Showing papers on "Contrast (vision) published in 2016"

Visual dysfunction in Parkinson's disease.

Abstract: Patients with Parkinson's disease have a number of specific visual disturbances. These include changes in colour vision and contrast sensitivity and difficulties with complex visual tasks such as mental rotation and emotion recognition. We review changes in visual function at each stage of visual processing from retinal deficits, including contrast sensitivity and colour vision deficits to higher cortical processing impairments such as object and motion processing and neglect. We consider changes in visual function in patients with common Parkinson's disease-associated genetic mutations including GBA and LRRK2 . We discuss the association between visual deficits and clinical features of Parkinson's disease such as rapid eye movement sleep behavioural disorder and the postural instability and gait disorder phenotype. We review the link between abnormal visual function and visual hallucinations, considering current models for mechanisms of visual hallucinations. Finally, we discuss the role of visuo-perceptual testing as a biomarker of disease and predictor of dementia in Parkinson's disease.

Optical and neural anisotropy in peripheral vision.

Abstract: Optical blur in the peripheral retina is known to be highly anisotropic due to nonrotationally symmetric wavefront aberrations such as astigmatism and coma. At the neural level, the visual system exhibits anisotropies in orientation sensitivity across the visual field. In the fovea, the visual system shows higher sensitivity for cardinal over diagonal orientations, which is referred to as the oblique effect. However, in the peripheral retina, the neural visual system becomes more sensitive to radially-oriented signals, a phenomenon known as the meridional effect. Here, we examined the relative contributions of optics and neural processing to the meridional effect in 10 participants at 0°, 10°, and 20° in the temporal retina. Optical anisotropy was quantified by measuring the eye's habitual wavefront aberrations. Alternatively, neural anisotropy was evaluated by measuring contrast sensitivity (at 2 and 4 cyc/deg) while correcting the eye's aberrations with an adaptive optics vision simulator, thus bypassing any optical factors. As eccentricity increased, optical and neural anisotropy increased in magnitude. The average ratio of horizontal to vertical optical MTF (at 2 and 4 cyc/deg) at 0°, 10°, and 20° was 0.96 ± 0.14, 1.41 ± 0.54 and 2.15 ± 1.38, respectively. Similarly, the average ratio of horizontal to vertical contrast sensitivity with full optical correction at 0°, 10°, and 20° was 0.99 ± 0.15, 1.28 ± 0.28 and 1.75 ± 0.80, respectively. These results indicate that the neural system's orientation sensitivity coincides with habitual blur orientation. These findings support the neural origin of the meridional effect and raise important questions regarding the role of peripheral anisotropic optical quality in developing the meridional effect and emmetropization.

The effect of transcranial direct current stimulation on contrast sensitivity and visual evoked potential amplitude in adults with amblyopia.

Abstract: Amblyopia is a neurodevelopmental disorder of vision that occurs when the visual cortex receives decorrelated inputs from the two eyes during an early critical period of development. Amblyopic eyes are subject to suppression from the fellow eye, generate weaker visual evoked potentials (VEPs) than fellow eyes and have multiple visual deficits including impairments in visual acuity and contrast sensitivity. Primate models and human psychophysics indicate that stronger suppression is associated with greater deficits in amblyopic eye contrast sensitivity and visual acuity. We tested whether transcranial direct current stimulation (tDCS) of the visual cortex would modulate VEP amplitude and contrast sensitivity in adults with amblyopia. tDCS can transiently alter cortical excitability and may influence suppressive neural interactions. Twenty-one patients with amblyopia and twenty-seven controls completed separate sessions of anodal (a-), cathodal (c-) and sham (s-) visual cortex tDCS. A-tDCS transiently and significantly increased VEP amplitudes for amblyopic, fellow and control eyes and contrast sensitivity for amblyopic and control eyes. C-tDCS decreased VEP amplitude and contrast sensitivity and s-tDCS had no effect. These results suggest that tDCS can modulate visual cortex responses to information from adult amblyopic eyes and provide a foundation for future clinical studies of tDCS in adults with amblyopia.

