OBLIGATORY EDGE ASSIGNMENT IN VISION - THE ROLE OF FIGURE AND PART SEGMENTATION IN SYMMETRY DETECTION

Symmetry detection within a shape can be effortless, perhaps because the 2 sides have matching parts according to D. D. Hoffman and W. A. Richards's (1984) minima rule. It becomes more difficult when figure-ground factors are manipulated to reverse the assignment of convexity and concavity for one of the sides. Under these conditions, symmetrical contours no longer have matching parts, but repeated contours do. Repetition now becomes easier to detect than symmetry, reversing the classic effect observed by E. Mach (1885/1959). It was also found here that symmetry perception is less efficient between the contours of 2 separate objects than within an object but that the reverse can apply for repetition detection. It was concluded that each contour in an image is obligatorily assigned to just one of the regions that it separates. This region becomes figural and acquires a shape description in terms of the component parts of its contours.