Showing papers on "Contrast (vision) published in 1985"

Neural dynamics of form perception: boundary completion, illusory figures, and neon color spreading.

Abstract: A real-time visual processing theory is used to analyse real and illusory contour formation, contour and brightness interactions, neon color spreading, complementary color induction, and filling-in of discounted illuminants and scotomas The theory also physically interprets and generalizes Land's retinex theory These phenomena are traced to adaptive processes that overcome limitations of visual uptake to synthesize informative visual representations of the external world Two parallel contour sensitive processes interact to generate the theory's brightness, color, and form estimates A boundary contour process is sensitive to orientation and amount of contrast but not to direction of contrast in scenic edges It synthesizes boundaries sensitive to the global configuration of scenic elements A feature contour process is insensitive to orientation but sensitive to both amount of contrast and to direction of contrast in scenic edges It triggers a diffusive filling-in of featural quality within perceptual domains whose boundaries are determined by completed boundary contours The boundary contour process is hypothesized to include cortical interactions initiated by hypercolumns in Area 17 of the visual cortex The feature contour process is hypothesized to include cortical interactions initiated by the cytochrome oxydase staining blobs in Area 17 Relevant data from striate and prestriate visual cortex, including data that support two predictions, are reviewed Implications for other perceptual theories and axioms of geometry are discussed

Contrast Sensitivity in Diabetics with and Without Background Retinopathy

Abstract: • Contrast sensitivity measurements were obtained from 64 patients with insulin-dependent (IDDM) and non-insulin-dependent (NIDDM) diabetes mellitus who had normal Snellen acuity and minimal or no visible diabetic retinopathy. Contrast thresholds were determined for stationary gratings at six spatial frequencies, ranging from 0.5 to 22.8 cycles/degree (c/deg), and for 1.0-c/deg gratings phase-alternated at 15 Hz. Data from each group of diabetic patients were compared with data from age-matched normal subjects. We found that (1) patients with IDDM and no retinopathy had normal contrast sensitivity, (2) patients with NIDDM and no retinopathy had abnormal contrast sensitivity at only one spatial frequency (22.8 c/deg), and (3) patients with NIDDM and background retinopathy had abnormal contrast sensitivity at all spatial frequencies tested. We also found a dissociation of Snellen acuity and contrast sensitivity, indicating that contrast sensitivity can be used as an early index of changes in the retina not demonstrated by measurements of visual acuity.

Quantification of Glaucomatous Visual Field Defects with Automated Perimetry

Abstract: A method to quantify different glaucomatous visual field defects is presented Three visual field indices are calculated: the short-term fluctuation, the mean defect, and the corrected loss variation The method was applied to visual fields tested with program JO on the Octopus automated perimeter The indices of 130 glaucoma suspects and 50 glaucoma patients were compared with 100 normal controls The indices provide good detectability of visual field defects and easy follow-up Invest Ophthalmol Vis Sci 26:176-181, 1985 Perimetry has been the standard test of visual function in glaucoma for decades In the last few years, it has been suggested that perimetric defects are preceded by other psychophysical disturbances such as color vision discrimination,' spatial and flicker contrast sensitivity 2 as well as by electrophysical functions such as the pattern VEP 3 Morphologic changes such as disc haemorrhages 45 and defects in the nerve fiber layer 6 also have been claimed to precede visual field damage 7 Histologically extensive nerve fiber loss has been determined in glaucoma suspects with so-called "normal visual fields" 8 The question arises, therefore, whether the differential light sensitivity that is measured in perimetry is, indeed, less sensitive to pressure damage than other visual functions or whether the methods applied to measure and analyze the differential light threshold are not sensitive enough Before one can study this question, though, one must reevaluate the methodology for measuring and analyzing visual fields in glaucoma patients The normal visual field has to be defined and one has to know (1) what the relevant parameters in a glaucomatous visual field are, (2) what their normal values are, and (3) how to compare the measured parameters with the normal values The purpose of this study is

Detection and discrimination of sinusoidal grating displacements

Abstract: Vertical sine-wave gratings of varying spatial frequency were stepped instantaneously to the right or to the left at differing phase angles (θ). Separate paradigms measured the contrast threshold for the detection of such a step and for the discrimination of the direction of the same step. By considering the grating before and after its displacement as a rotating phasor, we made the following predictions: (1) Contrast sensitivity for the detection of a displacement should rise as sin(θ). (2) Contrast sensitivity for the discrimination of the direction of the displacement should rise as sin(θ/2). Both predictions were confirmed using a range of spatial frequencies and phase angles. From the results of additional experiments, by measuring the discrimination of the direction thresholds as a function of contrast, we derived a nonlinear contrast response function for the motion system. This function appears to saturate fully at fairly low levels, in the neighborhood of 2 to 3% under the conditions examined. Our results suggest a direct connection among the contrast sensitivity, the contrast response function, and motion-hyperacuity thresholds.

