cut-off (3 dB or half-power point) and 6 dB per octave roll-off. The low pass filter was a two-pole Butterworth type with a 100 Hz cut-off (3 dB or half-power point) and 12 dB octave roll-off. Activity for each lead was displayed on the monitor of the acquisition computer using Snapshot-Snapstream software (HEM Data Corp.; Southfield, MI). The EEG was digitized on-line at 512 samples per second for each channel to eliminate the effects of aliasing. Raw data were stored for later analyses. Prior to the recording of each subject, a 10 Hz, 50uV peak-to-peak sine wave was input through each amplifier; this calibration signal was digitized for 30s and stored for subsequent analysis. Spectral analysis of the calibration signal and computation of power at the 9–11 Hz frequency band was accomplished. The power figures were used to calibrate the power derived from the subsequent spectral analysis of the EEG.
