Degraded illusory contour formation with non-uniform inducers in Kanizsa configurations: the role of contrast polarity

The neuronal and computational mechanisms involved in illusory contour form ation are thought to be sensitive to the orientation and magnitude of contr ast at luminance discontinuities, but insensitive to the sign of contrast a t such discontinuities. It is shown here that boundary formation in Kanizsa -type illusory figures exhibits sensitivity to the spatial distribution of inducing elements of opposite contrast polarity. Shape discrimination was u sed as an objective measure of the saliency of illusory figures, revealing pronounced degradation of illusory boundaries when contrast polarity revers ed at the intersections of orthogonally oriented edges within each inducer. These results suggest the previously unsuspected importance of image prope rties related to environmentally relevant constraints in perception of illu sory contours and occlusion. (C) 2000 Elsevier Science Ltd. All rights rese rved.