MOVEMENT PERCEPTION

process through which humans and other animals orient themselves to their own or others' physical movements. Most animals, including humans, move in search of food that itself often moves; they move to avoid predators and to mate. Animals must perceive their own movements to balance themselves and to move effectively; without such perceptual functions the chances for survival would be sharply reduced. process through which human beings and other animals orient themselves to their own or others' physical movements. The ability to perceive movement is essential to the survival of virtually all animals, including human beings. An animal needs to detect movement in order to hunt, to elude dangers, and to mate. Although movement is perceived primarily through vision, it can also be detected by several nonvisual senses. The processing of visual data is shared between eye and brain, in proportions that vary from species to species. In general, the eyes of lower animals are adapted to the job of detecting motions directly relevant to the animal's survival. For example, the eyes of frogs are able to register when an insect is moving near enough to be captured. The visual data do not have to be processed by the frog's brain before they are acted onthe eye itself triggers the proper reaction. In higher animals, the brain plays a more active part, permitting responses to a vastly wider range of stimuli. The eye serves primarily to collect visual data, which is transmitted to the brain for processing. Apparently, there are special cells in the visual area of the brain whose function is to detect movement. However, the outer area of the retinas of most higher animals is especially sensitive to movement, which stimulates a reflex turning of the eye in order to bring the moving object into the more acute centre of the field of vision. There are various mechanisms by which the visual system aids the perception of movement. Obviously, if the eye were held motionless, a moving object would soon move out of the centre of vision, if not out of the visual field altogether. But in fact the eyes reflexively follow a moving object in order to keep it in the centre of vision. When the eyes move to take in a stationary scene, the images on the retinas change exactly as if the scene were moving and the eyes were standing still. The scene is experienced as stationary, however, because the brain takes account of the signals to the eye muscles in its interpretation of the retinal image. If something interferes with the normal control over the eye muscles, perceptual errors can result whereby an involuntary movement of the eye produces an apparent motion in what is seen. Perception of movement also exploits the relation between the moving object and the stationary objects that surround it. These objects serve as a frame of reference that fixes the sense of what is moving and what is not. Without such a frame, a stationary object will look as if it is drifting. Side-to-side movement is easier to perceive than movement toward or away from the eye. Depth perception is gained by binocular vision (see space perception), changes in focusing, change in size of the retinal image, and variations in the object's brightness. Technological means can be used to induce the appearance of movement. Two phenomena permit the illusion of motion. First is the persistence of vision, where a visual stimulus continues to be registered by the brain for a very short time after the stimulus ends. Second is the so-called phi function. If two adjacent lights flash on and off alternately, we seem to see a single light shifting back and forth. This is because we tend to fill in gaps between closely spaced objects of vision. These two phenomena are exploited by motion pictures, which consist of rapid successions of still frames in which the moving objects are displaced a very short distance. Several nonvisual senses are also used to perceive movement. Although land animals can infer an object's motion from changes in its sounds' loudness or direction, sound as an indication of movement is generally more useful to fish and to bats. Another important source of data about the movements of our own bodies is kinesthesis. Kinesthetic sensations are produced by sense organs found in membranes at the joints and from the effort felt during consciously controlled motions. Another mode of perceiving our own movements involves the vestibular system, which consists of a series of enclosed cavities (vestibules) found in the inner ear, and responds mainly to movements of the head. The brain uses information from the vestibular system to maintain bodily balance and also to control the aforementioned compensatory eye movements. Additional reading An excellent general text for undergraduates is Robert Sekuler and Randolph Blake, Perception, 3rd ed. (1994), focusing on perception through the senses. Irvin Rock, An Introduction to Perception (1975), is a standard introductory text for experimental psychology students. Also of interest is Shimon Ullman, The Interpretation of Visual Motion (1979), an original and accessible account of how we connect successive views of a moving object. More advanced reading is presented in James E. Cutting, Perception with an Eye for Motion (1986). Louis Jolyon West The Editors of the Encyclopdia Britannica