ON THE ROLE OF 2ND-ORDER SIGNALS IN THE PERCEIVED DIRECTION OF MOTIONOF TYPE-II PLAID PATTERNS

Second-order Type I and Type II plaids were constructed by combining t wo random-dot gratings. Each component consisted of a dynamic random-d ot field, the contrast of which was modulated by a drifting sinusoidal grating. Orienting the two components suitably and interleaving at 12 0 Hz allowed us to produce a two-dimensional plaid pattern made from o ne-dimensional second-order components. The perceived direction of mot ion of both Type I and Type II plaids was measured as a function of st imulus duration. Type I plaids had a perceived direction close to the intersection of constraints/vector sum solution (which only coincide f or these patterns) at all durations. Type II plaids had a perceived di rection that moved away from the vector sum and toward the intersectio n of constraints solution as the duration of presentation increased. T hese results are similar in form to those found for plaids made from f irst-order (luminance-defined) components [Yo & Wilson (1992), Vision Research, 32, 135-147]. This suggests that a delay which operates spec ifically on second-order signals cannot be the sole cause for the chan ge in perceived direction of Type II plaids made from first-order comp onents [Wilson, Ferrera and Yo (1992), Visual Neuroscience, 9,79-97].