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Rumi Hisakata, Ikuya Murakami; Spatial scaling for the Rotating Snakes illusion. Journal of Vision 2010;10(7):828. doi: 10.1167/10.7.828.
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© ARVO (1962-2015); The Authors (2016-present)
In the Rotating Snakes illusion, vigorous motion is perceived in a static figure comprised of repetitive luminance micropatterns: black, dark-gray, white, and light-gray. Our previous study (Hisakata & Murakami, 2008) showed that the illusion strength increased with eccentricity, implying that motion processing units related to this illusion have preferred stimulus sizes that systematically vary with eccentricity. To investigate the quantitative details about the effect of eccentricity on the illusion, we measured the illusion strength while manipulating stimulus size and eccentricity. An array of micropatterns arranged as a ring with the strip width of 2 deg rotated about the fixation point. The background was filled with static random noise. After the stimulus ring was presented for 500 ms, subjects answered whether the ring appeared to rotate clockwise or counter-clockwise. The size of each micropattern was manipulated by changing the number of micropatterns per ring, and the eccentricity was manipulated by changing the radius of the stimulus ring. Results indicated that the illusion strength, i.e., the physical velocity that just nulled the illusory motion, decreased with decreasing size and reached the minimum at a particular size at each eccentricity. We applied the spatial scaling technique to the illusion strength and found that all data converged into a single function when both stimulus size and nulling velocity were scaled according to scaling factors as a linear function of eccentricity. Compared with the scaling factors in previous studies, the estimated scaling factors for the Rotating Snakes illusion were analogous with those estimated for contrast detection thresholds and motion detection thresholds, both believed to reflect cortical architecture at early stages of the visual system. These results suggest that processing units at early stages internally produce motion signals related to illusory motion from the retinal image of the stimulus for the Rotating Snakes illusion.
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