The radial motion and the leftward motion are physically equal in speed, but most observers say the radial motion appears to be faster.
Radial motion looks faster.
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. The perception and discrimination of speed in complex motion. This illusion shows that there are at least two hierarchically organized stages of the brain that process visual information on motion. Psychophysical evidence for a functional hierarchy of motion processing mechanisms. Animations by Peter Bex.