Biomedical subjects

S Pardhan

Publications and source records attributed to S Pardhan.

30 records · Page 2Linked to original sources

PubMed1993

Binocular performance in patients with unilateral cataract using the Regan test: binocular summation and inhibition with low-contrast charts.

Recent reports have shown that binocular contrast sensitivity is an important indicator for assessing the visual function of a unilateral cataract. Binocular sensitivity, assessed in terms of binocular summation and inhibition, is dependent on the difference between the two eyes. Maximum binocular summation, obtained when the sensitivities of two eyes are equal, decreases if they are made unequal. Further increases in the monocular difference produce binocular inhibition, defined as a binocular sensitivity which is lower than the monocular. The aim of this study was to investigate whether the binocular effects in unilateral cataract that have been shown using laboratory-based equipment can be demonstrated using commercially available Regan charts. Binocular scores were compared with monocular scores in 16 unilateral cataract patients and 16 aged-matched normals using a high-contrast chart (96%) and a low-contrast (11%) chart. Normal subjects showed binocular summation with both charts. The cataractous patients showed no summation at high contrast and binocular inhibition with low-contrast charts. The implications of binocular inhibition at low contrast are discussed.

Aged↗

PubMed1993

Contrast detection in noise: a new method for assessing the visual function in cataract.

The detectability of a signal decreases in the presence of externally added visual noise. A linear relation exists between the energy of a signal at threshold and the spectral density of the added visual noise. This relation, represented by contrast detection in noise function, allows a given loss in contrast sensitivity (CS) to be attributed to an increase in the internal intrinsic noise and/or a decrease in the detection efficiency of the observer. Intrinsic noise gives a measure of the random background noise within the visual system, whereas detection efficiency is a measure of how effectively the observer utilizes the available stimulus information. Recent work shows that neural dysfunction produces a decrease in the detection efficiency, whereas optical deficits produce a change in the intrinsic noise only. We investigated whether the CS loss in cataract can be attributed to either a change in intrinsic noise, in detection efficiency, or both. Contrast detection in noise functions was measured for 10 uniocular cataract patients. Comparison between the two eyes showed no significant difference in detection efficiency, although the intrinsic noise increased significantly in the cataractous eye. The data suggest that detection efficiency may provide a measure of neural visual function behind a cataract and, conversely, that intrinsic noise provides a measure of the visual effect of the cataract itself. We discuss the implications of intrinsic noise as a measure of cataract for both clinical assessment and research.

Aged↗

PubMed1992

Binocular contrast summation and inhibition in amblyopia. The influence of the interocular difference on binocular contrast sensitivity.

The monocular contrast sensitivity loss in amblyopia is well documented. We investigated the influence of interocular sensitivity difference on binocular contrast sensitivity in amblyopia. Monocular and binocular contrast sensitivity functions of six amblyopes (three strabismic and three anisometropic) were measured. The monocular contrast sensitivity loss depended on the type of amblyope. Anisometropic amblyopes generally showed high frequency losses. Strabismic amblyopes showed losses at both low and high spatial frequencies. Binocular performance was assessed in terms of binocular ratios (binocular/non-amblyopic). A binocular ratio greater than 1 indicates binocular summation (binocular > monocular) while a ratio less than 1 shows binocular inhibition (binocular < monocular). In all subjects, the binocular ratio depended on the difference between the amblyopic and the non-amblyopic eye. Minimal interocular difference produced binocular summation, the magnitude of which decreased as the difference between the two eyes increased. Further increases in the monocular difference produced binocular inhibition. Anisometropic amblyopes showed a greater degree of binocular summation at low spatial frequencies compared to strabismic amblyopes. Both types of amblyopes showed binocular inhibition at high spatial frequencies. Clinical implications of binocular summation and inhibition in amblyopia are discussed.

Adolescent↗

PubMed1991

The importance of measuring binocular contrast sensitivity in unilateral cataract.

Monocular and binocular contrast sensitivities were measured in patients with uniocular cataract. The cataractous eye showed a greater monocular loss at higher spatial frequencies compared to lower spatial frequencies. Binocular contrast sensitivity depended on the contrast sensitivity differences between the two eyes. At low spatial frequencies, where the monocular sensitivity difference was minimal, binocular summation was obtained. As the sensitivity difference increased at higher spatial frequencies, the binocular contrast sensitivity decreased steadily until it reached a level below the sensitivity of the cataractous eye, demonstrating binocular inhibition. The clinical implications of binocular inhibition obtained with uniocular cataract are discussed.

Aged↗

PubMed1991

Comparison of non-stereo polaroids and slides in detection of diabetic retinopathy.

Fundus photography plays an important role in detection and assessment of diabetic retinopathy. This study compared the detectability of diabetic retinopathy lesions on Polaroid prints and Ektachrome slides obtained with a non-mydriatic camera. The number of lesions detected with Ektachrome slides was higher compared to Polaroids. The method of viewing the Ektachrome slides was also shown to be important. Slides when projected onto a white screen revealed a higher number of lesions compared to the same slide viewed through a macroscope. If a non-mydriatic camera is used to screen diabetic retinopathy, judgement from a projected Ectachrome slide is recommended.

Adult↗

PubMed1990

Binocular inhibition: psychophysical and electrophysiological evidence.

Binocular summation, defined as an increase in the binocular response compared with the monocular, occurs when the sensitivities of the two eyes are equal. We investigated the psychophysical and electrophysiological binocular response to a difference in monocular retinal illuminance. Different levels of unequal monocular sensitivities were induced by means of neutral density filters placed in front of one eye. Both studies produced similar results. In the absence of filters, maximum binocular summation was produced. With increasing difference in monocular illuminance, the binocular response decreased steadily until it reached a level below the monocular. The clinical implications of binocular inhibition, a perceptual phenomenon similar to Fechner's Paradox, are discussed.

