The phi phenomenon is a perceptual illusion in which a disembodied perception of motion is produced by a succession of still images. Confusingly, the term "phi phenomena" is used to refer to both phi phenomenon and beta movement, a related illusion. This results in phi phenomenon often being confused with beta movement.
The apparent movement of phi phenomena is not fully understood, but it has led to significant research into visual perception. In particular, Max Wertheimer was led to his studies of perception and the formulation of new ideas that led to the founding of the school of Gestalt psychology through his discovery of this phenomenon. In addition to providing ways to research human abilities, illusions like the phi phenomenon intrigue and entertain people, reflecting our cognitive curiosity and endless appreciation for creativity that is part of human nature.
Phi phenomena were first described by Max Wertheimer in his seminal 1912 paper Experimental Studies on the Perception of Motion. This discovery so intrigued Wertheimer that he continued research on what he considered to be "pure movement"—movement that does not involve perceiving the movement of any object. Wertheimer claimed that movement is a primary sensation, and this became one of the fundamental principles in the school of Gestalt psychology he founded, significantly changing how psychologists approach the study of perception.
The classic phi phenomenon experiment involves a viewer or audience watching a screen, upon which the experimenter projects two images in succession. The first image depicts a line on the left side of the frame. The second image depicts a line 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.
At certain combinations of spacing and timing of the two images, a viewer will report a sensation of motion in the space between and around the two lines, even though the viewer also perceives two distinct lines and not the continuous motion of objects referred to as beta movement. The phi phenomenon looks like a moving zone or cloud of background color surrounding the flashing objects. Phi phenomenon generally takes place at much higher switching speeds than beta movement.
Computerized demonstrations of phi phenomena often show a circular group of smaller circles, which switch on and off in quick sequence. Phi is described as "pure movement" that always takes on the background color. In beta movement, it appears that the circles (or other figures) are moving. With phi, the circles appear stationary, but movement is perceived around them.
The color phi phenomenon is a more complex variation of beta movement, in which a perception of motion and color change is produced by a sequence of still images of different colors.
Gestalt psychologists, including Max Wertheimer, Kurt Koffka, and Wolfgang Kohler, believed that the apparent movement in the phi phenomenon is due to an electrical charge passing across the brain giving a sense of movement. This idea, and the simple design of experimental apparatus to test it, led to significant research into visual perception. Today, however, the electric charge in the brain is no longer considered a valid explanation of phi phenomena.
Although Gestalt psychologists believed that real and apparent movement were mediated by the same process, this is not necessarily the case. Some researchers have questioned this and claim to have found significant differences implying different processes. However, these findings are not conclusive.
Scientists do not yet fully understand what causes the illusion of apparent motion with beta movement or phi phenomenon. Still, most theories involve a physiological, rather than psychological explanation, and have to do with the various ways the brain and optic nerves communicate.
The discovery of phi phenomena played a significant role in Gestalt psychology, as it changed the way perception was studied. Illusions like the phi phenomenon continue to help scientists find new ways to study the way the human brain and visual system perceive and interpret information.
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