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Gerald Westheimer

Publications and source records attributed to Gerald Westheimer.

12 recordsLinked to original sources
PubMed2005

Specifying and controlling the optical image on the human retina.

A review covering the trends that led to the current state of knowledge in the areas of: (a) schematic models of the eye, and the definition of the retinal image in terms of first-order optics; (b) the description of the actual image on the retina and methods for accessing and characterizing it; (c) available procedures for controlling the quality of the retinal image in defined situations; and (d) intra-receptoral optical effects that cause differences between the light distribution on the retinal surface and at the level of interaction with photopigment molecules.

Humans↗
PubMed2005

Anisotropies in peripheral vernier acuity.

Vernier acuity for short horizontal, vertical and oblique target lines was measured in many locations in the periphery of the visual field in normal human observers. In the 10 deg periphery, the average alignment threshold with oblique vernier lines in eight locations for three observers was 2.29 times higher than that with vertical and horizontal target lines. This oblique effect was found everywhere in the visual field. Similar conclusions are drawn for configurations in which the lines were replaced by just their distal endpoints, but here, additionally, performance was distinctly better when the dot pair was collinear with the fixation point, i.e. oriented radially, than when it was oriented tangentially. Both for vernier lines and for dot pairs, in all observers, horizontal configurations showed somewhat better thresholds than vertical ones. These results suggest an inherent pattern of connectivity throughout the visual field favoring processing in the cardinal orientations over the obliques, the radial over the tangential and, to a limited extent, the horizontal over the vertical.

Adult↗
PubMed2004

The resolving power of the eye.

When the separation between the incoherent point sources is decreased, in the limit a decision whether the image distribution arises from one or two points or a short bar rests, in the absence of prior information, on the detection of an indentation. That, in essence, is the concept of resolution.

Discrimination, Psychological↗
PubMed2004

Center-surround antagonism in spatial vision: retinal or cortical locus?

Mach and Hering had early advanced a model of spatial visual processing featuring an antagonistic interaction between adjoining areas in the visual field. Spatial opponency was one of the first findings when single-unit studies of the retina were begun. Not long afterwards psychophysical experiments revealed a center-surround organization closely matching that found in the mammalian retina. It hinged on the demonstration of reduction of sensitivity in a small patch of the visual field when its surround was changed from dark to bright. Because such patterns inevitably produce borders, well-known phenomena of border interaction could be seen as providing alternative explanations, whose substrate would most likely be in the visual cortex. These competing viewpoints are discussed especially as they pertain to the recent demonstration of spatial differences in the center/surround organization between the normal and affected eyes of amblyopes. To the extent that most findings favor a retinal site for the psychophysically measured antagonism, and that evidence is accumulating for a direct effect on the mammalian retina of stimulus manipulation during visual development, the difference in spatial parameters of center/surround antagonism in amblyopia suggests that the dysfunction in amblyopia begins already in the retina.

Amblyopia↗
PubMed2003

The distribution of preferred orientations in the peripheral visual field.

Orientation discrimination was measured for line targets in the fovea and in locations along four meridians (vertical, horizontal, 45 degrees and 135 degrees ) in the peripheral visual field. In all locations, the performance for vertical and horizontal contours was on average almost twice as good as for oblique ones. In addition, especially in peripheral locations along oblique meridians, thresholds tend to be better for radially-oriented contours compared to tangential ones. These effects are robust to length of the test lines and to small changes in position and cannot therefore be due to cortical singularities (pinwheels). The results suggest that the advantage of horizontal and vertical contours, as well as possibly of radial ones, is a manifestation of a characteristic structural property of the organization of the visual pathway.

Adult↗
PubMed2003

Meridional anisotropy in visual processing: implications for the neural site of the oblique effect.

The contention is examined that the oblique effect, i.e., the well-known performance deficit in detecting orientation difference in oblique lines as compared to vertical and horizontal ones, has its origin in a relative deficiency of neurons with obliquely-oriented receptive fields in the primary visual cortex. Psychophysical observations demonstrate a prominent oblique effect also in visual tasks involving widely-separated elements and other stimuli that would elicit little or no response in oriented neurons in the visual cortex. Conversely, some tasks, e.g. position discrimination, exhibit no oblique effect even with short, high-contrast lines. When the comparison with the reference can be accomplished during a single brief exposure rather than sequential ones, thresholds for orientation differences between adjacent contours in oblique meridians are also elevated compared to those in the vertical and horizontal, but to a lesser extent. In one particular texture discrimination task some but not all observers have a conspicuous oblique effect. The discrimination only of the direction of streaming random dots, not of their speed, is poorer for motions in oblique meridians. The findings imply that the neural locus for the oblique effect is more central than the primary visual cortex.

