A new angle on contour integration: The role of corners

Contour integration refers to the binding of disjoint local segments into contiguous global shapes. One central tenet in the study of contour integration has been the dependency of contour visibility on curvature. When contours become increasingly jagged, contour salience deteriorates down to the point of invisibility. In this study, we show that the deterioration of contour visibility due to sharp changes in curvature can be easily remedied by inserting corner elements at the points of angular discontinuity. Corners render even highly bent contours as salient as straight ones. We find contour integration with corners to be psychophysically indistinguishable from the integration of perfectly straight contours. Corners thereby enable a more general form of good continuation, which no longer relies on smooth curvature but merely on the presence of sufficiently predictive signals of direction and directional change. This challenges established theories of human contour integration that rely on local interactions between orientation sensitive neurons early in the visual pathway, the so-called association field models. The capacity to seamlessly integrate orientation signals with vastly different complexities, such as straight lines and corners, likely places contour completion with other image composition mechanisms beyond primary visual cortex.