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Kaoru Amano, Tsunehiro Takeda; Increase of brain activity during the motion aftereffect investigated by magnetoencephalogram. Journal of Vision 2002;2(7):660. doi: 10.1167/2.7.660.
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Purpose: Several neuroimaging studies have reported the enhancement of MT+ activity during the motion aftereffect (MAE). However, only physically standstill test stimuli were used in those studies. Therefore the detail study to investigate the relationship between brain activity during the MAE and the change in perceived velocity caused by adaptation should be conducted. In the present study, we used magnetoencephalogram (MEG) to examine brain activity when subjects observed physically moving test stimuli after adaptation. Method: We used concentric half rings (1.1 c/deg), that moved in either contracting or expanding direction. MEG responses were recorded in synchrony with the change in velocity of motion. In the first experiment, after the presentation of the adapting stimuli at −3 or −0.75 deg/s for 5 s (MAE condition) or 0.6 s (control condition), we presented the test stimuli at −6, −3, −1.5, 0.75, 0, 0.75, 1.5, 3, or 6 deg/s for 1s. Positive and negative values of velocities indicate expansion and contraction of the stimuli, respectively. In the second experiment, subjects adjusted velocities of the test stimuli to null the MAE (perceive no illusory motion). It was designed to investigate whether the enhancement of brain activity during the MAE was directly related to the perception of illusory motion. Results and Conclusion:(1) MEG peak latency showed no statistically significant change by adaptation. (2) Brain activity in area MT+ clearly increased by adaptation and the enhancement was common to all test stimuli irrelevant to the change in perceived velocities by adaptation. (3) MEG intensity was not simply related to perceived velocities of the test stimuli. Our results suggest two possibilities. First, it is possible that the direction selective cells that are sensitive to the opposite direction to the adapting stimuli increase in firing rates during the MAE. Second, each direction selective cells would fire with increasing synchronization by adaptation.
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