Effect of age on contrast sensitivity function: uniocular and binocular findings.

Abstract: Monocular and binocular contrast sensitivity function for a range of spatial frequencies was measured in two groups of subjects with normal vision. Statistically significant differences in performance between the younger group (age 20-30 years) and the older group (age 50-87 years) were found at all spatial frequencies sampled between 0.40 and 19 X 25 cpd. In the age range 50-87 years there was a linear decline in contrast sensitivity with age for medium and high spatial frequencies, but sensitivity for low spatial frequencies was independent of age.

Loss of spatial phase relationships in extrafoveal vision.

Abstract: Objects in peripheral vision are not simply blurred but lack quality of form. Assuming that the visual system performs a (patchwise) Fourier analysis of the retinal image (for review see ref. 2), it has been suggested that this disadvantage of peripheral vision may be due to the inability to encode properly spatial phase relationships. This is of great interest for neurological research as certain visual pathologies imply alterations of perceived form. Previous attempts at measuring phase sensitivities failed to distinguish between the detection of phase-related changes in contrast and phase coding in the visual system. We separated these processing strategies by applying the iso-second-order texture paradigm of Julesz to the discrimination of compound gratings. Our results, reported here, show that the energy detection properties of both foveal and peripheral vision are comparable, however, independently of scale, peripheral vision ignores the relative position of image components.

Spatial Vision in Alzheimer's Disease: General Findings and a Case Report

Abstract: • Visual contrast sensitivity to sinusoidal gratings of five spatial frequencies was measured in 15 patients with Alzheimer's disease and in eight control subjects. Contrast sensitivity thresholds were elevated at all frequencies in 14 patients compared with control subjects. The 15th patient was unique: she had an impairment in object and face recognition so severe that she could not recognize her husband visually. Her sensitivity to low and intermediate frequencies was markedly reduced in relation to that of other patients, whereas her sensitivity to the highest frequency tested equaled theirs. These observations emphasize the importance of low spatial frequency information for visual object and face recognition.

Visibility of interference fringes near the resolution limit

Abstract: The contrast sensitivity of the visual system to interference fringes has been measured in the range from 10 to 65 cycles/deg with a forced-choice psychophysical procedure. Masking produced by the spatial-noise characteristic of coherent fields was avoided by diluting the interferometric field with a fixed amount of uniform, incoherent light. The loss of contrast sensitivity between 10 and 60 cycles/deg ranged from 0.85 to 1.5 log units depending on the observer. Despite these individual differences, the mean contrast sensitivity for six observers at 60 cycles/deg was more than a factor of 8 higher than the most sensitive previous estimates, suggesting that the neural visual system is much more sensitive to fine detail than previously believed. The most sensitive observer required only 4% contrast to detect a 60-cycle/deg interference fringe. Even the shallow interferometric contrast-sensitivity functions reported here are too steep to be explained solely by scattered light at the retina. It is argued that the optical properties of the photoreceptor mosaic make a negligible contribution to the contrast-sensitivity loss between 0 and 60 cycles/deg, and neural factors must be implicated.

Capabilities of monkey cortical cells in spatial-resolution tasks

Abstract: The performance of individual neurons in monkey striate cortex has been examined in three spatial-resolution tasks by making microelectrode recordings from single cells in anaesthetized, paralyzed animals. The statistical reliability of responses from cells was used to estimate threshold levels of performance. For each task (resolution acuity for high-contrast gratings, discrimination of gratings varying in spatial frequency, and localization ability, i.e., discrimination of spatial phase), performance approaching psychophysical thresholds was obtained from single cortical cells. The receptive-field organization underlying localization performance was examined in detail by the use of a linear model that relates localization ability to the sensitivity of the receptive field to luminance contrast. Calculations from this model agree well with direct measurements of localization performance and are comparable with psychophysical measurements of hyperacuity. Though it has been suggested that cells with nonoriented receptive fields in cortical layer ivcβ may be responsible for recovering fine-grain spatial information, our calculations indicate that these cells are poorer at localization than many other cells in the cortex.

