Effects of practice and the separation of test targets on foveal and peripheral stereoacuity.

The improvement in stereoacuity of two inexperienced, normal subjects was compared at foveal and at 2.5 degrees and 5 degrees peripheral target locations as a function of practice. Outlines of two squares differing only in binocular disparity were used as test stimuli and estimates of stereoacuity were obtained by application of the method of constant stimuli with feedback. The peripheral thresholds of both subjects improved 60-80% over the course of the first 3000-4000 responses at each stimulus location. Foveal improvement followed an identical time-course with a 73% improvement in one subject and only 23% in the other. This difference was reflected in the peripheral/foveal threshold ratios of the two subjects and underlines the necessity of ensuring the stability of thresholds. Stereoacuity measurements were also obtained using several different square separations at the fovea and at 2.5 degrees, 5 degrees and 10 degrees peripheral locations along the horizontal and vertical retinal meridians of two other normal subjects. Practice-stabilized disparity thresholds using optimal target separations revealed a steeper deterioration between the fovea and 2.5-5 degrees eccentricities than did measurements of the same subjects' minimum angles of resolution (MAR). The decrease of optimal stereoacuity at the more peripheral test locations was more gradual than has been previously reported but was not clearly related to that of the MAR.