Binocular single vision and perceptual processing.

Stimuli with small binocular disparities are seen as single, despite their differing visual directions for the two eyes. Such stimuli also yield stereopsis, but stereopsis and single vision can be dissociated. The occurrence of binocular single vision depends not only on the disparities of individual stimulus elements, but also on the geometrical relation of different parts of the pattern presented to each eye. A pair of vertical bars with opposite binocular disparities is seen as single if the pair is moderately widely spaced but not if it is narrow. Vertical alignment and identity in length of such bars also increase the occurrence of double vision. It is argued that these effects reflect the extraction of features of the monocular patterns, with these detected monocular features determining the binocular percept. Single and double vision of bars differing in orientation can be similarly analysed. The occurrence of relatively elaborate processing of monocular signals does not exclude the possibility that binocular interaction can occur between signals that have not been so processed. Multiple sites or types of binocular interaction are likely.