Classifying illusory contours: edges defined by "pacman" and monocular tokens.

Thresholds for the discrimination of orientation were measured in the human fovea for figures and borders delineated by solid lines and by "pacman" tokens as introduced by Kanizsa, as well as by contours induced by monocular tokens giving a stereoscopic depth illusion of a knife edge. Orientation discrimination of these illusory contours is poorer by a factor of approximately 2 than that of equivalent contours made of solid lines and is not much better than that for their supporting structures if taken alone. It is concluded that these kinds of illusory borders do not address the "border" or "edge" mechanism in the same way as real lines. Orientation discrimination and simultaneous orientation contrast (tilt illusion) were compared for a variety of illusory borders. The more robust the borders, i.e., the more sensitive to changes in orientation, the less their susceptibility to the tilt illusion.