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Danting Liu, George Sperling; The perceived motion direction of fast-moving Type-II plaids. Journal of Vision 2008;8(6):19. doi: 10.1167/8.6.19.
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© ARVO (1962-2015); The Authors (2016-present)
A plaid motion stimulus consists of two superimposed sinewave gratings moving in independent directions. Every plaid has a rigid interpretation as a translating single frame. Sometimes plaids components are perceived to move transparently in different directions, sometimes in the direction of the vector sum of the component direction-speed vectors, sometimes in the rigid direction. Whereas the vector sum direction always lies between the two component directions, in Type-II plaids, the rigid direction lies outside, making Type-II plaids useful for discriminating theories. We varied component-contrasts of Type-II plaids to produce different contrast ratios and used high temporal frequencies (e.g., 10 and 20 Hz) to exclude third-order motion (“feature tracking”). Explanations of previous plaid experiments have concentrated on “rigid” versus “vector sum” directions. With our plaids, 1 cycle/deg gratings within a circular Gaussian window, we find something quite different. At sufficiently high temporal frequencies, perceived plaid motion direction is entirely determined by the contrast ratio of the components. Whenever the contrast of the higher-contrast grating exceeds the lower by more than about 4:1, perceived direction is the direction of the higher-contrast component. For intermediate contrast ratios, all intermediate directions are systematically observed. For contrasts of the higher-contrast component ranging from 4 to 32%, the perceived plaid direction depends entirely on the components' contrast ratio, independent of overall contrast. However, when component temporal frequencies decrease towards the range of third-order motion, the direction of plaids with equal and near-equal-contrast components deviates increasingly towards the rigid direction, especially at higher overall contrasts. Conclusions: When only the first-order motion system is stimulated, perceived plaid direction is determined by the relative strengths of the components, strength increasing monotonically with grating contrast. Slower third-order motion processing influences perceived plaid motion towards the rigid direction. The vector sum direction is irrelevant.
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