The representation of illusory and real contours in human cortical visual areas revealed by functional magnetic resonance imaging

Illusory contours (perceived edges that exist in the absence of local stimu lus borders) demonstrate that perception is an active process, creating fea tures not present in the light patterns striking the retina. Illusory conto urs are thought to be processed using mechanisms that partially overlap wit h those of "real" contours, but questions about the neural substrate of the se percepts remain. Here, we employed functional magnetic resonance imaging to obtain physiological signals from human visual cortex while subjects vi ewed different types of contours, both real and illusory. We sampled these signals independently from nine visual areas, each defined by retinotopic o r other independent criteria. Using both within- and across-subject analysi s, we found evidence for overlapping sites of processing; most areas respon ded to most types of contours. However, there were distinctive differences in the strength of activity across areas and contour types. Two types of il lusory contours differed in the strength of activation of the retinotopic a reas, but both types activated crudely retinotopic visual areas, including V3A, V4v, V7, and V8, bilaterally. The extent of activation was largely inv ariant across a range of stimulus sizes that produce illusory contours perc eptually, but it was related to the spatial frequency of displaced-grating stimuli. Finally, there was a striking similarity in the pattern of results for the illusory contour-defined shape and a similar shape defined by ster eoscopic depth. These and other results suggest a role in surface perceptio n for this lateral occipital region that includes V3A, V4v, V7, and V8.