Showing papers on "Contrast (vision) published in 1984"

Illusory contours and cortical neuron responses

Abstract: Figures in which human observers perceive "illusory contours" were found to evoke responses in cells of area 18 in the visual cortex of alert monkeys. The cells responded as if the contours were formed by real lines or edges. Modifications that weakened the perception of contours also reduced the neuronal responses. In contrast, cells in area 17 were apparently unable to "see" these contours.

Perceived velocity of moving chromatic gratings

Abstract: Equiluminous red-green sine-wave gratings were drifted at a uniform rate in the bottom half of a 10-deg field. In the top half of the display was a sinusoidal-luminance grating of the same spatial frequency and 95% contrast that drifted in the opposite direction. Observers, while fixating a point in the display center, adjusted the speed of this upper comparison grating so that it appeared to match the velocity of the chromatic grating below. At low spatial frequencies, equiluminous gratings were appreciably slowed and sometimes stopped even though the individual bars of the grating could be easily resolved. The amount of slowing was proportionally greatest for gratings with slow drift rates. Blue-yellow sine-wave gratings showed similar effects. When luminance contrast was held constant, increasing chrominance modulation caused further decreases in apparent velocity, ruling out the possibility that the slowing was simply due to decreased luminance contrast. Perceived velocity appears to be a weighted average of luminance and chrominance velocity information.

A new contrast sensitivity vision test chart.

Abstract: A new contrast sensitivity vision chart has been tested and compared to an automated video-based vision tester on 83 observers whose ages ranged from 9 to 75 years. Good agreement was found between the contrast sensitivity measurements obtained from the vision chart and the automated tester for similar population and age variations. These results suggest that vision test charts can be developed to provide useful contrast sensitivity psychometric functions and yet be as simple to use as present eye charts.

Neural dynamics of brightness perception: features, boundaries, diffusion, and resonance.

Abstract: A real-time visual processing theory is used to unify the explanation of monocular and binocular brightness data. This theory describes adaptive processes which overcome limitations of the visual uptake process to synthesize informative visual representations of the external world. The brightness data include versions of the Craik-O’Brien-Cornsweet effect and its exceptions, Bergstrom’s demonstrations comparing the brightnesses of smoothly modulated and step-like luminance profiles, Hamada’s demonstrations of nonclassical differences between the perception of luminance decrements and increments, Fechner’s paradox, binocular brightness averaging, binocular brightness summation, binocular rivalry, and fading of stabilized images and ganzfelds. Familiar concepts such as spatial frequency analysis, Mach bands, and edge contrast are relevant but insufficient to explain the totality of these data. Two parallel contour-sensitive processes interact to generate the theory’s brightness, color, and form explanations. A boundary-contour process is sensitive to the orientation and amount of contrast but not to the direction of contrast in scenic edges. It generates contours that form the boundaries of monocular perceptual domains. The spatial patterning of these contours is sensitive to the global configuration of scenic elements. A feature-contour process is insensitive to the orientation of contrast, but is sensitive to both the amount of contrast and to the direction of contrast in scenic edges. It triggers a diffusive filling-in reaction of featural quality within perceptual domains whose boundaries are dynamically defined by boundary contours. The boundary-contour system is hypothesized to include the hypercolumns in visual striate cortex. The feature-contour system is hypothesized to include the blobs in visual striate cortex. These preprocessed monocular activity patterns enter consciousness in the theory via a process of resonant binocular matching that is capable of selectively lifting whole monocular patterns into a binocular representation of form-and-color-in-depth. This binocular process is hypothesized to occur in area V4 of the visual pre striate cortex.

Contrast sensitivity and visual disability in chronic simple glaucoma.

Abstract: A battery of vision tests was used to quantify visual defect in a group of 50 patients with chronic simple glaucoma. The vision tests were near and distance visual acuity, visual fields, and contrast sensitivity to static and temporally modulated sinusoidal grating patterns. Of these, static contrast sensitivity function appears to be the most sensitive method of measuring visual defect in glaucoma patients. The visual disability experienced by the glaucoma patients was quantified by means of a questionnaire, and the relationship between perceived visual disability and visual defect was examined. It was found that results from a group of tests, near visual acuity, visual field, and contrast sensitivity measures, are the best predictors of the difficulty experienced by patients in performing visually dependent daily activities.

