Anita J. Simmers

Deficits to global motion processing in human amblyopia.

We investigated global motion processing in a group of adult amblyopes using a method that allows us to factor out any influence of the known contrast sensitivity deficit. We show that there are independent global motion processing deficits in human amblyopia that are unrelated to the contrast sensitivity deficit, and that are more extensive for contrast-defined than for luminance-defined stimuli. We speculate that the site of these deficits must include the extra-striate cortex and in particular the dorsal pathway.

The influences of visibility and anomalous integration processes on the perception of global spatial form versus motion in human amblyopia.

Do amblyopes demonstrate general irregularities in processes of global image integration? Or are these anomalies stimulus specific? To address these questions we employed directly analogous global-orientation and global-motion stimuli using a method that allows us to factor out any influence of the low-level visibility loss [Simmers, A. J., Ledgeway, T., Hess, R. F., & McGraw, P. V. (2003). Deficits to global motion processing in human amblyopia. Vision Research 43, pp. 729-738]. The combination of orientation and motion coherence thresholds reported here provides comparable psychophysical measures of global processing by spatial-sensitive and motion-sensitive mechanisms in the amblyopic visual system. The results show deficits in both global-orientation and global-motion processing in amblyopia, which appear independent of any low-level visibility loss, but with the most severe deficit affecting the extraction of global motion. This provides evidence for the existence of a dominant temporal processing deficit in amblyopia.

Contour interaction for high and low contrast optotypes in normal and amblyopic observers.

We investigate the influence of stimulus contrast upon contour interaction in normal and amblyopic subjects. Using a computer generated acuity task, flanked and unflanked acuities were measured psychometrically at both high contrast (80%) and low contrast (6%), in a group of 19 normal and 11 amblyopic subjects. The crowding ratio for high contrast letters was found to be significantly higher than that for low contrast letters. The extent of the crowding zone was measured at high and low contrast by varying the separation of the optotype and flanking bars. The crowding zone measurement was repeated for the high contrast optotypes using dioptric blur. The position of the flanking contours was found to have a significant effect on letter resolution at high contrast but no significant effect was demonstrable at low contrast. With the addition of dioptric blur the effect of contour interaction became negligible at high contrast. These findings support the hypothesis that the crowding effect is: (1) similar in normal and amblyopic eyes when tested at threshold; (2) is contrast dependent appearing only for high contrast optotypes.

The shape and size of crowding for moving targets.

Our ability to identify alphanumeric characters can be impaired by the presence of nearby features, especially when the target is presented in the peripheral visual field, a phenomenon is known as crowding. We measured the effects of motion on acuity and on the spatial extent of crowding. In line with many previous studies, acuity decreased and crowding increased with eccentricity. Acuity also decreased for moving targets, but the absolute size of crowding zones remained relatively invariant of speed at each eccentricity. The two-dimensional shape of crowding zones was measured with a single flanking element on each side of the target. Crowding zones were elongated radially about central vision, relative to tangential zones, and were also asymmetrical: a more peripheral flanking element crowded more effectively than a more foveal one; and a flanking element that moved ahead of the target crowded more effectively than one that trailed behind it. These results reveal asymmetrical space-time dependent regions of visual integration that are radially organised about central vision.

The representation of global spatial structure in amblyopia

Visual processing is thought to involve initial local analyses that are subsequently integrated globally to derive functional representations of structure that extends over large areas of visual space. Amblyopia is a common deficit in spatial vision that could be based on either unreliable local estimates of image structure, irregularities in global image integration or a combination of errors at both these stages. The purpose of this study was to quantify the integration of local spatial information in amblyopia with global orientation discrimination and inter-ocular matching tasks. Stimuli were composed of pseudo-random arrays of highly visible and resolvable features (Gabor patches) whose local orientation and position were drawn from global distributions whose mean and variance statistics were systemically varied. Global orientation discrimination thresholds in both the amblyopic and fellow eye were elevated. The orientational and positional variances perceived by the amblyopic eye were matched by stimuli with higher variances perceived in the fellow eye. It would appear that amblyopes are able to integrate orientation information across visual space but the global representation of local structure shows greater variability compared to normal. It is this increased spatial uncertainty that underlies the spatial deficit in amblyopia.

Border distinctness in amblyopia

On the basis of the contrast sensitivity loss in amblyopia which mainly affects higher spatial frequencies, one would expect amblyopes to perceive sharp edges as blurred. We show that they perceive sharp edges as sharp and have veridical edge blur perception. Contrary to the currently accepted view, this suggests that the amblyopic visual system is not characterized by a blurred visual representation.