Motion perception: from phi to omega.

When human observers view dynamic random noise, such as television ''snow'', through a curved or annular aperture, they experience a compelling illusion that the noise is moving smoothly and coherently around the curve (the ''omega effect''). In several series of experiments, we have investigated the conditions under which this effect occurs and the possible mechanisms that might cause it. We contrast the omega effect with ''phi motion'', seen when an object suddenly changes position. Our conclusions are that the visual scene is first segmented into objects before a coherent velocity is assigned to the texture on each object''s surface. The omega effect arises because there are motion mechanisms that deal specifically with object rotation and these interact with pattern mechanisms sensitive to curvature.