Whole head system

MEG was acquired using a 306-channel (204 planar gradiometers and 102 magnetometers) whole-head system (Elekta-Neuromag, Helsinki, Finland) in a magnetically shielded room. The sampling rate was 1,000 Hz, with a band-pass filter of 0.03–330 Hz. EEG was simultaneously recorded using 19 scalp electrodes according to the international 10–20 system, although the sparse EEG data were not analyzed in the current study.

MEG was recorded continuously from a 306 triple sensor (204 planar gradiometers; 102 magnetometers) whole-head system (Elekta Neuromag, Helsinki, Finland) using a sampling rate of 1 kHz and an online band-bass filter between 0.1 and 300 Hz. The headshape of each individual participant was measured using a Polhemus FASTRAK 3D digitizer. Head position of the subject was recorded continuously using five localization coils (forehead, mastoids).
Data pre-processing was performed using the open-source Matlab toolbox Fieldtrip (www.fieldtriptoolbox.org; RRID:SCR_004849). First the continuous data were filtered (high-pass Butterworth filter at 1 Hz, low-pass Butterworth filter at 170 Hz, and DFT filter at 50, 100, and 150 Hz to remove line-noise artefacts in the signal), and downsampled to 256 Hz. Next, the data were epoched into segments of 1 s for subsequent analysis.
Rejection of trials containing artefacts and bad channels was performed using a semi-automatic procedure. First, outliers were rejected using a pre-screening based on the variance and range in each trial/channel. Then, algorithmically guided visual inspection of the raw data was performed to remove any remaining sources of noise.