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Stuart Anstis, Hiroyuki Ito; Vector summation in split-dot motion. Journal of Vision 2002;2(7):742. doi: 10.1167/2.7.742.
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Contrast affects motion strength. Two small, touching dots, of the same or different contrasts and luminance polarities, jumped back and forth along short, orthogonal, crossing paths. Dots were small enough to fuse into a single perceived motion (larger dots would give two transparent motions). Observers reported the perceived direction of the motion, which we found to vary all the way around the clock as the relative dot contrasts varied, even though the stimulus geometry never changed. We found that the visual system takes the vector sum of small moving dots of all contrast levels and polarities. The background luminance at which a black dot and a white dot made equal contributions to the motion percept was at the arithmetic (not geometrical) mean of the two luminances. On this background the two dots had equal Weber contrasts. We examine the role of reversed phi and static displacement.
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