Kinetic disruption of optical texture: The perception of depth at an edge

An examination of the changes in visual stimulation that result from changes in the occlusionofone surface or object by another suggests two hypotheses about the motion-carried information that is sufficient for the perception of the relative depth at an edge as well as the perception of occlusion. Ss' reports when shown displays containing only this information give strong support to the hypotheses. Terrestrial animals have presumably evolved to move about in an environment that consists of, among other things, extended surfaces with cliffs, caves, detachable objects, and trees. An important fact about such an environment is the occluding, covering, or hiding of one thing by another. That is, from a given point in space some objects and some surfaces will be temporarily hidden from view while others will be visible. Occluding edges and the spatial relationships implied by them are involved in the traditional study of interposition or superposition as a monocular cue for relative depth. However, starting with the conjecture of Helmholtz (1962, p. 284), extending through the studies of Chapanis and McCleary (1953), and Dinnerstein and Wertheimer (1957) there has been no real success in finding the sufficient proximal information for the perception of occlusion. The position taken here is that if only the static visual stimulus-the frozen pictorial aspect of stimulation-vis considered, there will probably continue to be little success. However, if the more representative situation of changes in superposition is considered, there may be a solution. There are, after all, good reasons for considering the motion-carried aspects of stimulation. Previous attempts at an analysis of such information have demonstrated that they might consistute an important source of information for the visual system (Gibson, 1966, 1968a;3 Hay, 1966; Johannson, 1964; Braunstein, 1966). The present study consists of an attempt to specify the changes in proximal stimulation that stem from changes in the location of two surfaces, one of which is partially occluding the other. Consider that as a consequence of an O's movement or motion of an object relative to an 0, there are changes in the surfaces or parts of surfaces that are projected to the station point at which the O's eye is located.f Figure I illustrates the nature of these changes for the case in which one surface moves past, and thereby covers, another surface. The letters and numbers represent elements of the surface texture and the optical texture (no particular definition of texture element is assumed; however, it is assumed that there is a one-to-one mapping from surface element to optical element). Note that as the right-hand surface progressively occludes the left-hand surface pans of the latter are deleted from the optic array. If the event were reversed then parts of the left-hand surface would be accreted to the array. This optical transformation of accretion/deletion of texture elements is defined with respect to the proximal array only and, for the present discussion, will be taken to be the progressive removal or addition of adjacent elements of the optical texture at some location in the optic array.