Motion perception is the process of inferring the speed and direction of objects that move in a visual scene given some visual input. It is the way in which the human mind processes information regarding the movement of objects. It also includes the idea of processing the motion of the human body through its surroundings. In addition to allowing the mind to perceive which object is moving through which space, motion perception is also significant in determining the distance between objects and their sizes. Motion perception in particular studies how the brain interprets movement around it and how the world is actually moving. Its components include the human body, mind and the visual systems that receive the information. A situation where motion perception may be observed is when one is watching a movie projected onto a screen through a film reel. The film reel itself is made up of hundreds of individual pictures in which there is no movement. However, when they are projected onto the screen at a high rate of speed, the motion perception abilities of the brain translates the single images the eyes see as having fluid movement. Motion perception may be divided into three categories.
First, we have motion parallax. Motion parallax provides perceptual cues about difference in distance and motion, and is associated with depth perception. For example, if you are riding in a car, objects that are close to you seem to go by really quickly (for example, a road sign that you pass), but objects that are further away appear to move much more slowly. Second, we have apparent motion. This is an optical illusion that makes a still object appear to move. It works by flashing pictures of a still image in different locations so quickly that the image seems to move from one location to the other. If you draw a picture of a stick person throwing a ball, and have consecutive pictures of the ball in different spots along its path until in the last picture it is on the ground, and then flip quickly through the pictures, it would appear the ball was actually thrown. Animation uses this phenomenon to give the impression that inanimate objects are moving as well. Lastly, we have background motion. Background motion is somewhat when two or more stimuli that are switched on and off in alternation can produce two different motion precepts.