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 perceive it. Many species use judgements of symmetry in various behaviours, including mate selection [1, 2 and 3]. Originally, however, symmetry perception may have developed as a tool for generating object-centered, rather than viewer-centered, descriptions of objects, facilitating recognition irrespective of position or orientation [4]. There is evidence that the visual system treats the orientation of axes-of-symmetry in the same way it treats the orientation of luminance-defined contours [5], suggesting that axes-of-symmetry act as ‘processing tokens’ [6]. We have investigated the characteristics of neural mechanisms giving rise 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 luminance-defined contours [7 and 8]. Induction of aftereffects during binocular rivalry resulted in a reduction of the magnitude of these effects, consistent with the aftereffects being mediated in extrastriate visual cortex, probably between visual areas V2 and MT [9]. In a second experiment in which the aftereffects were induced monocularly, their magnitudes were measured in the unadapted eye. Contraction effects transferred 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 about the orientation of axes-of-symmetry may be available as early as area V1, but that processing continues in extrastriate cortex.