Biomedical subjects

E A Essock

Publications and source records attributed to E A Essock.

At least 19 recordsLinked to original sources

PubMed1999

Interocular symmetry in nerve fiber layer thickness of normal eyes as determined by polarimetry.

PURPOSE: The existence of asymmetries between the two eyes in number and distribution of nerve fibers may provide an early anatomic indication of glaucomatous disruption if a symmetrical pattern of nerve fibers can be shown in normal eyes. Normal eyes were tested to determine whether a high degree of correspondence of regional nerve fiber layer thickness exists between fellow eyes. METHODS: Nerve fiber layer thickness was inferred from retardation shift measured by a scanning laser polarimeter (Laser Diagnostic Technologies, Inc.) in 40 volunteers without glaucoma or ocular disease (80 eyes). Total thickness and the pattern of nerve fiber thickness over 208 regions was compared between the paired eyes. RESULTS: Inferred nerve fiber layer thickness in companion eyes was highly correlated. Variation of the total nerve fiber thickness was much less between companion eyes than between eyes of different individuals. In terms of regional (i.e., local) interocular correspondence, the measurements in the inferior retina were observed to be highly similar in the two eyes, but a pronounced shift of the location of the superior peak (maximum thickness) in the left eye relative to the right was observed, as were differences between the two eyes in the thickness measurements at the nasal and temporal minima. CONCLUSION: Significant interocular asymmetries were detected in regions other than the interior hemiretina. These interocular asymmetries may reflect previously unrecognized anatomic nerve fiber differences or systematic angular bias in the instrument for one eye relative to the other. In contrast to the regional measures, the measures of total thickness were very similar between companion eyes, suggesting that the assessment of interocular asymmetries of total thickness determined by polarimetry may offer potential for detecting glaucoma.

Adult↗

PubMed1997

A motion illusion from two-dimensional periodic patterns.

The spatial properties of a motion illusion (the Ouchi illusion) that occurs in a stationary pattern were examined by means of a variety of two-dimensional periodic patterns (formed by multiplying pairs of various one-dimensional periodic functions). In two experiments, observers rated the magnitude of the illusion. The results showed that (1) patterns having large energy and steep saddle-shaped contrast gradients tended to generate stronger illusions, (2) the composite pattern made up of the sum of the fundamental and harmonic components exhibited a stronger illusion than either the fundamental or the harmonic pattern, (3) patterns possessing an element orientation and phase shift similar to those of a rectangular checkerboard, and with element sizes of 15-50 min in width and 4-8 min in height yielded a larger illusion, (4) equiluminant colors largely abolished the effect, and (5) blurring the boundary between the test and surround did not reduce the illusion. Interactions between spatially overlapping ON and OFF units was discussed as a possible underlying cause of this phenomenon.

Humans↗

PubMed1997

Apparent relative motion from a checkerboard surround.

To better understand the Ouchi illusion in which a stationary picture generates illusory relative motion, the spatial properties of the constituent elements of the rectangular checkerboard background were examined. Results of experiment 1 revealed that the largest illusion was obtained with elements of approximately 20-30 min in width and 4-6 min in height, an orientation of the constituents that was orthogonal to that of the test grating, and a phase shift of the alternate stripes that was close to 180 degrees. In experiment 2 it was found that the illusion increased in magnitude with increasing achromatic contrast but was minimal with a pattern of high chromatic contrast near isoluminance. In experiment 3, two test patches were presented simultaneously in the checkerboard background and were varied independently in their orientation to explore whether or not their motions were perceived as coherent (common fate). Patches having identical orientations, and nearly orthogonal to the surround, were synchronized more strongly than those having reflected orientations. Hysteresis related to the gain control of spatially overlapping visual units differing in their polarity (ON/OFF) was discussed as a possible cause of this phenomenon.

Contrast Sensitivity↗

PubMed1996

Psychophysical end-stopping associated with line targets.

Increment threshold for a small (e.g. 1' x 5') line target superimposed on backgrounds of various shapes and sizes was measured to provide a detailed map of the spatial interactions about line targets. This modified "Westheimer paradigm" indicated sensitization in the length direction as well as in the width direction around the line target. The effect of the adaptation field summed over an elongated, end-tapered central region, and showed strong end-zone antagonism beyond the ends of the elongated summation area, as well as flank antagonism to the sides. Secondary disinhibitory and inhibitory areas outside of the antagonistic surround were also demonstrated. When length of the test line was varied, the length of the summation region increased concomitantly, while the length of the end-zones remained fixed. End-zone antagonism was slightly weaker at oblique orientations. These results demonstrate a perceptual analog to neurophysiological end-stopping, and suggest a multilobed y-dimension weighting profile appropriate for models of spatial visual abilities.