Adult↗

PubMed1990

The effect of monocular defocus on binocular contrast sensitivity.

The monocular contrast sensitivity loss with defocus is well known. We measured binocular contrast sensitivity of sinewave gratings of 6 c/deg in the presence of different levels of monocular defocus. In the absence of defocus, the binocular sensitivity was about 42% higher than monocular. With increasing monocular defocus, the binocular sensitivity decreased steadily until it reached a level below the monocular, showing binocular inhibition. The clinical implications of binocular inhibition with monocular defocus are discussed.

Contrast Sensitivity↗

PubMed1990

Binocular contrast sensitivity with monocular glare disability.

Glare disability plays an important part in the investigation and assessment of cataract. We investigated the effect of monocular glare disability on binocular contrast sensitivity. The magnitude of binocular summation was measured at different degrees of monocular glare disability. In the absence of glare, maximum binocular summation was shown. With increasing glare disability, the degree of binocular summation decreased steadily until binocular inhibition was produced. Binocular inhibition is defined as a reduction in binocular sensitivity to reach a level below the monocular. The clinical implications of binocular inhibition with monocular glare disability are discussed.

Adult↗

PubMed1989

The effect of spatial frequency on binocular contrast inhibition.

Previous work has shown that binocular contrast summation, obtained with equal monocular sensitivities, remains constant over a range of spatial frequencies. We measured binocular contrast detection with a log 1.00 ND filter placed in front of one eye. For all eight subjects, the binocular contrast detection decreased to a level below that of the monocular detection, demonstrating a contrast analogy to Fechner's paradox. The degree of binocular inhibition, like summation, remained constant across the range of spatial frequencies.

Adult↗

PubMed1987

Binocular contrast detection with unequal monocular illuminance.

Binocular summation was measured in eight normal subjects by means of psychometric functions for contrast detection. An average 47% increase in binocular over monocular performance was obtained. Our data agreed with the simple summation model of Signal Detection Theory (Legge, 1984). Binocular psychometric functions were also measured when the sensitivity of one eye was decreased by means of a 1.0 neutral density (ND) filter. We found that binocular detectability in this case was reduced to below that of the better eye. This binocular inhibition was seen in all subjects. The slope of a contrast detection function gives a measure of the rate of change in detectability with contrast. If the slopes of two functions are equal, then the difference in detectability between these functions remains constant for all the contrast values used. When the slopes of the measured functions were analysed, no significant differences were found under any of the testing conditions. This indicates that the magnitude of summation (with equal monocular sensitivities), and of inhibition (with unequal monocular sensitivities), remains constant across the range of stimulus contrasts. The clinical implications of binocular inhibition are discussed.

Form Perception↗

PubMedDate not supplied

Comparison of videokeratoscope and autokeratometer measurements on ellipsoid surfaces and human corneas.

BACKGROUND: Various studies have compared the accuracy and repeatability of autokeratometers and videokeratoscopes using calibrated convex surfaces. We investigate the agreement between the Topcon KR-3500 autokeratometer and the EyeSys videokeratoscope on human corneas and calibrated convex surfaces. METHODS: Measurements were obtained from 30 convex ellipsoidal buttons and 20 right eyes of 20 young normal human subjects. Vertex radius and p-values were compared for the two instruments. RESULTS: The two instruments showed excellent agreement on convex buttons. The human data showed no such relationship. For vertex radius, a good level of agreement was obtained only for surfaces whose p-values were near unity. Repeatability was also calculated and was shown to be better with the Topcon autokeratometer than with the EyeSys videokeratoscope. Editing the EyeSys data to encompass the same corneal area as that of the Topcon improved its repeatability, although it did not reach the level of the Topcon autokeratometer. CONCLUSIONS: The Topcon autokeratometer and the EyeSys videokeratoscope showed reasonable agreement for surface topography on convex conicoidal plastic test buttons but not for human corneas. Alterations in the data-capture mechanisms of videokeratoscopes could improve their ability to accurately image paraboloidal surfaces.

Adolescent↗

PubMedDate not supplied

Measurement of corneal curvature in young and older normal subjects.

PURPOSE: A decrease in the vertex radius occurs with aging. The aim of this study was to investigate whether the degree of flattening from the vertex to the periphery of the cornea, ie, the surface profile, also changes with age. METHODS: A "p" value (a parameter that denotes the rate of flattening from the apex to the limbus) and vertex radius were measured in the horizontal meridian in a group of young and older subjects using the EyeSys videokeratoscope and the Topcon autokeratometer. RESULTS: With EyeSys data, horizontal vertex radius and p value were computed by means of plotting a graph of r2 (sagittal radius) versus y2 (perpendicular distance from the instrument optical axis). For Topcon data, previously derived equations were used. Mean vertex radii in the younger Topcon autokeratometer group was 7.91+/-0.31 and for the EyeSys videokeratoscope was 7.98+/-0.31. For the older group, Topcon mean vertex radii was 7.68+/-0.22 and EyeSys mean was 7.74+/-0.24. Mean p value in the younger group was 0.66+/-0.09 for the Topcon and 0.78+/-0.07 for the EyeSys. In the older group, mean vertex radii was 0.74+/-0.07 for the Topcon and 0.86+/-0.07 for the EyeSys. CONCLUSION: Vertex radius decreased with age, demonstrating a steepening of the cornea, and confirming previous results. The p value increases with age, indicating a shift toward a more spherical surface. The rate of change in the vertex radius and p value with age are predicted.

Adolescent↗