The role of memory in perceiving subjective contours.

Abstract

In the variety of subjective contours that are structured like Kanizsa's triangle, shapes consist partly of edges of elements that make up the patterns that give rise to the subjective contours and partly of the subjective contours. It was found that familiarity with the shape that fit the subjective-contour-inducing pattern in this fashion increased the likelihood that subjective con­ tours were perceived when the pattern was shown. Six containing patterns were constructed that did not readily yield subjective contours when the fitting shapes were not familiar. It was found that these containing patterns yielded subjective contours considerably more often when their fitting shapes had been made familiar than when they had not been. This result was obtained even though all subjects were acquainted with subjective contours before the contain­ ing patterns were shown, and even though they were asked whether or not they saw subjective contours when they saw the containing patterns. In a second experiment, subjects had no such set to experience subjective contours. When two of the containing patterns that had been used in the first experiment were shown, no subjective contours were perceived. However, after sub­ jects had been familiarized with a drawing of one of the fitting shapes, they saw that shape in the pattern that contained it and along with it the subjective contours. In recent years, subjective contours, also called illu­ sory contours, have been given much attention, and that is as it should be. Whenever percepts have features that do not have their counterparts in proximal stimulation, they are likely to reveal something about the perceptual processes involved. Subjective contours take many forms. They are discussed in a recent review of the large litera­ ture by Parks (1984). Our investigation deals with the kind of patterns that yield subjective contours most reliably when viewed by naive subjects, that is, by those who have not been specifically aware of subjective contours before. In these patterns, subjective contours are more or less straight connections between aligned edges in cutouts that are readily perceived as cutouts, where the subjective con­ tours are parts of edges of simple shapes, such as the up­ right triangle in Figure 1. The outline of the triangle con­ sists of real contours where it shares the edges of the cutouts in the three black disks. Where the outline of the triangle is not provided by stimulation-that is, in the straight connections between the ends of the real contours-subjective contours are seen. These parts ofthe triangle's outline are the perceived features that have no counterpart in stimulation. The pattern that gives rise to subjective contours-in the case of Figure 1 the three black disks and the interrupted outline of the upside-down triangle-has been called a subjective-contour-inducing array and will be referred to as a containingpattern. The figure that is composed in part of subjective contours, here