Human pattern-evoked electroretinogram

Abstract: We have recorded electroretinographic (ERG) responses to grating patterns whose spatial, temporal, and contrast parameters were varied. The resultant evoked potential is dependent on spatial frequency and it exhibits a spatial band-pass characteristic. The peak frequency is dependent on retinal eccentricity. These findings are independent of temporal frequency, contrast, or mean luminance in the photopic range. These results suggest that the pattern ERG originates from a postreceptoral site.

Low-contrast letter charts in early diabetic retinopathy, ocular hypertension, glaucoma, and Parkinson's disease.

Abstract: Diabetes can cause visual loss that is not detected by standard reading tests such as the Snellen test but can be detected by low-contrast letter charts. This visual loss is quite different from loss caused by refractive error. These low-contrast charts are diagnostically at least as sensitive as the sinewave grating contrast sensitivity test. They are inexpensive, and the test is brief and simple. Preliminary evidence is that patients with diabetes who have abnormal low-contrast chart results give abnormal intravenous fluorescein (IVF) test results, even though visual acuity is normal. Low-contrast charts also detect visual loss in patients with ocular hypertension, glaucoma, and Parkinson's disease, including patients with normal visual acuity.

Visual processing deficits as assessed by spatial frequency contrast sensitivity and backward masking in normal ageing and alzheimer's disease

Abstract: Visual functions of patients with senile dementia of the Alzheimer type were compared with those of young people and age-matched controls. Visual acuity and spatial frequency contrast sensitivity did not differ significantly between Alzheimer patients and normal elderly subjects, although both were impaired in comparison with young subjects. Alzheimer patients required more time than ageing controls to identify letters and were susceptible to the interfering effects of a backward pattern mask on letter recognition over a longer interval. The spatial extent over which the pattern mask was effective, as well as the time interval over which a homogeneous mask interfered with letter recognition, were equivalent in normal old people and Alzheimer patients. In all the masking tasks, young people performed better than the old. It is suggested that Alzheimer's disease affects later central visual functions more than early relatively peripheral ones.

Figure–ground segregation by motion contrast and by luminance contrast

Abstract: Some naturally camouflaged objects are invisible unless they move; their boundaries are then defined by motion contrast between object and background. We compared the visual detection of such camouflaged objects with the detection of objects whose boundaries were defined by luminance contrast. The summation field area is 0.16 deg2 , and the summation time constant is 750 msec for parafoveally viewed objects whose boundaries are defined by motion contrast; these values are, respectively, about 5 and 12 times larger than the corresponding values for objects defined by luminance contrast. The log detection threshold is proportional to the eccentricity for a camouflaged object of constant area. The effect of eccentricity on threshold is less for large objects than for small objects. The log summation field diameter for detecting camouflaged objects is roughly proportional to the eccentricity, increasing to about 20 deg at 32-deg eccentricity. In contrast to the 100:1 increase of summation area for detecting camouflaged objects, the temporal summation time constant changes by only 40% between eccentricities of 0 and 16 deg.

Image Enhancement For The Visually Impaired

Abstract: Application of image processing for the visually impaired is discussed. Image degradation in the low vision patient's visual system can be specified as a transfer function obtained by measurements of contrast sensitivity. The effectiveness of adaptive image enhancement for printed pictures is demonstrated using an optically simulated cataractous lens.

Perception of Lightness and Illumination in a World of One Reflectance

Abstract: Observers looked into a miniature room in which everything was painted matte white, or—in another room—matte black. They made both reflectance and illumination judgments for eight test spots. The test spots (which varied in luminance) were perceived as approximately equal in reflectance—not different, as conventional contrast theories would seem to require. The illumination matches made to the same points, however, closely paralleled the pattern of actual illumination levels, and this result is discussed as evidence that edges are classified as changes in either reflectance or illumination. The white room was correctly perceived as white, and the black room was perceived as middle gray; similar results were obtained even when the luminances in the black room were higher (owing to higher illumination) than the corresponding luminances in the white room. An explanation in terms of differences in gradient patterns is presented and supported with luminance profiles.