Visual BOLD Response in Late Blind Subjects with Argus II Retinal Prosthesis

Abstract: Retinal prosthesis technologies require that the visual system downstream of the retinal circuitry be capable of transmitting and elaborating visual signals. We studied the capability of plastic remodeling in late blind subjects implanted with the Argus II Retinal Prosthesis with psychophysics and functional MRI (fMRI). After surgery, six out of seven retinitis pigmentosa (RP) blind subjects were able to detect high-contrast stimuli using the prosthetic implant. However, direction discrimination to contrast modulated stimuli remained at chance level in all of them. No subject showed any improvement of contrast sensitivity in either eye when not using the Argus II. Before the implant, the Blood Oxygenation Level Dependent (BOLD) activity in V1 and the lateral geniculate nucleus (LGN) was very weak or absent. Surprisingly, after prolonged use of Argus II, BOLD responses to visual input were enhanced. This is, to our knowledge, the first study tracking the neural changes of visual areas in patients after retinal implant, revealing a capacity to respond to restored visual input even after years of deprivation.

Rhythmic modulation of visual contrast discrimination triggered by action

Abstract: Recent evidence suggests that ongoing brain oscillations may be instrumental in binding and integrating multisensory signals. In this experiment, we investigated the temporal dynamics of visual–motor integration processes. We show that action modulates sensitivity to visual contrast discrimination in a rhythmic fashion at frequencies of about 5 Hz (in the theta range), for up to 1 s after execution of action. To understand the origin of the oscillations, we measured oscillations in contrast sensitivity at different levels of luminance, which is known to affect the endogenous brain rhythms, boosting the power of alpha-frequencies. We found that the frequency of oscillation in sensitivity increased at low luminance, probably reflecting the shift in mean endogenous brain rhythm towards higher frequencies. Importantly, both at high and at low luminance, contrast discrimination showed a rhythmic motor-induced suppression effect, with the suppression occurring earlier at low luminance. We suggest that oscillations play a key role in sensory–motor integration, and that the motor-induced suppression may reflect the first manifestation of a rhythmic oscillation.

Visual dysfunction and its correlation with retinal changes in patients with Parkinson's disease: an observational cross-sectional study

Abstract: Objectives To evaluate visual dysfunction and its correlation with structural changes in the retina in patients with Parkinson's disease (PD). Methods Patients with PD (n=37) and controls (n=37) were included in an observational cross-sectional study, and underwent visual acuity (VA), colour vision (using the Farnsworth and Lanthony desaturated D15 colour tests) and contrast sensitivity vision (CSV; using the Pelli-Robson chart and CSV 1000E test) evaluation to measure visual dysfunction. Structural measurements of the retinal nerve fibre layer (RNFL), and macular and ganglion cell layer (GCL) thicknesses, were obtained using spectral domain optical coherence tomography (SD-OCT). Comparison of obtained data, and correlation analysis between functional and structural results were performed. Results VA (in all different contrast levels) and all CSV spatial frequencies were significantly worse in patients with PD than in controls. Colour vision was significantly affected based on the Lanthony colour test. Significant GCL loss was observed in the minimum GCL+inner plexiform layer. A clear tendency towards a reduction in several macular sectors (central, outer inferior, outer temporal and superior (inner and outer)) and in the temporal quadrant of the RNFL thickness was observed, although the difference was not significant. CSV was the functional parameter most strongly correlated with structural measurements in PD. Colour vision was associated with most GCL measurements. Macular thickness was strongly correlated with macular volume and functional parameters (r>0.70, p<0.05). Conclusions Patients with PD had visual dysfunction that correlated with structural changes evaluated by SD-OCT. GCL measurements may be reliable indicators of visual impairment in patients with PD.

Evaluating a New Suite of Luminance-Based Design Metrics for Predicting Human Visual Comfort in Offices with Daylight

Abstract: A new suite of visual comfort metrics is proposed and evaluated for their ability to explain the variability in subjective human responses in a mock private office environment with daylight. Participants (n = 48) rated visual comfort and preference factors, including 1488 discreet appraisals, and these subjective results were correlated against more than 2000 unique luminance-based metrics that were captured using high dynamic range photography techniques. Importantly, luminance-based metrics were more capable than illuminance-based metrics for fitting the range of human subjective responses to data from visual preference questionnaire items. No metrics based upon the entire scene ranked in the top 20 squared correlation coefficients, nor did any based upon illuminance or irradiance data, nor did any of the studied glare indices, luminance ratios, or contrast ratios. The standard deviation of window luminance was the metric that best fit human subjective responses to visual preference on seven of ...

