Isotrak 2

We acquired the data using a 306-channel VectorView system (Elekta Neuromag), with 1 magnetometer and 2 orthogonal planar gradiometers at each of the 102 locations. Subjects were seated in a magnetically shielded room. The data were sampled at 1000 Hz, and band pass filtered at 0.03–330 Hz. For the adults, we monitored head position at the start and at the end of each of the 4 runs using 4 head position indicator (HPI) coils fixed to the head. For the children, we recorded continuous head position throughout each run, such that we could correct for any head movements during our analyses. For each subject, we used a Polhemus Isotrak II to digitize the locations of fiducial markers (the nasion, left and right pre-auricular points), the HPI coils and a number of additional head points (these were subsequently used to aid our co-registration). Vertical and horizontal electro-oculograms were recorded and were subsequently used to remove trials contaminated by blinks or eye-movements.

Participants’ MEG activity was measured in a magnetically shielded room using a 306-channel whole scalp array (204 planar gradiometers and 102 magnetometers) from the Elekta Neuromag system (Elekta Neuromag Oy, Helsinki, Finland). ECG and EOG were recorded simultaneously. The recording sampling rate was 1000 Hz. Continuous head position indicator (cHPI) signals were recorded during the experiments to track the subject’s head movements.
Prior to experimentation, a 3D digitization system (Isotrak II^®, Polhemus, Colchester, VT, USA) was used to localize anatomical fiducial points for later co-registration with magnetic resonance acquisition (MRI). The relative positions of the cHPI emitters and of numerous points over the participant’s head surface were also measured. For the “FingerTap” experiment, a fiber optic-based sensor allowed the recording of the participant’s index tapping. It provided the onset/offset timing of the executed movement.
Individual anatomical 3D T1-weighted magnetic resonance acquisitions were performed using a 1.5 T MRI scanner for the “FingerTap” subjects (Magnetom Espree, Siemens, Erlangen, Germany) and a 3 T MRI scanner for the “FlexElbow” subjects (Achieva TX, Philips, Eindhoven, The Nederlands), with a resolution of 1 × 1 × 1.1 mm.