Discriminating the direction of second-order motion at short stimulus durations

We measured the ability of human observers to discriminate the direction of motion of different spatial patterns presented for durations ranging from 0.021 to 0.67 sec. The patterns were: (1) a vertical grating (spatial frequency 0.93 c/deg at 5% contrast); (2) a "beat" pattern made by adding vertical gratings of 6.3 and 5.4 c/deg both at 5% contrast moving in opposite directions (this pattern appears as a horizontally moving, 0.93 c/deg "beat"; i.e. spatial variation in the contrast of a stationary vertical grating of 5.8 c/deg); and (3) a "plaid" pattern made by adding gratings of 5.9 c/deg orientated +/- 81 deg from vertical (this pattern can also be expressed as a horizontally moving 1.9 c/deg beat in a horizontal grating of 5.8 c/deg). The direction of motion of the grating and the plaid pattern were discriminable at all durations tested. The direction of motion of the beat could only be discriminated at durations above approx. 200 msec. We suggest that this is a consequence of the fact that the moving beat is only visible to second-order mechanisms, and that second-order mechanisms for the analysis of motion operate more slowly than first-order mechanisms.