Muckli, Lars

2017-09-22

Visual processing in cortex relies on feedback projections contextualising feedforward information flow. Primary visual cortex (V1) has small receptive fields and processes feedforward information at a fine-grained spatial scale, whereas higher visual areas have larger, spatially invariant receptive fields. Therefore, feedback could provide coarse information about the global scene structure or alternatively recover fine-grained structure by targeting small receptive fields in V1. We tested if feedback signals generalise across different spatial frequencies of feedforward inputs, or if they are tuned to the spatial scale of the visual scene. Using a partial occlusion paradigm, functional magnetic resonance imaging (fMRI) and multivoxel pattern analysis (MVPA) we investigated whether feedback to V1 contains coarse or fine-grained information by manipulating the spatial frequency of the scene surround outside an occluded image portion. We show that feedback transmits both coarse and fine-grained information as it carries information about both low (LSF) and high spatial frequencies (HSF). Further, feedback signals containing LSF information are similar to feedback signals containing HSF information, even without a large overlap in spatial frequency bands of the HSF and LSF scenes. Lastly, we found that feedback carries similar information about the spatial frequency band across different scenes. We conclude that cortical feedback signals contain information which generalises across different spatial frequencies of feedforward inputs. Copyright © 2017 The Authors. Published by Elsevier Inc. All rights reserved.

Identifying Aquifer Heterogeneities using the Level Set Method

NASA Astrophysics Data System (ADS)

Lu, Z.; Vesselinov, V. V.; Lei, H.

2016-12-01

Material interfaces between hydrostatigraphic units (HSU) with contrasting aquifer parameters (e.g., strata and facies with different hydraulic conductivity) have a great impact on flow and contaminant transport in subsurface. However, the identification of HSU shape in the subsurface is challenging and typically relies on tomographic approaches where a series of steady-state/transient head measurements at spatially distributed observation locations are analyzed using inverse models. In this study, we developed a mathematically rigorous approach for identifying material interfaces among any arbitrary number of HSUs using the level set method. The approach has been tested first with several synthetic cases, where the true spatial distribution of HSUs was assumed to be known and the head measurements were taken from the flow simulation with the true parameter fields. These synthetic inversion examples demonstrate that the level set method is capable of characterizing the spatial distribution of the heterogeneous. We then applied the methodology to a large-scale problem in which the spatial distribution of pumping wells and observation well screens is consistent with the actual aquifer contamination (chromium) site at the Los Alamos National Laboratory (LANL). In this way, we test the applicability of the methodology at an actual site. We also present preliminary results using the actual LANL site data. We also investigated the impact of the number of pumping/observation wells and the drawdown observation frequencies/intervals on the quality of the inversion results. We also examined the uncertainties associated with the estimated HSU shapes, and the accuracy of the results under different hydraulic-conductivity contrasts between the HSU's.

Efficient use of bit planes in the generation of motion stimuli

NASA Technical Reports Server (NTRS)

Mulligan, Jeffrey B.; Stone, Leland S.

1988-01-01

The production of animated motion sequences on computer-controlled display systems presents a technical problem because large images cannot be transferred from disk storage to image memory at conventional frame rates. A technique is described in which a single base image can be used to generate a broad class of motion stimuli without the need for such memory transfers. This technique was applied to the generation of drifting sine-wave gratings (and by extension, sine wave plaids). For each drifting grating, sine and cosine spatial phase components are first reduced to 1 bit/pixel using a digital halftoning technique. The resulting pairs of 1-bit images are then loaded into pairs of bit planes of the display memory. To animate the patterns, the display hardware's color lookup table is modified on a frame-by-frame basis; for each frame the lookup table is set to display a weighted sum of the spatial sine and cosine phase components. Because the contrasts and temporal frequencies of the various components are mutually independent in each frame, the sine and cosine components can be counterphase modulated in temporal quadrature, yielding a single drifting grating. Using additional bit planes, multiple drifting gratings can be combined to form sine-wave plaid patterns. A large number of resultant plaid motions can be produced from a single image file because the temporal frequencies of all the components can be varied independently. For a graphics device having 8 bits/pixel, up to four drifting gratings may be combined, each having independently variable contrast and speed.

Validation of non-stationary precipitation series for site-specific impact assessment: comparison of two statistical downscaling techniques

NASA Astrophysics Data System (ADS)

Mullan, Donal; Chen, Jie; Zhang, Xunchang John

2016-02-01

Statistical downscaling (SD) methods have become a popular, low-cost and accessible means of bridging the gap between the coarse spatial resolution at which climate models output climate scenarios and the finer spatial scale at which impact modellers require these scenarios, with various different SD techniques used for a wide range of applications across the world. This paper compares the Generator for Point Climate Change (GPCC) model and the Statistical DownScaling Model (SDSM)—two contrasting SD methods—in terms of their ability to generate precipitation series under non-stationary conditions across ten contrasting global climates. The mean, maximum and a selection of distribution statistics as well as the cumulative frequencies of dry and wet spells for four different temporal resolutions were compared between the models and the observed series for a validation period. Results indicate that both methods can generate daily precipitation series that generally closely mirror observed series for a wide range of non-stationary climates. However, GPCC tends to overestimate higher precipitation amounts, whilst SDSM tends to underestimate these. This infers that GPCC is more likely to overestimate the effects of precipitation on a given impact sector, whilst SDSM is likely to underestimate the effects. GPCC performs better than SDSM in reproducing wet and dry day frequency, which is a key advantage for many impact sectors. Overall, the mixed performance of the two methods illustrates the importance of users performing a thorough validation in order to determine the influence of simulated precipitation on their chosen impact sector.

Improved specimen reconstruction by Hilbert phase contrast tomography.

PubMed

Barton, Bastian; Joos, Friederike; Schröder, Rasmus R

2008-11-01

The low signal-to-noise ratio (SNR) in images of unstained specimens recorded with conventional defocus phase contrast makes it difficult to interpret 3D volumes obtained by electron tomography (ET). The high defocus applied for conventional tilt series generates some phase contrast but leads to an incomplete transfer of object information. For tomography of biological weak-phase objects, optimal image contrast and subsequently an optimized SNR are essential for the reconstruction of details such as macromolecular assemblies at molecular resolution. The problem of low contrast can be partially solved by applying a Hilbert phase plate positioned in the back focal plane (BFP) of the objective lens while recording images in Gaussian focus. Images recorded with the Hilbert phase plate provide optimized positive phase contrast at low spatial frequencies, and the contrast transfer in principle extends to the information limit of the microscope. The antisymmetric Hilbert phase contrast (HPC) can be numerically converted into isotropic contrast, which is equivalent to the contrast obtained by a Zernike phase plate. Thus, in-focus HPC provides optimal structure factor information without limiting effects of the transfer function. In this article, we present the first electron tomograms of biological specimens reconstructed from Hilbert phase plate image series. We outline the technical implementation of the phase plate and demonstrate that the technique is routinely applicable for tomography. A comparison between conventional defocus tomograms and in-focus HPC volumes shows an enhanced SNR and an improved specimen visibility for in-focus Hilbert tomography.

The pointillism method for creating stimuli suitable for use in computer-based visual contrast sensitivity testing.

PubMed

Turner, Travis H

2005-03-30

An increasingly large corpus of clinical and experimental neuropsychological research has demonstrated the utility of measuring visual contrast sensitivity. Unfortunately, existing means of measuring contrast sensitivity can be prohibitively expensive, difficult to standardize, or lack reliability. Additionally, most existing tests do not allow full control over important characteristics, such as off-angle rotations, waveform, contrast, and spatial frequency. Ideally, researchers could manipulate characteristics and display stimuli in a computerized task designed to meet experimental needs. Thus far, 256-bit color limitation in standard cathode ray tube (CRT) monitors has been preclusive. To this end, the pointillism method (PM) was developed. Using MATLAB software, stimuli are created based on both mathematical and stochastic components, such that differences in regional luminance values of the gradient field closely approximate the desired contrast. This paper describes the method and examines its performance in sine and square-wave image sets from a range of contrast values. Results suggest the utility of the method for most experimental applications. Weaknesses in the current version, the need for validation and reliability studies, and considerations regarding applications are discussed. Syntax for the program is provided in an appendix, and a version of the program independent of MATLAB is available from the author.

Effect of contrast on human speed perception

NASA Technical Reports Server (NTRS)

Stone, Leland S.; Thompson, Peter

1992-01-01

This study is part of an ongoing collaborative research effort between the Life Science and Human Factors Divisions at NASA ARC to measure the accuracy of human motion perception in order to predict potential errors in human perception/performance and to facilitate the design of display systems that minimize the effects of such deficits. The study describes how contrast manipulations can produce significant errors in human speed perception. Specifically, when two simultaneously presented parallel gratings are moving at the same speed within stationary windows, the lower-contrast grating appears to move more slowly. This contrast-induced misperception of relative speed is evident across a wide range of contrasts (2.5-50 percent) and does not appear to saturate (e.g., a 50 percent contrast grating appears slower than a 70 percent contrast grating moving at the same speed). The misperception is large: a 70 percent contrast grating must, on average, be slowed by 35 percent to match a 10 percent contrast grating moving at 2 deg/sec (N = 6). Furthermore, it is largely independent of the absolute contrast level and is a quasilinear function of log contrast ratio. A preliminary parametric study shows that, although spatial frequency has little effect, the relative orientation of the two gratings is important. Finally, the effect depends on the temporal presentation of the stimuli: the effects of contrast on perceived speed appears lessened when the stimuli to be matched are presented sequentially. These data constrain both physiological models of visual cortex and models of human performance. We conclude that viewing conditions that effect contrast, such as fog, may cause significant errors in speed judgments.

Intercontinental gene flow among western arctic populations of Lesser Snow Geese

USGS Publications Warehouse

Shorey, Rainy I.; Scribner, Kim T.; Kanefsky, Jeannette; Samuel, Michael D.; Libants, Scot V.

2011-01-01

Quantifying the spatial genetic structure of highly vagile species of birds is important in predicting their degree of population demographic and genetic independence during changing environmental conditions, and in assessing their abundance and distribution. In the western Arctic, Lesser Snow Geese (Chen caerulescens caerulescens) provide an example useful for evaluating spatial population genetic structure and the relative contribution of male and female philopatry to breeding and wintering locales. We analyzed biparentally inherited microsatellite loci and maternally inherited mtDNA sequences from geese breeding at Wrangel Island (Russia) and Banks Island (Canada) to estimate gene flow among populations whose geographic overlap during breeding and winter differ. Significant differences in the frequencies of mtDNA haplotypes contrast with the homogeneity of allele frequencies for microsatellite loci. Coalescence simulations revealed high variability and asymmetry between males and females in rates and direction of gene flow between populations. Our results highlight the importance of wintering areas to demographic independence and spatial genetic structure of these populations. Male-mediated gene flow among the populations on northern Wrangel Island, southern Wrangel Island, and Banks Island has been substantial. A high rate of female-mediated gene flow from southern Wrangel Island to Banks Island suggests that population exchange can be achieved when populations winter in a common area. Conversely, when birds from different breeding populations do not share a common wintering area, the probability of population exchange is likely to be dramatically reduced.

Dyadic contrast function and quadratic forward model for radio frequency tomography

NASA Astrophysics Data System (ADS)

Picco, Vittorio

Radio Frequency Tomography is an underground imaging technology that aims to reconstruct extended, deeply buried objects such as tunnels or Underground Facilities (UGF). A network of sensors collects scattered electromagnetic field samples, which are processed to obtain 2D or 3D images of the complex dielectric permittivity profile of the volume under investigation. Unlike systems such as Synthetic Aperture Radar (SAR) or Ground Penetrating Radar (GPR) which normally employ wide-band pulses, RF Tomography uses Continuous Wave (CW) signals to illuminate the scene. The information about the target is not retrieved by relying on bandwidth but by exploiting spatial, frequency and/or polarization diversity. Interestingly, RF Tomography can be readily adapted to obtain images of targets in free space. In this context, in the Andrew Electromagnetics Laboratory of the University of Illinois at Chicago, a measurement system aimed to validate experimentally the performance of RF Tomography has been designed and built. Experimental data have been used to validate its forward model, different inversion algorithms, its performance in terms of resolution and the ability of the system to distinguish between metallic and non-metallic targets. In the specific case of imaging of metallic targets, this thesis proposes to extend the capabilities of RF Tomography by introducing a dyadic permittivity contrast. Electromagnetic scattering from a thin, wire-like object placed in free space with its main axis at an angle with respect to the incident electric field is studied. It is possible to show that for this configuration a fundamental difference exists between a metallic and a dielectric object. This phenomenon can be modeled into Maxwell's equations by using a dyadic permittivity contrast, as it is commonly done when studying crystals. As a result a new formulation of the RF Tomography forward model is obtained, based on a dyadic contrast function. Reconstruction of this dyad allows to estimate not only the location and shape, but also the spatial orientation of the target. In addition, this dissertation proposes an alternative modification of the forward model which removes some limitations caused by the Born approximation. Traditionally, the Born approximation is used to linearize the inherently non-linear forward model. This approximation is valid if the scatterer is small and does not interact strongly with other objects. A quadratic forward model represents a more correct formulation of the scattering phenomenon, and it allows to attempt quantitative reconstruction. Numerical results are presented to highlight the advantages that such a formulation provides over the Born approximation.

Black–white asymmetry in visual perception

PubMed Central

Lu, Zhong-Lin; Sperling, George

2012-01-01

With eleven different types of stimuli that exercise a wide gamut of spatial and temporal visual processes, negative perturbations from mean luminance are found to be typically 25% more effective visually than positive perturbations of the same magnitude (range 8–67%). In Experiment 12, the magnitude of the black–white asymmetry is shown to be a saturating function of stimulus contrast. Experiment 13 shows black–white asymmetry primarily involves a nonlinearity in the visual representation of decrements. Black–white asymmetry in early visual processing produces even-harmonic distortion frequencies in all ordinary stimuli and in illusions such as the perceived asymmetry of optically perfect sine wave gratings. In stimuli intended to stimulate exclusively second-order processing in which motion or shape are defined not by luminance differences but by differences in texture contrast, the black–white asymmetry typically generates artifactual luminance (first-order) motion and shape components. Because black–white asymmetry pervades psychophysical and neurophysiological procedures that utilize spatial or temporal variations of luminance, it frequently needs to be considered in the design and evaluation of experiments that involve visual stimuli. Simple procedures to compensate for black–white asymmetry are proposed. PMID:22984221

Effects of speckle/pixel size ratio on temporal and spatial speckle-contrast analysis of dynamic scattering systems: Implications for measurements of blood-flow dynamics.

PubMed

Ramirez-San-Juan, J C; Mendez-Aguilar, E; Salazar-Hermenegildo, N; Fuentes-Garcia, A; Ramos-Garcia, R; Choi, B

2013-01-01

Laser Speckle Contrast Imaging (LSCI) is an optical technique used to generate blood flow maps with high spatial and temporal resolution. It is well known that in LSCI, the speckle size must exceed the Nyquist criterion to maximize the speckle's pattern contrast. In this work, we study experimentally the effect of speckle-pixel size ratio not only in dynamic speckle contrast, but also on the calculation of the relative flow speed for temporal and spatial analysis. Our data suggest that the temporal LSCI algorithm is more accurate at assessing the relative changes in flow speed than the spatial algorithm.

Automatic evaluation of interferograms

NASA Technical Reports Server (NTRS)

Becker, F.

1982-01-01

A system for the evaluation of interference patterns was developed. For digitizing and processing of the interferograms from classical and holographic interferometers a picture analysis system based upon a computer with a television digitizer was installed. Depending on the quality of the interferograms, four different picture enhancement operations may be used: Signal averaging; spatial smoothing, subtraction of the overlayed intensity function and the removal of distortion-patterns using a spatial filtering technique in the frequency spectrum of the interferograms. The extraction of fringe loci from the digitized interferograms is performed by a foating-threshold method. The fringes are numbered using a special scheme after the removal of any fringe disconnections which appeared if there was insufficient contrast in the holograms. The reconstruction of the object function from the fringe field uses least squares approximation with spline fit. Applications are given.

Is attention based on spatial contextual memory preferentially guided by low spatial frequency signals?

PubMed

Patai, Eva Zita; Buckley, Alice; Nobre, Anna Christina

2013-01-01

A popular model of visual perception states that coarse information (carried by low spatial frequencies) along the dorsal stream is rapidly transmitted to prefrontal and medial temporal areas, activating contextual information from memory, which can in turn constrain detailed input carried by high spatial frequencies arriving at a slower rate along the ventral visual stream, thus facilitating the processing of ambiguous visual stimuli. We were interested in testing whether this model contributes to memory-guided orienting of attention. In particular, we asked whether global, low-spatial frequency (LSF) inputs play a dominant role in triggering contextual memories in order to facilitate the processing of the upcoming target stimulus. We explored this question over four experiments. The first experiment replicated the LSF advantage reported in perceptual discrimination tasks by showing that participants were faster and more accurate at matching a low spatial frequency version of a scene, compared to a high spatial frequency version, to its original counterpart in a forced-choice task. The subsequent three experiments tested the relative contributions of low versus high spatial frequencies during memory-guided covert spatial attention orienting tasks. Replicating the effects of memory-guided attention, pre-exposure to scenes associated with specific spatial memories for target locations (memory cues) led to higher perceptual discrimination and faster response times to identify targets embedded in the scenes. However, either high or low spatial frequency cues were equally effective; LSF signals did not selectively or preferentially contribute to the memory-driven attention benefits to performance. Our results challenge a generalized model that LSFs activate contextual memories, which in turn bias attention and facilitate perception.

Is Attention Based on Spatial Contextual Memory Preferentially Guided by Low Spatial Frequency Signals?

PubMed Central

Patai, Eva Zita; Buckley, Alice; Nobre, Anna Christina

2013-01-01

A popular model of visual perception states that coarse information (carried by low spatial frequencies) along the dorsal stream is rapidly transmitted to prefrontal and medial temporal areas, activating contextual information from memory, which can in turn constrain detailed input carried by high spatial frequencies arriving at a slower rate along the ventral visual stream, thus facilitating the processing of ambiguous visual stimuli. We were interested in testing whether this model contributes to memory-guided orienting of attention. In particular, we asked whether global, low-spatial frequency (LSF) inputs play a dominant role in triggering contextual memories in order to facilitate the processing of the upcoming target stimulus. We explored this question over four experiments. The first experiment replicated the LSF advantage reported in perceptual discrimination tasks by showing that participants were faster and more accurate at matching a low spatial frequency version of a scene, compared to a high spatial frequency version, to its original counterpart in a forced-choice task. The subsequent three experiments tested the relative contributions of low versus high spatial frequencies during memory-guided covert spatial attention orienting tasks. Replicating the effects of memory-guided attention, pre-exposure to scenes associated with specific spatial memories for target locations (memory cues) led to higher perceptual discrimination and faster response times to identify targets embedded in the scenes. However, either high or low spatial frequency cues were equally effective; LSF signals did not selectively or preferentially contribute to the memory-driven attention benefits to performance. Our results challenge a generalized model that LSFs activate contextual memories, which in turn bias attention and facilitate perception. PMID:23776509

Fundamental relationship between the noise properties of grating-based differential phase contrast CT and absorption CT: theoretical framework using a cascaded system model and experimental validation.

PubMed

Li, Ke; Bevins, Nicholas; Zambelli, Joseph; Chen, Guang-Hong

2013-02-01

Using a grating interferometer, a conventional x-ray cone beam computed tomography (CT) data acquisition system can be used to simultaneously generate both conventional absorption CT (ACT) and differential phase contrast CT (DPC-CT) images from a single data acquisition. Since the two CT images were extracted from the same set of x-ray projections, it is expected that intrinsic relationships exist between the noise properties of the two contrast mechanisms. The purpose of this paper is to investigate these relationships. First, a theoretical framework was developed using a cascaded system model analysis to investigate the relationship between the noise power spectra (NPS) of DPC-CT and ACT. Based on the derived analytical expressions of the NPS, the relationship between the spatial-frequency-dependent noise equivalent quanta (NEQ) of DPC-CT and ACT was derived. From these fundamental relationships, the NPS and NEQ of the DPC-CT system can be derived from the corresponding ACT system or vice versa. To validate these theoretical relationships, a benchtop cone beam DPC-CT/ACT system was used to experimentally measure the modulation transfer function (MTF) and NPS of both DPC-CT and ACT. The measured three-dimensional (3D) MTF and NPS were then combined to generate the corresponding 3D NEQ. Two fundamental relationships have been theoretically derived and experimentally validated for the NPS and NEQ of DPC-CT and ACT: (1) the 3D NPS of DPC-CT is quantitatively related to the corresponding 3D NPS of ACT by an inplane-only spatial-frequency-dependent factor 1∕f (2), the ratio of window functions applied to DPC-CT and ACT, and a numerical factor C(g) determined by the geometry and efficiency of the grating interferometer. Note that the frequency-dependent factor is independent of the frequency component f(z) perpendicular to the axial plane. (2) The 3D NEQ of DPC-CT is related to the corresponding 3D NEQ of ACT by an f (2) scaling factor and numerical factors that depend on both the attenuation and refraction properties of the image object, as well as C(g) and the MTF of the grating interferometer. The performance of a DPC-CT system is intrinsically related to the corresponding ACT system. As long as the NPS and NEQ of an ACT system is known, the corresponding NPS and NEQ of the DPC-CT system can be readily estimated using additional characteristics of the grating interferometer.

Model of human visual-motion sensing

NASA Technical Reports Server (NTRS)

Watson, A. B.; Ahumada, A. J., Jr.

1985-01-01

A model of how humans sense the velocity of moving images is proposed. The model exploits constraints provided by human psychophysics, notably that motion-sensing elements appear tuned for two-dimensional spatial frequency, and by the frequency spectrum of a moving image, namely, that its support lies in the plane in which the temporal frequency equals the dot product of the spatial frequency and the image velocity. The first stage of the model is a set of spatial-frequency-tuned, direction-selective linear sensors. The temporal frequency of the response of each sensor is shown to encode the component of the image velocity in the sensor direction. At the second stage, these components are resolved in order to measure the velocity of image motion at each of a number of spatial locations and spatial frequencies. The model has been applied to several illustrative examples, including apparent motion, coherent gratings, and natural image sequences. The model agrees qualitatively with human perception.

A forward model and conjugate gradient inversion technique for low-frequency ultrasonic imaging.

PubMed

van Dongen, Koen W A; Wright, William M D

2006-10-01

Emerging methods of hyperthermia cancer treatment require noninvasive temperature monitoring, and ultrasonic techniques show promise in this regard. Various tomographic algorithms are available that reconstruct sound speed or contrast profiles, which can be related to temperature distribution. The requirement of a high enough frequency for adequate spatial resolution and a low enough frequency for adequate tissue penetration is a difficult compromise. In this study, the feasibility of using low frequency ultrasound for imaging and temperature monitoring was investigated. The transient probing wave field had a bandwidth spanning the frequency range 2.5-320.5 kHz. The results from a forward model which computed the propagation and scattering of low-frequency acoustic pressure and velocity wave fields were used to compare three imaging methods formulated within the Born approximation, representing two main types of reconstruction. The first uses Fourier techniques to reconstruct sound-speed profiles from projection or Radon data based on optical ray theory, seen as an asymptotical limit for comparison. The second uses backpropagation and conjugate gradient inversion methods based on acoustical wave theory. The results show that the accuracy in localization was 2.5 mm or better when using low frequencies and the conjugate gradient inversion scheme, which could be used for temperature monitoring.

UCEPR: Ultrafast localized CEST-spectroscopy with PRESS in phantoms and in vivo.

PubMed

Liu, Zheng; Dimitrov, Ivan E; Lenkinski, Robert E; Hajibeigi, Asghar; Vinogradov, Elena

2016-05-01

Chemical exchange saturation transfer (CEST) is a contrast mechanism enhancing low-concentration molecules through saturation transfer from their exchangeable protons to bulk water. Often many scans are acquired to form a Z-spectrum, making the CEST method time-consuming. Here, an ultrafast localized CEST-spectroscopy with PRESS (UCEPR) is proposed to obtain the entire Z-spectrum of a voxel using only two scans, significantly accelerating CEST. The approach combines ultrafast nonlocalized CEST spectroscopy with localization using PRESS. A field gradient is applied concurrently with the saturation pulse producing simultaneous saturation of all Z-spectrum frequencies that are also spatially encoded. A readout gradient during data acquisition resolves the spatial dependence of the CEST responses into frequency. UCEPR was tested on a 3T scanner both in phantoms and in vivo. In phantoms, a fast Z-spectroscopy acquisition of multiple pH-variant iopamidol samples was achieved with four- to seven-fold acceleration as compared to the conventional CEST methods. In vivo, amide proton transfer (APT) in white matter of healthy human brain was measured rapidly in 48 s and with high frequency resolution (≤ 0.2 ppm). Compared with conventional CEST methods, UCEPR has the advantage of rapidly acquiring high-resolution Z-spectra. Potential in vivo applications include ultrafast localized Z-spectroscopy, quantitative, or dynamic CEST studies. © 2015 Wiley Periodicals, Inc.

Spatial Release From Masking in Simulated Cochlear Implant Users With and Without Access to Low-Frequency Acoustic Hearing

PubMed Central

Dietz, Mathias; Hohmann, Volker; Jürgens, Tim

2015-01-01

For normal-hearing listeners, speech intelligibility improves if speech and noise are spatially separated. While this spatial release from masking has already been quantified in normal-hearing listeners in many studies, it is less clear how spatial release from masking changes in cochlear implant listeners with and without access to low-frequency acoustic hearing. Spatial release from masking depends on differences in access to speech cues due to hearing status and hearing device. To investigate the influence of these factors on speech intelligibility, the present study measured speech reception thresholds in spatially separated speech and noise for 10 different listener types. A vocoder was used to simulate cochlear implant processing and low-frequency filtering was used to simulate residual low-frequency hearing. These forms of processing were combined to simulate cochlear implant listening, listening based on low-frequency residual hearing, and combinations thereof. Simulated cochlear implant users with additional low-frequency acoustic hearing showed better speech intelligibility in noise than simulated cochlear implant users without acoustic hearing and had access to more spatial speech cues (e.g., higher binaural squelch). Cochlear implant listener types showed higher spatial release from masking with bilateral access to low-frequency acoustic hearing than without. A binaural speech intelligibility model with normal binaural processing showed overall good agreement with measured speech reception thresholds, spatial release from masking, and spatial speech cues. This indicates that differences in speech cues available to listener types are sufficient to explain the changes of spatial release from masking across these simulated listener types. PMID:26721918

Blur-resistant perimetric stimuli.

