Shape Encoding and Subjective Contours

Abstract

Ullman [15] has investigated the shape of subjective contours (see for example [7]. [4]. [5], (121). In fact, the work is more generally applicable to other cases of pcrccptual shape completion, in which the visual system is not constrained by actual physical intensity changes. Examples include patterns foimcd from dots and incomplctcly line drawings and alphabetical characters. Ullman proposes that subjective contc,.us consist of two circles which meet smoothly and which arc tangential to the contrast boundaries from which they originate. The foim of the solution derives from a number of premises, one of which Ullman calls “the locality hypothesis”. ‘Ihis is “based in part on cxpcrimcntal obscrvntions, and partly on a theoretical consideration ” ([I 51 ~2). ‘I’hc“cxpcrimcntal observation” rcfcrrcd to is the following: suppose that A ’ is a point near A on the filled-in contour AB as shown in Figure 1. If the process by which AB was constructed is applied to A’B, it is claimed that