When the pedestal grating drifted in the preferred direction of the cell, responses to the test gratings were strongly reduced by the pedestal, regardless of whether the test and pedestal were separated or superimposed. These results support previous work suggesting gain control acts globally over MT receptive fields (Britten & Heuer, 1999; Majaj et al, SFN 2000). When the pedestal grating drifted in the null direction, test responses were reduced only when the test grating was superimposed on the pedestal and were largely unaffected when the gratings were separated. The difference between the results obtained with masks moving in the preferred and null directions suggests that gain control in MT is tuned for the direction of motion.

Moreover, the existence of a tuned normalization signal in MT that follows an untuned normalization stage in V1 may describe the phenomenon of local motion opponency (Qian et al. 1994) without the need to invoke an explicit opponent computation.