Contrast Sensitivity↗
PubMed2003

Visual acuity with reversed-contrast charts: I. Theoretical and psychophysical investigations.

When the contrast polarity of a visual acuity chart is reversed by using bright letters on a dark background instead of dark on light, the shape and height of the resolution feature in the retinal image is identical but inverted compared with the normal situation. However, the contrast is different in the two cases because the background light level, which is a dividing factor when contrast is calculated, is much less when only the letters are bright. To the extent that contrast is a limiting factor in visual acuity, reversed-contrast acuity would be expected to be better, and proportionally more so in eyes where light scatter and aberrations widen and flatten the point-spread function. In a careful psychophysical study of Landolt C resolution, the minimum angle of resolution was found to be significantly smaller for white letters on a dark background than for the traditional dark on bright situation.

Contrast Sensitivity↗
PubMed2003

Visual acuity with reversed-contrast charts: II. Clinical investigation.

Snellen visual acuity was measured in 106 patients ranging in age from 20 to 88 years in routine examinations in the general refraction clinic with two kinds of charts: the standard chart using black letters on a white background and a reversed-contrast display featuring white letters on a black background. The overall ratio of the white-on-black to the black-on-white Snellen fractions was 1.043. A scattergram relating this ratio to patient age revealed that the older the patient, the more the visual acuity was improved by switching to the reversed-contrast chart, with a regression line slope of 0.5 +/- 0.10. Impairment of the eye's optics, in particular by intraocular scatter causing a widening and flattening of the eye's point-spread function, explains these findings and suggests prognostic and therapeutic value of reversing the contrast polarity of displays.

Adult↗
PubMed2002

Orthogonal adaptation and orientation discrimination.

The change in apparent orientation of lines and gratings induced by surrounding or preceding patterns of a different orientation (the tilt illusion and tilt after-effect) has been abundantly documented, but there is no unanimity about the effect of such inducing patterns on orientation discrimination thresholds. In particular, because inducing contours that are almost orthogonal cause the direction of the tilt illusion to reverse, evidence for an improvement of orientation discrimination with orthogonal adaptation has been welcomed on theoretical ground as supporting concepts of inversion of polarity of neural connection between cortical cells with oriented receptive fields for large orientation differences. In careful psychophysical experiments on human observers with several kinds of test and orthogonal adaptation patterns the average ratio of adapted/unadapted discrimination thresholds in paired sets of data was 1.027+/-0.13, which does not differ significantly from unity and hence constitutes evidence that orthogonal adaptation does not improve orientation discrimination.

Adaptation, Physiological↗
PubMed2002

Time course of masking in spatial resolution tasks.

PURPOSE: Further evidence is presented that Landolt C acuity has an integration time extending over several hundred milliseconds, well beyond the Bunsen-Roscoe-Bloch critical duration within which the intensity-time reciprocity for visual thresholds holds. METHODS: To test whether all segments of a longer exposure contribute equally to the uptake of acuity information, the acuity impairment caused by short-duration masking stimuli was measured for a range of onset asynchronies. RESULTS: A 50-ms mask impairs resolution most when it is presented 50 to 100 ms after the onset of the target, and it no longer influences acuity when shown 50 ms after its extinction. CONCLUSION: Allowing for the well-established optimally-effective test-mask asynchrony of 50 ms or so, the results permit the conclusion that whereas acuity information is assimilated during the whole exposure of a target, the first 50 ms are more effective, and this fits with the initial peaking of impulse activity of neurons in the primate visual cortex to flashed visual stimuli.

Humans↗
PubMed2001

Updating the classical approach to visual acuity.

The classical account of visual acuity incorporates optical, anatomical and physiological components. Knowledge of these helps to put into perspective recent scientific advances that have clinical implications. They include the development of active optical devices for neutralising aberrations in peripheral zones of the pupil, with the potential of improving the eye's imagery beyond its customary limit. In another application of modern optical and electronic instrumentation, the quality of retinal images in young eyes has been compared with that in the aged. This has led to a better understanding of the choice of contrast patterns to enhance visual acuity in older patients.

Journal Article↗