The radial motion and the leftward motion are physically equal in speed, but most observers say the radial motion appears to be faster. See: Bex, P. J., & Makous, W. (1997). Radial motion looks faster. Vision Research, 37, 3399-3405. We speculated there that the greater apparent speed of the radial motion is a consequence of a tendency for the visual system to interpret the radial pattern as motion in depth. Since then, Clifford, Beardsley, & Vaina (Clifford, C. W. G., Beardsley, S. A. & Vaina, L. M. (1999). The perception and discrimination of speed in complex motion. Vision Research 39, 2213-27) have reported evidence that supports this speculation.

All 12 gratings are moving at the same physical speed, but tests show that when observers fixate the crosses in the center of each of the patterns, they judge the speed of the pattern labeled "Expansion" to be fastest. This illusion shows that there are at least two hierarchically organized stages of the brain that process visual information on motion. See: Bex, P. J., Metha, A. B., & Makous, W. (1998). Psychophysical evidence for a functional hierarchy of motion processing mechanisms. Journal of the Optical Society of America A, 15, 769-776.