Target crowding in foveal and peripheral stereoacuity.

Disparity thresholds were obtained for a single point target when surrounded by a hexagonal array of comparison point targets. The experiment was carried out in two observers at the fovea and at retinal eccentricities of 3, 6, and 9 degrees. In each case the array diameter for best stereoacuity was determined. Care was taken that the data represented the optimum performance of the observer for the level of training in peripheral stereo tasks and also for the distance from the horopter. The results show a steady rise with eccentricity both for the stereothreshold and for the minimum target separations needed for uncrowded stereo performance. Both increase by a factor of about 10 between the fovea and the 9 degrees periphery, where a clear zone of at least 2 degrees diameter between test and comparison targets is needed for best stereoacuity.