Figure-ground segregation and brightness perception at illusory contours: a newronal model

It has been shown in animals and man that illusory contours are represented at an early stage of visual processing. Animal studies revealed that neurons which signaled illusory contours usually also responded to contrast borders (bars, edges), and that the orientations of these contours are represented in similar cortical maps. In humans, illusory contour representations have been found at a comparable level of processing. Further, evidence of perception suggests that illusory contours often coincide with occluding contours and that mechanisms segregating figure and ground at such contours are also implemented at an early stage of processing. We studied this question in the visual cortex of the alert monkey by recording the responses of single neurons in stimulus conditions which defined illusory contours and the associated step in depth on the basis of occlusion cues (light and dark line-ends, or corners). In area V2, we found neurons sensitive to the figure-ground direction that human observers perceive at such contours. Most neurons showed this sensitivity independent of the contrast polarity that the stimuli induced at the contour, the remainder preferred a certain combination of figure-ground direction and contrast polarity. We explain these results in terms of a computational model using end-stopped operators for the detection of occlusion cues. In computer simulations we show that this model reproduces the figure-ground direction and the contrast polarity that human observers perceive at illusory (occluding) contours.