Curvy is the new straight: Kanizsa triangles

The cortical representation of figure and ground still seems to be a partly solved puzzle. Based on psychophysically mapped contrast sensitivity fields within closed boundaries, it has been suggested earlier that symmetry related surface representations might be relevant in addition to co-linear activation along the path of a contour (Kovacs and Julesz, 1994). Here we test the classic illusory figures (Kanizsa triangles) with a test probe that appears near the illusory edge. The task is to decide whether the probe appears inside or outside of the illusory triangle. Assuming an equilateral triangle, and that the illusory edge spans a straight path between the inducers, our results are surprisingly asymmetrical for inside and outside test probes. While there is no difficulty to judge outside targets, probes appearing on the alleged contour, or inside the assumed triangle are very clearly judged to be outside for up to a distance that is about 3\% of the illusory contour length. The bent illusory contours seem to be more curved for diagonal than for horizontal or vertical edges. We interpret these results as a new indication for the necessity to couple medialness structure (Aparajeya and Fol Leymarie, 2016) with lateral propagation of contour completion.