The effect of target size and eccentricity on reflex disparity vergence

This study examined the effects of stimulus size and eccentricity on reflex disparity vergence: the small, involuntary corrections of eye alignment wh ich serve to minimize the binocular disparity of fixated targets. Subjects were instructed to fixate steadily on a small, stationary mark superimposed on the center of a dynamic random dot stereogram. The stereogram was binoc ularly uncorrelated except for a fully correlated patch whose size and ecce ntricity were varied systematically across trials. The disparity of the pat ch was varied sinusoidally over time to stimulate vergence following moveme nts. The overall purpose was to determine the relative contributions of var ious held loci in controlling binocular fixation by finding the smallest pa tch which would reliably drive vergence against the effort to fixate steadi ly. Psychophysical thresholds for detection of the correlated patch stimuli were also measured for comparison to the oculomotor results. Results showe d that the smallest effective patch increased with eccentricity similarly f or both vergence responses and psychophysical detection, suggesting they de pend on a common, presumably cortical matching process. The dependence of r esponse on eccentricity is roughly consistent with changes in the cortical magnification factor, suggesting that the area of cortex stimulated may be the determining factor in vergence responses to this class of stimulus. (C) 1998 Elsevier Science Ltd. All rights reserved.