As a mechanism to detect differential motion, we have proposed a model of 'a motion contrast detector' and have shown that it can explain the perceptual change from motion capture to induced motion with increasing stimulus size and decreasing eccentricity. To further test the feasibility of the model, we examined the effect of surround motion on the motion aftereffect (MAE) elicited in the center. Using a drifting grating surrounded by another drifting grating, the duration of MAE in the center after adaptation was measured for various surround velocities (Expt 1). MAE was stronger when the surround moved oppositely to, than together with, the center. This finding was consistent with some previous reports. Using similar stimuli, MAE was measured at various stimulus sizes and eccentricities by the cancellation technique (Expt 2). The effect of surround modulation turned out to vary with both size and eccentricity. We examined if the apparent dependence on eccentricity could reflect a simpler effect of cortical size when the data were rescaled according to a linear scaling factor. We interpret our results in terms of motion contrast detectors, possibly located in the area MT.