Cortical activation to illusory shapes as measured with magnetoencephalography.

Spatiotemporal patterns of cortical activation during the perceptual grouping of elements to form illusory shapes were estimated using anatomically constrained magnetoencephalography. Subjects were shown an array of Kanizsa-style figures which were either aligned to form illusory squares or misaligned so that no illusory contour or shape was perceived. Differential activity is more pronounced in the right hemisphere. After a weakly significant modulation at approximately 110 ms in the occipital pole, a prominent peak appears at approximately 155 ms in the lateral occipital cortex. Modulation then appears to spread back from this location toward the occipital pole, as well as ventrally to involve ventral occipital and temporal cortices for the next 180 ms, eventually involving ventral orbitofrontal cortex at 325 ms. The prominent lateral occipital response is consistent with fMRI studies with similar stimuli which found activation in that region as well as in V3A, V4v, V7, and V8. Furthermore, the timing of this activation, after the occipital pole but before ventral temporal, is consistent with a putative role for this region in midlevel vision. The late ventral temporal response (235 ms) is centered in the lingual and fusiform areas implicated in object identification. The V1/V2 modulation at this time may reflect top-down modulation by lateral occipitotemporal and ventral temporal areas.