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We recently reported a new motion illusion where dots in expanding random dot patterns appear to move faster than those in rotation patterns despite having the same physical speed distributions. In the current paper, we compared expansion and rotation motion to translational motion and found that the perceived dot speed in translation patterns was between that of expansion and rotation. We also explored contraction motion and found subjects perceived dots in contracting patterns as moving slightly faster than those in expanding patterns and much faster than those in rotating patterns. Finally, we found that stimulus presentation order in a trial plays an important role in determining the magnitude of the speed illusion--the effect is greater when the subjectively faster stimulus is viewed second (e.g., expansion after rotation). The dependence on stimulus order is greatest when comparing complex motion patterns with large subjective speed differences. This phenomenon is unlikely to be explained in terms of channel fatigue or adaptation. more...

Using random dot stimuli well controlled for dot speed, we found that the moving features in expanding patterns appear to move faster than those in rotating patterns. The illusion is well correlated with the strength of the global motion signal. For example, in displays where the number of motion directions defining the patterns is reduced, the magnitude of the illusion decreases. Similarly, the strength of the effect diminishes as dot density is reduced. In patterns where only wedge-shaped segments of the stimuli are left exposed, the difference in perceived speed increases with the angular size of the wedge. Stimulus placement relative to the fixation point has little effect on the persistence of this phenomenon-expansion patterns appear to contain elements of greater speed, independent of stimulus eccentricity. These results argue against a local explanation for this perceptual illusion, suggesting that the global motion pattern of the stimulus, per se, is responsible. more...