ILLUSORY CONTOURS ACTIVATE SPECIFIC REGIONS IN HUMAN VISUAL-CORTEX - EVIDENCE FROM FUNCTIONAL MAGNETIC-RESONANCE-IMAGING

The neural basis for perceptual grouping operations in the human visua l system, including the processes which generate illusory contours, is fundamental to understanding human vision. We have employed functiona l magnetic resonance imaging to investigate these processes noninvasiv ely. Images were acquired on a GE Signa 1.5T scanner equipped for echo planar imaging with an in-plane resolution of 1.5 x 1.5 mm and slice thicknesses of 3.0 or 5.0 mm. Visual stimuli included nonaligned induc ers (pacmen) that created no perceptual contours, similar inducers at the corners of a Kanizsa square that created illusory contours, and a real square formed by continuous contours. Multiple contiguous axial s lices were acquired during baseline, visual stimulation, and poststimu lation periods, Activated regions were identified by a multistage stat istical analysis of the activation for each volume element sampled and were compared across conditions. Specific brain regions were activate d in extrastriate cortex when the illusory contours were perceived but not during conditions when the illusory contours were absent. These u nique regions were found primarily in the right hemisphere for all fou r subjects and demonstrate that specific brain regions are activated d uring the kind of perceptual grouping operations involved in illusory contour perception.