Proceedings of the Annual Meeting of the Cognitive Science Society

Visual images are ambiguous. Any given image, or collection of images, is consistent with an infinite number of possible states of the external world. Yet, the human visual system seems to have little difficulty in reducing this potential uncertainty to one, or perhaps a few perceptual interpretations. Many vision researchers have investigated what sort of constraints— assumptions about the external world and the images formed of it—that the visual system might be using to arrive at its perceptions. One important class of constraints are those based on genericity or general position. W e propose a theory of illusory contours in which general position assumptions are used to infer certain necessary conditions for the occurrence of illusory figures that appear to occlude their inducers. Experiments with human subjects are described. The results of these experiments suggest an important role for general position assumptions in understanding the perception of illusory contours. It is also demonstrated that parallelism of contours of "blob" type inducers is an important determinant of illusory contour strength.