Subjective contours and apparent depth.

The phenomenal appearance of contours in the absence of abrupt stimulus gradients was first discovered by F. Schumann. Recently such contours have been produced in Julesz patterns using binocular disparity. Analysis indicates that both monocular and binocular subjective contours result from the presence of depth cues in the stimulus array. Schumann (1904) investigated the physical parameters which affect the perception of contour. He reported that for a contour to be perceived the prerequisite was some relatively abrupt local change in brightness or color. In the course of these investigations, however, he noted that under certain favorable conditions contours could be made to appear in areas of the visual field where the physical stimulation was homogenous. As evidence of this, he presented the pattern shown in Figure 1. In the center of this pattern most observers see a white square, bounded by intermittent black and white on either side. Since the contours delineating this square are not actually present in the stimulation, these phenomena have been called "subjective contours " (Osgood, 1951). The subjective contours in the Schumann figure are somewhat unstable. Not all observers see these contour lines, and when they are carefully fixated, they tend to disappear. Their appearance is generally enhanced when the configuration is of small visual angle. All of these factors led Schumann to conclude that subjective contours were the result of some organizational 1 This work was partially supported by National