Comparing the Shape of Contrast Sensitivity Functions for Normal and Low Vision.

Abstract: Low Vision Comparing the Shape of Contrast Sensitivity Functions for Normal and Low Vision Susana T. L. Chung 1 and Gordon E. Legge 2 School of Optometry, University of California, Berkeley, Berkeley, California, United States Department of Psychology, University of Minnesota, Minneapolis, Minnesota, United States Correspondence: Susana T. L. Chung, 360 Minor Hall, School of Optometry, University of California, Berkeley, Berkeley, CA 94720-2020, USA; s.chung@berkeley.edu. Submitted: August 31, 2015 Accepted: December 9, 2015 Citation: Chung STL, Legge GE. Com- paring the shape of contrast sensitivity functions for normal and low vision. Invest Ophthalmol Vis Sci. 2016;57:198–207. DOI:10.1167/ iovs.15-18084 P URPOSE . The contrast sensitivity function (CSF) provides a detailed description of an individual’s spatial-pattern detection capability. We tested the hypothesis that the CSFs of people with low vision differ from a ‘‘normal’’ CSF only in their horizontal and vertical positions along the spatial frequency (SF) and contrast sensitivity (CS) axes. M ETHODS . Contrast sensitivity for detecting horizontal sinewave gratings was measured with a two temporal-interval forced-choice staircase procedure, for a range of SFs spanning 5 to 6 octaves, for 20 low-vision observers and five adults with normal vision. An asymmetric parabolic function was used to fit the aggregate data of the normal-vision observers, yielding the ‘‘normal template.’’ Each of the 20 low-vision CSFs was fit in two ways, by using a shape- invariant version of the normal template (with the width parameters fixed) that was shifted along the log-SF and log-CS axes, and by an unconstrained asymmetric parabolic function (‘‘free-fit’’). R ESULTS . The two fitting methods yielded values of the peak CS, the SF corresponding to peak CS, and the high cut-off SF that were highly correlated and in good agreement with each other. In addition, the width parameters of the low-vision CSFs were comparable with those of the normal template, implying that low-vision CSFs are similar in shape to the normal CSF. C ONCLUSIONS . The excellent agreement of parameters estimated by the two fitting methods suggests that low-vision CSFs can be approximated by a normal CSF shifted along the log-SF and log-CS axes to account for the impaired acuity and contrast sensitivity. Keywords: contrast sensitivity function, low vision, psychophysics, spatial vision ur ability to detect the presence of an object depends on the size of the object (larger is generally easier to detect), and also on the presence of any differences, such as a luminance difference, between the object and its background. The sensitivity to the relative difference in luminance of an object from its background is referred to as contrast sensitivity. Contrast sensitivity depends on object size. A complete representation of how contrast sensitivity depends on object size is referred to as the contrast sensitivity function (CSF), where the object size is usually specified in spatial frequency (c/deg) of a sinewave pattern. As such, the CSF provides a rich description of an individual’s spatial-pattern detection capabil- ity. Knowing the CSF of a person with low vision is often informative about their ability to see shapes and recognize objects in their daily lives. The gold standard for determining a CSF is to measure contrast thresholds for detecting sinusoidal gratings across a range of spatial frequencies using robust psychophysical techniques. 1,2 However, this method is time consuming, technically demanding, and requires a carefully calibrated display, and thus is not amenable for the determination of CSF for clinical patients. Recently, a method that uses a Bayesian adaptive procedure to select the spatial frequency and contrast of each trial to maximize the expected information gain has been developed for measuring CSFs. 3,4 The efficiency of this quick CSF method relies on its assumptions about the shape of the CSF and requires 100 trials to achieve good O agreement with a CSF measured with the conventional method. 4 The determination of CSF could be made more efficient if we could reduce the number of measurements required to estimate the full CSF, and link those measurements to standard clinical measures. In fact, an important simplifica- tion would exist if low-vision CSFs are similar in shape to normal CSFs, differing only in their positions on the spatial frequency (SF) and contrast sensitivity (CS) axes. In this paper, we test this hypothesis. There are several ways in which low-vision CSFs could differ in shape from normal-vision CSFs. For example, if contrast sensitivity loss occurs primarily at high spatial frequencies, then the fall-off of contrast sensitivity with high spatial frequencies would be steeper than observed in a normal-vision CSF (Fig. 1A). In the case that contrast sensitivity is equally affected across all spatial frequencies, the resulting CSF would appear as a vertical shift of a normal-vision CSF (Fig. 1B). Alternatively, if the reduction is not uniform across frequencies, then the resulting CSF may look like the one shown in Figure 1C. In addition, notches, representing CS loss only at SFs between the peak SF and the high SF cutoff (Fig. 1D), have been reported for patients with neurologic diseases such as multiple sclerosis. 5 Notches can also be due to uncorrected refractive errors 6 or imprecision of measurements 7 ; but because they are not the etiologies of low vision, they will not be considered for the purpose of this paper. Despite the fact that some low-vision CSFs may differ in shape from normal-vision CSFs, a previous iovs.arvojournals.org j ISSN: 1552-5783 This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/Journals/IOVS/934840/ on 01/21/2016

