DOI:10.1068/p3011 Depth capture by kinetic depth and by stereopsis

Vision exploits multiple sources of optical information to specify the three-dimensional layout of objects and their surfaces. One of these sources is stereoscopic visionöthe strong, immediate sense of 3-D provided by the slight parallax differences between left-eye and right-eye views (Wheatstone 1838). The very keen sense of depth (Ogle 1964) and 3-D shape (Julesz 1971) from stereopsis provides the most plausible raison d'eª tre for frontal eye placement in diverse species spanning mammals, fish, and birds (Fox 1978). Another reliable, accurate source of 3-D structure comes from the differential optic flow generated by contours and surface markings of moving 3-D objects (Wallach and O'Connell 1953). Termed kinetic depth (KD), the perception of volume and shape from motion can be as compelling and as accurate as that created by stereopsis (Rogers and Graham 1982). Indeed, under some conditions observers find it impossible to distinguish KD from kinetic stereopsis (Nawrot and Blake 1993). Besides yielding comparable impressions of 3-D shape, stereopsis and KD have other things in common. For example, both stereopsis and KD generate compelling impressions of surface structure within regions of the visual field where there is no explicit depth information specified by motion (Saidpour et al 1992) or by binocular disparity (Grimson 1981)öboth, in other words, support vivid depth and shape interpolation. Converging lines of psychophysical and neurophysiological evidence suggest that KD and at least some forms of stereopsis may involve similar neural mechanisms (see review by Freeman 1998). Psychophysically, it has been shown that adaptation to a stereo-scopically defined surface can temporarily bias the otherwise ambiguous appearance of a 3-D surface specified by KD (Rogers and Graham 1984; Nawrot and Blake 1989, 1991a). In a similar vein, KD can bias perception of surface depth from ambiguous or weak stereoscopic signals (Nawrot and Blake 1993). Neurophysiologically, several laboratories have identified neurons in primate visual areas whose responses are selective for both stereoscopic disparity and direction of motion (Maunsell and Van Essen 1983; Bradley et al 1995; DeAngelis et al 1998). In fact, the responses of some neurons in visual area MT wax and wane in synchrony with the behaving monkey's report of depth in a bistable KD display (Bradley et al 1998).