The Role of Illusory Contours in Visual Segmentation

Abstract

Many paradoxical percepts are expressions of adaptive brain designs aimed at achieving informative visual representations of the external world. Paradoxical percepts may therefore be used as probes and tests of the mechanisms that are hypothesized to instantiate these adaptive brain designs. Illusory contour percepts, in particular, provide numerous clues and constraints for a theory of boundary formation and textural segmentation, because they involve subtle interactions of form and color processing. A main theme of this chapter is the role of illusory contours in perceptual grouping processes. Our theory makes precise the sense in which percepts of illusory contours—or contour percepts that do not correspond to one-dimensional luminance differences in a scenic image—and percepts of “real contours” are both synthesized by the same mechanisms.This discussion clarifies why, despite the visual system’s manifestly adaptive design, illusory contours are so abundant in visual percepts. We also suggest how illusory contours that are at best marginally visible can have powerful effects on perceptual grouping and object recognition processes.