Beta Movement

Beta movement is an optical illusion in which two brief, successive projections of similar figures shown one after another at different nearby locations are perceived not as two separate figures but as a single figure moving smoothly from the first location to the second. The Belgian physicist Joseph Plateau laid early groundwork for the study of apparent motion with his invention of the phenakistiscope in 1833, and in 1875 Sigmund Exner demonstrated a related effect by showing that people perceive two spatially separated electrical sparks, flashed one after the other, as a single spark moving between the two locations. The psychologist Max Wertheimer's influential 1912 research, using an instrument called a tachistoscope to control the timing of brief visual flashes precisely, distinguished several different apparent-motion effects that occur at different flash frequencies and helped lay the foundations of Gestalt psychology, and Friedrich Kenkel coined the specific German term beta movement, or beta Bewegung, in 1913. Beta movement is frequently confused with the closely related but distinct phi phenomenon: in beta movement, which occurs at intermediate flash frequencies, observers perceive a single object moving between the two positions, whereas in the phi phenomenon, which occurs at faster flash frequencies, observers instead report a vague, shadowy sense of something passing between the two locations without perceiving an actual moving object; researchers have noted that neither beta movement nor the phi phenomenon fully explains the short-range apparent motion perceived in film, since the human visual system cannot distinguish between such short-range apparent motion and genuinely continuous real motion, even though long-range beta movement is recognized as a separate phenomenon from real motion.