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S. Tsujimura, Q. Zaidi; Higher sensitivity for relative motion is due to position tracking. Journal of Vision 2001;1(3):87. doi: https://doi.org/10.1167/1.3.87.
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© ARVO (1962-2015); The Authors (2016-present)
When are observers more sensitive to relative than to uniform motion? Is this due to position tracking or specialized motion energy mechanisms? Experiment 1 measured displacement thresholds for shear and uniform motion of two abutting vertical sinusoidal gratings (Contrast = 0.05,0.2; TF= 0.25,0.5,1,2,4 Hz; SF= 0.5,1,2 c/deg). Observers simultaneously indicated perceived directions of motion for both gratings (4AFC). Displacement thresholds were lower for shear than for uniform motion, with a greater difference at slower velocities. In Experiment 2 half-cycle presentations of the test stimuli above were added to steady stationary gratings of 4 times the contrast with the same spatial frequency and orientation. The composite grating has the same spatial frequency and orientation, and oscillates in phase during each half-cycle: the initial motion is in the test direction when the initial superposition is in-phase, and in the opposite direction when the initial superposition is in opposite-phase. Each moving test grating contributes unidirectional motion energy to the Fourier spectrum. When both the top and bottom superpositions are in opposite-phase, detection of the correct directions of the test gratings indicate the extraction of motion energy. For both shear and uniform motion, motion energy was extracted at velocities above 2 deg/sec. When the top superposition is in-phase and the bottom in opposite-phase or vice versa, uniform test motion leads to shearing composites, and shearing tests lead to uniform composites. Accuracy of position tracking, measured by correct detection of shear versus uniform motion of the compound, was greater for shear at velocities below 1 deg/sec. These results show that observers are more sensitive to relative than to uniform motion only for slow velocities. At these velocities both motions are subserved by position tracking. At higher velocities both motions are processed by motion energy mechanisms.
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