Visual Perception Survey Project -- Perceiving Motion and Events

When a moving object is viewed by an observer with eyes, head, and body fixed, its projected image moves across the retina. In this instance it is natural to equate object motion with image motion. But much of the time, our eyes, head, and body are in motion, and this fact invalidates any attempt to equate object motion directly with image motion. Image motion depends on both eye movements and object motion. As the eye moves in one direction, the image sweeps across the visual field in the opposite direction. But, there is image motion when there is no object motion at all. The opposite can also occur, as when a moving object is tracked by the eye: It's image on the retina will actually be stationary even though the object itself is in motion. For this reason, it is important to distinguish clearly between image motion and object always depends on eye, head, and body movements as well as image motion. An environmental object is considered to be moving if its position changes over time. Because real objects do not disappear from one place and reappear in another, these changes in object position are continuous events. Not all such events produce visual experiences of motion, however; some occur too rapidly, and others too slowly. In terms of space-time diagrams, the boundary conditions for continuous motion perception are defined by the slope or orientation of space-time contours with respect to the temporal axis. A moving object is perceived as stationary if the slope of its space-time contour is too shallow. If the slope of its space-time contour is too steep, it is perceived not as moving, but as spatially smeared across its trajectory. Adaptation and Aftereffects When an observer stares for a prolonged period at a field of image elements moving at a constant direction and speed, the visual system undergoes motion adaptation; that is, its response to the motion diminishes in intensity over time. It is related to motion aftereffects: by viewing of constant motion for a prolonged period, after the motion has stopped, subsequent motion perception is significantly altered. Apparent Motion Modern technology provides a host of situations in which realistic motion perception arises from the rapid presentation of completely static images. This technique rely on the fact that the human visual system can be fooled into perceiving continuous motion from a sequence of "snapshots" or "frames" presented …