Subjective contours at line terminations depend on scene layout analysis, not image processing

Subjective contours at abutting lines, commonly attributed to levels of the visual system where image properties are directly encoded, are shown to require an explanation at the level of visual surface understanding. By the use of stereoscopic displays, subjective contours could either be abolished where they were monocularly strong or be created where they were monocularly nonexistent. Furthermore, it was shown that subjective contours are determined by the boundaries of occluded rather than occluding regions. The results cannot be accounted for by adding a stereo component to existing image-based theories, such as those based on end-stopped cells. Instead, any plausible process must explicitly determine the position of bounding surface contours in the scene. Minimum requirements of such a process are given. Subjective contours are of two major varieties: those that complete existing luminance contours across gaps (Schumann, 1900; Kanizsa, 1974; see Figure 1, top) and those that form along a set of line terminators (Ehrenstein, 1941/1987), especially along two sets of abutting line terminators (Kanizsa, 1974; see Figure 1, bottom left). In both cases, it is generally agreed that subjective contours serve the function of delineating the boundaries of occluding surfaces when luminance contours are camouflaged in the image by the luminance of the surrounding field. With respect to mechanism, there is considerable disagreement. Several physiological findings of great interest, discussed in detail, subsequently have encouraged the continuing widespread view that subjective contours are the product of low-level physiological mechanisms acting at early levels of the visual system that directly encode image properties. This marries function and mechanism in an unusually persuasive and satisfying manner. It is the goal of this article to present a different view based on striking new findings. We used stereograms to manipulate the three-dimensional (3-D) placement of the contours of the abutting line effect. This is the simplest configuration giving rise to subjective contours and the one most readily explained by low-level theories (Treisman, Cavanagh, Fischer, Ramachandran, & von der Heydt, 1990). On the basis of our results, we argue that such subjective contours can be explained only at the level of scene analysis. We sketch out some of the complexities an adequate explanation must encompass. We also attempt to analyze in some detail why current image-based mechanisms cannot provide such an explanation.