Seeing motion behind occluders.

The visual system has no difficulty maintaining the identity of an object as it disappears and reappears behind stationary occluders. In the natural world, a moving object may differ from occluders by many characteristics (colour, depth, shape and so on). Scene segmentation based on these characteristics is thought to happen early in visual processing, and to influence how objects, including moving objects, are identified. What happens if the only characteristic distinguishing an object is its direction of motion? Experiments with random dot displays show that one dot moving in a constant trajectory is readily detected among identical dots in brownian motion. Detection declines sharply if the trajectory is intermittently broken, but improves if occluders obscure the breaks in the trajectory. It is not sufficient that these occluders be perceived as segmented from the rest of the display (such as by colour or depth). Rather, it is critical that the occluders do not contain motion that is similar in direction to that of the target trajectory. We conclude that detection of the trajectory is due to the integration of information within a network of low-level motion detectors and is not dependent on segmentation processes.