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 formation are thought to be sensitive to the orientation and magnitude of contrast at luminance discontinuities, but insensitive to the sign of contrast at 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 used as an objective measure of the saliency of illusory figures, revealing pronounced degradation of illusory boundaries when contrast polarity reversed at the intersections of orthogonally oriented edges within each inducer. These results suggest the previously unsuspected importance of image properties related to environmentally relevant constraints in perception of illusory contours and occlusion.