On the role of second-order signals in the perceived direction of motion of type II plaid patterns

Second-order Type I and Type II plaids were constructed by combining two random-dot gratings. Each component consisted of a dynamic 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 stimulus duration. Type I plaids had a perceived direction close to the intersection of constraints/vector sum solution (which only coincide for these patterns) at all durations. Type II plaids had a perceived direction that moved away from the vector sum and toward the intersection of constraints solution as the duration of presentation increased. These results are similar in form to those found for plaids made from first-order (luminance-defined) components [Yo & Wilson (1992), Vision Research, 32, 135-147]. This suggests that a delay which operates specifically on second-order signals cannot be the sole cause for the change in perceived direction of Type II plaids made from first-order components [Wilson, Ferrera & Yo (1992), Visual Neuroscience, 9, 79-97].