EVIDENCE THAT BOTH AREA V1 AND EXTRASTRIATE VISUAL-CORTEX CONTRIBUTE TO SYMMETRY PERCEPTION

Bilateral symmetry is common in nature and most animals seem able to p erceive it, Many species use judgements of symmetry in various behavio urs, including mate selection [1-3], Originally, however, symmetry per ception may have developed as a tool for generating object-centered, r ather than viewer-centered, descriptions of objects, facilitating reco gnition irrespective of position or orientation [4]. There is evidence that the visual system treats the orientation of axes-of symmetry in the same way st treats the orientation of luminance defined contours [ 5], suggesting that axes-of-symmetry act as 'processing tokens' [6], W e have investigated the characteristics of neural mechanisms giving ri !je to the perceived orientation of axes-of-symmetry. We? induced tilt aftereffects with symmetrical dot patterns, eliciting perceived angle expansion and contraction effects like those usually observed with lu minance defined contours [7,8]. Induction of aftereffects during binoc ular rivalry resulted in a reduction of the magnitude of these effects , consistent with the aftereffects being mediated in extrastriate visu al cortex, probably between visual areas V2 and MT [9], In a second ex periment in which the aftereffects were induced monocularly, their mag nitudes were measured in the unadapted eye. Contraction effects transf erred completely, suggesting that they are mediated by binocular cells . Expansion effects did not transfer completely, consistent with their having a monocular component. These data suggest that information abo ut the orientation of axes of symmetry may be available as early as ar ea V1, but that processing continues in extrastriate cortex.