Measurements are made of the optimal displacement for an abruptly displaced sinewave grating to elicit a motion aftereffect. At a fixed nominal eccentricity of 4 deg, spatial frequencies ranging from 0.2 to 1.2 c/deg are effective in producing an aftereffect. At any given spatial frequency, the optimal displacement is slightly less than one quarter spatial cycle. The range of effective spatial frequencies does not correspond to the range of optimal spatial frequencies reported for neurons in primate Area V1, but does correspond to that for Area V2.