PubMed

Horner, Douglas G; Dul, Mitchell W; Swanson, William H; Liu, Tiffany; Tran, Irene

2013-05-01

To develop perimetric stimuli that are resistant to the effects of peripheral defocus. One eye each was tested on subjects free of eye disease. Experiment 1 assessed spatial frequency, testing 12 subjects at eccentricities from 2 to 7 degrees using blur levels from 0 to 3 diopters (D) for two (Gabor) stimuli (spatial SD, 0.5 degrees; spatial frequencies, 0.5 and 1.0 cycles per degree [cpd]). Experiment 2 assessed stimulus size, testing 12 subjects at eccentricities from 4 to 7 degrees using blur levels 0 to 6 D for two Gaussians with SD of 0.5 and 0.25 degrees and a 0.5-cpd Gabor with SD of 0.5 degrees. Experiment 3 tested 13 subjects at eccentricities from fixation to 27 degrees using blur levels 0 to 6 D for Gabor stimuli at 56 locations; the spatial frequency ranged from 0.14 to 0.50 cpd with location, and SD was scaled accordingly. In experiment 1, blur by 3 D caused a small decline in log contrast sensitivity for the 0.5-cpd stimulus (mean ± SE, 0.09 ± 0.08 log units) and a larger (t = 7.7, p < 0.0001) decline for the 1.0-cpd stimulus (0.37 ± 0.13 log units). In experiment 2, blur by 6 D caused minimal decline for the larger Gaussian, by 0.17 ± 0.16 log units, and larger (t > 4.5, p < 0.001) declines for the smaller Gaussian (0.33 ± 0.16 log units) and the Gabor (0.36 ± 0.18 log units). In experiment 3, blur by 6 D caused declines by 0.27 ± 0.05 log units for eccentricities from 0 to 10 degrees, by 0.20 ± 0.04 log units for eccentricities from 10 to 20 degrees, and 0.13 ± 0.03 log units for eccentricities from 20 to 27 degrees. Experiments 1 and 2 allowed us to design stimuli for experiment 3 that were resistant to effects of peripheral defocus.

Eulerian frequency analysis of structural vibrations from high-speed video

DOE Office of Scientific and Technical Information (OSTI.GOV)

Venanzoni, Andrea; Siemens Industry Software NV, Interleuvenlaan 68, B-3001 Leuven; De Ryck, Laurent

An approach for the analysis of the frequency content of structural vibrations from high-speed video recordings is proposed. The techniques and tools proposed rely on an Eulerian approach, that is, using the time history of pixels independently to analyse structural motion, as opposed to Lagrangian approaches, where the motion of the structure is tracked in time. The starting point is an existing Eulerian motion magnification method, which consists in decomposing the video frames into a set of spatial scales through a so-called Laplacian pyramid [1]. Each scale — or level — can be amplified independently to reconstruct a magnified motionmore » of the observed structure. The approach proposed here provides two analysis tools or pre-amplification steps. The first tool provides a representation of the global frequency content of a video per pyramid level. This may be further enhanced by applying an angular filter in the spatial frequency domain to each frame of the video before the Laplacian pyramid decomposition, which allows for the identification of the frequency content of the structural vibrations in a particular direction of space. This proposed tool complements the existing Eulerian magnification method by amplifying selectively the levels containing relevant motion information with respect to their frequency content. This magnifies the displacement while limiting the noise contribution. The second tool is a holographic representation of the frequency content of a vibrating structure, yielding a map of the predominant frequency components across the structure. In contrast to the global frequency content representation of the video, this tool provides a local analysis of the periodic gray scale intensity changes of the frame in order to identify the vibrating parts of the structure and their main frequencies. Validation cases are provided and the advantages and limits of the approaches are discussed. The first validation case consists of the frequency content retrieval of the tip of a shaker, excited at selected fixed frequencies. The goal of this setup is to retrieve the frequencies at which the tip is excited. The second validation case consists of two thin metal beams connected to a randomly excited bar. It is shown that the holographic representation visually highlights the predominant frequency content of each pixel and locates the global frequencies of the motion, thus retrieving the natural frequencies for each beam.« less

A Novel Method of High Accuracy, Wavefront Phase and Amplitude Correction for Coronagraphy

NASA Technical Reports Server (NTRS)

Bowers, Charles W.; Woodgate, Bruce E.; Lyon, Richard G.

2003-01-01

Detection of extra-solar, and especially terrestrial-like planets, using coronagraphy requires an extremely high level of wavefront correction. For example, the study of Woodruff et al. (2002) has shown that phase uniformity of order 10(exp -4)lambda(rms) must be achieved over the critical range of spatial frequencies to produce the approx. 10(exp 10) contrast needed for the Terrestrial Planet Finder (TPF) mission. Correction of wavefront phase errors to this level may be accomplished by using a very high precision deformable mirror (DM). However, not only phase but also amplitude uniformity of the same scale (approx. 10(exp -4)) and over the same spatial frequency range must be simultaneously obtained to remove all residual speckle in the image plane. We present a design for producing simultaneous wavefront phase and amplitude uniformity to high levels from an input wavefront of lower quality. The design uses a dual Michelson interferometer arrangement incorporating two DM and a single, fixed mirror (all at pupils) and two beamsplitters: one with unequal (asymmetric) beam splitting and one with symmetric beam splitting. This design allows high precision correction of both phase and amplitude using DM with relatively coarse steps and permits a simple correction algorithm.

Electrocortical amplification for emotionally arousing natural scenes: The contribution of luminance and chromatic visual channels

PubMed Central

Miskovic, Vladimir; Martinovic, Jasna; Wieser, Matthias M.; Petro, Nathan M.; Bradley, Margaret M.; Keil, Andreas

2015-01-01

Emotionally arousing scenes readily capture visual attention, prompting amplified neural activity in sensory regions of the brain. The physical stimulus features and related information channels in the human visual system that contribute to this modulation, however, are not known. Here, we manipulated low-level physical parameters of complex scenes varying in hedonic valence and emotional arousal in order to target the relative contributions of luminance based versus chromatic visual channels to emotional perception. Stimulus-evoked brain electrical activity was measured during picture viewing and used to quantify neural responses sensitive to lower-tier visual cortical involvement (steady-state visual evoked potentials) as well as the late positive potential, reflecting a more distributed cortical event. Results showed that the enhancement for emotional content was stimulus-selective when examining the steady-state segments of the evoked visual potentials. Response amplification was present only for low spatial frequency, grayscale stimuli, and not for high spatial frequency, red/green stimuli. In contrast, the late positive potential was modulated by emotion regardless of the scene’s physical properties. Our findings are discussed in relation to neurophysiologically plausible constraints operating at distinct stages of the cortical processing stream. PMID:25640949

Temporal Statistics of Natural Image Sequences Generated by Movements with Insect Flight Characteristics

PubMed Central

Schwegmann, Alexander; Lindemann, Jens Peter; Egelhaaf, Martin

2014-01-01

Many flying insects, such as flies, wasps and bees, pursue a saccadic flight and gaze strategy. This behavioral strategy is thought to separate the translational and rotational components of self-motion and, thereby, to reduce the computational efforts to extract information about the environment from the retinal image flow. Because of the distinguishing dynamic features of this active flight and gaze strategy of insects, the present study analyzes systematically the spatiotemporal statistics of image sequences generated during saccades and intersaccadic intervals in cluttered natural environments. We show that, in general, rotational movements with saccade-like dynamics elicit fluctuations and overall changes in brightness, contrast and spatial frequency of up to two orders of magnitude larger than translational movements at velocities that are characteristic of insects. Distinct changes in image parameters during translations are only caused by nearby objects. Image analysis based on larger patches in the visual field reveals smaller fluctuations in brightness and spatial frequency composition compared to small patches. The temporal structure and extent of these changes in image parameters define the temporal constraints imposed on signal processing performed by the insect visual system under behavioral conditions in natural environments. PMID:25340761

Electrocortical amplification for emotionally arousing natural scenes: the contribution of luminance and chromatic visual channels.

PubMed

Miskovic, Vladimir; Martinovic, Jasna; Wieser, Matthias J; Petro, Nathan M; Bradley, Margaret M; Keil, Andreas

2015-03-01

Emotionally arousing scenes readily capture visual attention, prompting amplified neural activity in sensory regions of the brain. The physical stimulus features and related information channels in the human visual system that contribute to this modulation, however, are not known. Here, we manipulated low-level physical parameters of complex scenes varying in hedonic valence and emotional arousal in order to target the relative contributions of luminance based versus chromatic visual channels to emotional perception. Stimulus-evoked brain electrical activity was measured during picture viewing and used to quantify neural responses sensitive to lower-tier visual cortical involvement (steady-state visual evoked potentials) as well as the late positive potential, reflecting a more distributed cortical event. Results showed that the enhancement for emotional content was stimulus-selective when examining the steady-state segments of the evoked visual potentials. Response amplification was present only for low spatial frequency, grayscale stimuli, and not for high spatial frequency, red/green stimuli. In contrast, the late positive potential was modulated by emotion regardless of the scene's physical properties. Our findings are discussed in relation to neurophysiologically plausible constraints operating at distinct stages of the cortical processing stream. Copyright © 2015 Elsevier B.V. All rights reserved.

Role of inhibition in the specification of orientation selectivity of cells in the cat striate cortex.

PubMed

Bonds, A B

1989-01-01

Mechanisms supporting orientation selectivity of cat striate cortical cells were studied by stimulation with two superimposed sine-wave gratings of different orientations. One grating (base) generated a discharge of known amplitude which could be modified by the second grating (mask). Masks presented at nonoptimal orientations usually reduced the base-generated response, but the degree of reduction varied widely between cells. Cells with narrow orientation tuning tended to be more susceptible to mask presence than broadly tuned cells; similarly, simple cells generally showed more response reduction than did complex cells. The base and mask stimuli were drifted at different temporal frequencies which, in simple cells, permitted the identification of individual response components from each stimulus. This revealed that the reduction of the base response by the mask usually did not vary regularly with mask orientation, although response facilitation from the mask was orientation selective. In some sharply tuned simple cells, response reduction had clear local maxima near the limits of the cell's orientation-tuning function. Response reduction resulted from a nearly pure rightward shift of the response versus log contrast function. The lowest mask contrast yielding reduction was within 0.1-0.3 log unit of the lowest contrast effective for excitation. The temporal-frequency bandpass of the response-reduction mechanism resembled that of most cortical cells. The spatial-frequency bandpass was much broader than is typical for single cortical cells, spanning essentially the entire visual range of the cat. These findings are compatible with a model in which weak intrinsic orientation-selective excitation is enhanced in two stages: (1) control of threshold by nonorientation-selective inhibition that is continuously dependent on stimulus contrast; and (2) in the more narrowly tuned cells, orientation-selective inhibition that has local maxima serving to increase the slope of the orientation-tuning function.

A hexagonal orthogonal-oriented pyramid as a model of image representation in visual cortex

NASA Technical Reports Server (NTRS)

Watson, Andrew B.; Ahumada, Albert J., Jr.

1989-01-01

Retinal ganglion cells represent the visual image with a spatial code, in which each cell conveys information about a small region in the image. In contrast, cells of the primary visual cortex use a hybrid space-frequency code in which each cell conveys information about a region that is local in space, spatial frequency, and orientation. A mathematical model for this transformation is described. The hexagonal orthogonal-oriented quadrature pyramid (HOP) transform, which operates on a hexagonal input lattice, uses basis functions that are orthogonal, self-similar, and localized in space, spatial frequency, orientation, and phase. The basis functions, which are generated from seven basic types through a recursive process, form an image code of the pyramid type. The seven basis functions, six bandpass and one low-pass, occupy a point and a hexagon of six nearest neighbors on a hexagonal lattice. The six bandpass basis functions consist of three with even symmetry, and three with odd symmetry. At the lowest level, the inputs are image samples. At each higher level, the input lattice is provided by the low-pass coefficients computed at the previous level. At each level, the output is subsampled in such a way as to yield a new hexagonal lattice with a spacing square root of 7 larger than the previous level, so that the number of coefficients is reduced by a factor of seven at each level. In the biological model, the input lattice is the retinal ganglion cell array. The resulting scheme provides a compact, efficient code of the image and generates receptive fields that resemble those of the primary visual cortex.

Surface Currents and Winds at the Delaware Bay Mouth

DOE Office of Scientific and Technical Information (OSTI.GOV)

Muscarella, P A; Barton, N P; Lipphardt, B L

2011-04-06

Knowledge of the circulation of estuaries and adjacent shelf waters has relied on hydrographic measurements, moorings, and local wind observations usually removed from the region of interest. Although these observations are certainly sufficient to identify major characteristics, they lack both spatial resolution and temporal coverage. High resolution synoptic observations are required to identify important coastal processes at smaller scales. Long observation periods are needed to properly sample low-frequency processes that may also be important. The introduction of high-frequency (HF) radar measurements and regional wind models for coastal studies is changing this situation. Here we analyze synoptic, high-resolution surface winds andmore » currents in the Delaware Bay mouth over an eight-month period (October 2007 through May 2008). The surface currents were measured by two high-frequency radars while the surface winds were extracted from a data-assimilating regional wind model. To illustrate the utility of these monitoring tools we focus on two 45-day periods which previously were shown to present contrasting pictures of the circulation. One, the low-outflow period is from 1 October through 14 November 2007; the other is the high-outflow period from 3 March through 16 April 2008. The large-scale characteristics noted by previous workers are clearly corroborated. Specifically the M2 tide dominates the surface currents, and the Delaware Bay outflow plume is clearly evident in the low frequency currents. Several new aspects of the surface circulation were also identified. These include a map of the spatial variability of the M2 tide (validating an earlier model study), persistent low-frequency cross-mouth flow, and a rapid response of the surface currents to a changing wind field. However, strong wind episodes did not persist long enough to set up a sustained Ekman response.« less

Characterization of Tissue Structure at Varying Length Scales Using Temporal Diffusion Spectroscopy

PubMed Central

Gore, John C.; Xu, Junzhong; Colvin, Daniel C.; Yankeelov, Thomas E.; Parsons, Edward C.; Does, Mark D.

2011-01-01

The concepts, theoretical behavior and experimental applications of temporal diffusion spectroscopy are reviewed and illustrated. Temporal diffusion spectra are obtained by using oscillating gradient waveforms in diffusion-weighted measurements, and represent the manner in which various spectral components of molecular velocity correlations vary in different geometrical structures that restrict or hinder free movements. Measurements made at different gradient frequencies reveal information on the scale of restrictions or hindrances to free diffusion, and the shape of a spectrum reveals the relative contributions of spatial restrictions at different distance scales. Such spectra differ from other so-called diffusion spectra which depict spatial frequencies and are defined at a fixed diffusion time. Experimentally, oscillating gradients at moderate frequency are more feasible for exploring restrictions at very short distances, which in tissues correspond to structures smaller than cells. We describe the underlying concepts of temporal diffusion spectra and provide analytical expressions for the behavior of the diffusion coefficient as a function of gradient frequency in simple geometries with different dimensions. Diffusion in more complex model media that mimic tissues has been simulated using numerical methods. Experimental measurements of diffusion spectra have been obtained in suspensions of particles and cells, as well as in vivo in intact animals. An observation of particular interest is the increased contrast and heterogeneity observed in tumors using oscillating gradients at moderate frequency compared to conventional pulse gradient methods, and the potential for detecting changes in tumors early in their response to treatment. Computer simulations suggest that diffusion spectral measurements may be sensitive to intracellular structures such as nuclear size, and that changes in tissue diffusion properties may be measured before there are changes in cell density. PMID:20677208

Extreme climatic events drive mammal irruptions: regression analysis of 100-year trends in desert rainfall and temperature

PubMed Central

Greenville, Aaron C; Wardle, Glenda M; Dickman, Chris R

2012-01-01

Extreme climatic events, such as flooding rains, extended decadal droughts and heat waves have been identified increasingly as important regulators of natural populations. Climate models predict that global warming will drive changes in rainfall and increase the frequency and severity of extreme events. Consequently, to anticipate how organisms will respond we need to document how changes in extremes of temperature and rainfall compare to trends in the mean values of these variables and over what spatial scales the patterns are consistent. Using the longest historical weather records available for central Australia – 100 years – and quantile regression methods, we investigate if extreme climate events have changed at similar rates to median events, if annual rainfall has increased in variability, and if the frequency of large rainfall events has increased over this period. Specifically, we compared local (individual weather stations) and regional (Simpson Desert) spatial scales, and quantified trends in median (50th quantile) and extreme weather values (5th, 10th, 90th, and 95th quantiles). We found that median and extreme annual minimum and maximum temperatures have increased at both spatial scales over the past century. Rainfall changes have been inconsistent across the Simpson Desert; individual weather stations showed increases in annual rainfall, increased frequency of large rainfall events or more prolonged droughts, depending on the location. In contrast to our prediction, we found no evidence that intra-annual rainfall had become more variable over time. Using long-term live-trapping records (22 years) of desert small mammals as a case study, we demonstrate that irruptive events are driven by extreme rainfalls (>95th quantile) and that increases in the magnitude and frequency of extreme rainfall events are likely to drive changes in the populations of these species through direct and indirect changes in predation pressure and wildfires. PMID:23170202

Increased fire frequency promotes stronger spatial genetic structure and natural selection at regional and local scales in Pinus halepensis Mill

PubMed Central

González-Martínez, Santiago C.; Navascués, Miguel; Burgarella, Concetta; Mosca, Elena; Lorenzo, Zaida; Zabal-Aguirre, Mario; Vendramin, Giovanni G.; Verdú, Miguel; Pausas, Juli G.

2017-01-01

Background and Aims The recurrence of wildfires is predicted to increase due to global climate change, resulting in severe impacts on biodiversity and ecosystem functioning. Recurrent fires can drive plant adaptation and reduce genetic diversity; however, the underlying population genetic processes have not been studied in detail. In this study, the neutral and adaptive evolutionary effects of contrasting fire regimes were examined in the keystone tree species Pinus halepensis Mill. (Aleppo pine), a fire-adapted conifer. The genetic diversity, demographic history and spatial genetic structure were assessed at local (within-population) and regional scales for populations exposed to different crown fire frequencies. Methods Eight natural P. halepensis stands were sampled in the east of the Iberian Peninsula, five of them in a region exposed to frequent crown fires (HiFi) and three of them in an adjacent region with a low frequency of crown fires (LoFi). Samples were genotyped at nine neutral simple sequence repeats (SSRs) and at 251 single nucleotide polymorphisms (SNPs) from coding regions, some of them potentially important for fire adaptation. Key Results Fire regime had no effects on genetic diversity or demographic history. Three high-differentiation outlier SNPs were identified between HiFi and LoFi stands, suggesting fire-related selection at the regional scale. At the local scale, fine-scale spatial genetic structure (SGS) was overall weak as expected for a wind-pollinated and wind-dispersed tree species. HiFi stands displayed a stronger SGS than LoFi stands at SNPs, which probably reflected the simultaneous post-fire recruitment of co-dispersed related seeds. SNPs with exceptionally strong SGS, a proxy for microenvironmental selection, were only reliably identified under the HiFi regime. Conclusions An increasing fire frequency as predicted due to global change can promote increased SGS with stronger family structures and alter natural selection in P. halepensis and in plants with similar life history traits. PMID:28159988

Nonlinear acoustic detection of weathered, low compliance landmines

NASA Astrophysics Data System (ADS)

Sabatier, James M.; Alberts, W. C. Kirkpatrick; Korman, Murray S.

2005-09-01

Two potential impediments to acoustic landmine detection are soil weathering processes and low compliance landmines. To bury landmines, the soil within a mine diameter is removed and replaced such that bulk density, compression, and shear strength all decrease, leaving an acoustic scar detectable with the linear acoustic measurement technique. After a few soil wetting and drying cycles, this contrast is reduced. Linear acoustic mine detection measurements were made on a low impedance contrast landmine before the first rainfall on several occasions over the subsequent 5 years. During this period of time, both the spatial and frequency resolution had to be increased to maintain an on/off target velocity ratio that allowed detection. In some cases, the landmine remains undetectable. To address this, two-tone nonlinear acoustic measurements have been made on these landmines. When the landmine is detectable with linear acoustics, two tones are broadcast at the frequency where the on/off target velocity ratio is the largest. For the cases when the landmine is undetectable, a two-tone sweep is performed and the operator observes the real-time velocity FFT, noting nonlinear sidebands. Next, two-tone tests are conducted at these sidebands to determine nonlinear velocity profiles. [Work supported by U.S. Army RDECOM, NVESD.

Additional-singleton interference in efficient visual search: a common salience route for detection and compound tasks.

PubMed

Zehetleitner, Michael; Proulx, Michael J; Müller, Hermann J

2009-11-01

In efficient search for feature singleton targets, additional singletons (ASs) defined in a nontarget dimension are frequently found to interfere with performance. All search tasks that are processed via a spatial saliency map of the display would be predicted to be subject to such AS interference. In contrast, dual-route models, such as feature integration theory, assume that singletons are detected not via a saliency map, but via a nonspatial route that is immune to interference from cross-dimensional ASs. Consistent with this, a number of studies have reported absent interference effects in detection tasks. However, recent work suggests that the failure to find such effects may be due to the particular frequencies at which ASs were presented, as well as to their relative saliency. These two factors were examined in the present study. In contrast to previous reports, cross-dimensional ASs were found to slow detection (target-present and target-absent) responses, modulated by both their frequency of occurrence and saliency (relative to the target). These findings challenge dual-route models and support single-route models, such as dimension weighting and guided search.

Influence of 4,4’-azobis (4-cyanopentanoic acid) in Transmission and Reflection Gratings Stored in a PVA/AA Photopolymer

PubMed Central

Fernandez, Elena; Fuentes, Rosa; Belendez, Augusto; Pascual, Inmaculada

2016-01-01

Holographic transmission gratings with a spatial frequency of 2658 lines/mm and reflection gratings with a spatial frequency of 4553 lines/mm were stored in a polyvinyl alcohol (PVA)/acrylamide (AA) based photopolymer. This material can reach diffraction efficiencies close to 100% for spatial frequencies about 1000 lines/mm. However, for higher spatial frequencies, the diffraction efficiency decreases considerably as the spatial frequency increases. To enhance the material response at high spatial frequencies, a chain transfer agent, the 4,4’-azobis (4-cyanopentanoic acid), ACPA, is added to the composition of the material. Different concentrations of ACPA are incorporated into the main composition of the photopolymer to find the concentration value that provides the highest diffraction efficiency. Moreover, the refractive index modulation and the optical thickness of the transmission and reflection gratings were obtained, evaluated and compared to procure more information about the influence of the ACPA on them. PMID:28773322

Gap discrete breathers in strained boron nitride

NASA Astrophysics Data System (ADS)

Barani, Elham; Korznikova, Elena A.; Chetverikov, Alexander P.; Zhou, Kun; Dmitriev, Sergey V.

2017-11-01

Linear and nonlinear dynamics of hexagonal boron nitride (h-BN) lattice is studied by means of molecular dynamics simulations with the use of the Tersoff interatomic potentials. It is found that sufficiently large homogeneous elastic strain along zigzag direction opens a wide gap in the phonon spectrum. Extended vibrational mode with boron and nitrogen sublattices vibrating in-plane as a whole in strained h-BN has frequency within the phonon gap. This fact suggests that a nonlinear spatially localized vibrational mode with frequencies in the phonon gap, called discrete breather (also often termed as intrinsic localized mode), can be excited. Properties of the gap discrete breathers in strained h-BN are contrasted with that for analogous vibrational mode found earlier in strained graphene. It is found that h-BN modeled with the Tersoff potentials does not support transverse discrete breathers.

Biomedical imaging with THz waves

NASA Astrophysics Data System (ADS)

Nguyen, Andrew

2010-03-01

We discuss biomedical imaging using radio waves operating in the terahertz (THz) range between 300 GHz to 3 THz. Particularly, we present the concept for two THz imaging systems. One system employs single antenna, transmitter and receiver operating over multi-THz-frequency simultaneously for sensing and imaging small areas of the human body or biological samples. Another system consists of multiple antennas, a transmitter, and multiple receivers operating over multi-THz-frequency capable of sensing and imaging simultaneously the whole body or large biological samples. Using THz waves for biomedical imaging promises unique and substantial medical benefits including extremely small medical devices, extraordinarily fine spatial resolution, and excellent contrast between images of diseased and healthy tissues. THz imaging is extremely attractive for detection of cancer in the early stages, sensing and imaging of tissues near the skin, and study of disease and its growth versus time.

A test of the reward-contrast hypothesis.

PubMed

Dalecki, Stefan J; Panoz-Brown, Danielle E; Crystal, Jonathon D

2017-12-01

Source memory, a facet of episodic memory, is the memory of the origin of information. Whereas source memory in rats is sustained for at least a week, spatial memory degraded after approximately a day. Different forgetting functions may suggest that two memory systems (source memory and spatial memory) are dissociated. However, in previous work, the two tasks used baiting conditions consisting of chocolate and chow flavors; notably, the source memory task used the relatively better flavor. Thus, according to the reward-contrast hypothesis, when chocolate and chow were presented within the same context (i.e., within a single radial maze trial), the chocolate location was more memorable than the chow location because of contrast. We tested the reward-contrast hypothesis using baiting configurations designed to produce reward-contrast. The reward-contrast hypothesis predicts that under these conditions, spatial memory will survive a 24-h retention interval. We documented elimination of spatial memory performance after a 24-h retention interval using a reward-contrast baiting pattern. These data suggest that reward contrast does not explain our earlier findings that source memory survives unusually long retention intervals. Copyright © 2017 Elsevier B.V. All rights reserved.

