FIGURE-GROUND SEGREGATION AT CONTOURS - A NEURAL MECHANISM IN THE VISUAL-CORTEX OF THE ALERT MONKEY

An important task of vision is the segregation of figure and ground in situations of spatial occlusion, Psychophysical evidence suggests tha t the depth order at contours is defined early in visual processing. W e have analysed this process in the visual cortex of the alert monkey, The animals were trained on a visual fixation task which reinforced f oveal viewing. During periods of active Visual fixation, we recorded t he responses of single neurons in striate and prestriate cortex (areas V1, V2, and V3/V3A), The stimuli mimicked situations of spatial occlu sion, usually a uniform light (or dark) rectangle overlaying a grating texture of opposite contrast. The direction of figure and ground al t he borders of these rectangles was defined by the direction of the ter minating grating lines (occlusion cues), Neuronal responses were analy sed with respect to figure-ground direction and contrast polarity at s uch contours, Striate neurons often failed to respond to such stimuli, or were selective for contrast polarity; others were non-selective. S ome neurons preferred a certain combination of figure-ground direction and contrast polarity. These neurons were rare both in striate and pr estriate cortex. The majority of neurons signalled figure-ground direc tion independent of contrast polarity, These neurons were only found i n prestriate cortex. We explain these responses in terms of a model wh ich also explains neuronal signals of illusory contours, These results suggest that occlusion cues are used at an early level of processing to segregate figure and ground at contours.