Induced movement: the flying bluebottle illusion

Two small objects (flies) followed identical circular orbits. However, a large background that circled around behind them in different phases made one orbit look twice as large as the other (size illusion) or made the circles look like very thin horizontal or vertical ellipses with aspect ratios of 7.5:1 or more (shape illusion). The nature of the perceptual distortion depended upon the relative phase between the movements of the background and those of the flies. Brief snatches of the moving background that added up to a circular motion were also effective.