Neuromag 306 channel

All participants were tested with MEG at the Oxford Centre for Human Brain Activity using an Elekta NeuroMag (306 channel) MEG system. A magnetic Polhemus FastTrak 3D system (Vermont, United States) was used for head localisation. Relative positions of three anatomical landmarks (nasion, left and right auricular points) were measured in addition to relative positions of four head-position indicator coils.
MEG data were recorded in six separate blocks of 5−6 min each. In between blocks, participants had a small break, during which they remained seated in the MEG chair. During acquisition, an analogue bandpass filter between 0.03 and 300 Hz was applied and data were digitised at a sampling rate of 1000 Hz. ECG and horizontal and vertical EOG were recorded in addition to eye tracking data that were recorded with a video-based eye tracker (EyeLink 1000, SR Research, Ontario, Canada) with a sampling frequency of 1000 Hz. A bimanual fibre-optic response device was used to collect manual responses.
Stimuli were presented using MATLAB (The MathWorks, Inc., Natick, MA) and Psychtoolbox v.3.0 for MATLAB (Kleiner et al., 2007 ). The stimuli were back-projected (Panasonic PT D7700E, Panasonic, Osaka Japan) on a 58 × 46 cm screen placed 120 cm in front of the participant, with a spatial resolution of 1280 × 1024 and a refresh rate of 60 Hz.

The Elekta Neuromag 306-channel scanner (Elekta TRIUX ®) was used in this study to record neural activity at 2000 Hz with a low-pass filter set to 660 Hz. Each sensor element consists of a magnetometer and two orthogonal planar gradiometers. Four head position indicator (HPI) coils were placed on the participants’ scalp to indicate head position. Furthermore, coordinates of head points were digitized (Polhemus Fastrak®) for MRI co-registration, including nasion, anterior points in front of the ear points, and about 300 additional points on the scalp. These flags and head position points were used to register the MEG and MRI coordinate systems further. A pair of electrodes were placed and attached to the participants’ chests to capture electrocardiogram synchronized with the MEG recording. Two electrodes were attached above and below eye to detect electro-ocular activity. Five continuous minutes of MEG resting-state data were acquired per participant on the Elekta Neuromag 306-channel scanner. The participant was laid comfortably in the scanner and instructed to remain awake and close eyes, but without performing any specific task. MEG scan would be restarted if the participant falls asleep or has excessive head movements during the scan. Because NDPH was a persistent headache, all patients experienced headaches during the scan.