3d tracking system

MEG data were acquired using an Elekta Neuromag 306-channel system (204 planar gradiometers, 102 magnetometers) with a sampling rate of 1000 Hz. A band-pass filter of 0.03–330 Hz was applied during acquisition. Eye movements were monitored on-line with a MEG-compatible eye-tracker (EyeLink 1000, SR Research, Ottawa, ON, Canada) recording at 500 Hz. If participants broke fixation during trials, we reminded them to refrain from moving their eyes in the next break. The electrocardiogram and the vertical and horizontal electrooculograms (EOGs) were also recorded. Head position was monitored during the experiment with emitting coils affixed to the participant's head. The positions of these coils were digitized using a Polhemus 3D tracking system (Polhemus, EastTrach 3D). The Polhemus probe was used to obtain a set of ∼100 points to record the shape of the participant's head.
Each participant completed 6 task blocks with 40 trials each. These were collected during 2–4 successive MEG recording sessions lasting approximately 25 min each, depending on the duration of breaks and reaction times of individual participants.

Participants underwent MEG for eight to ten continuous minutes. Participants were instructed to remain still, visually fixating on a cross displayed 120 cm in front of them. Prior to MEG acquisition, a Polhemus 3D tracking system was used to record each participant’s head shape relative to three fiducial points, located on the nasion and preauricular landmarks. Five head position indicator (HPI) coils were fixed to the participant’s nasion and bilateral supra-orbital and posterior auricular regions. The locations of HPI coils and three anatomical fiducials were digitized using the tracking system (Polhemus, EastTrach 3D) to define the subject-specific Cartesian head coordinate system. These coils were localized within a scanner space continuously throughout MEG data acquisition. Participants underwent a T₁-weighted structural MRI scan with a Siemens Trio 3T (settings: 3-dimensional, whole-brain, magnetization-prepared rapid-acquisition gradient echo sequence, repetition time = 1900 ms, echo time = 4.7 ms, flip angle 8°, 1 mm isotropic resolution, 7 min acquisition time) within 1 month for MEG co-registration.