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The term Beta movement is an optical illusion whereby viewing a rapidly changing series of static images creates the illusion of a smoothly flowing scene. This occurs when the frame rate is greater than 10 to 12 separate images per second. The illusion of motion caused by animation and film relies on beta movement. The static images do not physically change but give the appearance of motion because of being rapidly changed faster than the eye can see.
This optical illusion is caused by the fact that the human optic nerve responds to changes in light at about 10 cycles per second, so changes about double of this are registered as motion instead of being separate distinct images.
One example of the beta movement effect would be a set of LEDs, as shown at the adjacent picture. The LEDs, electronically, are individually controlled, but our eyes and brains perceive them as a snake running clockwise around the four edges of the square picture. This is also seen commonly on LED displays.
Observations of what has come to be known as beta movement go back to the 19th century. In 1833, Joseph Plateau introduced what became known as the phenakistiscope, an early animation device that displayed apparent motion based on a stroboscopic effect. However, it was incorrectly assumed that the effect was due to persistence of vision. In 1875, Sigmund Exner showed that, under the right conditions, people will see two quick, spatially separated but stationary electrical sparks as a single moving object. In 1912, Max Wertheimer described in detail numerous experiments on apparent motion. Some of these experiments involved beta movement. However, Wertheimer's work became famous (indeed it launched Gestalt psychology) due to his demonstrations of the phi phenomenon—a different illusion of apparent motion.
Beta movement is sometimes confused with the phi phenomenon but they are quite different perceptually. One way to describe the difference is this. In beta movement, you see two stimuli, and , in succession, but you perceive the movement of a single object, , into position . In phi movement, you see the two stimuli and in succession, but what you perceive is the motion of something shadowy passing over and . (Unfortunately, that seems to be about as clear a description as one will find in the scientific literature.) There are many factors that determine whether one will experience beta movement or the phi phenomenon in a particular circumstance. They include the luminance of the stimuli in contrast to the background, the size of the stimuli, how far apart they are, how long each one is displayed, and precisely how much time passes between them (or the extent to which they overlap in time).
The names beta and phi are simply the letters "β" and "ϕ" from the Greek alphabet, and have no particular significance beyond separating the two phenomena.