Sensitivity of high-frequency Rayleigh-wave data revisited

USGS Publications Warehouse

Xia, J.; Miller, R.D.; Ivanov, J.

2007-01-01

Rayleigh-wave phase velocity of a layered earth model is a function of frequency and four groups of earth properties: P-wave velocity, S-wave velocity (Vs), density, and thickness of layers. Analysis of the Jacobian matrix (or the difference method) provides a measure of dispersion curve sensitivity to earth properties. Vs is the dominant influence for the fundamental mode (Xia et al., 1999) and higher modes (Xia et al., 2003) of dispersion curves in a high frequency range (>2 Hz) followed by layer thickness. These characteristics are the foundation of determining S-wave velocities by inversion of Rayleigh-wave data. More applications of surface-wave techniques show an anomalous velocity layer such as a high-velocity layer (HVL) or a low-velocity layer (LVL) commonly exists in near-surface materials. Spatial location (depth) of an anomalous layer is usually the most important information that surface-wave techniques are asked to provide. Understanding and correctly defining the sensitivity of high-frequency Rayleigh-wave data due to depth of an anomalous velocity layer are crucial in applying surface-wave techniques to obtain a Vs profile and/or determine the depth of an anomalous layer. Because depth is not a direct earth property of a layered model, changes in depth will result in changes in other properties. Modeling results show that sensitivity at a given depth calculated by the difference method is dependent on the Vs difference (contrast) between an anomalous layer and surrounding layers. The larger the contrast is, the higher the sensitivity due to depth of the layer. Therefore, the Vs contrast is a dominant contributor to sensitivity of Rayleigh-wave data due to depth of an anomalous layer. Modeling results also suggest that the most sensitive depth for an HVL is at about the middle of the depth to the half-space, but for an LVL it is near the ground surface. ?? 2007 Society of Exploration Geophysicists.

Correction for spatial averaging in laser speckle contrast analysis

PubMed Central

Thompson, Oliver; Andrews, Michael; Hirst, Evan

2011-01-01

Practical laser speckle contrast analysis systems face a problem of spatial averaging of speckles, due to the pixel size in the cameras used. Existing practice is to use a system factor in speckle contrast analysis to account for spatial averaging. The linearity of the system factor correction has not previously been confirmed. The problem of spatial averaging is illustrated using computer simulation of time-integrated dynamic speckle, and the linearity of the correction confirmed using both computer simulation and experimental results. The valid linear correction allows various useful compromises in the system design. PMID:21483623

Adaptive nonlocal means filtering based on local noise level for CT denoising

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Zhoubo; Trzasko, Joshua D.; Lake, David S.

2014-01-15

Purpose: To develop and evaluate an image-domain noise reduction method based on a modified nonlocal means (NLM) algorithm that is adaptive to local noise level of CT images and to implement this method in a time frame consistent with clinical workflow. Methods: A computationally efficient technique for local noise estimation directly from CT images was developed. A forward projection, based on a 2D fan-beam approximation, was used to generate the projection data, with a noise model incorporating the effects of the bowtie filter and automatic exposure control. The noise propagation from projection data to images was analytically derived. The analyticalmore » noise map was validated using repeated scans of a phantom. A 3D NLM denoising algorithm was modified to adapt its denoising strength locally based on this noise map. The performance of this adaptive NLM filter was evaluated in phantom studies in terms of in-plane and cross-plane high-contrast spatial resolution, noise power spectrum (NPS), subjective low-contrast spatial resolution using the American College of Radiology (ACR) accreditation phantom, and objective low-contrast spatial resolution using a channelized Hotelling model observer (CHO). Graphical processing units (GPU) implementation of this noise map calculation and the adaptive NLM filtering were developed to meet demands of clinical workflow. Adaptive NLM was piloted on lower dose scans in clinical practice. Results: The local noise level estimation matches the noise distribution determined from multiple repetitive scans of a phantom, demonstrated by small variations in the ratio map between the analytical noise map and the one calculated from repeated scans. The phantom studies demonstrated that the adaptive NLM filter can reduce noise substantially without degrading the high-contrast spatial resolution, as illustrated by modulation transfer function and slice sensitivity profile results. The NPS results show that adaptive NLM denoising preserves the shape and peak frequency of the noise power spectrum better than commercial smoothing kernels, and indicate that the spatial resolution at low contrast levels is not significantly degraded. Both the subjective evaluation using the ACR phantom and the objective evaluation on a low-contrast detection task using a CHO model observer demonstrate an improvement on low-contrast performance. The GPU implementation can process and transfer 300 slice images within 5 min. On patient data, the adaptive NLM algorithm provides more effective denoising of CT data throughout a volume than standard NLM, and may allow significant lowering of radiation dose. After a two week pilot study of lower dose CT urography and CT enterography exams, both GI and GU radiology groups elected to proceed with permanent implementation of adaptive NLM in their GI and GU CT practices. Conclusions: This work describes and validates a computationally efficient technique for noise map estimation directly from CT images, and an adaptive NLM filtering based on this noise map, on phantom and patient data. Both the noise map calculation and the adaptive NLM filtering can be performed in times that allow integration with clinical workflow. The adaptive NLM algorithm provides effective denoising of CT data throughout a volume, and may allow significant lowering of radiation dose.« less

Discriminative spatial-frequency-temporal feature extraction and classification of motor imagery EEG: An sparse regression and Weighted Naïve Bayesian Classifier-based approach.

PubMed

Miao, Minmin; Zeng, Hong; Wang, Aimin; Zhao, Changsen; Liu, Feixiang

2017-02-15

Common spatial pattern (CSP) is most widely used in motor imagery based brain-computer interface (BCI) systems. In conventional CSP algorithm, pairs of the eigenvectors corresponding to both extreme eigenvalues are selected to construct the optimal spatial filter. In addition, an appropriate selection of subject-specific time segments and frequency bands plays an important role in its successful application. This study proposes to optimize spatial-frequency-temporal patterns for discriminative feature extraction. Spatial optimization is implemented by channel selection and finding discriminative spatial filters adaptively on each time-frequency segment. A novel Discernibility of Feature Sets (DFS) criteria is designed for spatial filter optimization. Besides, discriminative features located in multiple time-frequency segments are selected automatically by the proposed sparse time-frequency segment common spatial pattern (STFSCSP) method which exploits sparse regression for significant features selection. Finally, a weight determined by the sparse coefficient is assigned for each selected CSP feature and we propose a Weighted Naïve Bayesian Classifier (WNBC) for classification. Experimental results on two public EEG datasets demonstrate that optimizing spatial-frequency-temporal patterns in a data-driven manner for discriminative feature extraction greatly improves the classification performance. The proposed method gives significantly better classification accuracies in comparison with several competing methods in the literature. The proposed approach is a promising candidate for future BCI systems. Copyright © 2016 Elsevier B.V. All rights reserved.

Indian Summer Monsoon Rainfall: Implications of Contrasting Trends in the Spatial Variability of Means and Extremes

PubMed Central

Ghosh, Subimal; Vittal, H.; Sharma, Tarul; Karmakar, Subhankar; Kasiviswanathan, K. S.; Dhanesh, Y.; Sudheer, K. P.; Gunthe, S. S.

2016-01-01

India’s agricultural output, economy, and societal well-being are strappingly dependent on the stability of summer monsoon rainfall, its variability and extremes. Spatial aggregate of intensity and frequency of extreme rainfall events over Central India are significantly increasing, while at local scale they are spatially non-uniform with increasing spatial variability. The reasons behind such increase in spatial variability of extremes are poorly understood and the trends in mean monsoon rainfall have been greatly overlooked. Here, by using multi-decadal gridded daily rainfall data over entire India, we show that the trend in spatial variability of mean monsoon rainfall is decreasing as exactly opposite to that of extremes. The spatial variability of extremes is attributed to the spatial variability of the convective rainfall component. Contrarily, the decrease in spatial variability of the mean rainfall over India poses a pertinent research question on the applicability of large scale inter-basin water transfer by river inter-linking to address the spatial variability of available water in India. We found a significant decrease in the monsoon rainfall over major water surplus river basins in India. Hydrological simulations using a Variable Infiltration Capacity (VIC) model also revealed that the water yield in surplus river basins is decreasing but it is increasing in deficit basins. These findings contradict the traditional notion of dry areas becoming drier and wet areas becoming wetter in response to climate change in India. This result also calls for a re-evaluation of planning for river inter-linking to supply water from surplus to deficit river basins. PMID:27463092

Indian Summer Monsoon Rainfall: Implications of Contrasting Trends in the Spatial Variability of Means and Extremes.

PubMed

Ghosh, Subimal; Vittal, H; Sharma, Tarul; Karmakar, Subhankar; Kasiviswanathan, K S; Dhanesh, Y; Sudheer, K P; Gunthe, S S

2016-01-01

India's agricultural output, economy, and societal well-being are strappingly dependent on the stability of summer monsoon rainfall, its variability and extremes. Spatial aggregate of intensity and frequency of extreme rainfall events over Central India are significantly increasing, while at local scale they are spatially non-uniform with increasing spatial variability. The reasons behind such increase in spatial variability of extremes are poorly understood and the trends in mean monsoon rainfall have been greatly overlooked. Here, by using multi-decadal gridded daily rainfall data over entire India, we show that the trend in spatial variability of mean monsoon rainfall is decreasing as exactly opposite to that of extremes. The spatial variability of extremes is attributed to the spatial variability of the convective rainfall component. Contrarily, the decrease in spatial variability of the mean rainfall over India poses a pertinent research question on the applicability of large scale inter-basin water transfer by river inter-linking to address the spatial variability of available water in India. We found a significant decrease in the monsoon rainfall over major water surplus river basins in India. Hydrological simulations using a Variable Infiltration Capacity (VIC) model also revealed that the water yield in surplus river basins is decreasing but it is increasing in deficit basins. These findings contradict the traditional notion of dry areas becoming drier and wet areas becoming wetter in response to climate change in India. This result also calls for a re-evaluation of planning for river inter-linking to supply water from surplus to deficit river basins.

Controlling Energy Radiations of Electromagnetic Waves via Frequency Coding Metamaterials.

PubMed

Wu, Haotian; Liu, Shuo; Wan, Xiang; Zhang, Lei; Wang, Dan; Li, Lianlin; Cui, Tie Jun

2017-09-01

Metamaterials are artificial structures composed of subwavelength unit cells to control electromagnetic (EM) waves. The spatial coding representation of metamaterial has the ability to describe the material in a digital way. The spatial coding metamaterials are typically constructed by unit cells that have similar shapes with fixed functionality. Here, the concept of frequency coding metamaterial is proposed, which achieves different controls of EM energy radiations with a fixed spatial coding pattern when the frequency changes. In this case, not only different phase responses of the unit cells are considered, but also different phase sensitivities are also required. Due to different frequency sensitivities of unit cells, two units with the same phase response at the initial frequency may have different phase responses at higher frequency. To describe the frequency coding property of unit cell, digitalized frequency sensitivity is proposed, in which the units are encoded with digits "0" and "1" to represent the low and high phase sensitivities, respectively. By this merit, two degrees of freedom, spatial coding and frequency coding, are obtained to control the EM energy radiations by a new class of frequency-spatial coding metamaterials. The above concepts and physical phenomena are confirmed by numerical simulations and experiments.

Early deficits in spatial memory and theta rhythm in experimental temporal lobe epilepsy.

PubMed

Chauvière, Laetitia; Rafrafi, Nadia; Thinus-Blanc, Catherine; Bartolomei, Fabrice; Esclapez, Monique; Bernard, Christophe

2009-04-29

Patients with temporal lobe epilepsy (TLE), the most common form of epilepsy in adults, often display cognitive deficits. The time course and underlying mechanisms of cognitive decline remain unknown during epileptogenesis (the process leading to epilepsy). Using the rat pilocarpine model of TLE, we performed a longitudinal study to assess spatial and nonspatial cognitive performance during epileptogenesis. In parallel, we monitored interictal-like activity (ILA) in the hippocampal CA1 region, as well as theta oscillations, a brain rhythm central to numerous cognitive processes. Here, we report that spatial memory was altered soon after pilocarpine-induced status epilepticus, i.e., already during the seizure-free, latent period. Spatial deficits correlated with a decrease in the power of theta oscillations but not with the frequency of ILA. Spatial deficits persisted when animals had spontaneous seizures (chronic stage) without further modification. In contrast, nonspatial memory performances remained unaffected throughout. We conclude that the reorganization of hippocampal circuitry that immediately follows the initial insult can affect theta oscillation mechanisms, in turn, resulting in deficits in hippocampus-dependent memory tasks. These deficits may be dissociated from the process that leads to epilepsy itself but could instead constitute, as ILA, early markers in at-risk patients and/or provide beneficial therapeutic targets.

Is there "neural efficiency" during the processing of visuo-spatial information in male humans? An EEG study.

PubMed

Capotosto, Paolo; Perrucci, M Gianni; Brunetti, Marcella; Del Gratta, Cosimo; Doppelmayr, Michael; Grabner, Roland H; Klimesch, Wolfgang; Neubauer, Aljoscha; Neuper, Christa; Pfurtscheller, Gert; Romani, Gian Luca; Babiloni, Claudio

2009-12-28

More intelligent persons (high IQ) typically present a higher cortical activity during tasks requiring the encoding of visuo-spatial information, namely higher alpha (about 10 Hz) event-related desynchronization (ERD; Doppelmayr et al., 2005). The opposite is true ("neural efficiency") during the retrieval of the encoded information, as revealed by both lower alpha ERD and/or lower theta (about 5 Hz) event-related synchronization (ERS; Grabner et al., 2004). To reconcile these contrasting results, here we evaluated the working hypothesis that more intelligent male subjects are characterized by a high cortical activity during the encoding phase. This deep encoding would explain the relatively low cortical activity for the retrieval of the encoded information. To test this hypothesis, electroencephalographic (EEG) data were recorded in 22 healthy young male volunteers during visuo-spatial information processing (encoding) and short-term retrieval of the encoded information. Cortical activity was indexed by theta ERS and alpha ERD. It was found that the higher the subjects' total IQ, the stronger the frontal theta ERS during the encoding task. Furthermore, the higher the subjects' total IQ, the lower the frontal high-frequency alpha ERD (about 10-12 Hz) during the retrieval task. This was not true for parietal counterpart of these EEG rhythms. These results reconcile previous contrasting evidence confirming that more intelligent persons do not ever show event-related cortical responses compatible with "neural efficiency" hypothesis. Rather, their cortical activity would depend on flexible and task-adapting features of frontal activation.

Development of an adaptive bilateral filter for evaluating color image difference

NASA Astrophysics Data System (ADS)

Wang, Zhaohui; Hardeberg, Jon Yngve

2012-04-01

Spatial filtering, which aims to mimic the contrast sensitivity function (CSF) of the human visual system (HVS), has previously been combined with color difference formulae for measuring color image reproduction errors. These spatial filters attenuate imperceptible information in images, unfortunately including high frequency edges, which are believed to be crucial in the process of scene analysis by the HVS. The adaptive bilateral filter represents a novel approach, which avoids the undesirable loss of edge information introduced by CSF-based filtering. The bilateral filter employs two Gaussian smoothing filters in different domains, i.e., spatial domain and intensity domain. We propose a method to decide the parameters, which are designed to be adaptive to the corresponding viewing conditions, and the quantity and homogeneity of information contained in an image. Experiments and discussions are given to support the proposal. A series of perceptual experiments were conducted to evaluate the performance of our approach. The experimental sample images were reproduced with variations in six image attributes: lightness, chroma, hue, compression, noise, and sharpness/blurriness. The Pearson's correlation values between the model-predicted image difference and the observed difference were employed to evaluate the performance, and compare it with that of spatial CIELAB and image appearance model.

Broadband interferometric characterization of divergence and spatial chirp.

PubMed

Meier, Amanda K; Iliev, Marin; Squier, Jeff A; Durfee, Charles G

2015-09-01

We demonstrate a spectral interferometric method to characterize lateral and angular spatial chirp to optimize intensity localization in spatio-temporally focused ultrafast beams. Interference between two spatially sheared beams in an interferometer will lead to straight fringes if the wavefronts are curved. To produce reference fringes, we delay one arm relative to another in order to measure fringe rotation in the spatially resolved spectral interferogram. With Fourier analysis, we can obtain frequency-resolved divergence. In another arrangement, we spatially flip one beam relative to the other, which allows the frequency-dependent beamlet direction (angular spatial chirp) to be measured. Blocking one beam shows the spatial variation of the beamlet position with frequency (i.e., the lateral spatial chirp).

A Theory of the Visual System Biology Underlying Development of Spatial Frequency Lateralization

ERIC Educational Resources Information Center

Howard, Mary F.; Reggia, James A.

2007-01-01

The spatial frequency hypothesis contends that performance differences between the hemispheres on various visuospatial tasks are attributable to lateralized processing of the spatial frequency content of visual stimuli. Hellige has proposed that such lateralization could arise during infant development from the earlier maturation of the right…

Spatiotemporal frequency tuning of BOLD and gamma band MEG responses compared in primary visual cortex.

PubMed

Muthukumaraswamy, Suresh D; Singh, Krish D

2008-05-01

In this study, the spatial and temporal frequency tuning characteristics of the MEG gamma (40-60 Hz) rhythm and the BOLD response in primary visual cortex were measured and compared. In an identical MEG/fMRI paradigm, 10 participants viewed reversing square wave gratings at 2 spatial frequencies [0.5 and 3 cycles per degree (cpd)] reversing at 5 temporal frequencies (0, 1 6, 10, 15 Hz). Three-dimensional images of MEG source power were generated with synthetic aperture magnetometry (SAM) and showed a high degree of spatial correspondence with BOLD responses in primary visual cortex with a mean spatial separation of 6.5 mm, but the two modalities showed different tuning characteristics. The gamma rhythm showed a clear increase in induced power for the high spatial frequency stimulus while BOLD showed no difference in activity for the two spatial frequencies used. Both imaging modalities showed a general increase of activity with temporal frequency, however, BOLD plateaued around 6-10 Hz while the MEG generally increased with a dip exhibited at 6 Hz. These results demonstrate that the two modalities may show activation in similar spatial locations but that the functional pattern of these activations may differ in a complex manner, suggesting that they may be tuned to different aspects of neuronal activity.

Design and validation of a mathematical breast phantom for contrast-enhanced digital mammography

NASA Astrophysics Data System (ADS)

Hill, Melissa L.; Mainprize, James G.; Jong, Roberta A.; Yaffe, Martin J.

2011-03-01

In contrast-enhanced digital mammography (CEDM) an iodinated contrast agent is employed to increase lesion contrast and to provide tissue functional information. Here, we present the details of a software phantom that can be used as a tool for the simulation of CEDM images, and compare the degree of anatomic noise present in images simulated using the phantom to that associated with breast parenchyma in clinical CEDM images. Such a phantom could be useful for multiparametric investigations including characterization of CEDM imaging performance and system optimization. The phantom has a realistic mammographic appearance based on a clustered lumpy background and models contrast agent uptake according to breast tissue physiology. Fifty unique phantoms were generated and used to simulate regions of interest (ROI) of pre-contrast images and logarithmically subtracted CEDM images using monoenergetic ray tracing. Power law exponents, β, were used as a measure of anatomic noise and were determined using a linear least-squares fit to log-log plots of the square of the modulus of radially averaged image power spectra versus spatial frequency. The power spectra for ROI selected from regions of normal parenchyma in 10 pairs of clinical CEDM pre-contrast and subtracted images were also measured for comparison with the simulated images. There was good agreement between the measured β in the simulated CEDM images and the clinical images. The values of β were consistently lower for the logarithmically subtracted CEDM images compared to the pre-contrast images, indicating that the subtraction process reduced anatomical noise.

Low-dose cone-beam CT via raw counts domain low-signal correction schemes: Performance assessment and task-based parameter optimization (Part II. Task-based parameter optimization).

PubMed

Gomez-Cardona, Daniel; Hayes, John W; Zhang, Ran; Li, Ke; Cruz-Bastida, Juan Pablo; Chen, Guang-Hong

2018-05-01

Different low-signal correction (LSC) methods have been shown to efficiently reduce noise streaks and noise level in CT to provide acceptable images at low-radiation dose levels. These methods usually result in CT images with highly shift-variant and anisotropic spatial resolution and noise, which makes the parameter optimization process highly nontrivial. The purpose of this work was to develop a local task-based parameter optimization framework for LSC methods. Two well-known LSC methods, the adaptive trimmed mean (ATM) filter and the anisotropic diffusion (AD) filter, were used as examples to demonstrate how to use the task-based framework to optimize filter parameter selection. Two parameters, denoted by the set P, for each LSC method were included in the optimization problem. For the ATM filter, these parameters are the low- and high-signal threshold levels p l and p h ; for the AD filter, the parameters are the exponents δ and γ in the brightness gradient function. The detectability index d' under the non-prewhitening (NPW) mathematical observer model was selected as the metric for parameter optimization. The optimization problem was formulated as an unconstrained optimization problem that consisted of maximizing an objective function d'(P), where i and j correspond to the i-th imaging task and j-th spatial location, respectively. Since there is no explicit mathematical function to describe the dependence of d' on the set of parameters P for each LSC method, the optimization problem was solved via an experimentally measured d' map over a densely sampled parameter space. In this work, three high-contrast-high-frequency discrimination imaging tasks were defined to explore the parameter space of each of the LSC methods: a vertical bar pattern (task I), a horizontal bar pattern (task II), and a multidirectional feature (task III). Two spatial locations were considered for the analysis, a posterior region-of-interest (ROI) located within the noise streaks region and an anterior ROI, located further from the noise streaks region. Optimal results derived from the task-based detectability index metric were compared to other operating points in the parameter space with different noise and spatial resolution trade-offs. The optimal operating points determined through the d' metric depended on the interplay between the major spatial frequency components of each imaging task and the highly shift-variant and anisotropic noise and spatial resolution properties associated with each operating point in the LSC parameter space. This interplay influenced imaging performance the most when the major spatial frequency component of a given imaging task coincided with the direction of spatial resolution loss or with the dominant noise spatial frequency component; this was the case of imaging task II. The performance of imaging tasks I and III was influenced by this interplay in a smaller scale than imaging task II, since the major frequency component of task I was perpendicular to imaging task II, and because imaging task III did not have strong directional dependence. For both LSC methods, there was a strong dependence of the overall d' magnitude and shape of the contours on the spatial location within the phantom, particularly for imaging tasks II and III. The d' value obtained at the optimal operating point for each spatial location and imaging task was similar when comparing the LSC methods studied in this work. A local task-based detectability framework to optimize the selection of parameters for LSC methods was developed. The framework takes into account the potential shift-variant and anisotropic spatial resolution and noise properties to maximize the imaging performance of the CT system. Optimal parameters for a given LSC method depend strongly on the spatial location within the image object. © 2018 American Association of Physicists in Medicine.

TiO2 nanoparticles as exogenous contrast agent for 1 µm swept source optical coherence tomography: an in vitro study

NASA Astrophysics Data System (ADS)

Kumar, Atul; Mondal, Indranil; Roy, Poulomi; Poddar, Raju

2018-03-01

Optical coherence tomography (OCT) is a rapidly evolving, robust technology that has profoundly changed the practice of medical imaging. Swept source OCT (SSOCT) combines the standard time domain and the spatially encoded frequency domain OCT. We have employed a high-speed SSOCT system that utilizes a swept source laser with an A-scan rate of 100 kHz and a central wavelength of 1060 nm for the imaging of the tissue. SSOCT at 1060 nm allows for high penetration in the tissue. TiO2 nanoparticles (NPs) are mostly used for various experimental purposes as an exogenous imaging contrast agent. The in vitro imaging of chicken breast tissue is performed with and without the application of TiO2 NPs for exogenous contrast. Characterization of the chemically synthesized TiO2 NPs was done with dynamic light scattering and a scanning electron microscope method. The effect of TiO2 is studied at different exposure times. A significant improvement in the contrast to noise ratio has been observed through the in vitro imaging of a TiO2 treated tissue.

Magsat equivalent source anomalies over the southeastern United States - Implications for crustal magnetization

NASA Technical Reports Server (NTRS)

Ruder, M. E.; Alexander, S. S.

1986-01-01

The Magsat crustal anomaly field depicts a previously-unidentified long-wavelength negative anomaly centered over southeastern Georgia. Examination of Magsat ascending and descending passes clearly identifies the anomalous region, despite the high-frequency noise present in the data. Using ancillary seismic, electrical conductivity, Bouguer gravity, and aeromagnetic data, a preliminary model of crustal magnetization for the southern Appalachian region is presented. A lower crust characterized by a pervasive negative magnetization contrast extends from the New York-Alabama lineament southeast to the Fall Line. In southern Georgia and eastern Alabama (coincident with the Brunswick Terrane), the model calls for lower crustal magnetization contrast of -2.4 A/m; northern Georgia and the Carolinas are modeled with contrasts of -1.5 A/m. Large-scale blocks in the upper crust which correspond to the Blue Ridge, Charlotte belt, and Carolina Slate belt, are modeled with magnetization contrasts of -1.2 A/m, 1.2 A/m, and 1.2 A/m respectively. The model accurately reproduces the amplitude of the observed low in the equivalent source Magsat anomaly field calculated at 325 km altitude and is spatially consistent with the 400 km lowpass-filtered aeromagnetic map of the region.

Multivariate spatial models of excess crash frequency at area level: case of Costa Rica.

