Abstract
Prior research has shown that some depth cues, such as stereo and shadows, can generate strong percepts of motion. Here we tested whether another cue, occlusion, can also generate perceived motion in depth. In our basic display, blue circles, outlined in black, appeared one at a time at random locations on a black background. Old circles remained in the image as new circles appeared. New circles opaquely covered portions of old circles where they overlapped: Regardless of their prior colors, image locations where the edge of a new circle fell were set to black, and image locations corresponding to the interior of a new circle were set to blue. Monocular viewing of this display generated a strong percept of motion in depth; the circles appeared to stack on top of each other, with new circles appearing closer to the viewer than older ones. The circles also appeared to grow in size as they moved, consistent with a looming percept. To test if this occlusion display relies upon non-cognitive, relatively low-level mechanisms, we measured an aftereffect of the motion on a stimulus that could be seen as moving either toward or away from the observer. Subjects viewed this neutral stimulus both prior to and following adaptation to the occlusion display. The neutral stimulus consisted of identical unfilled blue circles that appeared one at a time, at random locations on the black background. New circles were transparent relative to old circles: Image locations that corresponded to the interior of a new circle remained their prior color. Before viewing the illusion, the neutral stimulus was perceived to be moving toward the observer more often than away from the observer. After viewing the illusion for two minutes, the neutral stimulus was perceived to be moving away from the observer reliably more often than toward the observer. Our results suggest that occlusion can have a substantial effect on perceptual motion mechanisms.