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William Curran, Christopher P Benton; The role of apparent contrast in the speed tuning of direction repulsion. Journal of Vision 2003;3(9):398. doi: 10.1167/3.9.398.
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A number of studies have investigated the speed tuning of direction repulsion. In these, the shift in perceived motion direction of a target is measured as a function of the speed of a distractor. Such studies have all used relatively narrow ranges of distractor speeds. We sought to obtain a more complete measurement of this speed-tuning function. Using translating, superimposed target and distractor planes of Laplacian of Gaussian dots, we measured perceived repulsion of the target (speed 2.5 deg/sec) as a function of distractor speed (0.625 – 15.0 deg/sec). Our results revealed an inverted U-shaped speed-tuning function peaking at about 5 deg/sec. This is what would be predicted if global motion extraction was performed by the two independent speed-tuned channels proposed by Edwards, Badcock and Smith (Vis. Res., 38, 1573–1580). However, it is known that repulsion magnitude is inversely related to distractor contrast, and that contrast sensitivity falls off rapidly at high temporal frequencies. Thus it is possible that the observed attenuation of target repulsion in the presence of high-speed distractors was a consequence of reduced distractor visibility. We measured motion direction detection thresholds for distractors drawn from the same speed range as before. We then repeated the speed-tuning experiment, setting the contrast of the target and distractor planes to ten times their contrast detection thresholds. Following this manipulation of stimulus contrast, we found that the attenuation of direction repulsion at high distractor speeds was almost entirely eliminated. Thus the reduction of repulsion magnitude in the presence of a high-speed distractor can be largely explained in terms of the reduced apparent contrast of the distractor. These results suggest that, if direction repulsion occurs at the level of Edwards et al's proposed speed-tuned channels, the two channels cannot be wholly independent.
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