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Rumi Hisakata, Ikuya Murakami; Illusory position shift induced by plaid motion. Journal of Vision 2009;9(8):689. doi: 10.1167/9.8.689.
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© ARVO (1962-2015); The Authors (2016-present)
The position of a moving stimulus appears as shifted in the motion direction. This motion-induced position shift (MIPS) occurs for many kinds of motion, including random-dot kinematograms, second-order motion and illusory motion after adaptation (motion aftereffect). In this study, we investigated the relationship between the mechanism of this illusion and the motion integration process. For this purpose, a moving plaid was used as the stimulus. First, we examined whether the type-1 plaid induced the MIPS. The two component gratings (50% contrast, 5-cpd spatial frequency) of the plaid moved in ±22.5°, ±45°, or ±67.5° directions, with 0° indicating purely horizontal. The perceived speed of the plaid was equated to that of a vertical Gabor patch moving horizontally at 4 Hz. Because it was type 1, the plaid's pattern motion was faster than its component gratings. Two such plaids were presented at 4-deg eccentricity above and below the fixation point at the center of the monitor. The subject's task was to judge whether the upper stimulus was to the right or left compared to the lower stimulus. Surprisingly, the horizontally moving plaid appeared as shifted purely horizontally, and the MIPS induced by the plaid was larger than that induced by either component grating. Next, we used a pseudo plaid, which was composed of 12 small Gabor patches of random orientations, each moving in the direction that was consistent with a common horizontal global motion (Amano et al., 2008). Two such pseudo plaids were presented above and below, and the subject did the same task. Again, the pseudo plaid also induced the MIPS in the pattern-motion direction. From these results, we conclude that the mechanism of the MIPS is located after integration of local motions into a global one. We will discuss the contribution of the processing of motion integration to position perception.
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