Harold E. Bedell

Moving ahead through differential visual latency

The time it takes to transmit information along the human visual pathways introduces a substantial delay in the processing of images that fall on the retina. This visual latency might be expected to cause a moving object to be perceived at a position behind its actual one, disrupting the accuracy of visually guided motor actions such as catching or hitting, but this does not happen. It has been proposed that the perceived position of a moving object is extrapolated forwards in time to compensate for the delay in visual processing.

Identifying amblyopia using associated conditions, acuity, and nonacuity features

ABSTRACT Reduced visual acuity is the most notable feature of functional amblyopia—so much so that it is common to think that it is the only, main, or even the fundamental defect. To identify all amblyopic eyes by acuity alone would require an acuity criterion of 6/6 (20/20) or somewhat better acuity—a criterion that would necessarily include more normal than amblyopic eyes. The association of amblyopia with strabismus and anisometropia is sufficiently specific to make these two nonacuity features useful in identifying amblyopia. The diagnosis of amblyopia derives from a syndrome of features, most of which are nonacuity features. Two recently quantified nonacuity features—spatial uncertainty and distortion—are not only clinically useful but they may be the fundamental defects in strabismic amblyopia, with impaired resolution being fundamental in anisometropic amblyopia.

Contrast sensitivity for letter and grating targets under various stimulus conditions.

Measurement of visual acuity for letters of different contrasts has been suggested as a clinical way to evaluate contrast sensitivity in patients with vision abnormalities. If variable-contrast letter acuity provides information similar to the contrast sensitivity function (CSF), then comparable effects should be seen in stimulus manipulations which simulate decreased vision. Using both our own and published data, we compared the effects of diffusive blur, dioptric blur, and eccentric viewing on contrast sensitivity for letter and grating targets. A diffuser placed close to the eye reduces contrast sensitivity fairly evenly across all spatial frequencies, with similar results for letters and gratings. However, dioptric blur reduces sensitivity substantially more to letters than to comparably fine gratings. Eccentric viewing also produces a larger sensitivity loss for letters than for gratings. Because some stimulus manipulations produce dissimilar changes in contrast sensitivity for letters and gratings, it is questionable whether the results of one measure can be used to draw inferences about the other. It is proposed that local or relative phase discrimination has an important role in explaining the different responses to letter and grating targets.

Extraretinal information about eye position during involuntary eye movement: Optokinetic afternystagmus

Despite importance for theories of perception, controversy exists as to whether information is available to the perceptual system about involuntary as well as voluntary eye movements. We measured the perceived direction of targets flashed briefly in an otherwise dark field during the primary phase of optokinetic afternystagmus (OKAN), an involuntary eye movement that persists in darkness following optokinetic stimulation. Perceived direction was measured by unseen pointing in one experiment and by pointing made under visual control in a second experiment. Pointing was essentially veridical in both experiments, indicating that accurate extraretinal information about eye position (presumably, as efference copy) exists for OKAN. Illusory motion of visual targets, which can occur during involuntary oculomotor responses, therefore cannot be attributed to a lack of efference-copy signals for such eye movements.

Magnitude of lateral chromatic aberration across the retina of the human eye

Lateral chromatic aberration was measured in the right eyes of four adult observers as the physical misalignment between perceptually aligned short- and long-wavelength targets. The magnitude of aberration generally increased with retinal eccentricity but remained less than 10 arcmin within 40 deg of the fovea. At 60 deg, lateral chromatic aberration increased to approximately 30 arcmin but was reducible by refractive correction in the two observers retested. The results are consistent with previous reports of a sizable region of reasonably good optical quality extending into the retinal midperiphery. Within this region, lateral chromatic aberration approximates the average spacing between adjacent retinal cones, indicating that it does not substantially limit peripheral color vision.