Adaptation, Ocular↗

PubMed1996

Spatial scaling of end-stopped perceptive fields: differences in neural bases of end-zones, flanks and centers.

Length and width spatial interactions associated with a small test line centered on a rectangular background were measured at 0, 5 and 10 deg retinal eccentricities. Results indicated an elongated central region of summation with antagonistic flanks and end-zones comparable to earlier results [Yu, C. & Essock, E. A. (1996). Vision Research 36, 2883-2896]. The extent of the end-zones, flanks and centers (length and width) exhibited significantly different spatial scaling, which was steepest for the end-zones (E2 = 0.45 deg), less steep for the flanks (E2 = 0.77 deg) and least steep for the centers (E2 = 2.05 deg). Perceptive fields measured with concentric circular stimuli showed center and surround scaling equivalent to center and flank scaling, respectively, in line target experiments. These results suggest that: (1) psychophysical end-stopping and flank-inhibition reflect different underlying cortical neural processes; and (2) the spatial interactions apparent on the conventional Westheimer paradigm are partly governed by cortical factors.

Adult↗

PubMed1996

Binocular function in early glaucoma.

PURPOSE: This study investigated whether certain binocular mechanisms are disrupted in early glaucoma. METHODS: Glaucoma patients, suspects, and normals were tested on a battery of psychophysical tests consisting of flicker sensitivity (5 and 34 Hz), temporal cut-off frequency (CFF), contrast sensitivity (Pelli-Robson chart), and stereoacuity. Monocular channels were evaluated with tests of monocular flicker performance and spatial contrast sensitivity. Binocular summation on spatial and temporal tests was used to reflect integrity of binocular neural interactions. Stereoacuity was taken as a measure of performance of disparity processing mechanisms. RESULTS: The groups differed in terms of binocular flicker sensitivity at both temporal rates, binocular and monocular peak contrast sensitivity, and stereoacuity. Binocular summation of both spatial and temporal sensitivity was normal. The glaucoma suspect group was distinguishable from the age-matched normal group on binocular contrast sensitivity and stereoacuity. CONCLUSIONS: The binocular mechanisms that mediate stereoacuity appear to be heavily disrupted, whereas the binocular mechanisms that mediate central neural interaction of monocular inputs are normal. Although monocular spatiotemporal abilities are disrupted, the binocular processes combine the monocular input normally. In addition, our results suggest a benefit of binocular testing for routine assessment of glaucoma patients. The profound disruption of stereoacuity appears to result from disorder in the spatial sampling array at the ganglion-cell level similar to the disorder reported in the normal periphery and the central retina of strabismic amblyopes. These and previous findings were reviewed to evaluate the supposition of preferential M-pathway disruption in early glaucoma. Such a model can not be reconciled with the present findings. We conclude that measurements of temporal modulation sensitivity fit well with such a model, but that the current evidence of spatiotemporal contrast sensitivity disruption is less supportive of such a model.

Analysis of Variance↗

PubMed1992

An anisotropy of human tactile sensitivity and its relation to the visual oblique effect.

The ability of humans to detect striated stimuli on the distal phalanges was found to be highly anisotropic. Observers were much more sensitive to stripes presented in the proximal-distal orientation than to stripes in any other orientation. This tactile anisotropy was contrasted with the well-known visual anisotropy in which sensitivity is greatest for stripes at the horizontal and vertical orientations. We suggest that both the tactile anisotropy and the visual anisotropy are caused by corresponding anisotropies in the distribution of preferred orientations of orientation-selective neurons with in the respective modalities.

Fingers↗

PubMed1990

The influence of stimulus length on the oblique effect of contrast sensitivity.

Contrast sensitivity for grating of different orientations was measured at various stimulus lengths. The oblique effect of contrast sensitivity was observed to increase in magnitude as the length of the truncated gratings was increased from 0.5 to 5.6 deg. This finding refutes a purported distinction between the contrast sensitivity oblique effect and the orientation discrimination oblique effect. Thus, these results are consistent with the idea that the contrast sensitivity oblique effect and the orientation discrimination oblique effect share the same common underlying anisotropic basis. These results also show that grating summation differs at oblique and nonoblique orientations in terms of length summation as well as in terms of width summation.

Adult↗

PubMed1987

Reading with a macular scotoma. II. Retinal locus for scanning text.