Outline of A Theory of Brightness, Color, and form Perception

Abstract: This paper describes new concepts and mechanisms from a real-time visual processing theory that has been used to explain paradoxical data about brightness and form perception. These data include the Craik-O'Brien effect, the Land brightness and color demonstrations, the fading of stabilized images, neon color spreading, complementary color induction, completion of illusory contours, and binocular rivalry. Two functionally distinct contour processes interact to generate these brightness and form properties in the theory. A Boundary Contour process is sensitive to the amount of contrast but not to the direction of contrast in scenic edges. It includes a binocular matching stage that is sensitive to spatial scale, orientation, and binocular disparity, and whose outcome triggers a process of monocular contour completion. These completed contours form the boundaries of monocular perceptual domains. A Feature Contour process is sensitive to both the amount of contrast and to the direction of contrast in scenic edges. It triggers a diffusive filling-in reaction of featural quality within perceptual domains whose boundaries are dynamically defined by the completed boundary contours. The diffusive filling-in reactions take place within syncytia of cell compartments. These preprocessed monocular representations give rise to a percept via a process of binocular resonance. The percept takes the form of standing waves of patterned activity among multiple spatial scales. The Boundary Contour process is hypothesized to be analogous to interactions between the hypercolumns in area 17 of the visual cortex. The Feature Contour process is hypothesized to be analogous to interactions between the cytochrome oxydase staining blobs in area 17 and prestriate cortex in area 18.

Spatial contrast sensitivity. Importance of controlling the patient's visibility criterion.

Abstract: • Clinical spatial contrast sensitivity measurements are typically made using psychophysical methods that do not specify the response criterion being used by the patient in judging grating visibility. Results of this report show the necessity of such methods for (1) maximizing detectability of early contrast sensitivity deficits by minimizing normal sample variance, and (2) ensuring that changes in an individual's contrast sensitivity reflect changes in vision and not simply fluctuations in the patient's criterion for judging grating visibility.

The effect of glaucoma on central visual function.

Abstract: Glaucoma has traditionally been thought to affect peripheral visual function in its early stages and to spare central visual function until late in the disease process The basis for this assumption has been the reliance on Goldmann-type perimetry, a rather sensitive method for assessing the peripheral visual function, and on Snellen-type visual acuity measurements, a rather insensitive method of assessing central visual function This belief has persisted despite frequent complaints from patients with glaucoma that their central vision is disturbed Over the past two decades, several investigations of central visual functions and their anatomic substrate have challenged this assumption Histologic studies of the nerve fiber layer in eyes with glaucoma suggest that the number of ganglion cells subserving macular function is decreased even in early stages of the disease In addition, afferent pupillary defects (a gross measurement of macular nerve fiber function) may also be present in eyes with early glaucoma Several studies have demonstrated that color perception (largely mediated by the fovea) is defective in glaucoma Furthermore, defects in color perception may even precede the development of visual field abnormalities Seventy-eight percent of patients with early glaucomatous visual field defects were found to have a defect in color perception when tested with a desaturated D-15 color panel that tests only the central 15 degrees In addition, both chromatic and achromatic foveal perception channels are defective in eyes with glaucoma and even in some eyes of those with suspected glaucoma Contrast sensitivity has become recognized as an important component of visual function Partial loss of contrast sensitivity may cause a degradation in the quality of perception even though the Snellen visual acuity remains normal Although contrast sensitivity is not entirely a macular function, it has been shown that as little as 3 degrees of disturbance of the macula (eg, with macular degeneration or with an artificial central scotoma) will reduce the contrast sensitivity, suggesting that this modality is indeed mediated to a significant extent by this portion of the retina Spatial contrast sensitivity appears to be reduced in patients with glaucoma However, because of overlap and lack of a sharp cutoff measurement, present testing procedures fail to allow a clear distinction between the glaucomatous and normal populations Although reduced temporal contrast sensitivity has been demonstrated in glaucomatous eyes by others, I undertook a systematic investigation of this function in a large group of patients with glaucoma and with suspected glaucoma(ABSTRACT TRUNCATED AT 400 WORDS)

large-sample Norms for Contrast Sensitivity

Abstract: A portable microprocessor-controlled instrument automatically measured the static and dynamic contrast sensitivity functions (CSF's) of 265 observers for a normal population at the Dayton Air Fair and Air Force Museum. Repeat measures for six observers taken 6 months later show small, nonsystematic differences from original measurements. Median values of contrast sensitivity as well as regions encompassing 75 of 90% of our sample are shown.