Spatial and Temporal Properties of the Illusory Motion-Induced Position Shift for Drifting Stimuli

The perceived position of a stationary Gaussian window of a Gabor target shifts in the direction of motion of the Gabors carrier stimulus, implying the presence of interactions between the specialized visual areas that encode form, position, and motion. The purpose of this study was to examine the temporal and spatial properties of this illusory motion-induced position shift (MIPS). We measured the magnitude of the MIPS for a pair of horizontally separated (2 or 8deg) truncated-Gabor stimuli (carrier=1 or 4cpd sinusoidal grating, Gaussian envelope SD=18arc min, 50% contrast) or a pair of Gaussian-windowed random-texture patterns that drifted vertically in opposite directions. The magnitude of the MIPS was measured for drift speeds up to 16deg/s and for stimulus durations up to 453ms. The temporal properties of the MIPS depended on the drift speed. At low velocities, the magnitude of the MIPS increased monotonically with the stimulus duration. At higher velocities, the magnitude of the MIPS increased with duration initially, then decreased between approximately 45 and 75ms before rising to reach a steady-state value at longer durations. In general, the magnitude of the MIPS was larger when the truncated-Gabor or random-texture stimuli were more spatially separated, but was similar for the different types of carrier stimuli. Our results are consistent with a framework that suggests that perceived form is modulated dynamically during stimulus motion.

Changes in oculocentric visual direction induced by the recalibration of saccades

The association between perceived visual direction and retinal location has been considered to be specified at birth and unmodifiable. We found that small changes occurred in the visual direction associated with a peripheral retinal location when the amplitudes of saccadic eye movements to a target imaged at that location were experimentally increased or decreased. This result indicates that at least limited plasticity exists in the perceptual representation of retinal location.

Effect of retinal image motion on visual acuity and contour interaction in congenital nystagmus

This study determined how contour interaction (the degradation of visual acuity by the presence of nearby contours) is affected by the incessant retinal image motion that occurs in observers with congenital nystagmus (CN). Visual acuity was measured for single, high-contrast, black Landolt Cs, presented without and with flanking bars (contour-to-C separation = 1, 2, 5, or 10 multiples of the gap width of the C). Stimuli were presented against either a white or a black surround. For comparison, acuity was also determined in normal observers, with and without motion of the stimulus to simulate the retinal image motion in jerk CN. The results show that the peak magnitude of contour interaction (the maximal degradation in acuity attributable to contour interaction) is significantly larger in the observers with CN than in normals. When acuity targets are presented against a black surround, contour interaction also occurs over a wider spatial extent in the observers with CN. Imposed image motion increases the extent of contour interaction in normal observers, but not sufficiently to account fully for the results of the observers with CN. We suggest that the additional contour interaction found in observers with CN may be attributable to the presence of amblyopia. For a small contour-to-C separation, contour interaction is significantly greater when stimuli are presented against a black rather than a white surround. Consequently, single-letter acuity may be appreciably underestimated clinically when an adjustable window is used to isolate letters on a projected acuity chart.

Are the Dominant Eyes of Amblyopes Normal

Abstract The common assumption that the dominant eyes of unilateral amblyopes are normal eyes is shown to be unjustified. The dominant eyes of unilateral amblyopes are shown to have reduced sensitivity, reduced acuity, and a horizontal eccentricity of fixation when compared to eyes of persons with normal acuity and normal binocular fixation.

Gamma-range oscillations in backward-masking functions and their putative neural correlates

Backward-masking functions have been hitherto categorized into two types, commonly named Type A and Type B. The analysis of a model of Retino-Cortical Dynamics produces the prediction that spatially localized stimuli should reveal an oscillatory metacontrast function. The predicted new type of metacontrast masking function was investigated in a psychophysical experiment. The results show oscillatory metacontrast functions with significant power in the gamma range (30-70 Hz). A marked decrease in the oscillations is observed when the spatial extent of the stimuli is increased. The theoretical basis of the study relates the oscillations found in the metacontrast function to gamma-range oscillations observed in scalp and intracerebral recordings. The qualitative agreement between the model and data provides support for this putative relationship.