ABSTRACT
The visual system segments optical input into regions that are separated by perceived contours or boundaries. This rapid, seemingly automatic, early step in visual processing is difficult to characterize, largely because
·many perceived contours have no obvious correlates in the optical input. A contour in a pattern of luminances is generally defined as a spatial discontinuity in luminance. Although usually sufficient, however, such discontinuities are by no means necessary for sustaining perceived contours. Regions separated by visual contours also oc-
cur in the presence of: statistical differences in textural qualities such as orientation, shape, density, or color (Beck, 1966a, 1966b, 1972,1982, 1983;Beck,Prazdny, & Rosenfeld, 1983), binocular matching of elements of differing disparities (Julesz, ! 960), accretion and deletion of texture elements in moving displays (Kaplan, 1969), and classical "subjective contours" {Kanizsa, ! 955). The extent to which the types of perceived contours just named involve the same visual processes as those triggered by luminance contours is not obvious, although the former are certainly as perceptually real and generally as vivid as the latter.