Visual function measures in early and intermediate age-related macular degeneration.

Abstract: PURPOSE The objectives of this study were to evaluate 1) the feasibility of performing computerized tests of low luminance visual acuity (LLVA), cone-specific contrast (Cone Contrast Test [CCT]), contrast sensitivity, and microperimetry and 2) the test-retest repeatability of these outcomes in dry age-related macular degeneration (AMD). METHODS This prospective study enrolled 30 subjects at a single site (8 controls, 8 early AMD, and 12 intermediate AMD). Subjects underwent LLVA, contrast sensitivity, CCT, and microperimetry with eye tracking. Low luminance deficit was defined as best-corrected visual acuity minus LLVA in EDTRS letters. Follow-up testing was administered at approximately 1 month. RESULTS There was high test-retest repeatability at one month for all visual function metrics (intraclass correlations >0.7) except log contrast sensitivity (intraclass correlations 0.6). Compared with controls, patients with intermediate AMD showed significant deficits on best-corrected visual acuity, LLVA, low luminance deficit, percent-reduced threshold on microperimetry, and red CCT (P < 0.05), but not on contrast sensitivity, green and blue CCT. CONCLUSION This pilot study supports the feasibility and reliability of using LLVA, microperimetry, and CCT in early dry AMD. Our data suggest these measures can be used as alternative future clinical trial endpoints. A larger, prospective natural history study of alternative visual function measures in dry AMD is warranted.

Assessing Suppression in Amblyopic Children With a Dichoptic Eye Chart.

Abstract: Purpose Suppression has a key role in the etiology of amblyopia, and contrast-balanced binocular treatment can overcome suppression and improve visual acuity. Quantitative assessment of suppression could have a role in managing amblyopia. We describe a novel eye chart to assess suppression in children. Methods We enrolled 100 children (7-12 years; 63 amblyopic, 25 nonamblyopic with strabismus or anisometropia, 12 controls) in the primary cohort and 22 children (3-6 years; 13 amblyopic, 9 nonamblyopic) in a secondary cohort. Letters were presented on a dichoptic display (5 letters per line). Children wore polarized glasses so that each eye saw a different letter chart. At each position, the identity of the letter and its contrast on each eye's chart differed. Children read 8 lines of letters for each of 3 letter sizes. The contrast balance ratio was the ratio at which 50% of letters seen by the amblyopic eye were reported. Results Amblyopic children had significantly higher contrast balance ratios for all letter sizes compared to nonamblyopic children and controls, requiring 4.6 to 5.6 times more contrast in the amblyopic eye compared to the fellow eye (P < 0.0001). Amblyopic eye visual acuity was correlated with contrast balance ratio (r ranged from 0.49-0.57 for the 3 letter sizes). Change in visual acuity with amblyopia treatment was correlated with change in contrast balance ratio (r ranged from 0.43-0.62 for the 3 letter sizes). Conclusions Severity of suppression can be monitored as part of a routine clinical exam in the management of amblyopia in children.

