A Novel Speed Illusion Involving Expansion and Rotation Patterns
Bard J. Geesaman and Ning Qian, Vision Research, 1996, 36:3281-3292.
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Abstract
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.
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