Whole-head Measures of Sleep from MEG Signals
and the Ubiquitous <1Hz "Slow Oscillation"
Norman R. Simon¹, Bob Kemp², Ilonka Manshanden³,
and Fernando H. Lopes da Silva^4
1Department of Physics and Astonomy, University of Nebraska, Lincoln, NE 68588-0111, USA
²Sleep Centre, Westeinde Hospital, Den Haag, The Netherlands
³MEG-Centre KNAW, Free University Hospital,
Amsterdam, The Netherlands
⁴Institute of Neurobiology, University of Amsterdam, The Netherlands

The whole-head MEG sleep records for each of three subjects were separated into segments of 5.12 seconds and then grouped according to sleep phase ranging from drowsiness-sleep onset to deep slow-wave sleep. Spectral decomposition was employed to find "collective modes," defined as narrow frequency ranges at which all or nearly all of the MEG detectors displayed easily discernable power peaks. To follow these collective modes, we developed a normalized multichannel parameter s based upon the density of the MEG records and shown to track the classic progression of NREM sleep. The most prevalent collective mode appeared in the two lowest frequency bins at 0.49 and 0.98 Hz, respectively, almost certainly corresponding to the "slow oscillation" first described by Steriade et al. (1993). This mode was extremely strong in all subjects for all phases of true sleep and seemed to be present even in drowsiness and sleep onset. As sleep progressed, power in the two lowest bins grew rapidly, until, in the deepest slow-wave phase, these frequencies totally dominated the spectrum. The flow of power into the two lowest frequency bins was found to occur all over the head, but, particularly, at non-peripheral temporal locations in both hemispheres.