Figure‐Ground Segregation at Contours: a Neural Mechanism in the Visual Cortex of the Alert Monkey

Abstract

An important task of vision is the segregation of figure and ground in situations of spatial occlusion. Psychophysical evidence suggests that the depth order at contours is defined early in visual processing. We have analysed this process in the visual cortex of the alert monkey. The animals were trained on a visual fixation task which reinforced foveal viewing. During periods of active visual fixation, we recorded the responses of single neurons in striate and prestriate cortex (areas V1, V2, and V3/V3A). The stimuli mimicked situations of spatial occlusion, usually a uniform light (or dark) rectangle overlaying a grating texture of opposite contrast. The direction of figure and ground at the borders of these rectangles was defined by the direction of the terminating grating lines (occlusion cues). Neuronal responses were analysed with respect to figure-ground direction and contrast polarity at such contours. Striate neurons often failed to respond to such stimuli, or were selective for contrast polarity; others were non-selective. Some neurons preferred a certain combination of figure-ground direction and contrast polarity. These neurons were rare both in striate and prestriate cortex. The majority of neurons signalled figure-ground direction independent of contrast polarity. These neurons were only found in prestriate cortex. We explain these responses in terms of a model which 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.

Stereoscopic Illusory Contours—Cortical Neuron Responses and Human Perception

“…Thus, V2 neurons seem to depend on retinal disparity as has been reported for V1 neurons (Cumming & Parker, 2000), but in addition, include a mechanism for defining illusory contours and the direction of figure and ground at these contours. This agrees with earlier studies on illusory contours not requiring stereoscopic cues (Baumann, van der Zwan, & Peterhans, 1997), which showed that V2 neurons can be selective for certain figure -ground directions at these contours. Recently, comparable properties have been found for V2 neurons that signaled contours from random-dot stereograms (von der Heydt, , and in relation to overlapping squares or rectangles defined by luminance contrast .…”

Higher Order Visual Processing in Macaque Extrastriate Cortex

“…Subsequent studies have observed small fractions of V1 neurons selective for illusory contours, however. In some of these studies, it could be argued that luminance cues were present in some of the stimuli used (17,85), but in others that was not the case (150,243). Alternative explanations might relate to laminar positions in V1 (150), since end-stopped neurons are more frequent in superficial layers (98).…”

Coding of Border Ownership in Monkey Visual Cortex

“…Displays that produce the perception of illusory contours can also evoke responses when the actual stimulation is confined to areas outside the classical receptive field (von der Heydt et al, 1984;Peterhans and von der Heydt, 1989). These responses might be related to figure-ground mechanisms (von der Heydt et al, 1993;Baumann et al, 1997;Heitger et al, 1998).…”