The perception of apparent motion.

P roducers of motion pictures, tele­ vision programs and even neon signs have long banked on the fact that human beings have a quirk in their visual system. When it is con­ fronted with a rapid series of still im­ ages, the mind can "fill in" the gaps between "frames" and imagine that it sees an object in continuous motion. For instance, a series of neon arrows lighted up in succession are perceived as being a single arrow moving through space. The illusion of continuous mo­ tion is called apparent motion to dis­ tinguish it from "real" motion, which is perceived when an object moves continuously across a viewer's visual field. When Sir Laurence Olivier ap­ pears to be fencing in a film, he is in apparent motion, whereas a person walking across the theater in front of the screen is in real motion. In the century or so since the mo, tion picture was invented, filmmakers and television workers have learned to create many compelling illusions of motion, but their progress has been furthered mainly by rule-of-thumb empiricism. Psychological research is only now beginning to describe the mechanisms by which the visual sys­ tem-the retina and the brain-per­ ceives apparent motion.