Monocular symmetry is neither necessary nor sufficient for the dichoptic perception of bilateral symmetry

Barlow and Reeves [1979. Vision Research, 19, 783-793] showed that bilatera l symmetry detection in dot patterns is about equally efficient whether the displays are viewed monocularly or binocularly. If there is a binocular pr ocess which can be stimulated monocularly, this experiment does not indicat e whether symmetry detection occurs before or after the site of binocular i ntegration. This is so because the symmetrical patterns would have stimulat ed both monocular and binocular mechanisms under both viewing conditions. W e presented stereoscopic 20-dot patterns, ten dots to each eye, for 150 ms so that 'false fusion' rather than rivalry occurred. Any axis of symmetry i n the patterns was oriented at vertical (90 degrees) or +/- 1, 2, 3, or 4 d egrees from vertical. The task was to judge whether the axis was tilted lef t or right of vertical, using the method of constant stimulus differences. Three kinds of pattern were used: SSS patterns were symmetrical in each eye alone and also dichoptically; NNS patterns were random monocularly but dic hoptically symmetrical; and SSN patterns were symmetrical monocularly but d ichoptically non-symmetrical. Orientation judgements were accurate, and equ ally so, for SSS and NNS displays but were extremely poor under SSN conditi ons. A control experiment showed that the poor performance in the SSN condi tion was not due to the axes of symmetry being eccentric to the fixation po int. Thus, monocular symmetry is neither necessary nor sufficient for dicho ptic bilateral symmetry perception; and symmetry mechanisms have no access to monocular signals. (C) 2000 Elsevier Science Ltd. All rights reserved.