Influence of the spatial periodicity of moving gratings on motion response.

The present experiments examine the suprathreshold response of the motion or direction-selective portion of the human visual system by means of the motion aftereffect (MAE). The MAE was measured as a function of the contrast and spatial frequency of moving sinusoidal gratings. For spatial frequencies less than 1 cy/deg, the MAE speed was found to increase linearly with log contrast up to 80%. For spatial frequencies greater than 1 cy/deg, the rate of increase of the MAE speed with log contrast was not found to be linear over the entire range of contrast. The nonlinearity was greatest for the 8 and 10 cy/deg gratings, which showed very little increase in MAE speed with contrast above 25%. We conclude that the direction-specific mechanisms in human vision show a more limited contrast response to the high spatial frequencies than does the visual system as a whole.