Contrast Enhancement by Nonlinear Diffusion Filtering

Abstract: To enhance the visual quality of an image that is degraded by uneven light, an effective method is to estimate the illumination component and compress it. Some previous methods have either defects of halo artifacts or contrast loss in the enhanced image due to incorrect estimation. In this paper, we discuss this problem and propose a novel method to estimate the illumination. The illumination is obtained by iteratively solving a nonlinear diffusion equation. During the diffusion process, surround suppression is embedded in the conductance function to specially enhance the diffusive strength in textural areas of the image. The proposed estimation method has the following two merits: 1) the boundary areas are preserved in the illumination, and thus halo artifacts are prevented and 2) the textural details are preserved in the reflectance to not suffer from illumination compression, which contributes to the contrast enhancement in the result. Experimental results show that the proposed algorithm achieves excellent performance in artifact removal and local contrast enhancement.

Facial contrast is a cue for perceiving health from the face.

Abstract: How healthy someone appears has important social consequences. Yet the visual cues that determine perceived health remain poorly understood. Here we report evidence that facial contrast-the luminance and color contrast between internal facial features and the surrounding skin-is a cue for the perception of health from the face. Facial contrast was measured from a large sample of Caucasian female faces, and was found to predict ratings of perceived health. Most aspects of facial contrast were positively related to perceived health, meaning that faces with higher facial contrast appeared healthier. In 2 subsequent experiments, we manipulated facial contrast and found that participants perceived faces with increased facial contrast as appearing healthier than faces with decreased facial contrast. These results support the idea that facial contrast is a cue for perceived health. This finding adds to the growing knowledge about perceived health from the face, and helps to ground our understanding of perceived health in terms of lower-level perceptual features such as contrast. (PsycINFO Database Record

Residual spatial entropy-based image contrast enhancement and gradient-based relative contrast measurement

Abstract: This paper proposes a novel algorithm to achieve both global and local contrast enhancement using a new definition of residual spatial entropy of gray levels of an image. Residual spatial entropy is utilized to assign a weight to each gray level which is further used in mapping of input gray levels to output gray levels to achieve global contrast enhancement. A non-linear mapping is applied on transform-domain coefficients of image enhanced globally to perform local contrast enhancement. The algorithm allows to control levels of perceived global and local contrast. New definitions of full-reference relative contrast measures are also introduced. Experimental results show that proposed algorithm produces better or comparable contrast-enhanced images than several state-of-the-art algorithms.

Single image dehazing via multi-scale gradient domain contrast enhancement

Abstract: Outdoor images captured under bad weathers often suffer from low visibility. In this study, a novel method is presented to improve the visibility of a single input hazy image. On the basis of the observation that degradation of a hazy image occurs both in contrast and colour, the authors method aims at compensating the contrast and colour of the image, respectively. To achieve this, they propose a multi-scale gradient domain contrast enhancement approach that handles the different residual images rather than the entire image, and correct the attenuation of colour according to the estimated transmission. Since there is no need to recover the scene radiance by the degradation model, their method depends less on the accuracy of transmission and does not require the estimation of atmospheric light. Experiments on a variety types of hazy images show that their method yields accurate results with fine details and vivid colour, even better than other state-of-the-art dehazing methods.

Diffraction Contrast Tomography in the Laboratory – Applications and Future Directions

Abstract: LabDCT derives 3D crystallographic information via diffraction contrast tomography (DCT) within a commercial laboratory X-ray microscope (ZEISS Xradia 520 Versa) that uses a synchrotron-style detection system for tomography. The establishment of DCT into a laboratory setting opens the way for routine, non-destructive, time-evolution studies of grain structure over meaningful sample volumes. The combination of grain information with microstructural features such as cracks, porosity, and inclusions, all derived non-destructively in 3D, enables materials characterization of damage, deformation, and growth mechanisms. Here, we introduce LabDCT and demonstrate its capabilities through a selection of materials science

Visual Function in Older Eyes in Normal Macular Health: Association with Incident Early Age-Related Macular Degeneration 3 Years Later

