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Hiromasa Takemura, Satohiro Tajima, Ikuya Murakami; Motion integration and segregation modulated by surrounding motion. Journal of Vision 2011;11(11):739. doi: 10.1167/11.11.739.
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When two visual motion components are simultaneously presented in the same region of the visual field, they may be perceptually integrated or segregated depending on the directional difference (van Doorn & Koenderink, 1982). However, influences of surrounding motions on such motion integration/segregation are still unknown. In the present study, we examined the functional relationship between center-surround interaction and motion integration/segregation. We presented two vertical random-dot motions in similar directions (e.g. ±45 deg from upward) in the center and a purely vertical random-dot motion (up or down) in the surrounding annulus. Subjects were asked to judge whether the central stimulus was seen as one coherent motion or two transparent motions. Subjects were also asked to report the perceived directions of the central motions by directional matching. The central stimulus was more frequently categorized as two segregated motions when the surrounding stimulus was moving upward. On the contrary, the central stimulus was more frequently categorized as one coherent upward motion when the surrounding stimulus was moving downward. These influences of surrounding motion on the perceived direction and motion segregation of the center decreased at faster central motion speeds. Since motion streaks were more readily observed in the central stimulus at faster central speeds at which the influence of the surround became weaker, this result refutes the hypothesis that the influence of the surround is mediated by a perceptual bias on motion streaks. Furthermore, we found that these psychophysical results were qualitatively replicable by a population coding model including surround suppression and response normalization in area MT neurons (Tajima et al., 2010). The present results suggest that motion segregation and integration are determined based on the directional representation modulated by the surrounding motion. Surround modulation in population activities may play a key role in the perceptual switching between motion integration and segregation.
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