An illusion of relative motion dependent upon spatial frequency and orientation

Observers scanned a stationary pattern comprising a tilted sine-wave grating completely surrounding another grating of similar spatial frequency but tilted in the opposite direction (Fig. 2). They reported an illusory "sliding" motion of the inset grating with respect to the surround grating and the effect was clearly strongest for angles between the gratings of less than 60 degrees and for spatial frequencies between 6-11 cpd. In a second experiment, a similar pattern was moved (2.0 deg/sec) either up or down for a presentation time of 167 msec. Simultaneously, the inset grating was drifted at different speeds in each of its two directions. Using the method of constant stimuli, it was shown that the relative motion illusion could be cancelled by physically drifting the grating in the opposite direction to the illusory movement. The illusion arises because there is a failure to integrate two motion signals into the single motion vector which characterises rigid motion.