Functional organizations underlying illusory and kinetic contour processing in extrastriate visual cortex V4d of macaques

Abstract

Visual Illusions have proven to be a fruitful strategy to investigate the constructive nature of vision, however the neuron substrate underlying illusory and kinetic contour processing has been surprisingly difficult to localize in visual pathway. With our home made in vivo dual Optical Imaging rig, the functional organizations of V4d in 4 macaques have been systematically charted for abutting line illusory contour (IC) and white-noise defined kinetic grating (KG) stimuli. We have observed clear modular assemblies of cells in V4d responded to IC and KC stimuli. By closely examination of the spatial alignments of these orientation domains activated by moving luminance sinusoidal gratings (sLG), IC and KG through pixel by pixel comparison, we found that the orientation domains in V4d activated by IC stimuli were closely in register with those activated by sLG stimuli of the same orientation. In contrast, the movement of white-noise kinetic gratings (KG) produced orthogonal orientation domains instead. By directly comparison with real contour stimulus of sinusoidal gratings, the signal strengths were relatively much weaker for both IC and KG responses in V4d as expected, with KG the weakest. Our observations demonstrated directly that the functional organization and cortical mechanism in V4d for processing IC and sLG is possibly with the same neuronal mechanism in a manner of form-cue invariance; but not for KG stimuli that might involve different neuron populations from segregated processing channels. Thus these data indicate that extrastriate V4 of the ventral stream in macaque is more likely the key converging station for IC processing, but not for motion defined contours which might be represented in MT of dorsal stream or appear as an emergent feature that requires ongoing interactions with MT in macaque. Correspondence: [e-mail]