Phi Phenomenon

The phi phenomenon is a type of apparent movement or an illusion of movement. It is also known as stroboscopic movement. In fact the phi phenomenon consists of three types of apparent movement: beta, gamma, and delta movements. For reasons of space, we shall elaborate only on the first, the beta movement. As in the case of the tau and kappa illusions, the phi phenomenon illustrates the complex nature of the interrelations between the perceptions of movement, distance, and time.

The apparent movement occurs when two physically distinct stimuli—say, two light points—are alternately displayed at a frequency that exceeds a certain threshold value. Under such conditions, a continuous movement from one stimulus to the other is perceived. The apparent movement's characteristics depend on a number of factors: the physical spatial distance between the two stimuli; the physical intensity of the stimuli, which determines the amount of energy reaching the relevant sensory apparatus in the perceiver; and the duration or time span between appearance of one stimulus and the next. If the distance and intensity factors are kept constant, then when the time span between the first stimulus and the second is approximately 20 milliseconds, this will result in a perceptual experience of simultaneity. When the time span is increased to approximately 60 milliseconds, this will result in a perception of continuous movement between the two stimuli. At approximately 200 milliseconds, the resulting perception will be of one stimulus followed by the next, without any movement between them.

By 1915, the psychologist A. Korte had defined the conditions for apparent movement as follows:

• If the intensity of stimuli is kept constant, then the time span needed for optimal apparent movement changes in direct proportion to the changes in distance between the stimuli.
• If the time span between stimuli is kept constant, then the required distance between them in order to generate optimal apparent movement changes in direct proportion to stimuli intensity.
• If the distance between stimuli is kept constant, then the stimulus intensity required for optimal apparent movement is inversely proportionate to the time span between the first stimulus and the second one.

Some researchers, however, have questioned the relevance of light intensity—in the visual case—for apparent movement.

Beta movement is a continuous apparent movement between two, closely adjacent static light points that alternate lighting up. Continuous movement is perceived when the frequency at which the lights go on and off exceeds a certain threshold value, which depends on the distance between the two light points. Some researchers believe that light intensity is a significant factor here as well.

Beta movement is used, for instance, on billboards that are actually made up out of tiny static light points: Each couple of adjacent light points switches on and off at an appropriate frequency and thus the flowing effect we are all familiar with is created.

Cinema is the ultimate application of the beta phenomenon. The audience is presented with projections of static images at a frequency that [Page 988]ranges between 12 and 24 images per second, and this is what creates the experience of continuous movement.

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