Abstract: PURPOSE In older eyes in normal macular health, we examined associations between impaired photopic acuity, mesopic acuity, spatial contrast sensitivity, light sensitivity, and the presence of low luminance deficit (difference between photopic and mesopic acuity) at baseline and incident AMD 3 years later. Associations were compared with an association between delayed rod-mediated dark adaptation and incident AMD, previously reported for this cohort. METHODS Enrollees were 60 years or older. Eyes at step 1 in the AREDS nine-step classification system based on masked grading of color fundus photographs were included. Photopic and mesopic acuity, contrast sensitivity, and light sensitivity, and the presence of low luminance deficit, were measured at baseline. Demographic, lifestyle, general health, and blood markers were assessed at baseline as potential confounders. Three years later fundus grading was repeated to determine AMD presence. RESULTS For the analysis, 827 eyes of 467 persons were eligible. Impaired mesopic acuity at baseline was associated with incident AMD, age-adjusted rate ratio (RR) 1.57 (95% confidence interval [CI] 1.04-2.35), whereas impaired photopic acuity, contrast sensitivity and macular light sensitivity, and the presence of a low luminance deficit were not. The mesopic acuity association was slightly weaker than the association between abnormal dark adaptation and incident AMD (RR 1.85, 95% CI 1.07-3.20). CONCLUSIONS Impaired mesopic acuity in eyes in normal macular health is a risk factor for incident early AMD 3 years later, however, photopic acuity, contrast sensitivity, and light sensitivity, and the presence of a low luminance deficit are not risk factors.

Role of Image Contrast Enhancement Technique for Ophthalmologist as Diagnostic Tool for Diabetic Retinopathy

Abstract: Analysing the retinal colour fundus is a critical step before any proposed computerised automatic detection of eye disease, especially Diabetic Retinopathy (DR). The retinal colour fundus image contains noise and varying low contrast of the blood vessel against its surrounding background. It makes it difficult to analyse the proper order of the vessel's network for detecting DR disease progress. The invasive method Fluorescein Angiogram Fundus (FFA) resolves these problems, but is not recommended due to an agent injection that leads to other side effects on the patient's health, in the worst cases death. In this research work, we propose a new image enhancement method based on a morphological operation along with proposed threshold based stationary wavelet transform for retinal fundus images and Contrast Limited Adaptive Histogram Equalisation (CLAHE) for the vessel enhancement. The experimental results show much better results than the FFA images. Experimental results are based on three databases of retinal colour fundus images and FFA images. The performance is evaluated by measuring the contrast enhancement factor of retinal colour fundus images and FFA images. The results show that the proposed image enhancement method is superior to other non-invasive image enhancement methods as well as invasive methods, thus it will play an important role in imaging retinal blood vessels. An average contrast improvement factor of 5.63 on colour fundus images is achieved as well as 5.57 on FFA images. This significant contribution to the enhancement of the contrast of retinal colour fundus will be one primary tool to reduce the use of an invasive method.

Marked dissociation of photopic and mesopic contrast sensitivity even in normal observers

Abstract: Although contrast vision is not routinely tested, it is important: for instance, it predicts traffic incidents better than visual acuity. Mesopic contrast sensitivity (CS) testing approximates low-lighting conditions but entails dark adaptation, which can disrupt clinical routine. In receptor-specific diseases, a dissociation of photopic and mesopic sensitivity would be expected, but can photopic CS act as a surrogate measure for mesopic CS, at least for screening purposes? Photopic and mesopic contrast sensitivities were studied in three groups: 47 normal subjects, 23 subjects with glaucoma, and three subjects with cataract. Twenty-eight of the normal subjects were additionally tested with artificial blur. Photopic contrast sensitivity was assessed with both the Freiburg Acuity and Contrast Test (FrACT) and the Mars Letter Contrast Sensitivity Charts. Mesopic contrast sensitivity, without and with glare, was measured with the Mesoptometer IIb. Coefficients of repeatability and limits of agreement were calculated for all tests. Test–retest limits of agreement were ± 0.17 logCS for Mars, ± 0.21 logCS for FrACT, and ±0.20 logCS / ± 0.14 logCS for Mesoptometer IIb without and with glare, respectively. In terms of inter-test comparison, Mars and FrACT largely agreed, except for ceiling effects in the Mars test. While mesopic and photopic contrast sensitivities correlate significantly (r = 0.51, p < 0.01), only 27 % of the variance is in common. In particular, subjects with high photopic results may be nearly as likely to have low as well as high mesopic results. The photopic contrast sensitivity tests assessed here cannot serve as surrogate measures for current mesopic contrast sensitivity tests. Low photopic CS predicts low mesopic CS, but with normal photopic CS, mesopic CS can be normal or pathologic.

