CONTOUR INTEGRATION ACROSS POLARITIES AND SPATIAL GAPS - FROM LOCAL CONTRAST FILTERING TO GLOBAL GROUPING

This article introduces an experimental paradigm to selectively probe the multiple levels of visual processing that influence the formation of object contours, perceptual boundaries, and illusory contours. The experiments test the assumption that, to integrate contour information across space and contrast sign, a spatially short-range filtering pro cess that is sensitive to contrast polarity inputs to a spatially long -range grouping process that pools signals from opposite contrast pola rities. The stimuli consisted of thin subthreshold lines, flashed upon gaps between collinear inducers which potentially enable the formatio n of illusory contours. The subthreshold lines were composed of one or more segments with opposite contrast polarities. The polarity nearest to the inducers was varied to differentially excite the short-range f iltering process. The experimental results are consistent with neuroph ysiological evidence for cortical mechanisms of contour processing and with the Boundary Contour System-model, which identifies the short-ra nge filtering process with cortical simple cells, and the long-range g rouping process with cortical bipole cells. (C) 1997 Elsevier Science Ltd. All rights reserved.