The effect of smooth tracking and saccadic eye movements on the perception of size: The shrinking circle illusion

An illusory distortion is described in which the apparent diameter of the path of a target moving in a circle appears contracted at intermediate speeds. The apparent reduction of path diameter is highly correlated with smooth pursuit behaviour of the eye, and the distortion disappears with the emergence of saccadic movements at higher speeds. In the absence of tracking eye movement the magnitude of the distortion is greatly reduced. These data are discussed in terms of a possible interaction between smooth pursuit and saccadic eye movements in the perception of size, and possible differences in the utilization of information from the two eye movement systems.