The pattern of lateral interactions in the primary visual cortex, which has emerged from recent studies, conforms to the grouping rules of similarity, proximity, smoothness and closure. The goal of this paper is to understand the perceptual salience of oriented elements that are specifically organized to form a smooth contour. An overview of recent studies, in combination with new experimental results, is presented here to emphasis the idea that visual responses depend on input from both the center and the surround of the classical receptive field (CRF). It is assumed that normal lateral interactions produce a neuronal network that is formed by two antagonistic mechanisms: (i) excitation, that is spatially organized along the optimal orientation (collinear), and is predominant near the contrast threshold of the neuron, and (ii) inhibition, that is less selective and is distributed diffusely around the cell's response field. Thus, the inputs from the CRF and the anisotropic surround are summated non-linearly. The specificity of the facilitation and suppression along the collinear direction suggests the existence of second-order elongated collinear filters, which may increase the response similarity between neurons responding to elongated stimulus, thus may enhance the perceptual salience of anisotropic configurations such as contours. This causal connection is particularly evident in amblyopes, where abnormal development of the network results in the abnormal perception of contours.
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The author is thankful to the referees for their helpful and insightful suggestions. I would like to thank to Ariela Popple for her comments on the earlier version of thiss manuscript. I have reported here studies that were conducted in collaboration with Sagi, Tony Dov Norcia, Takuji Kasamatsu, Christopher Tyler, Dennis Levi, Ilona Kovacs, Mark Pettet, Kieko Mizobe and Yoram Bonneh. I would like to thank them for their contribution. This research was supported The National Institute for in Israel, Charles Smith Foundation.