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Shawn A. Collier, Alan B. Cobo-Lewis; The effects of spatial-frequency and contrast ratio manipulations differ with dioptic and dichoptic viewing of Type 2 plaids. Journal of Vision 2005;5(8):131. doi: 10.1167/5.8.131.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Recent research has supported a Bayesian model of motion combination for the perception of moving Type 2 plaids. When one component grating provides a more reliable motion signal than the other, perceived direction of the plaid is biased in the direction of the more reliable grating. Greater reliability of a grating may be achieved by increasing its spatial frequency (SF) or its contrast relative to the other grating. This predicts a bias of plaid motion toward the grating with higher SF or greater contrast. These biases were investigated under dioptic and dichoptic conditions. Methods: Naïve subjects viewed Type 2 plaids with gratings drifting in directions separated by 15 deg and speeds differing by a factor of sqrt(1.5). Either the SF ratio or contrast ratio of gratings varied from 0.54 to 1.84 in logarithmic steps. Stimuli were viewed dioptically or dichoptically. Subjects adjusted a pointer in the direction of perceived drift. Results: Under dioptic conditions, perceived drift of the plaid pattern was biased in the direction of the grating with the higher SF or greater contrast. When gratings were of equal SF or contrast, plaid direction was perceived between the vector-sum and the intersection of constraints (IOC) directions. Under dichoptic conditions and contrast manipulation, perceived plaid direction was biased in the direction of the component with greater contrast. However, there was little effect of SF ratio manipulation on the perceived direction of plaid motion with dichoptic viewing. Additionally, under dichoptic conditions, when gratings were of equal SF or contrast, plaid direction was perceived at the vector-sum direction. Conclusion: Although motion information is combined across eyes in dichoptic viewing, the motion combination rule seems to differ from dioptic presentation. Constraint lines weighted by reliability seem important for dioptic viewing, but vector-sum weighted primarily by contrast seems important in dichoptic viewing.
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