Illusory Contour Orientation Discrimination

Just noticeable differences (JNDs) in orientation for real lines and illusory contours were compared. JNDs in orientation of an illusory contour and of a real line differ by less than a factor two. JNDs in orientation of an illusory contour showed meridional variations similar to those obtained for a real line. By scaling measurements illusory contours are equally visible at all orientations, so meridional variations in illusory orientation discrimination reflect an anisotropy in orientation processing mechanisms. JNDs in orientation measured at an oblique reference orientation improve with practice for an illusory contour as well as for a real line. However while the effect of practice transfers from an illusory to a real contour, the reverse is not true. These results suggest that there are two paths for processing orientation: one activated only by real lines, the other concerned with both real and illusory contours.