To reveal the time and frequency characteristics of the magnetic field data, time frequency representations on each channel were performed by Fourier transpose with a time window of 500 msec, time shift of 75% overlap, and Hanning window. ERD and ERS were described by time-frequency maps divided by a spectrum of the baseline state, which was selected in time intervals of 500 msec before stimuli, so that the final data have values that represent ERS by more than one and ERD by less than one. To confirm time-frequency aspects topographically, time-frequency maps of all of channels were displayed. Time-frequency representations were performed to both power and phase-locking factor.
