Fig 6. Amplitude modulation of MEG signal intensity. A, Procedure for calculating amplitude modulation for each frequency band of the MEG signal intensity. Raw data were first bandpass-filtered into , , β, and bands. Data were then full-wave-rectified, and an envelope function was calculated to measure the variation in amplitude with time. Approximately 5 seconds of data are shown. B, Envelope function from Fig 6A for 1 subject. Approximately 7 minutes of data are shown. Note the similarity in appearance of low-frequency fluctuations to BOLD fluctuations shown in Fig 2.