PubMed

Aguero-Valverde, Jonathan

2013-10-01

Recently, areal models of crash frequency have being used in the analysis of various area-wide factors affecting road crashes. On the other hand, disease mapping methods are commonly used in epidemiology to assess the relative risk of the population at different spatial units. A natural next step is to combine these two approaches to estimate the excess crash frequency at area level as a measure of absolute crash risk. Furthermore, multivariate spatial models of crash severity are explored in order to account for both frequency and severity of crashes and control for the spatial correlation frequently found in crash data. This paper aims to extent the concept of safety performance functions to be used in areal models of crash frequency. A multivariate spatial model is used for that purpose and compared to its univariate counterpart. Full Bayes hierarchical approach is used to estimate the models of crash frequency at canton level for Costa Rica. An intrinsic multivariate conditional autoregressive model is used for modeling spatial random effects. The results show that the multivariate spatial model performs better than its univariate counterpart in terms of the penalized goodness-of-fit measure Deviance Information Criteria. Additionally, the effects of the spatial smoothing due to the multivariate spatial random effects are evident in the estimation of excess equivalent property damage only crashes. Copyright © 2013 Elsevier Ltd. All rights reserved.

Magnetoacoustic tomography with magnetic induction for high-resolution bioimepedance imaging through vector source reconstruction under the static field of MRI magnet.

PubMed

Mariappan, Leo; Hu, Gang; He, Bin

2014-02-01

Magnetoacoustic tomography with magnetic induction (MAT-MI) is an imaging modality to reconstruct the electrical conductivity of biological tissue based on the acoustic measurements of Lorentz force induced tissue vibration. This study presents the feasibility of the authors' new MAT-MI system and vector source imaging algorithm to perform a complete reconstruction of the conductivity distribution of real biological tissues with ultrasound spatial resolution. In the present study, using ultrasound beamformation, imaging point spread functions are designed to reconstruct the induced vector source in the object which is used to estimate the object conductivity distribution. Both numerical studies and phantom experiments are performed to demonstrate the merits of the proposed method. Also, through the numerical simulations, the full width half maximum of the imaging point spread function is calculated to estimate of the spatial resolution. The tissue phantom experiments are performed with a MAT-MI imaging system in the static field of a 9.4 T magnetic resonance imaging magnet. The image reconstruction through vector beamformation in the numerical and experimental studies gives a reliable estimate of the conductivity distribution in the object with a ∼ 1.5 mm spatial resolution corresponding to the imaging system frequency of 500 kHz ultrasound. In addition, the experiment results suggest that MAT-MI under high static magnetic field environment is able to reconstruct images of tissue-mimicking gel phantoms and real tissue samples with reliable conductivity contrast. The results demonstrate that MAT-MI is able to image the electrical conductivity properties of biological tissues with better than 2 mm spatial resolution at 500 kHz, and the imaging with MAT-MI under a high static magnetic field environment is able to provide improved imaging contrast for biological tissue conductivity reconstruction.

Influence of chemistry, interfacial width, and non-isothermal conditions on spatially heterogeneous activated relaxation and elasticity in glass-forming free standing films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mirigian, Stephen; Schweizer, Kenneth S.

Here, we employ the Elastically Collective Nonlinear Langevin Equation (ECNLE) theory of activated relaxation to study several questions in free standing thin films of glass-forming molecular and polymer liquids. The influence of non-universal chemical aspects on dynamical confinement effects is found to be relatively weak, but with the caveat that for the systems examined, the bulk ECNLE polymer theory does not predict widely varying fragilities. Allowing the film model to have a realistic vapor interfacial width significantly enhances the reduction of the film-averaged glass transition temperature, T g, in a manner that depends on whether a dynamic or pseudo-thermodynamic averagingmore » of the spatial mobility gradient is adopted. The nature of film thickness effects on the spatial profiles of the alpha relaxation time and elastic modulus is studied under non-isothermal conditions and contrasted with the corresponding isothermal behavior. Modest differences are found if a film-thickness dependent T g is defined in a dynamical manner. But, adopting a pseudo-thermodynamic measure of T g leads to a qualitatively new form of the alpha relaxation time gradient where highly mobile layers near the film surface coexist with strongly vitrified regions in the film interior. Consequently, the film-averaged shear modulus can increase with decreasing film thickness, despite the T g reduction and presence of a mobile surface layer. Such a behavior stands in qualitative contrast to the predicted mechanical softening under isothermal conditions. Spatial gradients of the elastic modulus are studied as a function of temperature, film thickness, probing frequency, and experimental protocol, and a rich behavior is found.« less

Influence of chemistry, interfacial width, and non-isothermal conditions on spatially heterogeneous activated relaxation and elasticity in glass-forming free standing films

DOE PAGES

Mirigian, Stephen; Schweizer, Kenneth S.

2017-02-02

Here, we employ the Elastically Collective Nonlinear Langevin Equation (ECNLE) theory of activated relaxation to study several questions in free standing thin films of glass-forming molecular and polymer liquids. The influence of non-universal chemical aspects on dynamical confinement effects is found to be relatively weak, but with the caveat that for the systems examined, the bulk ECNLE polymer theory does not predict widely varying fragilities. Allowing the film model to have a realistic vapor interfacial width significantly enhances the reduction of the film-averaged glass transition temperature, T g, in a manner that depends on whether a dynamic or pseudo-thermodynamic averagingmore » of the spatial mobility gradient is adopted. The nature of film thickness effects on the spatial profiles of the alpha relaxation time and elastic modulus is studied under non-isothermal conditions and contrasted with the corresponding isothermal behavior. Modest differences are found if a film-thickness dependent T g is defined in a dynamical manner. But, adopting a pseudo-thermodynamic measure of T g leads to a qualitatively new form of the alpha relaxation time gradient where highly mobile layers near the film surface coexist with strongly vitrified regions in the film interior. Consequently, the film-averaged shear modulus can increase with decreasing film thickness, despite the T g reduction and presence of a mobile surface layer. Such a behavior stands in qualitative contrast to the predicted mechanical softening under isothermal conditions. Spatial gradients of the elastic modulus are studied as a function of temperature, film thickness, probing frequency, and experimental protocol, and a rich behavior is found.« less

Flow environment and matrix structure interact to determine spatial competition in Pseudomonas aeruginosa biofilms.

PubMed

Nadell, Carey D; Ricaurte, Deirdre; Yan, Jing; Drescher, Knut; Bassler, Bonnie L

2017-01-13

Bacteria often live in biofilms, which are microbial communities surrounded by a secreted extracellular matrix. Here, we demonstrate that hydrodynamic flow and matrix organization interact to shape competitive dynamics in Pseudomonas aeruginosa biofilms. Irrespective of initial frequency, in competition with matrix mutants, wild-type cells always increase in relative abundance in planar microfluidic devices under simple flow regimes. By contrast, in microenvironments with complex, irregular flow profiles - which are common in natural environments - wild-type matrix-producing and isogenic non-producing strains can coexist. This result stems from local obstruction of flow by wild-type matrix producers, which generates regions of near-zero shear that allow matrix mutants to locally accumulate. Our findings connect the evolutionary stability of matrix production with the hydrodynamics and spatial structure of the surrounding environment, providing a potential explanation for the variation in biofilm matrix secretion observed among bacteria in natural environments.

Role of the mouse retinal photoreceptor ribbon synapse in visual motion processing for optokinetic responses.

PubMed

Sugita, Yuko; Araki, Fumiyuki; Chaya, Taro; Kawano, Kenji; Furukawa, Takahisa; Miura, Kenichiro

2015-01-01

The ribbon synapse is a specialized synaptic structure in the retinal outer plexiform layer where visual signals are transmitted from photoreceptors to the bipolar and horizontal cells. This structure is considered important in high-efficiency signal transmission; however, its role in visual signal processing is unclear. In order to understand its role in visual processing, the present study utilized Pikachurin-null mutant mice that show improper formation of the photoreceptor ribbon synapse. We examined the initial and late phases of the optokinetic responses (OKRs). The initial phase was examined by measuring the open-loop eye velocity of the OKRs to sinusoidal grating patterns of various spatial frequencies moving at various temporal frequencies for 0.5 s. The mutant mice showed significant initial OKRs with a spatiotemporal frequency tuning (spatial frequency, 0.09 ± 0.01 cycles/°; temporal frequency, 1.87 ± 0.12 Hz) that was slightly different from the wild-type mice (spatial frequency, 0.11 ± 0.01 cycles/°; temporal frequency, 1.66 ± 0.12 Hz). The late phase of the OKRs was examined by measuring the slow phase eye velocity of the optokinetic nystagmus induced by the sinusoidal gratings of various spatiotemporal frequencies moving for 30 s. We found that the optimal spatial and temporal frequencies of the mutant mice (spatial frequency, 0.11 ± 0.02 cycles/°; temporal frequency, 0.81 ± 0.24 Hz) were both lower than those in the wild-type mice (spatial frequency, 0.15 ± 0.02 cycles/°; temporal frequency, 1.93 ± 0.62 Hz). These results suggest that the ribbon synapse modulates the spatiotemporal frequency tuning of visual processing along the ON pathway by which the late phase of OKRs is mediated.

Role of the Mouse Retinal Photoreceptor Ribbon Synapse in Visual Motion Processing for Optokinetic Responses

PubMed Central

Sugita, Yuko; Araki, Fumiyuki; Chaya, Taro; Kawano, Kenji; Furukawa, Takahisa; Miura, Kenichiro

2015-01-01

The ribbon synapse is a specialized synaptic structure in the retinal outer plexiform layer where visual signals are transmitted from photoreceptors to the bipolar and horizontal cells. This structure is considered important in high-efficiency signal transmission; however, its role in visual signal processing is unclear. In order to understand its role in visual processing, the present study utilized Pikachurin-null mutant mice that show improper formation of the photoreceptor ribbon synapse. We examined the initial and late phases of the optokinetic responses (OKRs). The initial phase was examined by measuring the open-loop eye velocity of the OKRs to sinusoidal grating patterns of various spatial frequencies moving at various temporal frequencies for 0.5 s. The mutant mice showed significant initial OKRs with a spatiotemporal frequency tuning (spatial frequency, 0.09 ± 0.01 cycles/°; temporal frequency, 1.87 ± 0.12 Hz) that was slightly different from the wild-type mice (spatial frequency, 0.11 ± 0.01 cycles/°; temporal frequency, 1.66 ± 0.12 Hz). The late phase of the OKRs was examined by measuring the slow phase eye velocity of the optokinetic nystagmus induced by the sinusoidal gratings of various spatiotemporal frequencies moving for 30 s. We found that the optimal spatial and temporal frequencies of the mutant mice (spatial frequency, 0.11 ± 0.02 cycles/°; temporal frequency, 0.81 ± 0.24 Hz) were both lower than those in the wild-type mice (spatial frequency, 0.15 ± 0.02 cycles/°; temporal frequency, 1.93 ± 0.62 Hz). These results suggest that the ribbon synapse modulates the spatiotemporal frequency tuning of visual processing along the ON pathway by which the late phase of OKRs is mediated. PMID:25955222

Visual Multipoles And The Assessment Of Visual Sensitivity To Displayed Images

NASA Astrophysics Data System (ADS)

Klein, Stanley A.

1989-08-01

The contrast sensitivity function (CSF) is widely used to specify the sensitivity of the visual system. Each point of the CSF specifies the amount of contrast needed to detect a sinusoidal grating of a given spatial frequency. This paper describes a set of five mathematically related visual patterns, called "multipoles," that should replace the CSF for measuring visual performance. The five patterns (ramp, edge, line, dipole and quadrupole) are localized in space rather than being spread out as sinusoidal gratings. The multipole sensitivity of the visual system provides an alternative characterization that complements the CSF in addition to offering several advantages. This paper provides an overview of the properties and uses of the multipole stimuli. This paper is largely a summary of several unpublished manuscripts with excerpts from them. Derivations and full references are omitted here. Please write me if you would like the full manuscripts.

TrkB signalling pathway mediates the protective effects of exercise in the diabetic rat retina.

PubMed

Allen, Rachael S; Hanif, Adam M; Gogniat, Marissa A; Prall, Brian C; Haider, Raza; Aung, Moe H; Prunty, Megan C; Mees, Lukas M; Coulter, Monica M; Motz, Cara T; Boatright, Jeffrey H; Pardue, Machelle T

2018-05-01

Diabetic retinopathy is a leading cause of vision loss. Treatment options for early retinopathy are sparse. Exercise protects dying photoreceptors in models of retinal degeneration, thereby preserving vision. We tested the protective effects of exercise on retinal and cognitive deficits in a type 1 diabetes model and determined whether the TrkB pathway mediates this effect. Hyperglycaemia was induced in Long Evans rats via streptozotocin injection (STZ; 100 mg/kg). Following confirmed hyperglycaemia, both control and diabetic rats underwent treadmill exercise for 30 min, 5 days/week at 0 m/min (inactive groups) or 15 m/min (active groups) for 8 weeks. A TrkB receptor antagonist (ANA-12), or vehicle, was injected 2.5 h before exercise training. We measured spatial frequency and contrast sensitivity using optokinetic tracking biweekly post-STZ; retinal function using electroretinography at 4 and 8 weeks; and cognitive function and exploratory behaviour using Y-maze at 8 weeks. Retinal neurotrophin-4 was measured using ELISA. Compared with non-diabetic controls, diabetic rats showed significantly reduced spatial frequency and contrast sensitivity, delayed electroretinogram oscillatory potential and flicker implicit times and reduced cognitive function and exploratory behaviour. Exercise interventions significantly delayed the appearance of all deficits, except for exploratory behaviour. Treatment with ANA-12 significantly reduced this protection, suggesting a TrkB-mediated mechanism. Despite this, no changes in retinal neurotrohin-4 were observed with diabetes or exercise. Exercise protected against early visual and cognitive dysfunction in diabetic rats, suggesting that exercise interventions started after hyperglycaemia diagnosis may be a beneficial treatment. The translational potential is high, given that exercise treatment is non-invasive, patient controlled and inexpensive. © 2018 Federation of European Neuroscience Societies and John Wiley & Sons Ltd.

Does Retinal Neurodegeneration Seen in Diabetic Patients Begin in the Insulin Resistance Stage?

PubMed

Arıkan, Sedat; Erşan, İsmail; Eroğlu, Mustafa; Yılmaz, Mehmet; Tufan, Hasan Ali; Gencer, Baran; Kara, Selçuk; Aşık, Mehmet

2016-12-01

To investigate whether retinal neurodegeneration and impairment in contrast sensitivity (CS), which have been demonstrated to begin in diabetic patients before the presence of signs of diabetic retinal vasculopathy, also occur in the stage of insulin resistance. The average, minimum and sectoral (inferior, superior, inferonasal, superonasal, inferotemporal and superotemporal) thicknesses of the ganglion cell-inner plexiform layer (GCIPL) measured using optical coherence tomography were compared between an insulin-resistant group and control group in order to evaluate the presence of retinal neurodegeneration. The CS of the two groups was also compared according to the logarithmic values measured at spatial frequencies of 1.5, 3, 6, 12 and 18 cycles per degree in photopic light using functional acuity contrast test (FACT). Twenty-five eyes of 25 patients with insulin resistance (insulin resistant group) and 25 eyes of 25 healthy subjects (control group) were included in this study. There were no statistically significant differences between the two groups in any of the spatial frequencies in the FACT. The mean average GCIPL thickness and mean GCIPL thickness in the inferotemporal sector were significantly less in the insulin-resistant group when compared with the control group (mean average GCIPL thicknesses in the insulin-resistant and control groups were 83.6±4.7 µm and 86.7±3.7 µm respectively, p=0.01; mean inferotemporal GCIPL thicknesses in the insulin-resistant and control groups were 83±6.0 µm and 86.7±4.6 µm respectively, p=0.02). Although it may not lead to functional visual impairment such as CS loss, the retinal neurodegeneration seen in diabetic patients may begin in the insulin resistance stage.

A Preliminary Engineering Design of Intravascular Dual-Frequency Transducers for Contrast-Enhanced Acoustic Angiography and Molecular Imaging

PubMed Central

Ma, Jianguo; Martin, K. Heath; Dayton, Paul A.; Jiang, Xiaoning

2014-01-01

Current intravascular ultrasound (IVUS) probes are not optimized for contrast detection because of their design for high-frequency fundamental-mode imaging. However, data from transcutaneous contrast imaging suggests the possibility of utilizing contrast ultrasound for molecular imaging or vasa vasorum assessment to further elucidate atherosclerotic plaque deposition. This paper presents the design, fabrication, and characterization of a small-aperture (0.6 × 3 mm) IVUS probe optimized for high-frequency contrast imaging. The design utilizes a dual-frequency (6.5 MHz/30 MHz) transducer arrangement for exciting microbubbles at low frequencies (near their resonance) and detecting their broadband harmonics at high frequencies, minimizing detected tissue backscatter. The prototype probe is able to generate nonlinear microbubble response with more than 1.2 MPa of rarefractional pressure (mechanical index: 0.48) at 6.5 MHz, and is also able to detect microbubble response with a broadband receiving element (center frequency: 30 MHz, −6-dB fractional bandwidth: 58.6%). Nonlinear super-harmonics from microbubbles flowing through a 200-μm-diameter micro-tube were clearly detected with a signal-to-noise ratio higher than 12 dB. Preliminary phantom imaging at the fundamental frequency (30 MHz) and dual-frequency super-harmonic imaging results suggest the promise of small aperture, dual-frequency IVUS transducers for contrast-enhanced IVUS imaging. PMID:24801226

The Role of Low-Spatial Frequencies in Lexical Decision and Masked Priming

ERIC Educational Resources Information Center

Boden, C.; Giaschi, D.

2009-01-01

Spatial frequency filtering was used to test the hypotheses that low-spatial frequency information in printed text can: (1) lead to a rapid lexical decision or (2) facilitate word recognition. Adult proficient readers made lexical decisions in unprimed and masked repetition priming experiments with unfiltered, low-pass, high-pass and notch…

The Development of Spatial Frequency Biases in Face Recognition

ERIC Educational Resources Information Center

Leonard, Hayley C.; Karmiloff-Smith, Annette; Johnson, Mark H.

2010-01-01

Previous research has suggested that a mid-band of spatial frequencies is critical to face recognition in adults, but few studies have explored the development of this bias in children. We present a paradigm adapted from the adult literature to test spatial frequency biases throughout development. Faces were presented on a screen with particular…

A crowdful of letters: disentangling the role of similarity, eccentricity and spatial frequencies in letter crowding.

PubMed

Zahabi, Sacha; Arguin, Martin

2014-04-01

The present study investigated the joint impact of target-flanker similarity and of spatial frequency content on the crowding effect in letter identification. We presented spatial frequency filtered letters to neurologically intact non-dyslexic readers while manipulating target-flanker distance, target eccentricity and target-flanker confusability (letter similarity metric based on published letter confusion matrices). The results show that high target-flanker confusability magnifies crowding. They also reveal an intricate pattern of interactions of the spatial frequency content of the stimuli with target eccentricity, flanker distance and similarity. The findings are congruent with the notion that crowding results from the inappropriate pooling of target and flanker features and that this integration is more likely to match a response template at a subsequent decision stage with similar than dissimilar flankers. In addition, the evidence suggests that crowding from similar flankers is biased towards relatively high spatial frequencies and that crowding shifts towards lower spatial frequencies as target eccentricity is increased. Copyright © 2014 Elsevier B.V. All rights reserved.

Statistical model based iterative reconstruction (MBIR) in clinical CT systems. Part II. Experimental assessment of spatial resolution performance.

PubMed

Li, Ke; Garrett, John; Ge, Yongshuai; Chen, Guang-Hong

2014-07-01

Statistical model based iterative reconstruction (MBIR) methods have been introduced to clinical CT systems and are being used in some clinical diagnostic applications. The purpose of this paper is to experimentally assess the unique spatial resolution characteristics of this nonlinear reconstruction method and identify its potential impact on the detectabilities and the associated radiation dose levels for specific imaging tasks. The thoracic section of a pediatric phantom was repeatedly scanned 50 or 100 times using a 64-slice clinical CT scanner at four different dose levels [CTDIvol =4, 8, 12, 16 (mGy)]. Both filtered backprojection (FBP) and MBIR (Veo(®), GE Healthcare, Waukesha, WI) were used for image reconstruction and results were compared with one another. Eight test objects in the phantom with contrast levels ranging from 13 to 1710 HU were used to assess spatial resolution. The axial spatial resolution was quantified with the point spread function (PSF), while the z resolution was quantified with the slice sensitivity profile. Both were measured locally on the test objects and in the image domain. The dependence of spatial resolution on contrast and dose levels was studied. The study also features a systematic investigation of the potential trade-off between spatial resolution and locally defined noise and their joint impact on the overall image quality, which was quantified by the image domain-based channelized Hotelling observer (CHO) detectability index d'. (1) The axial spatial resolution of MBIR depends on both radiation dose level and image contrast level, whereas it is supposedly independent of these two factors in FBP. The axial spatial resolution of MBIR always improved with an increasing radiation dose level and/or contrast level. (2) The axial spatial resolution of MBIR became equivalent to that of FBP at some transitional contrast level, above which MBIR demonstrated superior spatial resolution than FBP (and vice versa); the value of this transitional contrast highly depended on the dose level. (3) The PSFs of MBIR could be approximated as Gaussian functions with reasonably good accuracy. (4) Thez resolution of MBIR showed similar contrast and dose dependence. (5) Noise standard deviation assessed on the edges of objects demonstrated a trade-off with spatial resolution in MBIR. (5) When both spatial resolution and image noise were considered using the CHO analysis, MBIR led to significant improvement in the overall CT image quality for both high and low contrast detection tasks at both standard and low dose levels. Due to the intrinsic nonlinearity of the MBIR method, many well-known CT spatial resolution and noise properties have been modified. In particular, dose dependence and contrast dependence have been introduced to the spatial resolution of CT images by MBIR. The method has also introduced some novel noise-resolution trade-off not seen in traditional CT images. While the benefits of MBIR regarding the overall image quality, as demonstrated in this work, are significant, the optimal use of this method in clinical practice demands a thorough understanding of its unique physical characteristics.

Population genetic studies of the polar bear (Ursus maritimus): A summary of available data and interpretation of results

USGS Publications Warehouse

Scribner, Kim T.; Garner, G.W.; Amstrup, Steven C.; Cronin, M.A.; Dizon, Andrew E.; Chivers, Susan J.; Perrin, William F.

1997-01-01

A summary of existing population genetics literature is presented for polar bears (Ursus maritimus) and interpreted in the context of the species' life-history characteristics and regional heterogeneity in environmental regimes and movement patterns. Several nongenetic data sets including morphology, contaminant levels, geographic variation in reproductive characteristics, and the location and distribution of open-water foraging habitat suggest some degree of spatial structuring. Eleven populations are recognized by the IUCN Polar Bear Specialist Group. Few genetics studies exist for polar bears. Interpretation and generalizations of regional variation in intra- and interpopulation levels of genetic variability are confounded by the paucity of data from many regions and by the fact that no single informative genetic marker has been employed in multiple regions. Early allozyme studies revealed comparatively low levels of genetic variability and no compelling evidence of spatial structuring. Studies employing mitochondrial DNA (mtDNA) also found low levels of genetic variation, a lack of phylogenetic structure, and no significant evidence for spatial variation in haplotype frequency. In contrast, microsatellite variable number of tandem repeat (VNTR) loci have revealed significant heterogeneity in allele frequency among populations in the Canadian Arctic. These regions are characterized by archipelgic patterns of sea-ice movements. Further studies using highly polymorphic loci are needed in regions characterized by greater polar bear dependency on pelagic sea-ice movements and in regions for which no data currently exist (i.e., Laptev and Novaya Zemlya/Franz Josef).

Plant community patterns in unburned and burned blackbrush (Coleogyne ramosissima) shrublands in the Mojave Desert

USGS Publications Warehouse

Brooks, Matthew L.; Matchett, John R.

2003-01-01

The blackbrush vegetation type is dominated by Coleogyne ramossisima, which is thought to preclude the coexistence of many other plant species. Fire can remove blackbrush cover and possibly increase plant species richness and evenness. Fire also may increase the frequency and cover of alien annual grasses, thereby intensifying landscape flammability. We tested these predictions in unburned and burned (6-14 years postfire) blackbrush at 3 sites spanning the range of this vegetation type in the Mojave Desert. Species richness in unburned blackbrush was similar to published values for other vegetation types in western North America, but richness varied significantly among the 3 sites and 4 spatial scales (1, 10, 100, and 1000 m2). Richness values declined in order from annual forbs, woody perennials, herbaceous perennials, annual grasses, cacti, to perennial grasses. Fire reduced Coleogyne cover, thus boosting species evenness. In contrast, species richness decreased after burning, although the results varied among spatial scales. Total cover was unaffected by fire because cover of woody perennials decreased, while cover of annual forbs, annual grasses, herbaceous perennials, and perennial grasses increased. Native species richness and cover decreased, whereas alien richness and cover increased after burning, especially where the alien forb Erodium cicutarium was present. Fire had no effect on frequency and variable effects on cover of alien annual grasses. These results indicate that in blackbrush species richness can vary among sites and local spatial scales, and effects of fire can vary among plant life-forms and between natives and aliens.

Intercontinental gene flow among western arctic populations of lesser snow geese

USGS Publications Warehouse

Shorey, Rainy I.; Scribner, K.T.; Kanefsky, Jeannette; Samuel, M.D.; Libants, S.V.

2011-01-01

Quantifying the spatial genetic structure of highly vagile species of birds is important in predicting their degree of population demographic and genetic independence during changing environmental conditions, and in assessing their abundance and distribution. In the western Arctic, Lesser Snow Geese (Chen caerulescens caerulescens) provide an example useful for evaluating spatial population genetic structure and the relative contribution of male and female philopatry to breeding and wintering locales. We analyzed biparentally inherited microsatellite loci and maternally inherited mtDNA sequences from geese breeding at Wrangel Island (Russia) and Banks Island (Canada) to estimate gene flow among populations whose geographic overlap during breeding and winter differ. Significant differences in the frequencies of mtDNA haplotypes contrast with the homogeneity of allele frequencies for microsatellite loci. Coalescence simulations revealed high variability and asymmetry between males and females in rates and direction of gene flow between populations. Our results highlight the importance of wintering areas to demographic independence and spatial genetic structure of these populations. Male-mediated gene flow among the populations on northern Wrangel Island, southern Wrangel Island, and Banks Island has been substantial. A high rate of female-mediated gene flow from southern Wrangel Island to Banks Island suggests that population exchange can be achieved when populations winter in a common area. Conversely, when birds from different breeding populations do not share a common wintering area, the probability of population exchange is likely to be dramatically reduced. ?? The Cooper Ornithological Society 2011.

