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Caterina Ripamonti; Illusory backward motion occurs only with a luminance component. Journal of Vision 2010;10(7):441. doi: 10.1167/10.7.441.
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For stimulus durations shorter that 35 msec, the perceived direction of motion of a stimulus composed of a moving 3-c/deg grating and a static 1-c/deg grating can appear reversed (Derrington and Henning, 1987), even though the direction of motion of the high frequency grating when presented alone is perceived correctly. We tested whether this illusory motion occurs also for stimuli composed of coloured gratings with and without a luminance component. Stimuli consisted of a moving 3-c/deg and a static 1-c/deg horizontal sinewave gratings. The high frequency grating moved at 6-c/deg or 12-c/deg. The gratings were seen through a circular aperture of 5 deg diameter surrounded by a uniform grey background. Stimulus duration was controlled by varying the standard deviation of a temporal Gaussian envelope. A 2-AFC paradigm was used to determine the perceived motion direction of the stimulus. When both gratings contained a luminance component, we found that for stimulus durations between 35 and 125 msec, the gratings appeared to slide on top of each other. The apparent motion of the steady low-frequency grating was in the opposite direction to the high-frequency grating. At durations shorter than 35 msec, the two gratings appeared as a single pattern moving in the opposite direction of the high frequency grating (illusory motion). Interestingly, when either or both gratings were isoluminant, only the high frequency grating was seen moving. Its perceived direction of motion was correct only for durations above 35 msec, but below 35 msec performance was change. In summary, illusory backward motion is only found with stimuli with a luminance component. We suggest that illusory backward motion is due to a higher-order feature tracking system that requires two luminance inputs.
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