Analysis of Perspective Reversal and Associated Apparent Motions Using a Perspective-Bound Movement Illusion

Abstract

The nature of perspective reversal was examined using among other techniques a previously undescribed movement illusion specific to the non-veridical perception of actual depth. The apparent movement of the illusion proved to be veridical parallax movement displaced spatially. Apparent changes in direction of rotation and apparent oscillation were shown to be consequences of perspective. Objects seen in reversed perspective illustrated spectacularly the size-distance invariance. Detailed analysis revealed that depth perception per se is veridical, and only the apparent relocations of parts are involved in perspective reversal. When a perspective reverses, O misperceives the location of the near and far parts of the object, but those parts “reverse” about the veridical center in situ and on a strictly 1:1 depth basis. Perspective changes occur only at a plane perpendicular to O in the depth dimension—never in the horizontal-vertical plane. Parts of a single figure may reverse independently of others, thereby forming a separate perceptual unit, the configuration of which is determined by O‘s position rather than by properties of the stimulus. More complex figures (e.g., a rectangular prism composed of three cubes), may be perceived as an entire Gestalt, or as various smaller independent units each reversing perspective independently as verified by the movement illusion. The analysis of the nature of perspective reversal suggests that depth perception is composed of at least two processes: first, the perception of absolute depth, and second, the spatial ordering of objects or points on objects. The first process seems not to be related to perspective reversal, but the second seems to be implicated as the critical one. The depth dimension necessarily is represented nonveridically as a flat projection on the retina, while those parts of the visual field that are perpendicular to O are represented veridically. When O “reconstructs” the depth dimension from this flat retinal image (whether the depth is real or only apparent in the stimulus), depending on how he interprets the order of the corresponding parts, he will perceive the object in true or false perspective.