Contrast sensitivity predicts age-related differences in highway-sign discriminability

Abstract: This study was conducted to determine if contrast sensitivity could predict age-related differences in the ability to discriminate simple road signs, as these differences have not been predicted by Snellen visual acuity. Contrast sensitivity, Snellen visual acuity, and discrimination distances for projected images of highway signs were measured for 7 older observers, ages 55 to 79, and 13 younger observers, ages 19 to 30. All subjects had 20/20 visual acuity or better, but the older group had significantly lower contrast sensitivity than did the younger group at three spatial frequencies: 3, 6, and 12 cycles/deg of visual angle. The older group required a significantly larger sign symbol in order to determine if it denoted a + or T intersection. Correlations between measures showed that highway-sign discrimination distance was significantly related to contrast sensitivity at two spatial frequencies, 1.5 and 12 cycles/deg, but discrimination distance was not related to visual acuity. Implication for highway-sign design and driver vision standards are discussed. (Author abstract)

Contrast and assimilation in the perception of brightness

Abstract: The rapid estimation of the brightness of objects is one of the nervous system's major visual tasks. Exactly how the eye and brain perform this basic task is still not understood. Two mechanisms that contribute to human perception of the brightness of objects have been identified previously: (i) the visual response to physical contrast and (ii) assimilation. Use of a unique visual display device allowed us to measure the relative importance of these two mechanisms. The present results reveal that assimilation is about half as effective as physical contrast in determining the apparent brightness of objects. These results imply that previous theories of vision--for instance, the retinex theory--will have to be revised; the importance of physical contrast must be weighted more strongly.

Assessment of the optical contributions to the age-related deterioration in vision.

Abstract: Contrast sensitivities were measured in human observers of different ages in response to sinusoidal grating patterns generated by laser interferometer and cathode ray tube (c.r.t.). The former method bypasses the effects of the optical media to assess directly the contrast sensitivity of the retina/visual system, while the over-all assessment including optical media is made to the c.r.t. display. The ratio of the two sets of values gives the contrast ratio of the optical media. The latter assessment does not include the effects of wide-angle light scattering which were measured by comparing the responses of eyes with a Perspex lens implant or without any lens at all (i.e. without the major source of scattering) with natural eyes. With increasing age, c.r.t. contrast sensitivities remained steady until the sixth decade when they declined while laser interference fringe contrast sensitivities declined continuously, apart from some abnormally low results for teenage observers. In contrast, the optical contrast did not vary systematically with age nor did its rate of change with spatial frequency vary. The latter was always less than the rate for laser interference fringe contrast sensitivities indicating that the retina/visual system always sets the limit to visual resolution. Wide-angle light scattering was not found to contribute significantly to the changes in ageing nor did reduced retinal illumination, as reproduced by reduced pupil diameter or viewing through a neutral density filter. We therefore conclude that the age-related deterioration is primarily caused by changes within the central nervous system rather than the optical media, the transmission quality of which remains unaffected.

The influence of age on the spatial and temporal contrast sensitivity function.

Abstract: A clinical technique for sampling the spatial and temporal CSF at low, intermediate and high frequencies is described. Reductions in contrast sensitivity with age were shown with the highest spatial and temporal frequency stimuli. The reductions can be almost completely accounted for by the reduction of pupil area with age, causing a reduction in retinal illuminance. The results show a significant deterioration in vision with age that is not revealed by standard visual acuity testing.

