Abstract

Absolute distance from the fixation point in the sagittal depth plane is the primary stimulus for fusion whereas relative disparities within and slightly beyond this range stimulate stereoscopic depth. Stereothreshold increases markedly with sagittal distance from the fixation plane. The Weber fraction describing the ratio of stereothreshold (sec of arc)/overall disparity pedestal (min of arc) increases at a rate of 0.125 with pedestal amplitudes up to 1.5°. Beyond this absolute disparity, targets appear diplopic at an unspecific distance. Stereothreshold and sensory fusion limits both increase with coarseness of fusion stimuli. The variations of stereo and diplopia thresholds with spatial frequency account for the simultaneous perception of diplopia and stereopsis in random dot stereograms. In contrast, both stereosensitivity and Panum’s fusional limit remain uniformly high at eccentric locations along the frontoparallel plane in comparison to monocular hyperacuity tasks such as Vernier acuity. Both stereosensitivity and the limits of fusion are influenced by relative position of disparate targets. As target separation decreases, stereothreshold and the fusion limit decrease according to a constant disparity/ separation gradient. At separations less than 1/4 degree, a crowding or depth averaging effect reduces stereosensitivity. Depth sensitivity appears to be enhanced at the optimal separation of 1/4 degree by a perceived depth repulsion between neighboring disparate targets. In addition to crowding and repulsion effects there is a third global interaction, in which ambiguous disparity features take on a depth appearance that is similar to or continuous with the depth stimulated by less ambiguous neighboring targets. The role that variations in binocular retinal correspondence may play in these global interactions is discussed.

© 1986 Optical Society of America