Level of processing in the perception of symmetrical forms viewed from different angles.

This study is concerned with the level of processing underlying the perception of symmetry about a vertical axis. Specifically, it asks whether the equality of the two sides of a symmetrical pattern must be present retinally or perceptually for the impression of symmetry to be realized. Sixty-four subjects were assigned to groups that viewed symmetrical and asymmetrical figures when the figures were in either a frontoparallel plane or one slanted by 65 deg from the line of sight. In the 65 deg condition, the objectively symmetrical stimuli projected an asymmetrical shape on the retina, and conversely the objectively asymmetrical stimuli produced a symmetrical retinal projection. From each viewing angle, patterns were observed under either full or reduced (monocular) depth cue conditions. Like their counterparts in the frontoparallel condition, observers in the 65 deg, full depth-cue condition identified the objective symmetry of the figures. By contrast, 65 deg, reduced depth-cue observers responded primarily to the retinal structure of the stimuli. The same pattern of responding was observed for both holistic and multielement patterns and for both 100 ms and 1 s exposures. These findings and the significant relationship obtained between phenomenal judgment of symmetry and a measure of shape constancy suggest that the perception of symmetrical figures depends upon the perception of the equality of their two halves, and is thus another example of perceptual causality.