Spatial Contrast Sensitivity Deficits in Monkeys Produced by Optically Induced Anisometropia

Abstract: An anisometropia was simulated in infant rhesus monkeys by securing a high-powered minus lens (—10 D) in front of one eye. The anisometropia rearing procedure was initiated at 30 days of age and was continued for durations of 30, 60, or 90 days. Behavioral measurements of spatial contrast sensitivity obtained when the animals were 9 months of age indicated that the monkeys treated for 30 days had equal or nearly equal contrast sensitivities and cut-off spatial frequencies in the two eyes. The 30-day monkeys also demonstrated normal binocular summation for threshold stimuli. In contrast, the monkeys treated for either 60 or 90 days showed a significant reduction in contrast sensitivity in the defocused eyes for spatial frequencies greater than 1.0 cycles/deg and failed to show an improvement in contrast sensitivity under binocular viewing conditions. The cut-off spatial frequencies obtained at moderate luminance levels for the defocused eyes of the 60- and 90-day monkeys were slightly more than 1.0 octave lower than the cut-offs for the nondeprived eyes and, like humans with anisometropic amblyopia, the deficits in the spatial resolving capacity of the defocused eyes were observed over a large range of background luminances. The results indicate that the lens-reared monkey is a promising model for anisometropic amblyopia in humans. Invest Ophthalmol Vis Sci 26:330-342, 1985 Functional amblyopia is a reduction in visual acuity that occurs in the absence of any ophthalmoscopically detectable anomalies of the eye, and in appropriate cases, is remediated by therapeutic measures. The three most common etiologies of functional amblyopia are anisometropia, strabismus, and sensory deprivation. 1 Although all three of these forms of amblyopia are characterized by decreased visual acuity, recent psychophysical investigations 2 " 7 have revealed substantial differences in the visual characteristics of humans with these three types of amblyopia. The different patterns of visual deficits observed in human amblyopes suggest that these three forms of functional amblyopia may be associated with different neural abnormalities. Since a thorough understanding of the neural basis of amblyopia is essential for the development of optimal clinical treatment and management procedures, investigators have worked to develop animal models of amblyopia that could be used in anatomic

Effect of resolution improvement on required count density in ECT imaging: a computer simulation.

Abstract: The effects of changes in spatial resolution and total number of counts on image quality were investigated for positron and single photon emission computed tomography (ECT) systems. A variety of high contrast phantoms were generated in a computer simulation and count density and spatial resolution were varied independently over a wide range. As system spatial resolution is improved, significantly fewer counts are needed to give images of comparable visual quality. Using 100% object contrast, it was found that the number of counts could be reduced by a factor of four for a 2 mm improvement in spatial resolution over a wide range of parameters. This is due to the fact that image contrast increases rapidly with spatial resolution improvements in high contrast objects such as those used in this simulation and typically encountered in brain and cardiac ECT studies.

The pattern evoked electroretinogram: its variability in normals and its relationship to amblyopia.

Abstract: Electroretinograms evoked by pattern stimuli (contrast reversing gratings) were measured under steady state conditions in the normal and amblyopic eyes of 14 amblyopic individuals having Snellen acuities in the range 20/100 to 20/600. These ERGs were measured as a function of spatial frequency, and compared with the psychophysical threshold losses to the same stimuli. In all cases the authors compared the normal and fellow amblyopic eye's response while taking into account the variability of right-left eye comparisons of normal individuals for these psychophysical and electrophysiological tests. When factors such as optical focus, fixation alignment, and fixation stability have been individually optimized, no pattern ERG deficit was observed in a spatial frequency range where there were obvious psychophysical deficits to the same stimuli. Our results do not substantiate previous claims of a pattern ERG anomaly in many severely amblyopic eyes. Invest Ophthalmol Vis Sci 26:1610-1623, 1985 In recent years, amblyopia has become of interest to physiologists and psychologists who see it as an interesting case of anomalous visual development. The present animal models of human amblyopia, although still developing, strongly suggest that the main site of amblyopia is at the level of the primary visual cortex. It remains unclear whether the retina is also affected in humans with amblyopia or in animals with artificial amblyopia of the strabismic and anisometopic forms. Although it should be possible to address this question for animals with artifically induced strabismic and anisometropic amblyopia, a number of conflicting answers have been found. The results of some workers 1 " 3

Pattern ERG in amblyopia.