Visual stimulus eccentricity affects human gamma peak frequency.

PubMed

van Pelt, Stan; Fries, Pascal

2013-09-01

The peak frequency of neuronal gamma-band synchronization has received much attention in recent years. Gamma peak frequency shifts to higher frequency values for higher contrast, faster moving, and attended stimuli. In monkey V1, gamma peak frequency for a drifting grating is higher for a parafoveal as compared to an eccentric stimulus (Lima et al., 2010). This effect might be due to the cortical magnification factor: the higher cortical magnification for parafoveal stimuli increases the velocity with which the cortical representations of the moving grating stripes move across the cortical surface. Since faster moving stimuli lead to higher gamma frequency, a faster moving cortical representation might do the same. This explanation predicts that the eccentricity effect on gamma peak frequency is absent for stationary stimuli. To test this, we investigated the effect of eccentricity on gamma peak frequency by recording magnetoencephalography in human subjects while they viewed moving or stationary gratings. We found that both the moving and the stationary stimuli induced lower peak frequencies for larger eccentricities, arguing against an explanation based on the cortical magnification factor. We further investigated whether this eccentricity effect was explained by differences in the size or the spatial frequency of the expected cortical activation. Neither of those explained the eccentricity effect. We propose that the different stimulus and top-down factors leading to higher gamma peak frequency all result in higher stimulus salience, that salience is translated into gamma peak frequency, and that gamma peak frequency might subserve the preferential processing of neuronal activity induced by salient stimuli. Copyright © 2013 Elsevier Inc. All rights reserved.

Comparison of transducers with different frequencies in breast contrast-enhanced ultrasound (CEUS) using SonoVue as contrast agent.

PubMed

Wang, Yong-Mei; Fan, Wei; Zhang, Kai; Zhang, Li; Tan, Zhen; Ma, Rong

2016-07-01

To explore the effectiveness of different transducers in breast contrast-enhanced ultrasound (CEUS) using SonoVue(®) (Bracco, Plan-Les-Ouates, Switzerland) as the contrast agent. Breast CEUS was performed in 51 patients with 51 breast lesions using a low-frequency transducer (probe C5-1) and a high-frequency transducer (probe L12-5) separately. All image processes were reviewed for the presence of local blood perfusion defects and surrounding vessels. McNemar's test was conducted to compare the detection effectiveness between these two transducers. Pathological results revealed 38 malignant and 13 benign lesions. The two transducers showed no difference in detecting benign lesions. Among malignant lesions, CEUS conducted by probe C5-1 (frequency range from 1 to 5 MHz) presented 23 (60.5%) lesions with local blood perfusion defects and 26 (68.4%) lesions with surrounding vessels. Meanwhile, probe L12-5 (frequency range from 5 to 12 MHz) showed only 12 (31.6%) lesions with local blood perfusion defects and 12 (31.6%) lesions with surrounding vessel. Probe C5-1 was more sensitive than probe L12-5 in detecting malignant CEUS characteristics (p-value < 0.05). The low-frequency transducer was more sensitive than the high-frequency transducer in breast CEUS using SonoVue as the contrast agent. A new contrast agent with a higher resonance frequency, specially designed for high-frequency transducers, may be helpful in improving the clinical value of breast CEUS. The first study comparing different frequency transducers in breast CEUS of the same patient lesions. We brought out the requirement for CEUS contrast agents which are more suitable for high-frequency examinations.

Supramodal Enhancement of Auditory Perceptual and Cognitive Learning by Video Game Playing.

PubMed

Zhang, Yu-Xuan; Tang, Ding-Lan; Moore, David R; Amitay, Sygal

2017-01-01

Medical rehabilitation involving behavioral training can produce highly successful outcomes, but those successes are obtained at the cost of long periods of often tedious training, reducing compliance. By contrast, arcade-style video games can be entertaining and highly motivating. We examine here the impact of video game play on contiguous perceptual training. We alternated several periods of auditory pure-tone frequency discrimination (FD) with the popular spatial visual-motor game Tetris played in silence. Tetris play alone did not produce any auditory or cognitive benefits. However, when alternated with FD training it enhanced learning of FD and auditory working memory. The learning-enhancing effects of Tetris play cannot be explained simply by the visual-spatial training involved, as the effects were gone when Tetris play was replaced with another visual-spatial task using Tetris-like stimuli but not incorporated into a game environment. The results indicate that game play enhances learning and transfer of the contiguous auditory experiences, pointing to a promising approach for increasing the efficiency and applicability of rehabilitative training.

Neural dynamics of motion processing and speed discrimination.

PubMed

Chey, J; Grossberg, S; Mingolla, E

1998-09-01

A neural network model of visual motion perception and speed discrimination is presented. The model shows how a distributed population code of speed tuning, that realizes a size-speed correlation, can be derived from the simplest mechanisms whereby activations of multiple spatially short-range filters of different size are transformed into speed-turned cell responses. These mechanisms use transient cell responses to moving stimuli, output thresholds that covary with filter size, and competition. These mechanisms are proposed to occur in the V1-->MT cortical processing stream. The model reproduces empirically derived speed discrimination curves and simulates data showing how visual speed perception and discrimination can be affected by stimulus contrast, duration, dot density and spatial frequency. Model motion mechanisms are analogous to mechanisms that have been used to model 3-D form and figure-ground perception. The model forms the front end of a larger motion processing system that has been used to simulate how global motion capture occurs, and how spatial attention is drawn to moving forms. It provides a computational foundation for an emerging neural theory of 3-D form and motion perception.

Grading of cervical intraepithelial neoplasia using spatial frequency for optical histology

NASA Astrophysics Data System (ADS)

Pu, Yang; Jagtap, Jaidip; Pradhan, Asima; Alfano, Robert R.

2014-03-01

It is important to detect cervical dysplasia, Cervical Intraepithelial Neoplasia (CIN). CIN is the potentially premalignant and abnormal squamous cells on surface of cervix. In this study, the spatial frequency spectra of pre-cancer cervical tissues are used to detect differences among different grades of human cervical tissues. Seven sets of thick tissue sections of human cervix of normal, CIN 1, CIN 2, and CIN 3 tissues are studied. The confocal microscope images of the stromal region of normal and CIN human tissues were analyzed using Fast Fourier Transform (FFT) to generate the spatial spectra. It is observed that higher frequency components exist in CIN tissues than those in normal tissue, as well as those in higher grade CIN tissue than those in lower grade CIN tissue. The width of the spatial frequency of different types of tissues is used to create a criterion for CIN grading by training a support vector machine (SVM) classifier. The results show that the randomness of tissue structures from normal to different stages of precancer in cervical tissue can be recognized by fingerprints of the spatial frequency. The efficacy of spatial frequency analysis for CIN grading is evaluated as excellent since high AUC (area under the ROC curve), sensitivity and specificity are obtained by the statistics study. This works lays the foundation of using spatial frequency spectra for a histology evaluation.

Blur adaptation: contrast sensitivity changes and stimulus extent.

PubMed

Venkataraman, Abinaya Priya; Winter, Simon; Unsbo, Peter; Lundström, Linda

2015-05-01

A prolonged exposure to foveal defocus is well known to affect the visual functions in the fovea. However, the effects of peripheral blur adaptation on foveal vision, or vice versa, are still unclear. In this study, we therefore examined the changes in contrast sensitivity function from baseline, following blur adaptation to small as well as laterally extended stimuli in four subjects. The small field stimulus (7.5° visual field) was a 30min video of forest scenery projected on a screen and the large field stimulus consisted of 7-tiles of the 7.5° stimulus stacked horizontally. Both stimuli were used for adaptation with optical blur (+2.00D trial lens) as well as for clear control conditions. After small field blur adaptation foveal contrast sensitivity improved in the mid spatial frequency region. However, these changes neither spread to the periphery nor occurred for the large field blur adaptation. To conclude, visual performance after adaptation is dependent on the lateral extent of the adaptation stimulus. Copyright © 2015 The Authors. Published by Elsevier Ltd.. All rights reserved.

Monitoring in-vitro bovine embryo development during the first days after fertilization (Conference Presentation)

NASA Astrophysics Data System (ADS)

Kandel, Mikhail E.; Rubessa, Marcello; Fernandes, Daniel; Nguyen, Tan H.; Wheeler, Matthew B.; Popescu, Gabriel

2016-03-01

Conventional label-based contrast enhancement techniques (e.g., fluorescence) frequently modify the genetic makeup of tagged cells, making them poor candidates for use in in-vitro fertilization applications. Instead, we choose a label-free form of contrast, based on interferometric imaging, sensitive to optical path length differences. Compared to, single HeLa cells, typical mammalian ova and embryos are more than an order of magnitude thicker. As a result, regions of large phase variation lead to phase wrapping and an overall reduction in signal intensity occurs due to multiple scattering. These effects manifest themselves in low-spatial frequencies (blurs), with the desired details buried in the background. We present a phase shifting interferometer that yields the derivative of the phase, a quantity whose value is particularly sensitive to local variations and fine details. We demonstrate that our new real-time imaging platform is valuable in measuring the multiday development of bovine embryos. Reconstructing the derivative of the image phase and amplitude, we characterize the motion of previously low-contrast structures, which are relevant for embryo viability tests.

Spatial organisation of ecologically-relevant high frequency flow properties and implications for habitat assessment.

NASA Astrophysics Data System (ADS)

Trinci, G.; Harvey, G.; Henshaw, A.; Bertoldi, W.

2016-12-01

Turbulence plays a crucial role in the life cycle of river plants and animals. Turbulent flow facilitates access to food, maintenance of adequate oxygen levels, removal of wastes, locomotion and predator evasion, but can also act as a stressor, leading to dislodgement from habitats, increased energy costs, physiological damage and even mortality. Despite this, hydraulic habitat assessments for river appraisal and restoration design have largely focused on temporally and spatially averaged flow properties rather than more complex descriptors of turbulence (turbulence intensity, and the periodicity, orientation and scale of coherent flow structures) that are known to directly influence aquatic organisms. Contrasting relationships between turbulence and mean flow velocity have been reported and there is a pressing need to improve understanding of the hydraulic environment provided by mesoscale river features, such as geomorphic units (e.g. riffles, pools, steps), upon which river management and restoration often focuses. We undertook high frequency velocity surveys within three river reaches (low, medium and high gradient) using a 3-dimensional Acoustic Doppler Velocimeter, combined with detailed surveys of bed topography and visual assessments of the spatial organisation of geomorphic units. Using a combination of multivariate statistical analysis (Principal Components Analysis, Cluster Analysis and GLMs) and geostatistics (semi-variance), the paper explores the spatial organisation of key turbulence parameters across the reaches and linkages with mean flow velocity and characteristic roughness elements. The ability of `higher order' turbulence properties to distinguish between visually identified geomorphic units is also assessed. The findings provide insights into scales of variability in turbulence properties that have direct ecological relevance, helping to inform river assessment and restoration efforts.

Species dispersal rates alter diversity and ecosystem stability in pond metacommunities.

PubMed

Howeth, Jennifer G; Leibold, Mathew A

2010-09-01

Metacommunity theory suggests that relationships between diversity and ecosystem stability can be determined by the rate of species dispersal among local communities. The predicted relationships, however, may depend upon the relative strength of local environmental processes and disturbance. Here we evaluate the role of dispersal frequency and local predation perturbations in affecting patterns of diversity and stability in pond plankton metacommunities. Pond metacommunities were composed of three mesocosm communities: one of the three communities maintained constant "press" predation from a selective predator, bluegill sunfish (Lepomis macrochirus); the second community maintained "press" conditions without predation; and the third community experienced recurrent "pulsed" predation from bluegill sunfish. The triads of pond communities were connected at either no, low (0.7%/d), or high (20%/d) planktonic dispersal. Richness and composition of zooplankton and stability of plankton biomass and ecosystem productivity were measured at local and regional spatial scales. Dispersal significantly affected diversity such that local and regional biotas at the low dispersal rate maintained the greatest number of species. The unimodal local dispersal-diversity relationship was predator-dependent, however, as selective press predation excluded species regardless of dispersal. Further, there was no effect of dispersal on beta diversity because predation generated local conditions that selected for distinct community assemblages. Spatial and temporal ecosystem stability responded to dispersal frequency but not predation. Low dispersal destabilized the spatial stability of producer biomass but stabilized temporal ecosystem productivity. The results indicate that selective predation can prevent species augmentation from mass effects but has no apparent influence on stability. Dispersal rates, in contrast, can have significant effects on both species diversity and ecosystem stability at multiple spatial scales in metacommunities.

The influence of channel bed disturbance on benthic Chlorophyll a: A high resolution perspective

NASA Astrophysics Data System (ADS)

Katz, Scott B.; Segura, Catalina; Warren, Dana R.

2018-03-01

This study explores how spatial dynamics and frequency of bed mobility events in a headwater stream affect the spatial and temporal variability in stream benthic algal abundance and ultimately the resilience of benthic algae to stream scouring events of different magnitudes. We characterized spatial variability in sediment transport for nine separate flow events (0.1-1.7 of bankfull flow), coupling high resolution (< 0.1 m2) two-dimensional shear stress values with detailed measurements of the channel substrate. The stream bed was categorized into regions of high and low disturbance based on potential mobility of different grain sizes. High resolution (< 0.25 m2), in situ measurements of benthic Chlorophyll-a concentrations (Chl-a) were taken on 18 sampling dates before and after high flow events in regions of the streambed with contrasting disturbance to understand how benthic algal communities respond to sediment transport disturbance through space and time. According to the modeling results, the percentage of the channel likely to be disturbed varied greatly across the different flows and considered grain sizes between 7.7 and 70.4% for the lowest and highest flow events respectively. Mean shear stress in the channel bed across all sampling dates explained 49% of the variance in Chl-a. Over the 18 sampling dates - encompassing post-disturbance impacts and subsequent recovery - Chl-a differed between disturbance level categories defined based on the relative movement of the median grain size on 14 occasions. However, low disturbance locations were not always associated with higher Chl-a. The algal Chl-a biomass at any given time was a function of the stage of algal recovery following a high flow event and the magnitude of the disturbance itself - impacting algal loss during the event. Resistance of the algal communities to bed disturbance and resilience to recovery following a flow event varied spatially. Areas with low shear stress were less susceptible to scour during moderate disturbance events but were slower to recover when scour occurred. In contrast, high shear stress areas responded rapidly to flood events with rapid declines, but also recovered more quickly and appeared to have high potential for maximum accrual within our study reach. Ultimately, timing along with the inverse relationship between resiliency and disturbance frequency highlights the complexity of these processes and the importance of studying the interactions between geomorphic and ecological processes with high resolution across spatial and temporal scales.

A microarray analysis of retinal transcripts that are controlled by image contrast in mice

PubMed Central

Brand, Christine; Schaeffel, Frank

2007-01-01

Purpose The development of myopia is controlled by still largely unknown retinal signals. The aim of this study was to investigate the changes in retinal mRNA expression after different periods of visual deprivation in mice, while controlling for retinal illuminance. Methods Each group consisted of three male C57BL/6 mice. Treatment periods were 30 min, 4 h, and 6+6 h. High spatial frequencies were filtered from the retinal image by frosted diffusers over one eye while the fellow eyes were covered by clear neutral density (ND) filters that exhibited similar light attenuating properties (0.1 log units) as the diffusers. For the final 30 min of the respective treatment period mice were individually placed in a clear Perspex cylinder that was positioned in the center of a rotating (60 degrees) large drum. The inside of the drum was covered with a 0.1 cyc/degree vertical square wave grating. This visual environment was chosen to standardize illuminances and contrasts seen by the mice. Labeled cRNA was prepared and hybridized to Affymetrix GeneChip® Mouse Genome 430 2.0 arrays. Alterations in mRNA expression levels of candidate genes with potential biological relevance were confirmed by semi-quantitative real-time reverse transcription polymerase chain reaction (RT-PCR). Results In all groups, Egr-1 mRNA expression was reduced in diffuser-treated eyes. Furthermore, the degradation of the spatial frequency spectrum also changed the cFos mRNA level, with reduced expression after 4 h of diffuser treatment. Other interesting candidates were Akt2, which was up-regulated after 30 min of deprivation and Mapk8ip3, a neuron specific JNK binding and scaffolding protein that was temporally regulated in the diffuser-treated eyes only. Conclusions The microarray analysis demonstrated a pattern of differential transcriptional changes, even though differences in the retinal images were restricted to spatial features. The candidate genes may provide further insight into the biochemical short-term changes following retinal image degradation in mice. Because deprivation of spatial vision leads to increased eye growth and myopia in both animals and humans, it is believed some of the identified genes play a role in myopia development. PMID:17653032

Speed, spatial, and temporal tuning of rod and cone vision in mouse.

PubMed

Umino, Yumiko; Solessio, Eduardo; Barlow, Robert B

2008-01-02

Rods and cones subserve mouse vision over a 100 million-fold range of light intensity (-6 to 2 log cd m(-2)). Rod pathways tune vision to the temporal frequency of stimuli (peak, 0.75 Hz) and cone pathways to their speed (peak, approximately 12 degrees/s). Both pathways tune vision to the spatial components of stimuli (0.064-0.128 cycles/degree). The specific photoreceptor contributions were determined by two-alternative, forced-choice measures of contrast thresholds for optomotor responses of C57BL/6J mice with normal vision, Gnat2(cpfl3) mice without functional cones, and Gnat1-/- mice without functional rods. Gnat2(cpfl3) mice (threshold, -6.0 log cd m(-2)) cannot see rotating gratings above -2.0 log cd m(-2) (photopic vision), and Gnat1-/- mice (threshold, -4.0 log cd m(-2)) are blind below -4.0 log cd m(-2) (scotopic vision). Both genotypes can see in the transitional mesopic range (-4.0 to -2.0 log cd m(-2)). Mouse rod and cone sensitivities are similar to those of human. This parametric study characterizes the functional properties of the mouse visual system, revealing the rod and cone contributions to contrast sensitivity and to the temporal processing of visual stimuli.

Relaxometry imaging of superparamagnetic magnetite nanoparticles at ambient conditions

NASA Astrophysics Data System (ADS)

Finkler, Amit; Schmid-Lorch, Dominik; Häberle, Thomas; Reinhard, Friedemann; Zappe, Andrea; Slota, Michael; Bogani, Lapo; Wrachtrup, Jörg

We present a novel technique to image superparamagnetic iron oxide nanoparticles via their fluctuating magnetic fields. The detection is based on the nitrogen-vacancy (NV) color center in diamond, which allows optically detected magnetic resonance (ODMR) measurements on its electron spin structure. In combination with an atomic-force-microscope, this atomic-sized color center maps ambient magnetic fields in a wide frequency range from DC up to several GHz, while retaining a high spatial resolution in the sub-nanometer range. We demonstrate imaging of single 10 nm sized magnetite nanoparticles using this spin noise detection technique. By fitting simulations (Ornstein-Uhlenbeck process) to the data, we are able to infer additional information on such a particle and its dynamics, like the attempt frequency and the anisotropy constant. This is of high interest to the proposed application of magnetite nanoparticles as an alternative MRI contrast agent or to the field of particle-aided tumor hyperthermia.

Under-sampling trajectory design for compressed sensing based DCE-MRI.

PubMed

Liu, Duan-duan; Liang, Dong; Zhang, Na; Liu, Xin; Zhang, Yuan-ting

2013-01-01

Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) needs high temporal and spatial resolution to accurately estimate quantitative parameters and characterize tumor vasculature. Compressed Sensing (CS) has the potential to accomplish this mutual importance. However, the randomness in CS under-sampling trajectory designed using the traditional variable density (VD) scheme may translate to uncertainty in kinetic parameter estimation when high reduction factors are used. Therefore, accurate parameter estimation using VD scheme usually needs multiple adjustments on parameters of Probability Density Function (PDF), and multiple reconstructions even with fixed PDF, which is inapplicable for DCE-MRI. In this paper, an under-sampling trajectory design which is robust to the change on PDF parameters and randomness with fixed PDF is studied. The strategy is to adaptively segment k-space into low-and high frequency domain, and only apply VD scheme in high-frequency domain. Simulation results demonstrate high accuracy and robustness comparing to VD design.

Pattern reversal responses in man and cat: a comparison.

PubMed

Schuurmans, R P; Berninger, T

1984-01-01

In 42 enucleated and arterially perfused cat eyes, graded potentials were recorded from the retina (ERG) and from the optic nerve ( ONR ) in response to checker-board stimuli, reversing at a low temporal frequency in a square wave mode. The ERG and ONR responses show an almost perfect duplication of the response to each reversal of the pattern and exhibit, in contrast to luminance responses, striking similarities in response characteristics such as amplitude, wave shape and time course. Furthermore, the amplitude versus check size plots coincide in both responses. In cat, pattern reversal responses can be recorded from 74 to 9 min of arc, correlating to the cat's visual resolution. In man, almost identical responses can be recorded for the pattern ERG. However, in accordance with the difference in visual resolution in man and cat, a parallel shift for the human pattern reversal ERG response to higher spatial frequencies is observed.

Figure/ground segregation from temporal delay is best at high spatial frequencies.

PubMed

Kojima, H

1998-12-01

Two experiments investigated the role of spatial frequency in performance of a figure/ground segregation task based on temporal cues. Figure orientation was much easier to judge when figure and ground portions of the target were defined exclusively by random texture composed entirely of high spatial frequencies. When target components were defined by low spatial frequencies only, the task was nearly impossible except with long temporal delay between figure and ground. These results are inconsistent with the hypothesis that M-cell activity is primarily responsible for figure/ground segregation from temporal delay. Instead, these results point to a distinction between temporal integration and temporal differentiation. Additionally, the present results can be related to recent work on the binding of spatial features over time.

First- and second-order contrast sensitivity functions reveal disrupted visual processing following mild traumatic brain injury.

PubMed

Spiegel, Daniel P; Reynaud, Alexandre; Ruiz, Tatiana; Laguë-Beauvais, Maude; Hess, Robert; Farivar, Reza

2016-05-01

Vision is disrupted by traumatic brain injury (TBI), with vision-related complaints being amongst the most common in this population. Based on the neural responses of early visual cortical areas, injury to the visual cortex would be predicted to affect both 1(st) order and 2(nd) order contrast sensitivity functions (CSFs)-the height and/or the cut-off of the CSF are expected to be affected by TBI. Previous studies have reported disruptions only in 2(nd) order contrast sensitivity, but using a narrow range of parameters and divergent methodologies-no study has characterized the effect of TBI on the full CSF for both 1(st) and 2(nd) order stimuli. Such information is needed to properly understand the effect of TBI on contrast perception, which underlies all visual processing. Using a unified framework based on the quick contrast sensitivity function, we measured full CSFs for static and dynamic 1(st) and 2(nd) order stimuli. Our results provide a unique dataset showing alterations in sensitivity for both 1(st) and 2(nd) order visual stimuli. In particular, we show that TBI patients have increased sensitivity for 1(st) order motion stimuli and decreased sensitivity to orientation-defined and contrast-defined 2(nd) order stimuli. In addition, our data suggest that TBI patients' sensitivity for both 1(st) order stimuli and 2(nd) order contrast-defined stimuli is shifted towards higher spatial frequencies. Copyright © 2016 The Authors. Published by Elsevier Ltd.. All rights reserved.

Increases in tropical rainfall driven by changes in frequency of organized deep convection.

PubMed

Tan, Jackson; Jakob, Christian; Rossow, William B; Tselioudis, George

2015-03-26

Increasing global precipitation has been associated with a warming climate resulting from a strengthening of the hydrological cycle. This increase, however, is not spatially uniform. Observations and models have found that changes in rainfall show patterns characterized as 'wet-gets-wetter' and 'warmer-gets-wetter'. These changes in precipitation are largely located in the tropics and hence are probably associated with convection. However, the underlying physical processes for the observed changes are not entirely clear. Here we show from observations that most of the regional increase in tropical precipitation is associated with changes in the frequency of organized deep convection. By assessing the contributions of various convective regimes to precipitation, we find that the spatial patterns of change in the frequency of organized deep convection are strongly correlated with observed change in rainfall, both positive and negative (correlation of 0.69), and can explain most of the patterns of increase in rainfall. In contrast, changes in less organized forms of deep convection or changes in precipitation within organized deep convection contribute less to changes in precipitation. Our results identify organized deep convection as the link between changes in rainfall and in the dynamics of the tropical atmosphere, thus providing a framework for obtaining a better understanding of changes in rainfall. Given the lack of a distinction between the different degrees of organization of convection in climate models, our results highlight an area of priority for future climate model development in order to achieve accurate rainfall projections in a warming climate.

Practical implementation of channelized hotelling observers: effect of ROI size

NASA Astrophysics Data System (ADS)

Ferrero, Andrea; Favazza, Christopher P.; Yu, Lifeng; Leng, Shuai; McCollough, Cynthia H.

2017-03-01

Fundamental to the development and application of channelized Hotelling observer (CHO) models is the selection of the region of interest (ROI) to evaluate. For assessment of medical imaging systems, reducing the ROI size can be advantageous. Smaller ROIs enable a greater concentration of interrogable objects in a single phantom image, thereby providing more information from a set of images and reducing the overall image acquisition burden. Additionally, smaller ROIs may promote better assessment of clinical patient images as different patient anatomies present different ROI constraints. To this end, we investigated the minimum ROI size that does not compromise the performance of the CHO model. In this study, we evaluated both simulated images and phantom CT images to identify the minimum ROI size that resulted in an accurate figure of merit (FOM) of the CHO's performance. More specifically, the minimum ROI size was evaluated as a function of the following: number of channels, spatial frequency and number of rotations of the Gabor filters, size and contrast of the object, and magnitude of the image noise. Results demonstrate that a minimum ROI size exists below which the CHO's performance is grossly inaccurate. The minimum ROI size is shown to increase with number of channels and be dictated by truncation of lower frequency filters. We developed a model to estimate the minimum ROI size as a parameterized function of the number of orientations and spatial frequencies of the Gabor filters, providing a guide for investigators to appropriately select parameters for model observer studies.

