Maxfilter

Neuromagnetic responses were recorded with the helmet-shaped 306-channel sensor array (“Vectorview,” Neuromag Elekta Oy, Helsinki, Finland). In this study, data from 204 planar gradiometers were used for analyses. Prior to the MEG recording, the positions of HPI coils were digitized together with fiducial points using the 3D digitizer “FASTRAK” (Polhemus, Colchester, VT, United States) and were used to assess a subject’s head position inside the MEG helmet every 4 ​ms. Later, offline position correction procedure was applied to the recorded data to compensate for a head movement.
The spatiotemporal signal space separation method (tSSS) implemented by “MaxFilter” (Elekta Neuromag Oy software) was used to suppress interference signals generated outside the brain. An electrooculogram (EOG) was recorded using four electrodes placed at the outer canthi of the eyes as well as above and below the left eye. The MEG signals were recorded with a band-pass filter of 0.1–330 ​Hz, digitized at 1000 ​Hz, and stored for offline analysis.

Neuromagnetic activities were recorded with the helmet-shaped 306-channel detector array (“Vectorview,” Neuromag Elekta Oy, Helsinki, Finland). In this study, data from 204 planar gradiometers were used for analyses.
Prior to the MEG session, the positions of HPI coils were digitized together with fiducial points using the 3D digitizer “FASTRAK” (Polhemus, Colchester, VT, United States) and were used to assess a subject’s head position inside the MEG helmet every 4 ms. Later, offline position correction procedure was applied to the recorded data to compensate for a head movements. The mean change in the MEG sensor locations during the experiment ranged from 3 to 12 mm across subjects. The spatiotemporal signal space separation method (tSSS) implemented by “MaxFilter” (Elekta Neuromag Oy software) was used to suppress interference signals generated outside the brain. An electrooculogram (EOG) was recorded using four electrodes placed at the outer canthi of the eyes as well as above and below the left eye. The MEG signals were recorded with a band-pass filter of 0.1–330 Hz, digitized at 1000 Hz, and stored for offline analysis.

Data were pre-processed in two steps. First, magnetic interferences originating outside of the MEG helmet were suppressed by using Signal Space Separation (Taulu and Simola, 2006 (link)) provided by MaxFilter (Elekta-Neuromag Oy; Helsinki, Finland). The median head position of each participant over the three experimental runs was used as reference for the other two runs. In the majority of cases, the second run was the reference run. Second, PCA was performed to remove components accounting for ECG and EOG variance using Graph (Elekta-Neuromag Oy; Helsinki, Finland). The average cardiac and blink artifacts were computed on the basis of ECG and EOG recordings. Components were manually checked for each sensor type (gradiometers and magnetometers) and saved as separate matrices (for detailed procedure, see: Graph">http://www.unicog.org/pm/pmwiki.php/MEG/RemovingArtifactsWithPCAAndGraph).