Errors in direction-of-motion discrimination with complex stimuli.

The direction of apparent motion in a complex pattern comprising a static 1-cycle/degree (c/deg) grating and a moving 3-c/deg grating changes with stimulus duration. At durations longer than about 150 msec, motion is seen almost veridically; the motion of the 3-c/deg grating, which is seen correctly, merely induces in the 1-c/deg grating a weak apparent motion in the opposite direction. At shorter durations, however, the only motion seen is in the opposite direction from that which, in fact, occurs. The reversed apparent motion is both compelling and consistent; it is reported both by naive and by experienced observers, and, although it only occurs for certain ranges of spatial frequency, contrast and duration, the ranges are substantial. The reversal appears to be almost independent of the temporal frequency and the spatial phase of the stimulus; it occurs both for discrete and for continuous motion. It seems likely that the apparent motion with short duration stimuli reveals properties of local visual movement detection previously unknown and difficult to account for within the framework of current models of motion perception.