Practical implementation of Channelized Hotelling Observers: Effect of ROI size.

PubMed

Ferrero, Andrea; Favazza, Christopher P; Yu, Lifeng; Leng, Shuai; McCollough, Cynthia H

2017-03-01

Fundamental to the development and application of channelized Hotelling observer (CHO) models is the selection of the region of interest (ROI) to evaluate. For assessment of medical imaging systems, reducing the ROI size can be advantageous. Smaller ROIs enable a greater concentration of interrogable objects in a single phantom image, thereby providing more information from a set of images and reducing the overall image acquisition burden. Additionally, smaller ROIs may promote better assessment of clinical patient images as different patient anatomies present different ROI constraints. To this end, we investigated the minimum ROI size that does not compromise the performance of the CHO model. In this study, we evaluated both simulated images and phantom CT images to identify the minimum ROI size that resulted in an accurate figure of merit (FOM) of the CHO's performance. More specifically, the minimum ROI size was evaluated as a function of the following: number of channels, spatial frequency and number of rotations of the Gabor filters, size and contrast of the object, and magnitude of the image noise. Results demonstrate that a minimum ROI size exists below which the CHO's performance is grossly inaccurate. The minimum ROI size is shown to increase with number of channels and be dictated by truncation of lower frequency filters. We developed a model to estimate the minimum ROI size as a parameterized function of the number of orientations and spatial frequencies of the Gabor filters, providing a guide for investigators to appropriately select parameters for model observer studies.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Ke; Chen, Guang-Hong, E-mail: gchen7@wisc.edu; Garrett, John

Purpose: Statistical model based iterative reconstruction (MBIR) methods have been introduced to clinical CT systems and are being used in some clinical diagnostic applications. The purpose of this paper is to experimentally assess the unique spatial resolution characteristics of this nonlinear reconstruction method and identify its potential impact on the detectabilities and the associated radiation dose levels for specific imaging tasks. Methods: The thoracic section of a pediatric phantom was repeatedly scanned 50 or 100 times using a 64-slice clinical CT scanner at four different dose levels [CTDI{sub vol} =4, 8, 12, 16 (mGy)]. Both filtered backprojection (FBP) and MBIRmore » (Veo{sup ®}, GE Healthcare, Waukesha, WI) were used for image reconstruction and results were compared with one another. Eight test objects in the phantom with contrast levels ranging from 13 to 1710 HU were used to assess spatial resolution. The axial spatial resolution was quantified with the point spread function (PSF), while the z resolution was quantified with the slice sensitivity profile. Both were measured locally on the test objects and in the image domain. The dependence of spatial resolution on contrast and dose levels was studied. The study also features a systematic investigation of the potential trade-off between spatial resolution and locally defined noise and their joint impact on the overall image quality, which was quantified by the image domain-based channelized Hotelling observer (CHO) detectability index d′. Results: (1) The axial spatial resolution of MBIR depends on both radiation dose level and image contrast level, whereas it is supposedly independent of these two factors in FBP. The axial spatial resolution of MBIR always improved with an increasing radiation dose level and/or contrast level. (2) The axial spatial resolution of MBIR became equivalent to that of FBP at some transitional contrast level, above which MBIR demonstrated superior spatial resolution than FBP (and vice versa); the value of this transitional contrast highly depended on the dose level. (3) The PSFs of MBIR could be approximated as Gaussian functions with reasonably good accuracy. (4) Thez resolution of MBIR showed similar contrast and dose dependence. (5) Noise standard deviation assessed on the edges of objects demonstrated a trade-off with spatial resolution in MBIR. (5) When both spatial resolution and image noise were considered using the CHO analysis, MBIR led to significant improvement in the overall CT image quality for both high and low contrast detection tasks at both standard and low dose levels. Conclusions: Due to the intrinsic nonlinearity of the MBIR method, many well-known CT spatial resolution and noise properties have been modified. In particular, dose dependence and contrast dependence have been introduced to the spatial resolution of CT images by MBIR. The method has also introduced some novel noise-resolution trade-off not seen in traditional CT images. While the benefits of MBIR regarding the overall image quality, as demonstrated in this work, are significant, the optimal use of this method in clinical practice demands a thorough understanding of its unique physical characteristics.« less

Spatial-frequency composite watermarking for digital image copyright protection

NASA Astrophysics Data System (ADS)

Su, Po-Chyi; Kuo, C.-C. Jay

2000-05-01

Digital watermarks can be classified into two categories according to the embedding and retrieval domain, i.e. spatial- and frequency-domain watermarks. Because the two watermarks have different characteristics and limitations, combination of them can have various interesting properties when applied to different applications. In this research, we examine two spatial-frequency composite watermarking schemes. In both cases, a frequency-domain watermarking technique is applied as a baseline structure in the system. The embedded frequency- domain watermark is robust against filtering and compression. A spatial-domain watermarking scheme is then built to compensate some deficiency of the frequency-domain scheme. The first composite scheme is to embed a robust watermark in images to convey copyright or author information. The frequency-domain watermark contains owner's identification number while the spatial-domain watermark is embedded for image registration to resist cropping attack. The second composite scheme is to embed fragile watermark for image authentication. The spatial-domain watermark helps in locating the tampered part of the image while the frequency-domain watermark indicates the source of the image and prevents double watermarking attack. Experimental results show that the two watermarks do not interfere with each other and different functionalities can be achieved. Watermarks in both domains are detected without resorting to the original image. Furthermore, the resulting watermarked image can still preserve high fidelity without serious visual degradation.

Attention enhances contrast appearance via increased input baseline of neural responses

PubMed Central

Cutrone, Elizabeth K.; Heeger, David J.; Carrasco, Marisa

2014-01-01

Covert spatial attention increases the perceived contrast of stimuli at attended locations, presumably via enhancement of visual neural responses. However, the relation between perceived contrast and the underlying neural responses has not been characterized. In this study, we systematically varied stimulus contrast, using a two-alternative, forced-choice comparison task to probe the effect of attention on appearance across the contrast range. We modeled performance in the task as a function of underlying neural contrast-response functions. Fitting this model to the observed data revealed that an increased input baseline in the neural responses accounted for the enhancement of apparent contrast with spatial attention. PMID:25549920

On the assessment of spatial resolution of PET systems with iterative image reconstruction

NASA Astrophysics Data System (ADS)

Gong, Kuang; Cherry, Simon R.; Qi, Jinyi

2016-03-01

Spatial resolution is an important metric for performance characterization in PET systems. Measuring spatial resolution is straightforward with a linear reconstruction algorithm, such as filtered backprojection, and can be performed by reconstructing a point source scan and calculating the full-width-at-half-maximum (FWHM) along the principal directions. With the widespread adoption of iterative reconstruction methods, it is desirable to quantify the spatial resolution using an iterative reconstruction algorithm. However, the task can be difficult because the reconstruction algorithms are nonlinear and the non-negativity constraint can artificially enhance the apparent spatial resolution if a point source image is reconstructed without any background. Thus, it was recommended that a background should be added to the point source data before reconstruction for resolution measurement. However, there has been no detailed study on the effect of the point source contrast on the measured spatial resolution. Here we use point source scans from a preclinical PET scanner to investigate the relationship between measured spatial resolution and the point source contrast. We also evaluate whether the reconstruction of an isolated point source is predictive of the ability of the system to resolve two adjacent point sources. Our results indicate that when the point source contrast is below a certain threshold, the measured FWHM remains stable. Once the contrast is above the threshold, the measured FWHM monotonically decreases with increasing point source contrast. In addition, the measured FWHM also monotonically decreases with iteration number for maximum likelihood estimate. Therefore, when measuring system resolution with an iterative reconstruction algorithm, we recommend using a low-contrast point source and a fixed number of iterations.

Effect of chronic and acute low-frequency repetitive transcranial magnetic stimulation on spatial memory in rats.

PubMed

Li, Wei; Yang, Yuye; Ye, Qing; Yang, Bo; Wang, Zhengrong

2007-03-15

Repetitive transcranial magnetic stimulation (rTMS) is a novel, non-invasive neurological and psychiatric tool. The low-frequency (1 Hz or less) rTMS is likely to play a particular role in its mechanism of action with different effects in comparison with high-frequency (>1 Hz) rTMS. There is limited information regarding the effect of low-frequency rTMS on spatial memory. In our study, each male Wistar rat was daily given 300 stimuli (1.0 T, 200 micros) at a rate of 0.5 Hz or sham stimulation. We investigated the effects of chronic and acute rTMS on reference/working memory process in Morris water maze test with the hypothesis that the effect would differ by chronic or acute condition. Chronic low-frequency rTMS impaired the retrieval of spatial short- and long-term spatial reference memory but not acquisition process and working memory, whereas acute low-frequency rTMS predominantly induced no deficits in acquisition or short-term spatial reference memory as well as working memory except for long-term reference memory. In summary, chronic 0.5 Hz rTMS disrupts spatial short- and long-term reference memory function, but acute rTMS differently affects reference memory. Chronic low-frequency rTMS may be used to modulate reference memory. Treatment protocols using low-frequency rTMS in neurological and psychiatric disorders need to take into account the potential effect of chronic low-frequency rTMS on memory and other cognitive functions.

Spatial Resolution and Refractive Index Contrast of Resonant Photonic Crystal Surfaces for Biosensing

PubMed Central

Triggs, G. J.; Fischer, M.; Stellinga, D.; Scullion, M. G.; Evans, G. J. O.; Krauss, T. F.

2015-01-01

By depositing a resolution test pattern on top of a Si3N4 photonic crystal resonant surface, we have measured the dependence of spatial resolution on refractive index contrast Δn. Our experimental results and finite-difference time-domain (FDTD) simulations at different refractive index contrasts show that the spatial resolution of our device reduces with reduced contrast, which is an important consideration in biosensing, where the contrast may be of order 10−2. We also compare 1-D and 2-D gratings, taking into account different incidence polarizations, leading to a better understanding of the excitation and propagation of the resonant modes in these structures, as well as how this contributes to the spatial resolution. At Δn = 0.077, we observe resolutions of 2 and 6 μm parallel to and perpendicular to the grooves of a 1-D grating, respectively, and show that for polarized illumination of a 2-D grating, resolution remains asymmetrical. Illumination of a 2-D grating at 45° results in symmetric resolution. At very low index contrast, the resolution worsens dramatically, particularly for Δn < 0.01, where we observe a resolution exceeding 10 μm for our device. In addition, we measure a reduction in the resonance linewidth as the index contrast becomes lower, corresponding to a longer resonant mode propagation length in the structure and contributing to the change in spatial resolution. PMID:26356353

Controlling Energy Radiations of Electromagnetic Waves via Frequency Coding Metamaterials

PubMed Central

Wu, Haotian; Liu, Shuo; Wan, Xiang; Zhang, Lei; Wang, Dan; Li, Lianlin

2017-01-01

Metamaterials are artificial structures composed of subwavelength unit cells to control electromagnetic (EM) waves. The spatial coding representation of metamaterial has the ability to describe the material in a digital way. The spatial coding metamaterials are typically constructed by unit cells that have similar shapes with fixed functionality. Here, the concept of frequency coding metamaterial is proposed, which achieves different controls of EM energy radiations with a fixed spatial coding pattern when the frequency changes. In this case, not only different phase responses of the unit cells are considered, but also different phase sensitivities are also required. Due to different frequency sensitivities of unit cells, two units with the same phase response at the initial frequency may have different phase responses at higher frequency. To describe the frequency coding property of unit cell, digitalized frequency sensitivity is proposed, in which the units are encoded with digits “0” and “1” to represent the low and high phase sensitivities, respectively. By this merit, two degrees of freedom, spatial coding and frequency coding, are obtained to control the EM energy radiations by a new class of frequency‐spatial coding metamaterials. The above concepts and physical phenomena are confirmed by numerical simulations and experiments. PMID:28932671

Improvement of Speckle Contrast Image Processing by an Efficient Algorithm.

PubMed

Steimers, A; Farnung, W; Kohl-Bareis, M

2016-01-01

We demonstrate an efficient algorithm for the temporal and spatial based calculation of speckle contrast for the imaging of blood flow by laser speckle contrast analysis (LASCA). It reduces the numerical complexity of necessary calculations, facilitates a multi-core and many-core implementation of the speckle analysis and enables an independence of temporal or spatial resolution and SNR. The new algorithm was evaluated for both spatial and temporal based analysis of speckle patterns with different image sizes and amounts of recruited pixels as sequential, multi-core and many-core code.

Ultrafast 2D IR microscopy

PubMed Central

Baiz, Carlos R.; Schach, Denise; Tokmakoff, Andrei

2014-01-01

We describe a microscope for measuring two-dimensional infrared (2D IR) spectra of heterogeneous samples with μm-scale spatial resolution, sub-picosecond time resolution, and the molecular structure information of 2D IR, enabling the measurement of vibrational dynamics through correlations in frequency, time, and space. The setup is based on a fully collinear “one beam” geometry in which all pulses propagate along the same optics. Polarization, chopping, and phase cycling are used to isolate the 2D IR signals of interest. In addition, we demonstrate the use of vibrational lifetime as a contrast agent for imaging microscopic variations in molecular environments. PMID:25089490

Application of full field optical studies for pulsatile flow in a carotid artery phantom

PubMed Central

Nemati, M.; Loozen, G. B.; van der Wekken, N.; van de Belt, G.; Urbach, H. P.; Bhattacharya, N.; Kenjeres, S.

2015-01-01

A preliminary comparative measurement between particle imaging velocimetry (PIV) and laser speckle contrast analysis (LASCA) to study pulsatile flow using ventricular assist device in a patient-specific carotid artery phantom is reported. These full-field optical techniques have both been used to study flow and extract complementary parameters. We use the high spatial resolution of PIV to generate a full velocity map of the flow field and the high temporal resolution of LASCA to extract the detailed frequency spectrum of the fluid pulses. Using this combination of techniques a complete study of complex pulsatile flow in an intricate flow network can be studied. PMID:26504652

Dynamic accommodation with simulated targets blurred with high order aberrations

PubMed Central

Gambra, Enrique; Wang, Yinan; Yuan, Jing; Kruger, Philip B.; Marcos, Susana

2010-01-01

High order aberrations have been suggested to play a role in determining the direction of accommodation. We have explored the effect of retinal blur induced by high order aberrations on dynamic accommodation by measuring the accommodative response to sinusoidal variations in accommodative demand (1–3 D). The targets were blurred with 0.3 and 1 μm (for a 3-mm pupil) of defocus, coma, trefoil and spherical aberration. Accommodative gain decreased significantly when 1-μm of aberration was induced. We found a strong correlation between the relative accommodative gain (and phase lag) and the contrast degradation imposed on the target at relevant spatial frequencies. PMID:20600230

Modulation transfer function measurement technique for small-pixel detectors

NASA Technical Reports Server (NTRS)

Marchywka, Mike; Socker, Dennis G.

1992-01-01

A modulation transfer function (MTF) measurement technique suitable for large-format, small-pixel detector characterization has been investigated. A volume interference grating is used as a test image instead of the bar or sine wave target images normally used. This technique permits a high-contrast, large-area, sinusoidal intensity distribution to illuminate the device being tested, avoiding the need to deconvolve raw data with imaging system characteristics. A high-confidence MTF result at spatial frequencies near 200 cycles/mm is obtained. We present results at several visible light wavelengths with a 6.8-micron-pixel CCD. Pixel response functions are derived from the MTF results.

Direct optical measurement of the on-shot incoherent focal spot and intensity contrast on the OMEGA EP laser

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dorrer, C.; Consentino, A.; Irwin, D.

Characterizing the prepulse temporal contrast of optical pulses is required to understand their interaction with matter. Light with relatively low intensity can interact with the target before the main high-intensity pulse. Estimating the intensity contrast, instead of the spatially averaged power contrast, is important to understand intensity-dependent laser–matter interactions. A direct optical approach to determining the on-shot intensity of the incoherent pedestal on an aberrated high-intensity laser system is presented. The spatially resolved focal spot of the incoherent pedestal preceding the main coherent pulse and the intensity contrast are calculated using experimental data. Furthermore, this technique is experimentally validated onmore » one of the chirped pulse amplification beamlines of the OMEGA EP Laser System. The intensity contrast of a 1-kJ, 10-ps laser pulse is shown to be ~10× higher than the power contrast because of the larger spatial extent of the incoherent focal spot relative to the coherent focal spot.« less

Direct optical measurement of the on-shot incoherent focal spot and intensity contrast on the OMEGA EP laser

DOE PAGES

Dorrer, C.; Consentino, A.; Irwin, D.

2016-05-18

Characterizing the prepulse temporal contrast of optical pulses is required to understand their interaction with matter. Light with relatively low intensity can interact with the target before the main high-intensity pulse. Estimating the intensity contrast, instead of the spatially averaged power contrast, is important to understand intensity-dependent laser–matter interactions. A direct optical approach to determining the on-shot intensity of the incoherent pedestal on an aberrated high-intensity laser system is presented. The spatially resolved focal spot of the incoherent pedestal preceding the main coherent pulse and the intensity contrast are calculated using experimental data. Furthermore, this technique is experimentally validated onmore » one of the chirped pulse amplification beamlines of the OMEGA EP Laser System. The intensity contrast of a 1-kJ, 10-ps laser pulse is shown to be ~10× higher than the power contrast because of the larger spatial extent of the incoherent focal spot relative to the coherent focal spot.« less

Face Recognition Is Affected by Similarity in Spatial Frequency Range to a Greater Degree Than Within-Category Object Recognition

ERIC Educational Resources Information Center

Collin, Charles A.; Liu, Chang Hong; Troje, Nikolaus F.; McMullen, Patricia A.; Chaudhuri, Avi

2004-01-01

Previous studies have suggested that face identification is more sensitive to variations in spatial frequency content than object recognition, but none have compared how sensitive the 2 processes are to variations in spatial frequency overlap (SFO). The authors tested face and object matching accuracy under varying SFO conditions. Their results…

Multispectral medical image fusion in Contourlet domain for computer based diagnosis of Alzheimer's disease.

PubMed

Bhateja, Vikrant; Moin, Aisha; Srivastava, Anuja; Bao, Le Nguyen; Lay-Ekuakille, Aimé; Le, Dac-Nhuong

2016-07-01

Computer based diagnosis of Alzheimer's disease can be performed by dint of the analysis of the functional and structural changes in the brain. Multispectral image fusion deliberates upon fusion of the complementary information while discarding the surplus information to achieve a solitary image which encloses both spatial and spectral details. This paper presents a Non-Sub-sampled Contourlet Transform (NSCT) based multispectral image fusion model for computer-aided diagnosis of Alzheimer's disease. The proposed fusion methodology involves color transformation of the input multispectral image. The multispectral image in YIQ color space is decomposed using NSCT followed by dimensionality reduction using modified Principal Component Analysis algorithm on the low frequency coefficients. Further, the high frequency coefficients are enhanced using non-linear enhancement function. Two different fusion rules are then applied to the low-pass and high-pass sub-bands: Phase congruency is applied to low frequency coefficients and a combination of directive contrast and normalized Shannon entropy is applied to high frequency coefficients. The superiority of the fusion response is depicted by the comparisons made with the other state-of-the-art fusion approaches (in terms of various fusion metrics).

Multispectral medical image fusion in Contourlet domain for computer based diagnosis of Alzheimer’s disease

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bhateja, Vikrant, E-mail: bhateja.vikrant@gmail.com, E-mail: nhuongld@hus.edu.vn; Moin, Aisha; Srivastava, Anuja

Computer based diagnosis of Alzheimer’s disease can be performed by dint of the analysis of the functional and structural changes in the brain. Multispectral image fusion deliberates upon fusion of the complementary information while discarding the surplus information to achieve a solitary image which encloses both spatial and spectral details. This paper presents a Non-Sub-sampled Contourlet Transform (NSCT) based multispectral image fusion model for computer-aided diagnosis of Alzheimer’s disease. The proposed fusion methodology involves color transformation of the input multispectral image. The multispectral image in YIQ color space is decomposed using NSCT followed by dimensionality reduction using modified Principal Componentmore » Analysis algorithm on the low frequency coefficients. Further, the high frequency coefficients are enhanced using non-linear enhancement function. Two different fusion rules are then applied to the low-pass and high-pass sub-bands: Phase congruency is applied to low frequency coefficients and a combination of directive contrast and normalized Shannon entropy is applied to high frequency coefficients. The superiority of the fusion response is depicted by the comparisons made with the other state-of-the-art fusion approaches (in terms of various fusion metrics).« less

Dual-Frequency Piezoelectric Transducers for Contrast Enhanced Ultrasound Imaging

PubMed Central

Martin, K. Heath; Lindsey, Brooks D.; Ma, Jianguo; Lee, Mike; Li, Sibo; Foster, F. Stuart; Jiang, Xiaoning; Dayton, Paul A.

2014-01-01

For many years, ultrasound has provided clinicians with an affordable and effective imaging tool for applications ranging from cardiology to obstetrics. Development of microbubble contrast agents over the past several decades has enabled ultrasound to distinguish between blood flow and surrounding tissue. Current clinical practices using microbubble contrast agents rely heavily on user training to evaluate degree of localized perfusion. Advances in separating the signals produced from contrast agents versus surrounding tissue backscatter provide unique opportunities for specialized sensors designed to image microbubbles with higher signal to noise and resolution than previously possible. In this review article, we describe the background principles and recent developments of ultrasound transducer technology for receiving signals produced by contrast agents while rejecting signals arising from soft tissue. This approach relies on transmitting at a low-frequency and receiving microbubble harmonic signals at frequencies many times higher than the transmitted frequency. Design and fabrication of dual-frequency transducers and the extension of recent developments in transducer technology for dual-frequency harmonic imaging are discussed. PMID:25375755

Dual-frequency piezoelectric transducers for contrast enhanced ultrasound imaging.

PubMed

Martin, K Heath; Lindsey, Brooks D; Ma, Jianguo; Lee, Mike; Li, Sibo; Foster, F Stuart; Jiang, Xiaoning; Dayton, Paul A

2014-11-04

For many years, ultrasound has provided clinicians with an affordable and effective imaging tool for applications ranging from cardiology to obstetrics. Development of microbubble contrast agents over the past several decades has enabled ultrasound to distinguish between blood flow and surrounding tissue. Current clinical practices using microbubble contrast agents rely heavily on user training to evaluate degree of localized perfusion. Advances in separating the signals produced from contrast agents versus surrounding tissue backscatter provide unique opportunities for specialized sensors designed to image microbubbles with higher signal to noise and resolution than previously possible. In this review article, we describe the background principles and recent developments of ultrasound transducer technology for receiving signals produced by contrast agents while rejecting signals arising from soft tissue. This approach relies on transmitting at a low-frequency and receiving microbubble harmonic signals at frequencies many times higher than the transmitted frequency. Design and fabrication of dual-frequency transducers and the extension of recent developments in transducer technology for dual-frequency harmonic imaging are discussed.

Analysis of marine multi-channel seismic data using a 2D continuous wavelet transform

NASA Astrophysics Data System (ADS)

Vuong, A. K.; Zhang, J.; Gibson, R. L.; Sager, W. W.

2011-12-01

Marine multi-channel seismic (MCS) profiles provide important constraints on crustal structure beneath the sea floor. MCS data usually provide good images of the upper part of the oceanic crust, especially in sedimentary layers. In contrast, it is often difficult to interpret deeper layers, especially those within the igneous basement, which is often nearly seismically transparent. That difference in interpretability occurs because sediments typically have continuous, well-layered and easily-traced structural features, whereas volcanic materials are characterized by smaller features with poorer lateral continuity and often with weak impedance contrasts. Since the basement tends to create weaker reflections, the signal-to-noise ratio decreases, creating additional difficulties that can be exacerbated by the presence of multiples generated by the sea floor and other sources of noise. However, it is still important to characterize the basement accurately to better understand oceanic crust formation and associated basaltic volcanism. We analyzed marine MCS data collected by R/V Marcus G. Langseth across the TAMU Massif of Shatsky Rise in the Northwest Pacific. The seismic data from this experiment display the typical problems in imaging basement features. Therefore, we seek to facilitate interpretation by applying 2-D continuous wavelet transforms to the data. Conventional Fourier methods transform 2-D seismic data from space and time domains to wavenumber and frequency, but the results are global in that there is no knowledge of temporal or spatial variations in frequency or wavenumber content. In contrast, wavelet transforms provide estimates of the local frequency and wavenumber content of the seismic image. The transform achieves this result by utilizing a localized, 2D wavelet function instead of the infinite sines and cosines applied in Fourier transforms. We utilize an anisotropic Mexican hat wavelet, where the horizontal and vertical scales are related to wavelength and period of the data, respectively. When analyzing the Shatsky Rise data set, we find, for example, that much of the noise in the seismic image of the basement is at small wavelengths corresponding to several traces, about 25 m. Using the wavelet transforms, we can extract reflection events at longer wavelengths corresponding to expected features in the subsurface. Observing reflections at a certain wavelength provides an estimate of the size scale of the associated geologic structures. The results at a frequency of 31.25 Hz, near the dominant frequency of the data, provide images of reflectors in the deep part of oceanic crust with scales from 200 m to 2000 m that are much easier to interpret than in the original seismic image. In particular, at scales from 200 m to 1000 m, we can see many reflectors with consistent with sizes and locations for localized magma intrusions into the oceanic crust. However, for spatial scales of about 2000 m, only a few reflectors are observed, suggesting there are fewer intrusions of this dimension. These features can also be examined at a range of frequencies to provide additional insights, and the wavelet transform can also be generalized to estimate dips of reflectors.

Apparatus for direct-to-digital spatially-heterodyned holography

DOEpatents

Thomas, Clarence E.; Hanson, Gregory R.

