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Simon J. Cropper, David R. Badcock; Perception of direction of motion reflects the early integration of first and second-order stimulus spatial properties. Journal of Vision 2008;8(4):12. doi: 10.1167/8.4.12.
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Second-order Type I and Type II plaids were constructed by combining two orientation-filtered random-dot gratings. Each component consisted of a dynamic filtered random-dot field, the contrast of which was modulated by a drifting sinusoidal grating. Orienting the two components suitably and interleaving at 120 Hz allowed us to produce a two-dimensional plaid pattern made from one-dimensional second-order components. The perceived direction of motion of both Type I and Type II plaids was measured as a function of the orientation content of the carrier, the contrast, and the duration of the stimulus. Type I plaids had a perceived direction close to the intersection of constraints/vector sum solution (which coincide for Type I patterns) for all conditions when the motion was visible. Type II plaids had a perceived direction that moved away from the vector sum and toward the intersection of constraints solution as the orientation bandwidth of the carrier increased. The data explain discrepancies in previous work using comparable stimuli and are consistent with recent evidence that the previously considered parallel pathways of form and motion have a strong influence upon one another from early stages of cortical visual processing.
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