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William Curran, Colin Clifford, Christopher Benton; Adaptation precedes inhibition for motion direction interactions. Journal of Vision 2008;8(6):1033. doi: 10.1167/8.6.1033.
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© ARVO (1962-2015); The Authors (2016-present)
Neural adaptation and inhibition are thought to underlie the direction aftereffect (DAE) and direction repulsion, respectively. With these phenomena one over-estimates the difference between a test's motion direction and that of a previous adaptor (DAE) or simultaneous distractor (direction repulsion). Two sets of experiments addressed whether these phenomena reflect neural activity at the same or different stages of motion processing. In the first series of experiments we used a recently reported phenomenon, the binary direction aftereffect, to demonstrate that the DAE precedes direction repulsion. The binary direction aftereffect was induced by adapting to a transparent, mixed-speed stimulus in which fast dots moved 25 deg off vertical and slow dots moved 25 deg to the other side of vertical. When viewing a post-adaptation test stimulus containing fast and slow dots moving vertically, both speed sets were perceived as moving in an off-vertical direction away from their respective adaptor speeds. When a single-speed test stimulus was used, the speed-specific aftereffect was significantly reduced. This difference in DAE magnitude between mixed- and single-speed test stimuli was consistent with direction repulsion occurring subsequent to direction adaptation within the test stimulus. The second series of experiments had observers adapt to a direction repulsion stimulus, and then judge the direction of a uni-directional test stimulus. The test stimulus direction was half way between one of the adaptor directions and its perceived direction. If the DAE precedes direction repulsion, one would predict that apparent test direction would be determined by the physical directions of the adaptor. If direction repulsion precedes the DAE, apparent test direction would be determined by the perceived directions of the adaptor. Observers' responses were consistent with a DAE induced by the physical adapting directions. The results of these experiments demonstrate that the DAE precedes direction repulsion in the visual motion processing hierarchy.
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