2006-12-12

An apparatus operable to record a spatially low-frequency heterodyne hologram including spatially heterodyne fringes for Fourier analysis includes: a laser; a beamsplitter optically coupled to the laser; an object optically coupled to the beamsplitter; a focusing lens optically coupled to both the beamsplitter and the object; a digital recorder optically coupled to the focusing lens; and a computer that performs a Fourier transform, applies a digital filter, and performs an inverse Fourier transform. A reference beam and an object beam are focused by the focusing lens at a focal plane of the digital recorder to form a spatially low-frequency heterodyne hologram including spatially heterodyne fringes for Fourier analysis which is recorded by the digital recorder, and the computer transforms the recorded spatially low-frequency heterodyne hologram including spatially heterodyne fringes and shifts axes in Fourier space to sit on top of a heterodyne carrier frequency defined by an angle between the reference beam and the object beam and cuts off signals around an original origin before performing the inverse Fourier transform.

High source levels and small active space of high-pitched song in bowhead whales (Balaena mysticetus).

PubMed

Tervo, Outi M; Christoffersen, Mads F; Simon, Malene; Miller, Lee A; Jensen, Frants H; Parks, Susan E; Madsen, Peter T

2012-01-01

The low-frequency, powerful vocalizations of blue and fin whales may potentially be detected by conspecifics across entire ocean basins. In contrast, humpback and bowhead whales produce equally powerful, but more complex broadband vocalizations composed of higher frequencies that suffer from higher attenuation. Here we evaluate the active space of high frequency song notes of bowhead whales (Balaena mysticetus) in Western Greenland using measurements of song source levels and ambient noise. Four independent, GPS-synchronized hydrophones were deployed through holes in the ice to localize vocalizing bowhead whales, estimate source levels and measure ambient noise. The song had a mean apparent source level of 185±2 dB rms re 1 µPa @ 1 m and a high mean centroid frequency of 444±48 Hz. Using measured ambient noise levels in the area and Arctic sound spreading models, the estimated active space of these song notes is between 40 and 130 km, an order of magnitude smaller than the estimated active space of low frequency blue and fin whale songs produced at similar source levels and for similar noise conditions. We propose that bowhead whales spatially compensate for their smaller communication range through mating aggregations that co-evolved with broadband song to form a complex and dynamic acoustically mediated sexual display.

High Source Levels and Small Active Space of High-Pitched Song in Bowhead Whales (Balaena mysticetus)

PubMed Central

Tervo, Outi M.; Christoffersen, Mads F.; Simon, Malene; Miller, Lee A.; Jensen, Frants H.; Parks, Susan E.; Madsen, Peter T.

2012-01-01

The low-frequency, powerful vocalizations of blue and fin whales may potentially be detected by conspecifics across entire ocean basins. In contrast, humpback and bowhead whales produce equally powerful, but more complex broadband vocalizations composed of higher frequencies that suffer from higher attenuation. Here we evaluate the active space of high frequency song notes of bowhead whales (Balaena mysticetus) in Western Greenland using measurements of song source levels and ambient noise. Four independent, GPS-synchronized hydrophones were deployed through holes in the ice to localize vocalizing bowhead whales, estimate source levels and measure ambient noise. The song had a mean apparent source level of 185±2 dB rms re 1 µPa @ 1 m and a high mean centroid frequency of 444±48 Hz. Using measured ambient noise levels in the area and Arctic sound spreading models, the estimated active space of these song notes is between 40 and 130 km, an order of magnitude smaller than the estimated active space of low frequency blue and fin whale songs produced at similar source levels and for similar noise conditions. We propose that bowhead whales spatially compensate for their smaller communication range through mating aggregations that co-evolved with broadband song to form a complex and dynamic acoustically mediated sexual display. PMID:23300591

Image routing via atomic spin coherence

PubMed Central

Wang, Lei; Sun, Jia-Xiang; Luo, Meng-Xi; Sun, Yuan-Hang; Wang, Xiao-Xiao; Chen, Yi; Kang, Zhi-Hui; Wang, Hai-Hua; Wu, Jin-Hui; Gao, Jin-Yue

2015-01-01

Coherent storage of optical image in a coherently-driven medium is a promising method with possible applications in many fields. In this work, we experimentally report a controllable spatial-frequency routing of image via atomic spin coherence in a solid-state medium driven by electromagnetically induced transparency (EIT). Under the EIT-based light-storage regime, a transverse spatial image carried by the probe field is stored into atomic spin coherence. By manipulating the frequency and spatial propagation direction of the read control field, the stored image is transferred into a new spatial-frequency channel. When two read control fields are used to retrieve the stored information, the image information is converted into a superposition of two spatial-frequency modes. Through this technique, the image is manipulated coherently and all-optically in a controlled fashion. PMID:26658846

The effects of longitudinal chromatic aberration and a shift in the peak of the middle-wavelength sensitive cone fundamental on cone contrast

PubMed Central

Rucker, F. J.; Osorio, D.

2009-01-01

Longitudinal chromatic aberration is a well-known imperfection of visual optics, but the consequences in natural conditions, and for the evolution of receptor spectral sensitivities are less well understood. This paper examines how chromatic aberration affects image quality in the middle-wavelength sensitive (M-) cones, viewing broad-band spectra, over a range of spatial frequencies and focal planes. We also model the effects on M-cone contrast of moving the M-cone fundamental relative to the long- and middle-wavelength (L- and M-cone) fundamentals, while the eye is accommodated at different focal planes or at a focal plane that maximizes luminance contrast. When the focal plane shifts towards longer (650 nm) or shorter wavelengths (420 nm) the effects on M-cone contrast are large: longitudinal chromatic aberration causes total loss of M-cone contrast above 10 to 20 c/d. In comparison, the shift of the M-cone fundamental causes smaller effects on M-cone contrast. At 10 c/d a shift in the peak of the M-cone spectrum from 560 nm to 460 nm decreases M-cone contrast by 30%, while a 10 nm blue-shift causes only a minor loss of contrast. However, a noticeable loss of contrast may be seen if the eye is focused at focal planes other than that which maximizes luminance contrast. The presence of separate long- and middle-wavelength sensitive cones therefore has a small, but not insignificant cost to the retinal image via longitudinal chromatic aberration. This aberration may therefore be a factor limiting evolution of visual pigments and trichromatic color vision. PMID:18639571

Estimation of Soil Moisture Under Vegetation Cover at Multiple Frequencies

NASA Astrophysics Data System (ADS)

Jadghuber, Thomas; Hajnsek, Irena; Weiß, Thomas; Papathanassiou, Konstantinos P.

2015-04-01

Soil moisture under vegetation cover was estimated by a polarimetric, iterative, generalized, hybrid decomposition and inversion approach at multiple frequencies (X-, C- and L-band). Therefore the algorithm, originally designed for longer wavelength (L-band), was adapted to deal with the short wavelength scattering scenarios of X- and C-band. The Integral Equation Method (IEM) was incorporated together with a pedo-transfer function of Dobson et al. to account for the peculiarities of short wavelength scattering at X- and C-band. DLR's F-SAR system acquired fully polarimetric SAR data in X-, C- and L-band over the Wallerfing test site in Lower Bavaria, Germany in 2014. Simultaneously, soil and vegetation measurements were conducted on different agricultural test fields. The results indicate a spatially continuous inversion of soil moisture in all three frequencies (inversion rates >92%), mainly due to the careful adaption of the vegetation volume removal including a physical constraining of the decomposition algorithm. However, for X- and C-band the inversion results reveal moisture pattern inconsistencies and in some cases an incorrectly high inversion of soil moisture at X-band. The validation with in situ measurements states a stable performance of 2.1- 7.6vol.% at L-band for the entire growing period. At C- and X-band a reliable performance of 3.7-13.4vol.% in RMSE can only be achieved after distinct filtering (X- band) leading to a loss of almost 60% in spatial inversion rate. Hence, a robust inversion for soil moisture estimation under vegetation cover can only be conducted at L-band due to a constant availability of the soil signal in contrast to higher frequencies (X- and C-band).

A Mouse Model of Visual Perceptual Learning Reveals Alterations in Neuronal Coding and Dendritic Spine Density in the Visual Cortex.

PubMed

Wang, Yan; Wu, Wei; Zhang, Xian; Hu, Xu; Li, Yue; Lou, Shihao; Ma, Xiao; An, Xu; Liu, Hui; Peng, Jing; Ma, Danyi; Zhou, Yifeng; Yang, Yupeng

2016-01-01

Visual perceptual learning (VPL) can improve spatial vision in normally sighted and visually impaired individuals. Although previous studies of humans and large animals have explored the neural basis of VPL, elucidation of the underlying cellular and molecular mechanisms remains a challenge. Owing to the advantages of molecular genetic and optogenetic manipulations, the mouse is a promising model for providing a mechanistic understanding of VPL. Here, we thoroughly evaluated the effects and properties of VPL on spatial vision in C57BL/6J mice using a two-alternative, forced-choice visual water task. Briefly, the mice underwent prolonged training at near the individual threshold of contrast or spatial frequency (SF) for pattern discrimination or visual detection for 35 consecutive days. Following training, the contrast-threshold trained mice showed an 87% improvement in contrast sensitivity (CS) and a 55% gain in visual acuity (VA). Similarly, the SF-threshold trained mice exhibited comparable and long-lasting improvements in VA and significant gains in CS over a wide range of SFs. Furthermore, learning largely transferred across eyes and stimulus orientations. Interestingly, learning could transfer from a pattern discrimination task to a visual detection task, but not vice versa. We validated that this VPL fully restored VA in adult amblyopic mice and old mice. Taken together, these data indicate that mice, as a species, exhibit reliable VPL. Intrinsic signal optical imaging revealed that mice with perceptual training had higher cut-off SFs in primary visual cortex (V1) than those without perceptual training. Moreover, perceptual training induced an increase in the dendritic spine density in layer 2/3 pyramidal neurons of V1. These results indicated functional and structural alterations in V1 during VPL. Overall, our VPL mouse model will provide a platform for investigating the neurobiological basis of VPL.

The influence of L-opsin gene polymorphisms and neural ageing on spatio-chromatic contrast sensitivity in 20-71 year olds.

PubMed

Dees, Elise W; Gilson, Stuart J; Neitz, Maureen; Baraas, Rigmor C

2015-11-01

Chromatic contrast sensitivity may be a more sensitive measure of an individual's visual function than achromatic contrast sensitivity. Here, the first aim was to quantify individual- and age-related variations in chromatic contrast sensitivity to a range of spatial frequencies for stimuli along two complementary directions in color space. The second aim was to examine whether polymorphisms at specific amino acid residues of the L- and M-opsin genes (OPN1LW and OPN1MW) known to affect spectral tuning of the photoreceptors could influence spatio-chromatic contrast sensitivity. Chromatic contrast sensitivity functions were measured in 50 healthy individuals (20-71 years) employing a novel pseudo-isochromatic grating stimulus. The spatio-chromatic contrast sensitivity functions were found to be low pass for all subjects, independent of age and color vision. The results revealed a senescent decline in spatio-chromatic contrast sensitivity. There were considerable between-individual differences in sensitivity within each age decade for individuals 49 years old or younger, and age did not predict sensitivity for these age decades alone. Forty-six subjects (including a color deficient male and eight female carriers) were genotyped for L- and M-opsin genes. The Ser180Ala polymorphisms on the L-opsin gene were found to influence the subject's color discrimination and their sensitivity to spatio-chromatic patterns. The results expose the significant role of neural and genetic factors in the deterioration of visual function with increasing age. Copyright © 2015 Elsevier Ltd. All rights reserved.

Understanding the exposure-time effect on speckle contrast measurements for laser displays

NASA Astrophysics Data System (ADS)

Suzuki, Koji; Kubota, Shigeo

2018-02-01

To evaluate the influence of exposure time on speckle noise for laser displays, speckle contrast measurement method was developed observable at a human eye response time using a high-sensitivity camera which has a signal multiplying function. The nonlinearity of camera light sensitivity was calibrated to measure accurate speckle contrasts, and the measuring lower limit noise of speckle contrast was improved by applying spatial-frequency low pass filter to the captured images. Three commercially available laser displays were measured over a wide range of exposure times from tens of milliseconds to several seconds without adjusting the brightness of laser displays. The speckle contrast of raster-scanned mobile projector without any speckle-reduction device was nearly constant over various exposure times. On the contrary to this, in full-frame projection type laser displays equipped with a temporally-averaging speckle-reduction device, some of their speckle contrasts close to the lower limits noise were slightly increased at the shorter exposure time due to the noise. As a result, the exposure-time effect of speckle contrast could not be observed in our measurements, although it is more reasonable to think that the speckle contrasts of laser displays, which are equipped with the temporally-averaging speckle-reduction device, are dependent on the exposure time. This discrepancy may be attributed to the underestimation of temporal averaging factor. We expected that this method is useful for evaluating various laser displays and clarify the relationship between the speckle noise and the exposure time for a further verification of speckle reduction.

Effects of Genetic Drift and Gene Flow on the Selective Maintenance of Genetic Variation

PubMed Central

Star, Bastiaan; Spencer, Hamish G.

2013-01-01

Explanations for the genetic variation ubiquitous in natural populations are often classified by the population–genetic processes they emphasize: natural selection or mutation and genetic drift. Here we investigate models that incorporate all three processes in a spatially structured population, using what we call a construction approach, simulating finite populations under selection that are bombarded with a steady stream of novel mutations. As expected, the amount of genetic variation compared to previous models that ignored the stochastic effects of drift was reduced, especially for smaller populations and when spatial structure was most profound. By contrast, however, for higher levels of gene flow and larger population sizes, the amount of genetic variation found after many generations was greater than that in simulations without drift. This increased amount of genetic variation is due to the introduction of slightly deleterious alleles by genetic drift and this process is more efficient when migration load is higher. The incorporation of genetic drift also selects for fitness sets that exhibit allele-frequency equilibria with larger domains of attraction: they are “more stable.” Moreover, the finiteness of populations strongly influences levels of local adaptation, selection strength, and the proportion of allele-frequency vectors that can be distinguished from the neutral expectation. PMID:23457235

Preclinical evaluation of spatial frequency domain-enabled wide-field quantitative imaging for enhanced glioma resection

NASA Astrophysics Data System (ADS)

Sibai, Mira; Fisher, Carl; Veilleux, Israel; Elliott, Jonathan T.; Leblond, Frederic; Roberts, David W.; Wilson, Brian C.

2017-07-01

5-Aminolevelunic acid-induced protoporphyrin IX (PpIX) fluorescence-guided resection (FGR) enables maximum safe resection of glioma by providing real-time tumor contrast. However, the subjective visual assessment and the variable intrinsic optical attenuation of tissue limit this technique to reliably delineating only high-grade tumors that display strong fluorescence. We have previously shown, using a fiber-optic probe, that quantitative assessment using noninvasive point spectroscopic measurements of the absolute PpIX concentration in tissue further improves the accuracy of FGR, extending it to surgically curable low-grade glioma. More recently, we have shown that implementing spatial frequency domain imaging with a fluorescent-light transport model enables recovery of two-dimensional images of [PpIX], alleviating the need for time-consuming point sampling of the brain surface. We present first results of this technique modified for in vivo imaging on an RG2 rat brain tumor model. Despite the moderate errors in retrieving the absorption and reduced scattering coefficients in the subdiffusive regime of 14% and 19%, respectively, the recovered [PpIX] maps agree within 10% of the point [PpIX] values measured by the fiber-optic probe, validating its potential as an extension or an alternative to point sampling during glioma resection.

First-in-human pilot study of a spatial frequency domain oxygenation imaging system

NASA Astrophysics Data System (ADS)

Gioux, Sylvain; Mazhar, Amaan; Lee, Bernard T.; Lin, Samuel J.; Tobias, Adam M.; Cuccia, David J.; Stockdale, Alan; Oketokoun, Rafiou; Ashitate, Yoshitomo; Kelly, Edward; Weinmann, Maxwell; Durr, Nicholas J.; Moffitt, Lorissa A.; Durkin, Anthony J.; Tromberg, Bruce J.; Frangioni, John V.

2011-08-01

Oxygenation measurements are widely used in patient care. However, most clinically available instruments currently consist of contact probes that only provide global monitoring of the patient (e.g., pulse oximetry probes) or local monitoring of small areas (e.g., spectroscopy-based probes). Visualization of oxygenation over large areas of tissue, without a priori knowledge of the location of defects, has the potential to improve patient management in many surgical and critical care applications. In this study, we present a clinically compatible multispectral spatial frequency domain imaging (SFDI) system optimized for surgical oxygenation imaging. This system was used to image tissue oxygenation over a large area (16×12 cm) and was validated during preclinical studies by comparing results obtained with an FDA-approved clinical oxygenation probe. Skin flap, bowel, and liver vascular occlusion experiments were performed on Yorkshire pigs and demonstrated that over the course of the experiment, relative changes in oxygen saturation measured using SFDI had an accuracy within 10% of those made using the FDA-approved device. Finally, the new SFDI system was translated to the clinic in a first-in-human pilot study that imaged skin flap oxygenation during reconstructive breast surgery. Overall, this study lays the foundation for clinical translation of endogenous contrast imaging using SFDI.

Defective chromatic and achromatic visual pathways in developmental dyslexia: Cues for an integrated intervention programme.

PubMed

Bonfiglio, Luca; Bocci, Tommaso; Minichilli, Fabrizio; Crecchi, Alessandra; Barloscio, Davide; Spina, Donata Maria; Rossi, Bruno; Sartucci, Ferdinando

2017-01-01

As well as obtaining confirmation of the magnocellular system involvement in developmental dyslexia (DD); the aim was primarily to search for a possible involvement of the parvocellular system; and, furthermore, to complete the assessment of the visual chromatic axis by also analysing the koniocellular system. Visual evoked potentials (VEPs) in response to achromatic stimuli with low luminance contrast and low spatial frequency, and isoluminant red/green and blue/yellow stimuli with high spatial frequency were recorded in 10 dyslexic children and 10 age- and sex-matched, healthy subjects. Dyslexic children showed delayed VEPs to both achromatic stimuli (magnocellular-dorsal stream) and isoluminant red/green and blue/yellow stimuli (parvocellular-ventral and koniocellular streams). To our knowledge, this is the first time that a dysfunction of colour vision has been brought to light in an objective way (i.e., by means of electrophysiological methods) in children with DD. These results give rise to speculation concerning the need for a putative approach for promoting both learning how to read and/or improving existing reading skills of children with or at risk of DD. The working hypothesis would be to combine two integrated interventions in a single programme aimed at fostering the function of both the magnocellular and the parvocellular streams.

Insensitivity to Fearful Emotion for Early ERP Components in High Autistic Tendency Is Associated with Lower Magnocellular Efficiency.

PubMed

Burt, Adelaide; Hugrass, Laila; Frith-Belvedere, Tash; Crewther, David

2017-01-01

Low spatial frequency (LSF) visual information is extracted rapidly from fearful faces, suggesting magnocellular involvement. Autistic phenotypes demonstrate altered magnocellular processing, which we propose contributes to a decreased P100 evoked response to LSF fearful faces. Here, we investigated whether rapid processing of fearful facial expressions differs for groups of neurotypical adults with low and high scores on the Autistic Spectrum Quotient (AQ). We created hybrid face stimuli with low and high spatial frequency filtered, fearful, and neutral expressions. Fearful faces produced higher amplitude P100 responses than neutral faces in the low AQ group, particularly when the hybrid face contained a LSF fearful expression. By contrast, there was no effect of fearful expression on P100 amplitude in the high AQ group. Consistent with evidence linking magnocellular differences with autistic personality traits, our non-linear VEP results showed that the high AQ group had higher amplitude K2.1 responses than the low AQ group, which is indicative of less efficient magnocellular recovery. Our results suggest that magnocellular LSF processing of a human face may be the initial visual cue used to rapidly and automatically detect fear, but that this cue functions atypically in those with high autistic tendency.

Effect of Grade I and II Intraventricular Hemorrhage on Visuocortical Function in Very Low Birth Weight Infants

PubMed Central

Madan, Ashima; Norcia, Anthony M.; Hou, Chuan; Pettet, Mark W.; Good, William V.

2015-01-01

The neurological outcome for infants with Grade I/II intraventricular hemorrhage (IVH) is debated. The aim of this study was to determine whether very low birth weight infants (VLBW, < 1500 g) with Grade I /II (IVH) have altered visuocortical activity compared with infants with no IVH. We assessed the quantitative swept parameter Visual Evoked Potential (sVEP) responses evoked by three different visual stimuli. Data from 52 VLBW infants were compared with data from 13 infants with Grade I or II IVH, enrolled at 5 – 7 months corrected age. Acuity thresholds and suprathreshold response amplitudes were compared. Grating Acuity (GA), Contrast Sensitivity (CS) and Vernier Acuity (VA) were each worse in the Grade I/ II IVH compared with the no IVH groups (8.24 cpd in IVH group vs 13.07 cpd in no IVH group for GA; 1.44% vs 1.18% for CS and 1.55 arcmin vs 0.58 arcmin for VA). The slopes of the response amplitude for CS and VA were significantly lower in IVH infants. The spatial frequency tuning function was shifted downward on the spatial frequency axis, without a change in slope. These results indicate that Grade I/II IVH are associated with deleterious effects on cortical vision development and function. PMID:22371027

First-in-human pilot study of a spatial frequency domain oxygenation imaging system

PubMed Central

Gioux, Sylvain; Mazhar, Amaan; Lee, Bernard T.; Lin, Samuel J.; Tobias, Adam M.; Cuccia, David J.; Stockdale, Alan; Oketokoun, Rafiou; Ashitate, Yoshitomo; Kelly, Edward; Weinmann, Maxwell; Durr, Nicholas J.; Moffitt, Lorissa A.; Durkin, Anthony J.; Tromberg, Bruce J.; Frangioni, John V.

2011-01-01

Oxygenation measurements are widely used in patient care. However, most clinically available instruments currently consist of contact probes that only provide global monitoring of the patient (e.g., pulse oximetry probes) or local monitoring of small areas (e.g., spectroscopy-based probes). Visualization of oxygenation over large areas of tissue, without a priori knowledge of the location of defects, has the potential to improve patient management in many surgical and critical care applications. In this study, we present a clinically compatible multispectral spatial frequency domain imaging (SFDI) system optimized for surgical oxygenation imaging. This system was used to image tissue oxygenation over a large area (16×12 cm) and was validated during preclinical studies by comparing results obtained with an FDA-approved clinical oxygenation probe. Skin flap, bowel, and liver vascular occlusion experiments were performed on Yorkshire pigs and demonstrated that over the course of the experiment, relative changes in oxygen saturation measured using SFDI had an accuracy within 10% of those made using the FDA-approved device. Finally, the new SFDI system was translated to the clinic in a first-in-human pilot study that imaged skin flap oxygenation during reconstructive breast surgery. Overall, this study lays the foundation for clinical translation of endogenous contrast imaging using SFDI. PMID:21895327

Preservation of visual cortical function following retinal pigment epithelium transplantation in the RCS rat using optical imaging techniques.

PubMed

Gias, Carlos; Jones, Myles; Keegan, David; Adamson, Peter; Greenwood, John; Lund, Ray; Martindale, John; Johnston, David; Berwick, Jason; Mayhew, John; Coffey, Peter

2007-04-01

The aim of this study was to determine the extent of cortical functional preservation following retinal pigment epithelium (RPE) transplantation in the Royal College of Surgeons (RCS) rat using single-wavelength optical imaging and spectroscopy. The cortical responses to visual stimulation in transplanted rats at 6 months post-transplantation were compared with those from age-matched untreated dystrophic and non-dystrophic rats. Our results show that cortical responses were evoked in non-dystrophic rats to both luminance changes and pattern stimulation, whereas no response was found in untreated dystrophic animals to any of the visual stimuli tested. In contrast, a cortical response was elicited in most of the transplanted rats to luminance changes and in many of those a response was also evoked to pattern stimulation. Although the transplanted rats did not respond to high spatial frequency information we found evidence of preservation in the cortical processing of luminance changes and low spatial frequency stimulation. Anatomical sections of transplanted rat retinas confirmed the capacity of RPE transplantation to rescue photoreceptors. Good correlation was found between photoreceptor survival and the extent of cortical function preservation determined with optical imaging techniques. This study determined the efficacy of RPE transplantation to preserve visual cortical processing and established optical imaging as a powerful technique for its assessment.

Neutral processes contribute to patterns of spatial variation for flower colour in the Mediterranean Iris lutescens (Iridaceae)

PubMed Central

Wang, Hui; Talavera, María; Min, Ya; Flaven, Elodie; Imbert, Eric

2016-01-01

Background and Aims Flower colour polymorphism in plants has been used as a classic model for understanding the importance of neutral processes vs. natural selection in population differentiation. However, current explanations for the maintenance of flower colour polymorphism mainly rely on balancing selection, while neutral processes have seldom been championed. Iris lutescens (Iridaceae) is a widespread species in the northern Mediterranean basin, which shows a stable and striking purple–yellow flower colour polymorphism. To evaluate the roles of neutral processes in the spatial variation for flower colour in this species, patterns of neutral genetic variation across its distribution range were quantified, and phenotypic differentiation was compared with neutral genetic differentiation. Methods Genetic diversity levels and population genetic structure were investigated through the genotyping of a collection of 1120 individuals in 41 populations ranging from Spain to France, using a set of eight newly developed microsatellite markers. In addition, phenotypic differentiation for flower colour was also quantified by counting colour morph frequency in each population, and measuring the reflectance spectra of sampled individuals. Key Results Populations in Spain present a sharp colour transition from solely purple to solely yellow. The results provide evidence that genetic drift through limited gene flow is important in the evolution of monomorphic populations. In contrast, most populations in France are polymorphic with both phenotypes, and the colour frequencies vary geographically without any spatial gradients observed. A pattern of isolation by distance is detected in France, and gene flow between adjacent populations seems to be an important factor maintaining populations polymorphic. Conclusions Overall, neutral processes contribute to patterns of spatial variation for flower colour in I. lutescens, but it cannot be excluded that natural selection is also operating. An interaction between neutral processes and natural selection is suggested to explain the spatial variation for flower colour in I. lutescens. PMID:27084922

Solutions to the cocktail party problem in insects: selective filters, spatial release from masking and gain control in tropical crickets.

