The identification of visual contours and surfaces is central to visual scene segmentation. One view of image construction argues that object contours are first identified and then surfaces are filled in. Although there are psychophysical and single-unit data to suggest that the filling-in view is correct, the underlying circuitry is unknown. Here we examine specific spike-timing relationships between border and surface responses in cat visual cortical areas 17 and 18. With both real and illusory (Cornsweet) brightness contrast stimuli, we found a border-to-surface shift in the relative timing of spike activity. This shift was absent when borders were absent and could be reversed with relocation of the stimulus border, indicating that the direction of information flow is highly dependent on stimulus conditions. Furthermore, this effect was seen predominantly in 17-18, and not 17-17, interactions. These results demonstrate a border-to-surface mechanism at early stages of visual processing and emphasize the importance of interareal circuitry in vision.
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