MOTION DETECTION ON FLASHED, STATIONARY PEDESTAL GRATINGS - EVIDENCE FOR AN OPPONENT-MOTION MECHANISM

Contrast thresholds were measured for discriminating left vs right mot ion of a vertical, 1 c/deg luminance grating lasting for one cycle of motion, This test was presented on a 1 c/deg stationary grating (pedes tal) of twice-threshold, hashed for the duration of the test motion, L u and Sperling [(1995), Vision Research, 35, 2697-2722] argue that the visual system detects the underlying, first-order motion of the test and is immune to the presence of the stationary pedestal (and the 'fea ture wobble' which it induces), On the contrary, we observe that the s tationary pedestal has large effects on motion detection at 7 and 15 H z, and smaller effects at 0.9-3.7 Hz, evidenced by a spatial phase dep endency between the stationary pedestal and moving test, At 15 Hz the motion threshold drops as much as five-fold, with the stationary pedes tal in the optimal spatial phase (i.e., pedestal and test spatially in phase at middle of motion), and the perceived direction of the test m otion reverses with the pedestal in the opposite phase. Phase dependen cy was also explored using a very brief (similar to 1 msec) static ped estal presented with the moving test, The pedestal of Lu and Sperling (flashed for the duration of the test) has a broad spectrum of left an d right moving components which interact with the moving test, The ped estal effects can he explained by the visual system's much higher sens itivity to the difference of the contrast of right vs left moving comp onents than to either component alone. (C) 1998 Elsevier Science Ltd. All rights reserved.