Neuronal correlates of real and illusory contour perception: functional anatomy with PET

Illusory contours provide a striking example of the visual system's ability to extract a meaningful representation of the surroundings from fragmented visual stimuli. Psychophysical and neurophysiological data suggest that il lusory contours are processed in early visual cortical areas, and neuroimag ing studies in humans have shown that Kanizsa-type illusory contours activa te early retinotopic visual areas that are also activated by real contours, It is not known whether other types of illusory contours are processed by the same mechanisms, nor is it clear to what extent attentional effects may have influenced these results, as no attempt was made to match the salienc e of real and illusory stimuli in previous imaging studies. It therefore re mains an open question whether there are any brain regions specifically inv olved in the perception of illusory contours. To address these questions, w e have used O-15-butanol positron emission tomography (PET) and a novel kin d of illusory contour stimulus that is induced only by aligned line ends. B y employing a form discrimination task that was matched for attention and s timulus salience across conditions we were able to directly contrast percep tion of real and illusory contours. We found that the regions activated by illusory contour perception were the same as those activated by real contou rs, Only one region, located in the right fusiform gyrus, was significantly more strongly activated by perception of illusory contours than by real co ntours. In addition, a principal component analysis suggested that illusory contour perception is associated with a change in the correlation between V1 and V2. We conclude that different kinds of illusory contours are proces sed by the same cortical regions and that these regions overlap extensively with those involved in processing of real contours. At the regional level, perception of illusory contours thus appears to differ from perception of real contours by the degree of involvement of higher visual areas as well a s by the nature of interaction between early visual areas.