The perceived speed of a grating pattern has often been reported to slow as the contrast of the pattern is reduced (though there are some contradictory reports). The mechanism of this perceived slowing has not yet been established nor have the conditions under which the effect occurs (or does not occur). We have therefore examined a range of stimuli that differ upon such aspects as one versus two dimensions, periodic versus nonperiodic, and whether the stimuli occur within a static window. We have also examined a range of stimulus speeds, different types of motion, and simultaneous versus successive presentations. We have found evidence for contrast-induced changes in perceived speed in all our stimuli, and thus suggest that none of the stimulus factors listed above is critical in producing the effect. Though the pattern of results is complex and shows substantial intersubject variation, we generally found that slowly moving patterns presented simultaneously produced the greatest decrease in perceived speed with decreasing contrast. On the other hand faster speeds and successive presentation produced more veridical matches or even an increase in perceived speed with decreasing contrast.
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