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Beta movement is a perceptual illusion, described by Max Wertheimer in his 1912 Experimental Studies on the Seeing of Motion, whereby two or more still images are combined by the brain into surmised motion. This is often erroneously referred to as the phi phenomenon, which is a different, related illusion.
The classic beta phenomenon experiment involves a viewer or audience watching a screen, upon which the experimenter projects two images in succession. The first image depicts a ball on the left side of the frame. The second image depicts a ball on the right side of the frame. The images may be shown quickly, in rapid succession, or each frame may be given several seconds of viewing time. Once both images have been projected, the experimenter asks the viewer or audience to describe what they saw.
Generally, audiences will claim that they saw a ball move from left to right. They did not, in fact, see this, but the cognitive process of perception links the two images in time and causality.
The beta phenomenon can also create the illusion of motion toward and away from an audience. When the first image is of a large object, and the second is of a small object (or vice-versa), the audience will generally report that the object moved away from them. Additionally, if the first frame depicts a brightly-colored object against a solid background, and the second depicts the same object but in colors similar to the background, the audience will report that the object moved away from them.