Apparent swinging motion from a 2-D sinusoidal pattern

A new motion illusion is reported that is observed on a 2-D sinusoidal pattern composed of two 1-D sinusoids, in which the constituent elements of the middle column appear to swing relative to the two flanking columns when the point of fixation is slowly moved back and forth about the middle column. To better understand the underlying mechanisms of the apparent swinging motion, the spatial properties of a 2-D sinusoidal pattern were examined in terms of spatial frequency, orientation, and contrast. Thirty-four subjects rated the magnitude of the motion. The apparent swinging was greatest when the two 1-D components had spatial frequencies of 1-2 cycles deg-1, relative orientations between 15 degrees and 30 degrees, and high contrasts. A spatiotemporal interaction between spatially overlapping visual units differing in polarity (Khang and Essock, 1997 Perception 26 585-597, 831-846) and the resultant shift in the global-motion signal was proposed as a likely cause of the apparent swinging motion.