Methods

The MEG signals were recorded in a magnetically shielded room with a 122-channel whole-scalp Neuromag-122 neuromagnetometer. This device collects data at 61 locations over the scalp, using orthogonal double-loop pick-up coils that couple strongly to a local source just underneath, thus making the measurement ``near-sighted'' hamalainen.

One of the authors served as the subject and was seated under the magnetometer. He kept his head immobile during the measurement. He was asked to blink and make horizontal saccades, in order to produce typical ocular artifacts. Moreover, to produce myographic artifacts, the subject was asked to bite his teeth for as long as 20 seconds. Yet another artifact was created by placing a digital watch one meter away from the helmet into the shieded room. Finally, to produce breathing artifacts, a piece of metal was placed next to the navel.

  

Figure: Samples of MEG signals, showing artifacts produced by blinking, saccades, biting and cardiac cycle. For each of the 6 positions shown, the two orthogonal directions of the sensors are plotted.

Vertical and horizontal electro-oculograms (VEOG and HEOG) and electrocardiogram (ECG) between both wrists were recorded simultaneously with the MEG, in order to guide and ease the identification of the independent components. The bandpass-filtered MEG (0.03-90 Hz), VEOG, HEOG, and ECG (0.1-100 Hz) signals were digitized at 297 Hz, and further digitally low-pass filtered, with a cutoff frequency of 45 Hz and downsampled by a factor of 2. The total length of the recording was 2 minutes. A second set of recordings was performed, to assess the reproducibility of the results.

Figure  presents a subset of 12 spontaneous MEG signals from the frontal, temporal and occipital areas. Due to the dimension of the data (122 magnetic signals were recorded), it is impractical to plot all MEG signals (the complete set is available on the internet -- see reference list for the adress [VigárioVigário1997a]). Also both EOG channels and the electrocardiogram are presented.