The effect of contrast upon perceived speed: a general phenomenon?

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 contradictor y reports). The mechanism of this perceived slowing has not yet been establ ished nor have the conditions under which the effect occurs (or does not oc cur). We have therefore examined a range of stimuli that differ upon such a spects as one versus two dimensions, periodic versus nonperiodic, and wheth er the stimuli occur within a static window. We have also examined a range of stimulus speeds, different types of motion, and simultaneous versus succ essive presentations. We have found evidence for contrast-induced changes i n perceived speed in all our stimuli, and thus suggest that none of the sti mulus 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 pr oduced the greatest decrease in perceived speed with decreasing contrast. O n the other hand faster speeds and successive presentation produced more ve ridical matches or even an increase in perceived speed with decreasing cont rast.