Depth release of illusory contour shape in the Ehrenstein grid

In the Ehrenstein grid, bright illusory patches delineated by illusory contours are seen. In order to assess whether the illusory patches possess shape constancy, the Ehrenstein grid was viewed straight on and at various angles of slant with respect to the observer. Observers matched the apparent shape of the illusory patches with a circle or an ellipse defined by real lines in a reference stimulus. Results show that, when viewed at a slanted angle, the shape of the bright patches was deformed and became oval. Such deformation was much less when the illusory contours were replaced by real contours. We label this dissociation of shape perception from depth cues depth release to contrast it with the previously described phenomenon of depth capture in which depth cues displace the illusory patches in depth. As a common explanation for both effects, it is proposed that the illusory contours induced by line ends or line ends themselves provide only weak or ineffective depth signals.