Tilt aftereffects with subjective contours.

Abstract

CELLS in the visual system respond well to spatial discontinuities in luminance (edges), but they are poorly activated by diffuse light1. It has been proposed2 that the selective response of edge-detectors to properties such as orientation and motion underlies form and pattern perception. In these terms, local discontinuity within the visual display is a necessary condition for perception of an edge. There are, however, anomalous contour effects3 (see examples in Fig. 1a and 16) in which edges are clearly visible at sites where the visual stimulus is homogeneous. Such edges have been called subjective or cognitive contours on the supposition that cues to apparent depth within the display lead the observer to infer that he is seeing one plane located in front of, and interrupting, another plane4,5. In these terms, “Since every plane must have an edge, the bounding contour is supplied by the perceptual system … a subjective contour is simply the edge of a subjective plane, and a subjective plane is a surface which ought to be present on the basis of available depth cues, but is not except in the mind of the perceiver.”6