PubMed

Schmidt, Arne K D; Römer, Heiner

2011-01-01

Insects often communicate by sound in mixed species choruses; like humans and many vertebrates in crowded social environments they thus have to solve cocktail-party-like problems in order to ensure successful communication with conspecifics. This is even more a problem in species-rich environments like tropical rainforests, where background noise levels of up to 60 dB SPL have been measured. Using neurophysiological methods we investigated the effect of natural background noise (masker) on signal detection thresholds in two tropical cricket species Paroecanthus podagrosus and Diatrypa sp., both in the laboratory and outdoors. We identified three 'bottom-up' mechanisms which contribute to an excellent neuronal representation of conspecific signals despite the masking background. First, the sharply tuned frequency selectivity of the receiver reduces the amount of masking energy around the species-specific calling song frequency. Laboratory experiments yielded an average signal-to-noise ratio (SNR) of -8 dB, when masker and signal were broadcast from the same side. Secondly, displacing the masker by 180° from the signal improved SNRs by further 6 to 9 dB, a phenomenon known as spatial release from masking. Surprisingly, experiments carried out directly in the nocturnal rainforest yielded SNRs of about -23 dB compared with those in the laboratory with the same masker, where SNRs reached only -14.5 and -16 dB in both species. Finally, a neuronal gain control mechanism enhances the contrast between the responses to signals and the masker, by inhibition of neuronal activity in interstimulus intervals. Thus, conventional speaker playbacks in the lab apparently do not properly reconstruct the masking noise situation in a spatially realistic manner, since under real world conditions multiple sound sources are spatially distributed in space. Our results also indicate that without knowledge of the receiver properties and the spatial release mechanisms the detrimental effect of noise may be strongly overestimated.

Neutral processes contribute to patterns of spatial variation for flower colour in the Mediterranean Iris lutescens (Iridaceae).

PubMed

Wang, Hui; Talavera, María; Min, Ya; Flaven, Elodie; Imbert, Eric

2016-05-01

Flower colour polymorphism in plants has been used as a classic model for understanding the importance of neutral processes vs. natural selection in population differentiation. However, current explanations for the maintenance of flower colour polymorphism mainly rely on balancing selection, while neutral processes have seldom been championed. Iris lutescens (Iridaceae) is a widespread species in the northern Mediterranean basin, which shows a stable and striking purple-yellow flower colour polymorphism. To evaluate the roles of neutral processes in the spatial variation for flower colour in this species, patterns of neutral genetic variation across its distribution range were quantified, and phenotypic differentiation was compared with neutral genetic differentiation. Genetic diversity levels and population genetic structure were investigated through the genotyping of a collection of 1120 individuals in 41 populations ranging from Spain to France, using a set of eight newly developed microsatellite markers. In addition, phenotypic differentiation for flower colour was also quantified by counting colour morph frequency in each population, and measuring the reflectance spectra of sampled individuals. Populations in Spain present a sharp colour transition from solely purple to solely yellow. The results provide evidence that genetic drift through limited gene flow is important in the evolution of monomorphic populations. In contrast, most populations in France are polymorphic with both phenotypes, and the colour frequencies vary geographically without any spatial gradients observed. A pattern of isolation by distance is detected in France, and gene flow between adjacent populations seems to be an important factor maintaining populations polymorphic. Overall, neutral processes contribute to patterns of spatial variation for flower colour in I. lutescens, but it cannot be excluded that natural selection is also operating. An interaction between neutral processes and natural selection is suggested to explain the spatial variation for flower colour in I. lutescens. © The Author 2016. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oup.com.

Solutions to the Cocktail Party Problem in Insects: Selective Filters, Spatial Release from Masking and Gain Control in Tropical Crickets

PubMed Central

Schmidt, Arne K. D.; Römer, Heiner

2011-01-01

Background Insects often communicate by sound in mixed species choruses; like humans and many vertebrates in crowded social environments they thus have to solve cocktail-party-like problems in order to ensure successful communication with conspecifics. This is even more a problem in species-rich environments like tropical rainforests, where background noise levels of up to 60 dB SPL have been measured. Principal Findings Using neurophysiological methods we investigated the effect of natural background noise (masker) on signal detection thresholds in two tropical cricket species Paroecanthus podagrosus and Diatrypa sp., both in the laboratory and outdoors. We identified three ‘bottom-up’ mechanisms which contribute to an excellent neuronal representation of conspecific signals despite the masking background. First, the sharply tuned frequency selectivity of the receiver reduces the amount of masking energy around the species-specific calling song frequency. Laboratory experiments yielded an average signal-to-noise ratio (SNR) of −8 dB, when masker and signal were broadcast from the same side. Secondly, displacing the masker by 180° from the signal improved SNRs by further 6 to 9 dB, a phenomenon known as spatial release from masking. Surprisingly, experiments carried out directly in the nocturnal rainforest yielded SNRs of about −23 dB compared with those in the laboratory with the same masker, where SNRs reached only −14.5 and −16 dB in both species. Finally, a neuronal gain control mechanism enhances the contrast between the responses to signals and the masker, by inhibition of neuronal activity in interstimulus intervals. Conclusions Thus, conventional speaker playbacks in the lab apparently do not properly reconstruct the masking noise situation in a spatially realistic manner, since under real world conditions multiple sound sources are spatially distributed in space. Our results also indicate that without knowledge of the receiver properties and the spatial release mechanisms the detrimental effect of noise may be strongly overestimated. PMID:22163041

Dynamics of phytoplankton productivity and exopolysaccharides (EPS and TEP) pools in the Seine Estuary (France, Normandy) over tidal cycles and over two contrasting seasons.

PubMed

Morelle, Jérôme; Schapira, Mathilde; Claquin, Pascal

2017-10-01

Exopolysaccharides (EPS) play an important role in the carbon flux and may be directly linked to phytoplankton and microphytobenthos production, most notably in estuarine systems. However the temporal and spatial dynamics of estuarine EPS are still not well understood, nor how primary productivity triggers this variability at these different scales. The aim of this study was to investigate the primary productivity of phytoplankton and EPS dynamics in the Seine estuary over a tidal cycle in three different haline zones over two contrasted seasons. The other objectives was to investigate the origin of pools of soluble carbohydrates (S-EPS) and transparent exopolymeric particles (TEP) in phytoplankton, microphytobenthos or other compartments. High frequency measurements of productivity were made in winter and summer 2015. Physical and chemical parameters, biomass and EPS were measured at hourly intervals in sub-surface waters and just above the water sediment-interface. Our results confirmed that high frequency measurements improve the accuracy of primary productivity estimations and associated carbon fluxes in estuaries. The photosynthetic parameters were shown to be strongly controlled by salinity and by the concentrations of suspended particle matter at the smallest temporal and at spatial scales. At these scales, our results showed an inverse relationship between EPS concentrations and biomass and productivity, and a positive relationship with sediment resuspension. Additionally, the distribution of EPS appears to be linked to hydrodynamics with the tide at daily scale and with the winter at seasonal scale. At spatial scale, the maximum turbidity zone played an important role in the distribution of TEP. Our results suggest that, in the Seine estuary, between 9% and 33% of the S-EPS pool in the water column can be attributed to phytoplankton excretion, while only 0.4%-1.6% (up to 6.14% in exceptional conditions) originates from the microphytobenthos compartments. Most EPS was attributed to remobilization of detrital carbon pools in the maximum turbidity zone and in the sediment or allochthonous origin. Copyright © 2017 Elsevier Ltd. All rights reserved.

Role of ictal baseline shifts and ictal high-frequency oscillations in stereo-electroencephalography analysis of mesial temporal lobe seizures.

PubMed

Wu, Shasha; Kunhi Veedu, Hari Prasad; Lhatoo, Samden D; Koubeissi, Mohamad Z; Miller, Jonathan P; Lüders, Hans O

2014-05-01

To assess the role of ictal baseline shifts (IBS) and ictal high-frequency oscillations (iHFOs) in intracranial electroencephalography (EEG) presurgical evaluation by analysis of the spatial and temporal relationship of IBS, iHFOs with ictal conventional stereo-electroencephalography (icEEG) in mesial temporal lobe seizures (MTLS). We studied 15 adult patients with medically refractory MTLS who underwent monitoring with depth electrodes. Seventy-five ictal EEG recordings at 1,000 Hz sampling rate were studied. Visual comparison of icEEG, IBS, and iHFOs were performed using Nihon-Kohden Neurofax systems (acquisition range 0.016-300 Hz). Each recorded ictal EEG was analyzed with settings appropriate for displaying icEEG, IBS, and iHFOs. IBS and iHFOs were observed in all patients and in 91% and 81% of intracranial seizures, respectively. IBS occurred before (22%), at (57%), or after (21%) icEEG onset. In contrast, iHFOs occurred at (30%) or after (70%) icEEG onset. The onset of iHFOs was 11.5 s later than IBS onset (p < 0.0001). All of the earliest onset of IBS and 70% of the onset of iHFOs overlapped with the ictal onset zone (IOZ). Compared with iHFOs, interictal HFOs (itHFOs) were less correlated with IOZ. In contrast to icEEG, IBS and iHFOs had smaller spatial distributions in 70% and 100% of the seizures, respectively. An IBS dipole was observed in 66% of the seizures. Eighty-seven percent of the dipoles had a negative pole at the anterior/medial part of amygdala/hippocampus complex (A-H complex) and a positive pole at the posterior/lateral part of the A-H complex. The results suggest that evaluation of IBS and iHFOs, in addition to routine icEEG, helps in more accurately defining the IOZ. This study also shows that the onset and the spatial distribution of icEEG, IBS, and iHFOs do not overlap, suggesting that they reflect different cellular or network dynamics. Wiley Periodicals, Inc. © 2014 International League Against Epilepsy.

Seasonal variations of the backscattering coefficient measured by radar altimeters over the Antarctic Ice Sheet

NASA Astrophysics Data System (ADS)

Ibrahime Adodo, Fifi; Remy, Frédérique; Picard, Ghislain

2018-05-01

Spaceborne radar altimeters are a valuable tool for observing the Antarctic Ice Sheet. The radar wave interaction with the snow provides information on both the surface and the subsurface of the snowpack due to its dependence on the snow properties. However, the penetration of the radar wave within the snowpack also induces a negative bias on the estimated surface elevation. Empirical corrections of this space- and time-varying bias are usually based on the backscattering coefficient variability. We investigate the spatial and seasonal variations of the backscattering coefficient at the S (3.2 GHz ˜ 9.4 cm), Ku (13.6 GHz ˜ 2.3 cm) and Ka (37 GHz ˜ 0.8 cm) bands. We identified that the backscattering coefficient at Ku band reaches a maximum in winter in part of the continent (Region 1) and in the summer in the remaining (Region 2), while the evolution at other frequencies is relatively uniform over the whole continent. To explain this contrasting behavior between frequencies and between regions, we studied the sensitivity of the backscattering coefficient at three frequencies to several parameters (surface snow density, snow temperature and snow grain size) using an electromagnetic model. The results show that the seasonal cycle of the backscattering coefficient at Ka frequency is dominated by the volume echo and is mainly driven by snow temperature evolution everywhere. In contrast, at S band, the cycle is dominated by the surface echo. At Ku band, the seasonal cycle is dominated by the volume echo in Region 1 and by the surface echo in Region 2. This investigation provides new information on the seasonal dynamics of the Antarctic Ice Sheet surface and provides new clues to build more accurate corrections of the radar altimeter surface elevation signal in the future.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Matsui, H.; Koike, Makoto; Takegawa, Nobuyuki

The new particle formation (NPF)-explicit version of the WRF-chem model, which we developed recently, can calculate the growth and sink of nucleated clusters explicitly with 20 aerosol size bins from 1 nm to 10 μm. In this study, the model is used to understand spatial and temporal variations of the frequency of NPF events and the concentrations of aerosols (condensation nuclei, CN) and cloud condensation nuclei (CCN) within the boundary layer in East Asia in spring 2009. Model simulations show distinct north-south contrast in the frequency and mechanism of NPF in East Asia. NPF mostly occurred over limited periods andmore » regions between 30° and 45°N, such as northeast China, Korea, and Japan, including regions around active volcanoes (Miyakejima and Sakurajima). At these latitudes, NPF was considerably suppressed by high concentrations of preexisting particles under stagnant air conditions associated with high-pressure systems, while nucleation occurred more extensively on most days during the simulation period. Conversely, neither nucleation nor NPF occurred frequently south of 30°N because of lower SO2 emissions and H2SO4 concentrations. The period-averaged NPF frequency was 3 times higher at latitudes of 30° - 45°N than at latitudes of 20° - 30°N. The north-south contrast of NPF frequency is validated by surface measurements in outflow regions in East Asia. The period- and domain-averaged contribution of secondary particles is estimated to be 44% for CN (> 10 nm) and 26% for CCN at a supersaturation of 1.0% in our simulation, though the contribution is highly sensitive to the magnitudes and size distributions of primary aerosol emissions and the coefficients in the nucleation parameterizations.« less

Image enhancement by non-linear extrapolation in frequency space

NASA Technical Reports Server (NTRS)

Anderson, Charles H. (Inventor); Greenspan, Hayit K. (Inventor)

1998-01-01

An input image is enhanced to include spatial frequency components having frequencies higher than those in an input image. To this end, an edge map is generated from the input image using a high band pass filtering technique. An enhancing map is subsequently generated from the edge map, with the enhanced map having spatial frequencies exceeding an initial maximum spatial frequency of the input image. The enhanced map is generated by applying a non-linear operator to the edge map in a manner which preserves the phase transitions of the edges of the input image. The enhanced map is added to the input image to achieve a resulting image having spatial frequencies greater than those in the input image. Simplicity of computations and ease of implementation allow for image sharpening after enlargement and for real-time applications such as videophones, advanced definition television, zooming, and restoration of old motion pictures.

Effect of scanning velocity on femtosecond laser-induced periodic surface structures on HgCdTe crystal

NASA Astrophysics Data System (ADS)

Gu, Hongan; Dai, Ye; Wang, Haodong; Yan, Xiaona; Ma, Guohong

2017-12-01

In this paper, a femtosecond laser line-scanning irradiation was used to induce the periodic surface microstructure on HgCdTe crystal. Low spatial frequency laser induced periodic surface structures of 650-770 nm and high spatial frequency laser induced periodic surface structures of 152-246 nm were respectively found with different scanning speeds. The evolution process from low spatial frequency laser induced periodic surface structures to high spatial frequency laser induced periodic surface structures is characterized by scanning electron microscope. Their spatial periods deduced by using a two-dimensional Fourier transformation partly agree with the predictions of the Sipe-Drude theory. Confocal micro-Raman spectral show that the atomic arrangement of induced low spatial frequency laser-induced structures are basically consistent with the crystal in the central area of laser-scanning line, however a new peak at 164 cm-1 for the CdTe-like mode becomes evident due to the Hg vaporization when strong laser ablation happens. The obtained surface periodic ripples may have applications in fabricating advanced infrared detector.

Uncomfortable images in art and nature.

PubMed

Fernandez, Dominic; Wilkins, Arnold J

2008-01-01

The ratings of discomfort from a wide variety of images can be predicted from the energy at different spatial scales in the image, as measured by the Fourier amplitude spectrum of the luminance. Whereas comfortable images show the regression of Fourier amplitude against spatial frequency common in natural scenes, uncomfortable images show a regression with disproportionately greater amplitude at spatial frequencies within two octaves of 3 cycles deg(-1). In six studies, the amplitude in this spatial frequency range relative to that elsewhere in the spectrum explains variance in judgments of discomfort from art, from images constructed from filtered noise, and from art in which the phase or amplitude spectra have been altered. Striped patterns with spatial frequency within the above range are known to be uncomfortable and capable of provoking headaches and seizures in susceptible persons. The present findings show for the first time that, even in more complex images, the energy in this spatial-frequency range is associated with aversion. We propose a simple measurement that can predict aversion to those works of art that have reached the national media because of negative public reaction.

Uncomfortable images in art and nature

PubMed Central

Fernandez, Dominic; Wilkins, Arnold J.

2008-01-01

We find that the ratings of discomfort from a wide variety of images can be predicted from the energy at different spatial scales in the image, as measured by the Fourier amplitude spectrum of the luminance. Whereas comfortable images show the regression of Fourier amplitude against spatial frequency common in natural scenes, uncomfortable images show a regression with disproportionately greater amplitude at spatial frequencies within two octaves of 3 cycles per degree. In six studies, the amplitude at this spatial frequency relative to that 3 octaves below explains variance in judgments of discomfort from art, from images constructed from filtered noise and from art in which the phase or amplitude spectra have been altered. Striped patterns with spatial frequency within the above range are known to be uncomfortable and capable of provoking headaches and seizures in susceptible persons. The present findings show for the first time that even in more complex images the energy in this spatial frequency range is associated with aversion. We propose a simple measurement that can predict aversion to those works of art that have reached the national media because of negative public reaction. PMID:18773732

qCSF in Clinical Application: Efficient Characterization and Classification of Contrast Sensitivity Functions in Amblyopia

PubMed Central

Hou, Fang; Huang, Chang-Bing; Lesmes, Luis; Feng, Li-Xia; Tao, Liming; Zhou, Yi-Feng; Lu, Zhong-Lin

2010-01-01

Purpose. The qCSF method is a novel procedure for rapid measurement of spatial contrast sensitivity functions (CSFs). It combines Bayesian adaptive inference with a trial-to-trial information gain strategy, to directly estimate four parameters defining the observer's CSF. In the present study, the suitability of the qCSF method for clinical application was examined. Methods. The qCSF method was applied to rapidly assess spatial CSFs in 10 normal and 8 amblyopic participants. The qCSF was evaluated for accuracy, precision, test–retest reliability, suitability of CSF model assumptions, and accuracy of amblyopia screening. Results. qCSF estimates obtained with as few as 50 trials matched those obtained with 300 Ψ trials. The precision of qCSF estimates obtained with 120 and 130 trials, in normal subjects and amblyopes, matched the precision of 300 Ψ trials. For both groups and both methods, test–retest sensitivity estimates were well matched (all R > 0.94). The qCSF model assumptions were valid for 8 of 10 normal participants and all amblyopic participants. Measures of the area under log CSF (AULCSF) and the cutoff spatial frequency (cutSF) were lower in the amblyopia group; these differences were captured within 50 qCSF trials. Amblyopia was detected at an approximately 80% correct rate in 50 trials, when a logistic regression model was used with AULCSF and cutSF as predictors. Conclusions. The qCSF method is sufficiently rapid, accurate, and precise in measuring CSFs in normal and amblyopic persons. It has great potential for clinical practice. PMID:20484592

Local introduction and heterogeneous spatial spread of dengue-suppressing Wolbachia through an urban population of Aedes aegypti

PubMed Central

Schmidt, Tom L.; Barton, Nicholas H.; Rašić, Gordana; Turley, Andrew P.; Montgomery, Brian L.; Iturbe-Ormaetxe, Inaki; Cook, Peter E.; Ryan, Peter A.; Ritchie, Scott A.; Hoffmann, Ary A.; O’Neill, Scott L.

2017-01-01

Dengue-suppressing Wolbachia strains are promising tools for arbovirus control, particularly as they have the potential to self-spread following local introductions. To test this, we followed the frequency of the transinfected Wolbachia strain wMel through Ae. aegypti in Cairns, Australia, following releases at 3 nonisolated locations within the city in early 2013. Spatial spread was analysed graphically using interpolation and by fitting a statistical model describing the position and width of the wave. For the larger 2 of the 3 releases (covering 0.97 km2 and 0.52 km2), we observed slow but steady spatial spread, at about 100–200 m per year, roughly consistent with theoretical predictions. In contrast, the smallest release (0.11 km2) produced erratic temporal and spatial dynamics, with little evidence of spread after 2 years. This is consistent with the prediction concerning fitness-decreasing Wolbachia transinfections that a minimum release area is needed to achieve stable local establishment and spread in continuous habitats. Our graphical and likelihood analyses produced broadly consistent estimates of wave speed and wave width. Spread at all sites was spatially heterogeneous, suggesting that environmental heterogeneity will affect large-scale Wolbachia transformations of urban mosquito populations. The persistence and spread of Wolbachia in release areas meeting minimum area requirements indicates the promise of successful large-scale population transformation. PMID:28557993

Local introduction and heterogeneous spatial spread of dengue-suppressing Wolbachia through an urban population of Aedes aegypti.

PubMed

Schmidt, Tom L; Barton, Nicholas H; Rašić, Gordana; Turley, Andrew P; Montgomery, Brian L; Iturbe-Ormaetxe, Inaki; Cook, Peter E; Ryan, Peter A; Ritchie, Scott A; Hoffmann, Ary A; O'Neill, Scott L; Turelli, Michael

2017-05-01

Dengue-suppressing Wolbachia strains are promising tools for arbovirus control, particularly as they have the potential to self-spread following local introductions. To test this, we followed the frequency of the transinfected Wolbachia strain wMel through Ae. aegypti in Cairns, Australia, following releases at 3 nonisolated locations within the city in early 2013. Spatial spread was analysed graphically using interpolation and by fitting a statistical model describing the position and width of the wave. For the larger 2 of the 3 releases (covering 0.97 km2 and 0.52 km2), we observed slow but steady spatial spread, at about 100-200 m per year, roughly consistent with theoretical predictions. In contrast, the smallest release (0.11 km2) produced erratic temporal and spatial dynamics, with little evidence of spread after 2 years. This is consistent with the prediction concerning fitness-decreasing Wolbachia transinfections that a minimum release area is needed to achieve stable local establishment and spread in continuous habitats. Our graphical and likelihood analyses produced broadly consistent estimates of wave speed and wave width. Spread at all sites was spatially heterogeneous, suggesting that environmental heterogeneity will affect large-scale Wolbachia transformations of urban mosquito populations. The persistence and spread of Wolbachia in release areas meeting minimum area requirements indicates the promise of successful large-scale population transformation.

Predicting spatial patterns of plant recruitment using animal-displacement kernels.

PubMed

Santamaría, Luis; Rodríguez-Pérez, Javier; Larrinaga, Asier R; Pias, Beatriz

2007-10-10

For plants dispersed by frugivores, spatial patterns of recruitment are primarily influenced by the spatial arrangement and characteristics of parent plants, the digestive characteristics, feeding behaviour and movement patterns of animal dispersers, and the structure of the habitat matrix. We used an individual-based, spatially-explicit framework to characterize seed dispersal and seedling fate in an endangered, insular plant-disperser system: the endemic shrub Daphne rodriguezii and its exclusive disperser, the endemic lizard Podarcis lilfordi. Plant recruitment kernels were chiefly determined by the disperser's patterns of space utilization (i.e. the lizard's displacement kernels), the position of the various plant individuals in relation to them, and habitat structure (vegetation cover vs. bare soil). In contrast to our expectations, seed gut-passage rate and its effects on germination, and lizard speed-of-movement, habitat choice and activity rhythm were of minor importance. Predicted plant recruitment kernels were strongly anisotropic and fine-grained, preventing their description using one-dimensional, frequency-distance curves. We found a general trade-off between recruitment probability and dispersal distance; however, optimal recruitment sites were not necessarily associated to sites of maximal adult-plant density. Conservation efforts aimed at enhancing the regeneration of endangered plant-disperser systems may gain in efficacy by manipulating the spatial distribution of dispersers (e.g. through the creation of refuges and feeding sites) to create areas favourable to plant recruitment.

Grid cell spatial tuning reduced following systemic muscarinic receptor blockade

PubMed Central

Newman, Ehren L.; Climer, Jason R.; Hasselmo, Michael E.

2014-01-01

Grid cells of the medial entorhinal cortex exhibit a periodic and stable pattern of spatial tuning that may reflect the output of a path integration system. This grid pattern has been hypothesized to serve as a spatial coordinate system for navigation and memory function. The mechanisms underlying the generation of this characteristic tuning pattern remain poorly understood. Systemic administration of the muscarinic antagonist scopolamine flattens the typically positive correlation between running speed and entorhinal theta frequency in rats. The loss of this neural correlate of velocity, an important signal for the calculation of path integration, raises the question of what influence scopolamine has on the grid cell tuning as a read out of the path integration system. To test this, the spatial tuning properties of grid cells were compared before and after systemic administration of scopolamine as rats completed laps on a circle track for food rewards. The results show that the spatial tuning of the grid cells was reduced following scopolamine administration. The tuning of head direction cells, in contrast, was not reduced by scopolamine. This is the first report to demonstrate a link between cholinergic function and grid cell tuning. This work suggests that the loss of tuning in the grid cell network may underlie the navigational disorientation observed in Alzheimer's patients and elderly individuals with reduced cholinergic tone. PMID:24493379

The impact of variation in low-frequency interaural cross correlation on auditory spatial imagery in stereophonic loudspeaker reproduction

NASA Astrophysics Data System (ADS)

Martens, William

2005-04-01

Several attributes of auditory spatial imagery associated with stereophonic sound reproduction are strongly modulated by variation in interaural cross correlation (IACC) within low frequency bands. Nonetheless, a standard practice in bass management for two-channel and multichannel loudspeaker reproduction is to mix low-frequency musical content to a single channel for reproduction via a single driver (e.g., a subwoofer). This paper reviews the results of psychoacoustic studies which support the conclusion that reproduction via multiple drivers of decorrelated low-frequency signals significantly affects such important spatial attributes as auditory source width (ASW), auditory source distance (ASD), and listener envelopment (LEV). A variety of methods have been employed in these tests, including forced choice discrimination and identification, and direct ratings of both global dissimilarity and distinct attributes. Contrary to assumptions that underlie industrial standards established in 1994 by ITU-R. Recommendation BS.775-1, these findings imply that substantial stereophonic spatial information exists within audio signals at frequencies below the 80 to 120 Hz range of prescribed subwoofer cutoff frequencies, and that loudspeaker reproduction of decorrelated signals at frequencies as low as 50 Hz can have an impact upon auditory spatial imagery. [Work supported by VRQ.