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Davis M. Glasser, Duje Tadin; Asymmetric Effects of Spatial Suppression on First- and Second-Order Motion. Journal of Vision 2011;11(11):721. doi: 10.1167/11.11.721.
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Increasing the size of a high-contrast moving grating makes its motion more difficult to discriminate (Tadin et al., 2003). This counterintuitive effect, termed spatial suppression, is believed to reflect antagonistic center-surround mechanisms. It is unknown, however, whether spatial suppression differentially affects first-order and second-order motion processing. Converging evidence from psychophysics and neuroimaging suggests that while first- and second-order motion signals are usually strongly correlated in natural stimuli, they are processed by separate mechanisms. Thus, we hypothesized that spatial suppression may not equally impact the perception of different orders of motion signals. Here, we investigated the effect of stimulus size on first- and second-order motion perception using compound grating stimuli (Nishida & Sato, 1995). The stimuli were high-contrast 2f + 3f gratings, which contain first- and second-order motion information moving in opposite directions. Varying the stimulus onset asynchrony (SOA) between frames changes which signal dominates perception. Motion is perceived in the first-order direction for shorter SOAs and in the second-order direction for longer SOAs. We found that this established relationship changes when the stimulus size is varied. Specifically, increasing stimulus size decreases the influence of first-order motion information, even at low SOAs that normally favor first-order direction. This result suggests that spatial suppression has a considerably stronger effect on first- than second-order motion perception.
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