Brightness preserving contrast enhancement of medical images using adaptive gamma correction and homomorphic filtering

Abstract: In this paper we propose an efficient method to enhance contrast of medical images. Image contrast enhancement is a pre-processing step that improves efficiency of other image dealing applications such as pattern recognition and computer vision. The proposed method enhances contrast and preserves brightness of given image using the gamma correction and homomorphic filtering. This method uses two step processing, in first step global contrast of image is enhanced using the gamma correction and weighted probability distribution of luminance pixels and then in second step homomorphic filtering is used for image sharpening, in order to preserve image brightness this filtering if followed by image normalization. Experiments results show that the proposed method is able to enhance contrast of medical images without much affecting its brightness. This makes the proposed algorithm to be a flexible tool for medical image enhancement.

Stereoscopic depth constancy

Abstract: 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'.

Visual acuity and contrast sensitivity following Descemet stripping automated endothelial keratoplasty

Abstract: Purpose To describe improvements in visual acuity and contrast sensitivity following Descemet stripping automated endothelial keratoplasty (DSAEK). Methods We analysed 128 consecutive patients (128 eyes) with Fuchs endothelial dystrophy (FED) and bullous keratopathy (BK) who underwent DSAEK at a single tertiary referral centre from January 2006 to September 2009. Our main outcome measures were best-corrected visual acuity (BCVA) and contrast sensitivity over 24 months follow-up. Results Median age was 67 (60–73) years with 55% women (n=70) and majority Chinese (74%, n=94) in our Asian population. There were no significant differences between demographics between the FED (48%, 61 eyes) and BK (52%, 67 eyes) groups. Forward multivariate linear regression adjusted for age, gender, donor graft thickness and diagnosis (FED vs BK) revealed that preoperative visual acuity was the most significant factor associated with visual acuity at 1 year (B=0.032, p=0.03, R 2 =0.122). Visual recovery was longer in eyes with BK, with a significantly better visual acuity in the FED group at 12 months (mean logarithm of the minimum angle of resolution BCVA BK: 0.27±0.1 vs FED: 0.22±0.9; p=0.001), but not significant at 24 months (p=0.154). Contrast sensitivity significantly improved more in the first 6 months in the FED when compared with the BK group (at 3.0, 6.0 and 12.0 cycles per degree, p Conclusions Our study suggests that while there was significant improvement in contrast sensitivity and visual acuity after DSAEK, poor preoperative visual acuity was associated with an inferior visual outcome and the time to recovery was longer in eyes with BK.

Single sand-dust image restoration using information loss constraint

Abstract: Outdoor images captured during sand–dust weather condition typically yield poor contrast and colour shift. A novel method for single sand–dust images restoration is introduced in this paper, which relies on the atmospheric scattering model and information loss constraint. To compensate the colour shift and achieve proper luminance, the proposed atmospheric light is changing with the content of the local scenes, which is initially estimated on the basis of the general scattering model and the grey-world assumption. Then, the initial atmospheric light is updated and the coarse transmission is estimated under the information loss constraint. Next, the fast guide filter is exploited in the post-refinement process to inhibit the halo artefacts. Comparison experiments demonstrate that the proposed algorithm is straightforward and efficient, the contrast and colour shift of different kinds of sand-dust images can be well compensated, especially, nice colour fidelity and proper luminance can be maintained.

An Orientation Map for Motion Boundaries in Macaque V2

Abstract: The ability to extract the shape of moving objects is fundamental to visual perception. However, where such computations are processed in the visual system is unknown. To address this question, we used intrinsic signal optical imaging in awake monkeys to examine cortical response to perceptual contours defined by motion contrast (motion boundaries, MBs). We found that MB stimuli elicit a robust orientation response in area V2. Orientation maps derived from subtraction of orthogonal MB stimuli aligned well with the orientation maps obtained with luminance gratings (LGs). In contrast, area V1 responded well to LGs, but exhibited a much weaker orientation response to MBs. We further show that V2 direction domains respond to motion contrast, which is required in the detection of MB in V2. These results suggest that V2 represents MB information, an important prerequisite for shape recognition and figure-ground segregation.

Predicting individual contrast sensitivity functions from acuity and letter contrast sensitivity measurements.

