Stereopsis loses dominance over relative size as target separation increases.

Binocular disparity produces less stereoscopic depth if the targets are separated by several degrees. It is thus possible that separation decreases the influence of stereopsis as a relative depth cue. Here, four experiments tested the strength of disparity in determining the direction of relative depth in the face of strongly conflicting relative size for a range of target separations. Under conditions of natural fixation-permitting sequential stereopsis-disparity dominated completely at small separations (0.42 degrees) but gradually gave way to relative size domination at large separations. However, when brief presentations prevented changes in fixation, disparity completely dominated at a separation of 0.5 degrees while relative size mostly dominated by 0.75 degrees - 1 degrees of separation. By varying target separation at different retinal eccentricities, we showed that separation per se was the critical factor in the dominance switch. Stereoacuity as a function of target separation for the same observers did not predict the switch from disparity to relative size. Stereoscopic dominance was found for the same small separations that are immune to stereoscopic reversals (Gillam, 1993 Perception 22 1025-1036). Our results suggest that relative disparity has a compulsory influence on perceived depth at small separations, suggesting a different mechanism from the one operating at larger separations.