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 bilateral symmetry detection in dot patterns is about equally efficient whether the displays are viewed monocularly or binocularly. If there is a binocular process which can be stimulated monocularly, this experiment does not indicate whether symmetry detection occurs before or after the site of binocular integration. This is so because the symmetrical patterns would have stimulated both monocular and binocular mechanisms under both viewing conditions. We 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 in the patterns was oriented at vertical (90 degrees ) or +/-1, 2, 3, or 4 degrees from vertical. The task was to judge whether the axis was tilted left 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 dichoptically symmetrical; and SSN patterns were symmetrical monocularly but dichoptically non-symmetrical. Orientation judgements were accurate, and equally so, for SSS and NNS displays but were extremely poor under SSN conditions. A control experiment showed that the poor performance in the SSN condition was not due to the axes of symmetry being eccentric to the fixation point. Thus, monocular symmetry is neither necessary nor sufficient for dichoptic bilateral symmetry perception; and symmetry mechanisms have no access to monocular signals.