Contrast Sensitivity: The Visual Rehabilitation of the Patient With Macular Degeneration

Abstract: • Contrast sensitivity functions (CSFs) were determined on a large group of patients with macular degeneration to better assess qualitative aspects of their residual vision. Contrast sensitivity was also determined through the telescopic low-vision aids of selected patients. In general, the patients have a substantial loss of contrast sensitivity for all spatial frequencies. The preferred eye of these patients appeared to be related to the peak of the CSF more so than to the Snellen visual or resolution acuity. Through the low-vision aid, not only was there the predictable increase in resolution acuity but also a notable increase in peak contrast sensitivity. These findings indicate that contrast sensitivity testing provides useful information that should be considered in the visual rehabilitation of the patient with macular degeneration.

VEPs in humans reveal high and low spatial contrast mechanisms.

Abstract: The effect of contrast on visual evoked potential (VEP) amplitude was examined in nine observers. A 6.0 cycles/deg (cpd) grating was modulated in an "on-off" mode at 7.5 Hz. The VEP response contains significant first and second harmonic components: their growth with contrast is parallel, each function consisting of two limbs. The data are consistent with the hypothesis that the pattern VEP obtained with "on-off" presentation may reflect the contributions of "low" and "high" contrast neuronal populations demonstrated in physiological studies of the primate.

An analysis of contrast sensitivity in identical twins with keratoconus.

Abstract: We studied a pair of 36-year-old identical twins with keratoconus, one of whom had undergone bilateral penetrating keratoplasties. Both patients had final best corrected Snellen visual acuity of approximately 20/20. However, contrast sensitivity functions for the unoperated twin were abnorma

The Effect of Penetrating Keratoplasty on Contrast Sensitivity in Keratoconus

Abstract: • Complaints of visual distortion often precede a decrement in visual acuity in keratoconus. We studied seven patients with keratoconus who had undergone keratoplasty in one eye and whose Snellen visual acuity measurements were equal in the grafted and nongrafted eyes. Subjects were examined clinically; we then measured contrast thresholds for seven spatial frequencies of computer-generated sinusoidal gratings after optimal contact lens correction. Results indicate that nongrafted keratoconic eyes demonstrate abnormal contrast sensitivity, even with good visual acuity measurements. Eyes with clear corneal grafts and with visual acuities comparable to nongrafted eyes fell within the 95% confidence limits of the normal contrast sensitivity curve. The findings confirm that corneal distortion or opacity in the optimally corrected keratoconic eye may account for notable visual dysfunction and that keratoplasty improves not only Snellen visual acuity but contrast sensitivity as well.

X-ray examination apparatus incorporating image subtraction

Abstract: An X-ray examination apparatus incorporating digital image subtraction in which from a series of images to be formed of an object with and without contrast medium those images are selected for subtraction which result in ultimate images which combine a low motional unsharpness with adequate contrast. Images are also formed on at least one of the edges of the contrast bolus, so that in each position several contrast images can be formed with a single contrast bolus.

Pattern Visual Evoked Potentials and Spatial Vision in Retrobulbar Neuritis and Multiple Sclerosis

Abstract: • In an attempt to resolve some of the reported disagreements between visual evoked potential (VEP) recordings and Snellen visual acuity in patients with multiple sclerosis, we compared these test results with sine-wave grating—contrast sensitivity curves. Disease that depressed visual sensitivity for high spatial frequencies, sparing low spatial frequencies, was associated with depressed visual acuity and attenuated small-check VEPs in the affected eye, while largecheck VEPs were not attenuated. When visual sensitivity to all spatial frequencies was depressed, both large-check and small-check VEPs were attenuated, and Snellen acuity was reduced. In general, abnormalities in the contrast sensitivity curve predicted abnormalities in VEP amplitude, but VEP delay was less accurately predicted.

Transient myopia after visual work

Abstract: After viewing at near distances for several hours observers may become temporarily myopic. This effect, known as transient myopia after visual work, was studied by measuring contrast sensitivity to a sinusoidally modulated grating pattern. After 3 hours of editing text at a visual display terminal, a significant decrease in sensitivity was found in five subjects out of seven. The results show interpersonal differences in the strength and the time course of work-induced myopia. Measurement of transient myopia by means of contrast sensitivity has the advantage of being rapid enough that even short-term effects can be followed. The decrease in sensitivity can be shown to be a purely optical effect of the eye.