Motion Perception
One explanation of motion perception involves a form of
unconscious inference. That is, when we walk around or move our head in a particular way, we unconsciously expect that images of stationary objects will move on our retina. We discount such movement on the retina as due to our own bodily motion and perceive the objects as stationary.
In contrast, when we are moving and the image of an object does not move on our retina, we perceive that object as moving.
Movement is also at the heart of a set of observations of considerable significance in the historical development of Gestalt theory. These observations concern circumstances in which people perceive movement in the absence of actual physical motion of the stimulus. One familiar instance of this class of events is referred to as the phi phenomenon. In simplest form, the phi phenomenon can be demonstrated by successively turning two adjacent lights on and off. Given appropriate temporal and spatial relations between the two lights, an observer will perceive the first light as if it were moving from its location to that of the second light. People may perceive motion when none actually exists. The motion-picture screen, for example, presents a series of briefly flashed, still images; the movement people see is a creation of their own perceptual systems.