To elucidate how patients with macular scotomas use residual functional retina for inspecting visual detail and reading, we tested three patients with dense macular scotomas using a scanning laser ophthalmoscope that allows an examiner to view and record stimuli on the retina while the patient views them. Using standard psychophysical techniques, we determined the retinal position of scotomas, the retinal areas used for fixating and inspecting acuity targets, and the retinal area used for reading simple, three-letter, nonsense syllables. We found that each patient used a single, idiosyncratic retinal area, immediately adjacent to the scotoma, for fixating, inspecting acuity targets, and scanning simple, nonsense-syllable text. This preferred retinal locus (PRL) was at different retinal eccentricities (relative to the foveola) for each patient and was not always as close as possible to the foveola. There appears to be no simple rule by which patients "select" a particular PRL. Plots of text placement on the retina revealed considerable differences in patients' abilities to execute an orderly text scan. Two patients read text more rapidly with a novel retinal area than with their PRL, suggesting that the PRL may not be optimal for text reading.

Eye Movements↗

PubMed1986

Areas of spatial interaction for a hyperacuity stimulus.

We report that the ability to detect a small vernier offset (less than 5 sec of arc in many individuals) between two small spots of light separated by a narrow gap can be disrupted by presenting additional targets in close proximity to the vernier stimulus. A rectangular background of light, centered on the vernier stimulus, elevates offset threshold by a factor of two when the total width of the background is 3-4 min arc. Backgrounds narrower or wider than 3-4 min have less or no effect on vernier threshold. These areas of spatial interaction extend to either side of a vertically-oriented vernier stimulus, or above and below a horizontally-oriented stimulus, and their extent is dependent upon the gap size of the two-dot stimulus. The effect of the presence of the background on vernier threshold cannot be accounted for by spatial interval cues nor by changes in the visibility of the stimulus. Two alternative interpretations of the results are presented to develop a description of the underlying mechanisms which produce hyperacuity spatial interactions.

Distance Perception↗

PubMed1986

Reading with a macular scotoma. I. Retinal location of scotoma and fixation area.

To investigate how patients with macular scotomas use residual functional retinal areas to inspect visual detail, a scanning laser ophthalmoscope (SLO) was used to map the retinal locations of scotomas and areas used to fixate. Three patients with dense macular scotomas of at least 20 months duration and with no explicit low vision training were tested. SLO stimuli were produced by computer modulation of the scanned laser beam, and could be placed on known retinal loci by direct observation of the retina on a television monitor. Videotaped SLO images were analyzed to produce retinal maps that are corrected for shifts of stimulus position due to fixational eye movement, thus showing the true retinal locations of scotomas and fixation loci. Major findings were as follows: 1) each patient used a single, idiosyncratic retinal area, immediately adjacent to the scotoma to fixate, and did not attempt to use the nonfunctional foveola, 2) fixation stability with the eccentric fixation locus was as good as, or better than, that of ocularly normal subjects trying to fixate at comparable eccentricities, 3) fixation stability was not systematically related to clinical visual acuity, and 4) there is good agreement as to the shape and overall size of SLO and standard clinical tangent screen scotoma maps for these three patients.

Adult↗

PubMed1985

Functional visual effects of lesions located near the optic nerve head.

Three patients with retinal lesions near or impinging on the optic nerve head are presented. Three different functional disruptions are demonstrated by quantitative layer-by-layer perimetry. In one, a patient who primarily exhibited subretinal neovascularization, inner retinal functions were anomalous; but the overlying nerve fiber bundle was as yet not affected. In the second patient, an individual with possible histoplasmosis causing an outer retinal/choroidal lesion, the overlying nerve fiber bundle was affected causing apparent retrograde changes in inner retinal function at remote visual field locations; but another aspect of optic nerve function (time-dependent response) was not anomalous. These apparently retrograde changes were essentially identical to functional changes previously demonstrated in patients with established open-angle glaucoma. In the third patient, with presumed toxoplasmosis, there were clear-out time-dependent anomalous responses consistent with optic nerve involvement, as previously reported in patients with optic neuritis. There were also modest, time-stable, nerve fiber bundle defects present in this third patient. Together, these results demonstrate that lesions in the area of the disc can have very different functional consequences, and that quantitative psychophysical techniques can elucidate these differences.

Adult↗

PubMed1985

Joint application of hyperacuity perimetry and gap tests to assess visual function behind cataracts: initial trials.

We have developed two related clinical tests that offer potential in better assessing visual function behind an ocular opacity. The tests are resistant to the effects of opacities because they utilize a localization task (vernier acuity) rather than a resolution task. In this paper we report the initial trials in which we apply the two tests together. Results from six case studies (some with actual opacities and some with simulated opacities) illustrate both the conduct of the tests and their potential capabilities in detecting even very modest retinal dysfunction in the presence of opacities. These techniques offer considerable potential in the separation of a given acuity loss observed in the presence of cataract into a portion due to the opacity and a portion due to retinal dysfunction.

