A sensation of movement in the absence of actual movement, especially a class of visual illusions that arise when two visual stimuli a few centimetres apart are exposed or displayed in an alternating pattern, first studied systematically in 1912 by the German Gestalt psychologist Max Wertheimer (1880–1943), who distinguished the following stages (Stadien). If each stimulus is exposed for about 50 milliseconds, then with an interstimulus interval below about 25 milliseconds the stimuli, though possibly flickering, appear to be simultaneous (this is called the Simultanstadium); with an interstimulus interval over about 400 milliseconds there is no illusion and the stimuli are seen to alternate (the Sukzessivstadium); but between approximately 25 and 400 milliseconds, under certain conditions defined by Korte's laws, powerful illusions of movement are created: first partial movement when the interstimulus interval is relatively short, then optimal movement when the interval is increased, and finally phi movement when the interval is increased further. These phenomena underlie the effects created by illuminated signboards with arrows or other elements that appear to move, and they also explain the motion pictures of cinema and television. The concepts of alpha movement, beta movement, gamma movement, and delta movement, denoting different forms of apparent movement, were introduced by other German Gestalt psychologists between 1913 and 1915. Apparent movement is also called apparent motion, phenomenal motion, or the phi phenomenon, but in careful usage this latter term is reserved for the phenomenon of phi movement in particular. See also autokinetic effect, iconic store, motion capture, phantom grating, random-dot kinematogram, Ternus phenomenon, vection, waterfall illusion.