WHY 2 EYES ARE BETTER THAN ONE FOR JUDGMENTS OF HEADING

ARE two eyes needed for judging direction of self-motion? Traditional analyses stress that the pattern of optic flow in one eye is sufficien t(1-5). The main difficulty is how to deal with the eye or head rotati on. Extraretinal signals help(6-8), but humans can also discount the e ffect of rotation purely on the basis of monocular flow(6,7,9-12) prov ided the scene contains depth(6,9,10). Depth differences give rise to changing binocular disparities when the observer moves. These disparit ies are ignored in monocular theories of judgements of heading. Using computer generated displays, we investigated whether stereoscopic pres entation improves heading judgements for conditions that pose problems to the monocular observer. We found that adding disparities to simula ted ego-motion through a cloud of dots made heading judgements up to f our times more tolerant to motion noise. The same improvement was foun d when the disparities specify the initial distances throughout the mo tion sequence. We conclude that binocular disparities improve judgemen ts of heading by imposing a depth order on the elements of the scene, not because they provide additional information on the elements' motio n in depth.