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Shawn A. Collier, Alan B. Cobo-Lewis, Nicole A. Thibodeau; Spatial-frequency ratio in Type 2 plaids drives perceived direction from vector-sum to IOC and back again. Journal of Vision 2004;4(8):563. doi: 10.1167/4.8.563.
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Purpose: The perceived direction of moving Type 2 plaids is usually between the veridical intersection-of-constraints (IOC) direction and the errant vector-sum direction. A recent analysis of a Bayesian model of motion extraction predicts that when one of a plaid's component gratings gives a more reliable motion signal than the other, perceived direction should be biased toward the grating with the more reliable motion signal. This predicts a pattern where perceived direction is close to IOC when gratings have similar spatial frequency (SF), but might be biased away from IOC when either grating has a higher SF than the other. Methods: Naïve subjects viewed Type 2 plaids whose component gratings drifted in directions separated by 15 deg and whose speeds differed by a factor of sqrt(1.5). Aperture diameter was 6.2 deg, a spatial Gaussian window had sigma = 1.55 deg, duration was 3 sec, and a fixation marker was present. Each grating had a contrast of 0.1. Overall SF (geometric mean) was 1 or 3 cpd, but SF ratio varied from 0.54 to 1.84, in logarithmic steps. Subjects pointed an arrow in the direction of perceived drift. Results: When the gratings were of equal SF, perceived direction was close to the IOC direction. When the faster grating had higher SF, perceived direction was also substantially biased, as predicted. When the slower grating had higher SF, perceived direction was substantially biased, as predicted. Conclusions: A Bayesian model of motion extraction suggests that IOC and vector-sum directions can arise from a motion analysis that weights motion signals according to their reliability. The analysis indicates that reliability depends on SF. By manipulating the relative reliability of a plaid's gratings, we were able to affect the perceived direction from near IOC to near component (vector-sum) directions, confirming a prediction of the model.
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