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Beta movement is an optical illusion, first described by Max Wertheimer in 1912, whereby a series of static images on a screen 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 is thought to rely on beta movement and the phi phenomenon, but the exact causes are still unclear. 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.
Examples of use
One example of the beta movement effect would be a set of LEDs, as shown at the picture on the right. 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.
The classic beta phenomenon experiment involved a viewer watching a screen, upon which the experimenter projected two images in succession. The first image depicted a ball on the left side of the frame. The second image depicted a ball on the right side of the frame. In the experiment, the images were first held steady, then switched between the two frames. The experimenter asked then what the audience thought they saw.
The beta phenomenon is often confused with the phi phenomenon but they are quite different physiologically. The phi phenomenon can be considered to be an apparent movement caused by luminous impulses in sequence, (that is to say, it is lights going on and off at regular intervals), whereas the beta movement is an apparent movement caused by lights that do not move, but seem to.
Beta and phi
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.
- Wertheimer, M. (1912). Experimentelle Studien über das Sehen von Bewegung. Zeitschrift für Psychologie, 61, 161–265.
- Pizlo, Filip J. (2000). "Phi is not Beta". www.psych.purdue.edu. Retrieved 14 July 2011.