Sequential stereopsis: a simple demonstration

Most people strongly prefer to use back-and-forth eye movements in order to discriminate 3-dimensional distances among targets that are widely separated from each other in direction. This viewing strategy permits sequential stereopsis: a comparison between the foveally-seen pre-saccadic disparity of one target with post-saccadic disparity of the other. This note describes a simple and qualitatively compelling demonstration of the usefulness of sequential stereopsis, in a situations in which classical stereopsis, with steady fixation, is greatly degraded. Targets of high-spatial-frequency texture are used, with details that can be resolved foveally before and after saccades, but that are unresolvable in peripheral vision. Back-and-forth eye movements between such textured targets, separated by 8-10 deg from each other, led to estimates of threshold that average less than 45 sec arc disparity (corresponding to about 0.18% of viewing distance): some of the best performances ever reported for targets so widely separated.