CLASSIFYING ILLUSORY CONTOURS - EDGES DEFINED BY PACMAN AND MONOCULARTOKENS

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 indu ced by monocular tokens giving a stereoscopic depth illusion of a knif e edge. Orientation discrimination of these illusory contours is poore r by a factor of similar to 2 than that of equivalent contours made of solid lines and is not much better than that for their supporting str uctures if taken alone. It is concluded that these kinds of illusory b orders do not address the ''border'' or ''edge'' mechanism in the same way as real lines. Orientation discrimination and simultaneous orient ation contrast (tilt illusion) were compared for a variety of illusory borders. The more robust the borders, i.e., the more sensitive to cha nges in orientation, the less their susceptibility to the tilt illusio n.