Anatomy and physiology of a neural mechanism defining depth order and contrast polarity at illusory contours

We studied the anatomy and physiology of neurons in monkey visual cortex, w hich contribute to mechanisms segregating figure and ground at contours bas ed on information provided by occlusion cues. First, we defined the locatio n of neurons sensitive to occluding (illusory) contours. These neurons were found most frequently in the pale cytochrome oxidase stripes of area V2 bu t rarely in V1. In area V2, they were found in all laminae and with similar frequencies. The few neurons recorded in area V1 concentrated in the upper laminae. Second, we studied the properties and anatomical location of neur ons sensitive to occlusion cues (dark and light line-ends, corners). These neurons had end-stopped receptive fields and were found with similar freque ncies in both areas. In area V1, they concentrated in the upper laminae. In area V2, they were found in all laminae and cytochrome oxidase stripes. Th ese neurons responded to short stimuli of optimal length (bars, edges) and to stimuli terminating in their receptive field (line-ends, corners). Overa ll, about half of these neurons detected the direction of such terminations and about 60% were selective for certain types of termination. In summary, our results suggest that in monkey visual cortex, occlusion cues are repre sented in areas V1 and V2, whereas grouping mechanisms detecting occluding contours concentrate in area V2.