Biomedical subjects

E A Essock

Publications and source records attributed to E A Essock.

30 records · Page 2Linked to original sources

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↗

PubMed1984

The resistance of selected hyperacuity configurations to retinal image degradation.

Traditional visual acuity is based on resolution of stimulus features, whereas hyperacuity (ie, vernier acuity) is based on relative localization of stimulus features. Since resolution acuity is influenced severely by optical degradation, it is often not a suitable measure of the status of the retinal/neural visual system in conditions of optical degradation. In the present study, the authors investigate the effect of optical degradation on various relative localization tasks. Thresholds for three types of hyperacuity stimuli (line vernier, two-dot vernier, and line tilt) were measured under various degrees of image degradation, produced by viewing the targets through ground glass. The results indicate that when a hyperacuity stimulus is optically degraded, relative localization threshold increases only slightly for certain separations of the comparison features. In comparison with resolution acuity, hyperacuity threshold at the optimum feature separation is quite resistant to image degradation. This finding demonstrates a potential for the clinical application of hyperacuity as a test of visual function in the presence of cataracts and other media opacities.

Cataract↗

PubMed1982

Anisotropies of perceived contrast and detection speed.

Gratings of different orientations were compared in terms of both apparent contrast and detection speed. Magnitude estimates demonstrated that oblique gratings appear perceptually to have lower contrasts than horizontal or vertical gratings of the same physical contrast. This anisotropy of perceived contrast holds across a wide suprathreshold range of physical contrasts. Even when gratings of the different orientations are equated in terms of perceived contrast, an oblique effect of detection time remains. The magnitude of this residual anisotropy of detection speed decreases as contrast is increased from threshold, such that this second anisotropy is observed only at a restricted range of lower contrasts.

Adult↗

PubMed1982

The oblique effects of pattern and flicker sensitivity: implications for mixed physiological input.

The inferior contrast sensitivity for oblique gratings has been previously demonstrated at high spatial frequencies with an absolute criterion. In this study contrast sensitivity at oblique and main axis orientations was obtained under separate pattern and flicker threshold criteria across a range of both spatial and temporal frequencies. The anisotropy of contrast sensitivity was not observed at low spatial frequencies under any stimulus or criterion conditions. At high spatial frequencies the anisotropy was observed for both pattern and flicker thresholds, although the flicker anisotropy was less pronounced. If, as previously suggested, this anisotropy is mediated by X cells, then these psychophysical findings suggest a mixed model in which both pattern and flicker systems receive input from both X and Y-cell pathways.

Adult↗

PubMed1981

Cross-species correspondence of spatial contrast sensitivity functions.

Spatial contrast sensitivity functions (CSFs) have been obtained for at least 9 species, including man. In the present paper, the shapes and octave band widths of these functions are compared. For most species, the shape of the CSF was an inverted-U, and the full width at half amplitude of the CSFs varied less than one octave. These similarities suggest that there is a close correspondence of the CSFs of these diverse animals; the major difference is the location of each CSF in the spatial frequency domain.

Animals↗

PubMed1981

Discrimination of orientation by human infants.

Successive discrimination of suprathreshold gratings of different orientations by two-month-old infants was tested with the use of a type of familiarization-novelty paradigm. Infants showed clear discrimination between horizontally and vertically oriented gratings. Infants failed, however, to discriminate mirror-image obliques from each other and also failed to discriminate nonmirror-image obliques. Considered along with previous demonstrations in children and adults of a greater confusability of oblique grating pairs relative to that of the horizontal and vertical pair, these findings indicate that this differential confusability of orientation is, at least in part, manifest in humans by two months of age.

Child Development↗

PubMed1980

The oblique effect of stimulus identification considered with respect to two classes of oblique effects.

Two classes of oblique effects are proposed. Oblique effects demonstrated in paradigms reflecting the basic functioning of the visual system are termed class 1, and those obtained in paradigms reflecting stimulus encoding and memory are termed class 2. The present experiments examine the class 2 oblique effect that has been obtained on reaction time (RT) tasks. Three RT tasks with different response requirements (identification, detection, and classification) were conducted to determine the basic conditions necessary for the production of the class 2 RT oblique effect. The results indicate that the source of the class 2 oblique effect obtained in RT paradigms is a greater confusability between the 45 degrees oblique lines than between the horizontal and vertical lines when identification of orientation is required.

Discrimination Learning↗