Abstract: The authors have recorded pattern ERGs in 62 amblyopic children. In 12 who were occluded up until the morning of the test, the occluded eye response was reduced, and the ratio ([response amplitude of amblyopic eye]/[response amplitude of fellow eye]) was greater than unity. In the remainder, the ratio was considerably less than unity. Ratios of less than 1.0 were found for anisometropic, esotropic, exotropic, and microtropic amblyopes. The ratio was not related to visual acuity or to squint but was higher in those children whose vision was improved by orthoptic treatment. In a group of older children recalled to the clinic, the ratio was 1.0 in those who had maintained equal visual acuity and was less than unity in those children whose acuity had regressed after treatment, or had not improved during treatment. The reduction of the pattern ERG in amblyopes occurs without a corresponding reduction in the focal ERG. In adult amblyopes, the relationship between loss of acuity, loss of grating contrast sensitivity, and the reduction in the PERG is complex and may differ according to the type of amblyopia.

Dopaminergic drugs improve human visual contrast sensitivity.

Abstract: Effects of dopaminergic drugs (L-dopa plus benserazide, or nomifensine) on human visual function have been tested in healthy volunteers by measuring the psychophysical contrast sensitivity for sinusoidal gratings of various spatial frequencies. After drug administration the contrast sensitivity improved in all subjects over a limited range of medium to high spatial frequencies.

Growth of the teleost eye: novel solutions to complex constraints

Abstract: The cichlid fish, Haplochromis burtoni, is highly dependent on vision for survival in its natural habitat. As is true of most teleost fishes, the eyes continue to grow throughout life without any obvious changes in visual capability. In H. burtoni, for example, retinal area may increase by 27 × in just 6 months. During growth, there is no obvious change in the visual sensitivity, visual acuity or lens quality which must all be appropriate for the enlarging eye. This requires that during growth competing constraints be met. For example, to maintain visual acuity, the number of ganglion cells per visual angle subtended on the retina must remain the same as must the convergence ratio of the cones onto those ganglion cells. In contrast, to maintain visual sensitivity, the number of rod photoreceptors per unit retinal area must remain the same. These requirements are in conflict since a larger eye may preserve acuity with fewer cells per unit area in a larger retina. In addition, the lens properties must remain the same as the animal increases in size so that the image available is of similar quality throughout life. Experiments have been performed to reveal the adaptations during growth which allow the fish to preserve its image of the world throughout life.

Human contrast sensitivity: regional retinal differences.

Abstract: Contrast sensitivity functions of foveal and of perifoveal upper and lower hemiretinal regions were measured in a population of twenty subjects. Foveal stimuli yielded consistently higher contrast sensitivities as well as a shift of the maximal sensitivity towards higher spatial frequencies as compared to perifoveal stimuli. The upper hemiretinal area was more sensitive at all spatial frequencies tested than the corresponding lower hemiretinal area. The statistical comparisons were highly significant, indicating not only regional retinal differences in visual acuity as reflected by the different contrast threshold levels at the highest spatial frequency, but also global differences between the upper and lower hemiretina systems not restricted to certain spatial frequency channels.

Perception of Movement and Correlation in Stroboscopically Presented Noise Patterns

Abstract: The detection of spatiotemporal correlation in visual displays has been studied with stroboscopically presented random-noise patterns and with a signal-to-noise ratio paradigm in which the moving pattern was masked with spatiotemporal white noise. These methods reveal the ability of the visual system to detect correlation of spatiotemporal structures, rather than luminance contrast. The effects of stroboscopic rate, exposure duration, target size, and the extent of discrete spatial shifts were studied in both the central and the peripheral visual field. Evidence for orientation-selective and speed-selective mechanisms was found, as well as for extensive spatiotemporal integration. Bounds on parameters of spatial and temporal correlation and integration were obtained. The results are similar to those reported earlier, and also extend them. Their relation to results obtained through other paradigms (eg the motion aftereffect) is explored.

Hemispheric symmetry in contrast and orientation sensitivity.

Abstract: This experiment was designed to determine whether there were hemispheric and/or hemiretinal (nasal/temporal) differences in contrast sensitivity and the oblique effect. Contrast sensitivity functions were measured in the left and right eyes for vertically (90°) and obliquely (45°) oriented sinusoidal gratings presented in the right and left visual fields. There were no hemispheric differences in contrast sensitivity for vertically or obliquely oriented gratings. However, sensitivity was lower for obliquely oriented gratings. Thus, the cerebral hemispheres do not appear to differ in sensitivity to contrast or in the magnitude of the oblique effect. The implications of these results are discussed in terms of the role of spatial frequency channels in information processing asymmetries between the left and right cerebral hemispheres.