Aged↗

PubMed1985

Temporal modulation of the background affects the sensitization response of X- and Y-cells in the dLGN of cat.

The responses of X- and Y-cells to a small flashing test probe modulated at 2 Hz were measured as a function of the diameter of a concentric circular background. The background was either a static homogeneous disk, a flickered homogeneous disk, a static radially-vaned disk, or a rotating vaned disk, all of equivalent space- and time-averaged luminance. Variation of background diameter produced systematic changes in response that reflected general X- and Y-cell receptive field differences. Initial increases in background diameter decrease the response to the test probe ("desensitization") of both X- and Y-cells to a minimum level at a diameter that approximates the size of the receptive field center. Further increases in the background diameter result in an increase of the response to the test probe ("sensitization") that is very large for X-cells and much less for Y-cells. Temporal modulation of the background does not alter the desensitization of X- or Y-cells, but strongly reduces the sensitization of X-cells. Temporal modulation of the background by flicker and by motion produced equivalent effects. These results are compared to similar human psychophysical tests used clinically. Implications concerning the mediation of psychophysical sensitization are also considered.

Action Potentials↗

PubMed1984

Relating vernier acuity and Snellen acuity in specific clinical populations.

The relationship between vernier acuity and Snellen acuity in a group of cataract patients was examined. The vernier stimulus consisted of two small spots of light, vertically separated by a variable-sized gap. The Snellen stimulus was a standard projected Snellen chart. The form of the relation between vernier and Snellen acuity was found to depend upon the vernier gap size used. Specifically, when the smaller gaps of 4 or 8 min of arc are used, the two types of acuity are linearly related. When the gap is 16 or 32 min of arc, (or if best vernier performance irrespective of gap is considered) vernier acuity is related to Snellen acuity by a power function with an exponent less than one, within this clinical population. Thus, with increasing degrees of retinal image degradation caused by cataract, optimum vernier acuity is impaired at a slower rate than Snellen acuity. Our results in cataract patients are compared to results obtained by others within a population of strabismic and anisometropic amblyopes.

Aged↗

PubMed1984

Hyperacuity perimetry. Assessment of macular function through ocular opacities.

"Hyperacuity perimetry" provides localized measures of visual performance across the central visual field that are resistant to the retinal image degradation caused by media opacities. A normal observer, viewing a vernier stimulus through an artificial opacity (ground glass), exhibits a pronounced performance improvement with foveal viewing, relative to viewing at visual field points just 2 degrees to 4 degrees parafoveal. Patients with central lesions exhibit anomalous hyperacuity perimetric profiles, even in the presence of a simulated ocular opacity. We conclude that in patients with media opacities, hyperacuity perimetry can preoperatively evaluate the distribution and size of areas of visual loss across the central visual field and the stability and location of primary fixation. This potentially powerful test of macular function behind media opacities is as easy to apply as any static visual field measurement.

Adult↗

PubMed1984

Retinal boundaries and visual function in gyrate atrophy.

Three patients with gyrate atrophy of the retina and choroid underwent fundus photoperimetry, the combination of fundus photography and visual field analysis within the same instrument, to examine the correspondence between the funduscopically observed, scalloped margin of remaining retina and the functional limit of the peripheral visual field. The results suggest that near the border between remaining retina and atrophic areas, visual sensitivity falls off quite rapidly, producing perimetric isopters that conform closely to the scalloped retinal margin. In some instances, visual function can be demonstrated in islands of remaining retina that are totally surrounded by atrophic areas. This study demonstrates the usefulness of fundus photoperimetry in following the course of degenerative retinal diseases.

Adult↗

PubMed1984

The effects of image degradation by cataract on vernier acuity.

The resistance of certain types of hyperacuity tasks (eg, vernier acuity) to optical degradation of the retinal image has been demonstrated previously. This suggests that such a task might provide a means of filling the clinical need for a reliable test of retinal function in the presence of cataracts and other ocular opacities. In the present study, we measure several aspects of vernier acuity performance in cataract patients. These data provide an early basis for the development of a vernier acuity test to detect retinal/neural visual loss prior to cataract surgery. We report the results of measurements in 15 cataract patients using a two-dot vernier acuity task with variable separation between the two dots (gap). The data indicate that the shape and mean level of the vernier threshold-versus-gap function co-vary with the functional severity of the opacity as corroborated by Snellen acuity measures. Furthermore, the optical effects of the cataracts we studied are very similar to the effects of degrading the visual stimulus by interposing ground glass for a normal observer. We conclude that the shape of the vernier threshold-versus-gap function is a good indicator of the functional severity of the cataract in patients with otherwise normal vision.

Aged↗