Neural correlates of shape perception : neuroimaging studies.

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The present study aimed to elucidate the neural correlates to the perceptual contextual effects by using neuroimaging technique. We examined the brain activities in the retinotopic areas of the visual cortex during subjects perceived the various stimuli evoking contextual effects and the functional relationship among these areas in the processing of the visual stimuli.We have carried out fMRI measurements for the visual phenomena of the contextual effects such as chromatic induction effect, masking effect and amodal completion. We identified the retinotopic areas on the basis of the fMRI measurement using phase-encoding stimuli. The brain activities were measured during subject perceived the chromatic induction, masking, and amodal completion and the results were analyzed and compared among the retinotopic areas. The results showed that there was difference in the spatio-temporal characteristics of the brain activities between the early and higher-order visual cortical areas. In general, the spatial extent of the contextual effect is characterized as local for early processes such as V1, V2 and V3. On the other hand, the spatial extent of the contextual effect is characterized as global for higher-stage such as V4. Psychophysical measurement of the chromatic induction and masking effect showed that the differential effects of the visual context correspond to the local and global processing of the visual information.