Abstract: Contrast sensitivity (CS) is widely used as a measure of visual function in both basic research and clinical evaluation. There is conflicting evidence on the extent to which measuring the full contrast sensitivity function (CSF) offers more functionally relevant information than a single measurement from an optotype CS test, such as the Pelli-Robson chart. Here we examine the relationship between functional CSF parameters and other measures of visual function, and establish a framework for predicting individual CSFs with effectively a zero-parameter model that shifts a standard-shaped template CSF horizontally and vertically according to independent measurements of high contrast acuity and letter CS, respectively. This method was evaluated for three different CSF tests: a chart test (CSV-1000), a computerized sine-wave test (M&S Sine Test), and a recently developed adaptive test (quick CSF). Subjects were 43 individuals with healthy vision or impairment too mild to be considered low vision (acuity range of -0.3 to 0.34 logMAR). While each test demands a slightly different normative template, results show that individual subject CSFs can be predicted with roughly the same precision as test-retest repeatability, confirming that individuals predominantly differ in terms of peak CS and peak spatial frequency. In fact, these parameters were sufficiently related to empirical measurements of acuity and letter CS to permit accurate estimation of the entire CSF of any individual with a deterministic model (zero free parameters). These results demonstrate that in many cases, measuring the full CSF may provide little additional information beyond letter acuity and contrast sensitivity.

High Contrast Imaging in the Visible: First Experimental Results at the Large Binocular Telescope

Abstract: In February 2014, the SHARK-VIS (System for High contrast And coronography from R to K at VISual bands) Forerunner, a high contrast experimental imager operating at visible wavelengths, was installed at LBT (Large Binocular Telescope). Here we report on the first results obtained by recent on-sky tests. These results show the extremely good performance of the LBT ExAO (Extreme Adaptive Optics) system at visible wavelengths, both in terms of spatial resolution and contrast achieved. Similarly to what was done by (Amara et al. 2012), we used the SHARK-VIS Forerunner data to quantitatively assess the contrast enhancement. This is done by injecting several different synthetic faint objects in the acquired data and applying the ADI (angular differential imaging) technique. A contrast of the order of $5 \times 10^{-5}$ is obtained at 630 nm for angular separations from the star larger than 100 mas. These results are discussed in light of the future development of SHARK-VIS and compared to those obtained by other high contrast imagers operating at similar wavelengths.

Vision-targeted health related quality of life in older adults: patient-reported visibility problems in low luminance activities are more likely to decline than daytime activities

Abstract: Commonly used vision-targeted health-related quality of life questionnaires almost exclusively focus items on vision under daytime conditions. Older adults even when in good eye health frequently report experiencing vision problems at night and under low environmental light levels, and psychophysical studies also document these visibility problems. Here we compare the progression of self-reported low luminance visibility problems and self-reported visibility problems under daytime conditions in older adults. Trained interviewers administered two questionnaires to older adults in normal eye health: the National Eye Institute Visual Function Questionnaire – 25 (NEI VFQ-25) where items are almost entirely focused on difficulties in daytime activities, and the Low Luminance Questionnaire (LLQ) where items are focused on difficulties seeing at night and under low luminance conditions. The following visual functions were also measured: visual acuity, low luminance visual acuity, low luminance deficit, contrast sensitivity, light sensitivity in the macula, and rod-mediated dark adaptation. The protocol was repeated 3 years later. Scores on the NEI VFQ-25 composite and its subscales were unchanged between baseline and 3-year follow-up, whereas scores on the LLQ composite and 5 of 6 subscales significantly decreased (corresponding to less functionality) at the 3-year follow-up. Participants were more likely to display a ≥ 5 point decrease on the LLQ composite than on the NEI VFQ-25 over 3 years. Visual functional tests were largely unrelated to changes in NEI VFQ-25 and LLQ scores from baseline to follow-up. Older adults’ vision-targeted quality of life as measured by questionnaire is more likely to exhibit a practically significant decrease over 3 years using a questionnaire that focused on low luminance activities (LLQ) than one focused on daytime activities (NEI VFQ-25). That the results of visual functional testing did not correspond to older adults’ decline in self-reported problems in low luminance activities emphasizes the importance of questionnaires in understanding visual difficulties from the patients’ own perspective.