Two-stroke: a new illusion of visual motion based on the time course of neural responses in the human visual system.

A sequence of static images presented in rapid succession can create a powerful impression of visual movement, a fact exploited by the visual media (television and cinema) and by animators. A new illusion of movement called "two-stroke" is described, in which repeated presentation of a two-frame pattern displacement can create an impression of continuous forward motion, without the inclusion of any additional pattern displacements. The illusion can be explained by a biphasic temporal impulse response that modifies the stimulus delivered to motion energy sensors. It offers a basis for further research on temporal and motion responses in the visual system as well as a tool for animators and graphic artists to create consistent apparent movement from minimal external stimulation.