The razor's edge: a dichotomy between monohedral and dihedral edge perception.

The "motion-induced contour" is seen in an image where a motion sequence specifies a dihedral edge. Five experiments examined the role of dihedral angle size in the perception of this illusory contour. Observers used free magnitude estimation of contour perceptibility; the data were analyzed by a repeated measures ANOVA and Newman-Keuls procedure. It was found that planar motion is the motion-induced contour in the "dihedral edges" where the dihedral angles are 45, 90, or 135 deg. However, the "monohedral edge," where the dihedral angle is zero, does not show a superiority of three-dimensional motion over planar motion. The difference between these two classes of edges is described in terms of ecological optics.