The motion aftereffect (MAE) is a modification of motion perception following prolonged observation of a regularly moving stimulus (adaptation to the stimulus). The MAE involves the apparent motion of a stationary stimulus in the direction opposite to a previously observed one. Many psychophysical studies on the MAE have been reported to investigate the mechanisms of motion perception. On the other hand, some noninvasive functional brain imaging studies have reported the brain activity in area MT/V5 during the MAE. However, only static stimuli were used in these studies; therefore insufficient study on the relationship between brain activity and velocity perception during the MAE has been conducted. So we investigated this relationship by using both moving and static stimuli presented after adaptation, which were perceived to move with a velocity different from the physical velocity by the MAE. We used concentric half rings that moved in either a contracting or an expanding direction as visual stimuli. MEG responses were recorded in synchrony with the instance of change in the direction of motion or of the termination of motion under adapted or nonadapted states. Perceived velocities were measured before and after the velocity changes of the stimuli. We also measured the MEG responses when subjects' perceptions were equal (subjectively static) between adapted and nonadapted conditions to investigate the effect of adaptation on brain activity. Subjectively static stimuli were defined by psychophysical measurement in a preliminary session. The results: (1) MEG peak latency did not change by adaptation; (2) There exists a brain area whose activity increases by adaptation; (3) Measured MEG intensity was not simply related to the perceived velocity, and an effect of adaptation on the intensity is significant. These results with previous electrophysiological studies suggest the possibility that the direction-selective cells sensitive to the direction opposite to adaptation stimuli increase in firing rates.
