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Rebecca A. Champion, Stephen T. Hammett, Peter G. Thompson; Perceived direction of plaid motion is not predicted by component speeds. Journal of Vision 2006;6(6):1043. doi: https://doi.org/10.1167/6.6.1043.
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© ARVO (1962-2015); The Authors (2016-present)
It has been shown previously that the perceived direction of a plaid with components of unequal contrast is biased towards the direction of the higher contrast component (Stone, Watson and Mulligan, 1990). It was proposed that this effect is due to the influence of contrast on the perceived speed of the plaid components and this lead to the conclusion that perceived plaid direction is computed by the intersection of constraints (IOC) of the perceived speeds of the components rather than their physical speeds. We tested this proposal at a wider range of component speeds, across which the effect of contrast on perceived speed is seen to reverse. Plaid stimuli were composed of two orthogonal sinusoidal gratings (1 c/deg), of unequal contrast (60% and 30%) drifting at speeds ranging from 2 – 16 deg/sec. Subjects judged whether the plaid was drifting to the left or right of vertical and the direction error was estimated for each component speed. In addition the relative perceived speeds of the individual grating components were estimated to allow prediction of plaid direction errors based on contrast induced errors in component perceived speed. We find that although predicted errors are increasingly biased toward the lower contrast component as component speed increases, the observers' perceived direction is increasingly biased towards the higher contrast component as component speed increases. This result is not predicted by an IOC model using either physical or perceived component speed.
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