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Yu-Chin Chai, Bart Farell; Effects of reference stimuli on motion sensitivity. Journal of Vision 2007;7(9):401. doi: 10.1167/7.9.401.
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The presence of a reference stimulus can alter spatial judgments of a target stimulus. Moreover, the similarity of reference and target stimuli can affect the precision of these judgments. For example, a spatial frequency difference between reference and target can scale stereoacuity by the frequency ratio (Farell, VSS 2003). Motion and stereo are similar in that they take input as a single stimulus at two different locations, spatiotemporal and retinal, respectively. We ask here whether motion sensitivity, like stereoacuity, shows a dependence on the relative spatial frequencies of reference and target stimuli. We measured thresholds for discriminating the phase of one cycle of displacement (first leftward then rightward vs. first rightward then leftward) of sinusoidal gratings. The target was annular (a radial Gabor) and the reference was a foveally centered Gabor patch. Motion thresholds measured the amplitude of the instantaneous to-and-fro phase shift of the surrounding sinusoidal carrier that allowed 84% correct left-right vs. right-left discriminations. The central Gabor reference was stationary. We varied spatial frequencies of target and reference stimuli independently and also varied reference orientation. Target and reference contrasts were constant multiple of contrast detection thresholds. Motion thresholds varied with the relative spatial frequencies and orientations of the target and reference gratings. For a given target frequency, threshold was highest when the reference frequency was at its lowest value. Threshold decreased as the reference frequency increased and was smallest when reference and target frequencies matched. Orientation differences raised thresholds; the increase was proportional to the reference period in the direction of the target's motion. These effects were generally smaller for motion than for stereo, with some qualitative differences as well. The data are consistent with an analysis of motion, including relative motion, that takes place within frequency and orientation channels.
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