Comparative power spectral analysis of simultaneous elecroencephalographic and magnetoencephalographic recordings in humans suggests non-resistive extracellular media
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Abstract
The resistive or non-resistive nature of the extracellular space in the brain is still
debated, and is an important issue for correctly modeling extracellular potentials.
Here, we first show theoretically that if the medium is resistive, the frequency scaling
should be the same for electroencephalogram (EEG) and magnetoencephalogram (MEG) signals
at low frequencies (<10 Hz). To test this prediction, we analyzed the spectrum of
simultaneous EEG and MEG measurements in four human subjects. The frequency scaling
of EEG displays coherent variations across the brain, in general between 1/f and 1/f^2,
and tends to be smaller in parietal/temporal regions. In a given region, although
the variability of the frequency scaling exponent was higher for MEG compared to EEG,
both signals consistently scale with a different exponent. In some cases, the scaling
was similar, but only when the signal-to-noise ratio of the MEG was low. Several methods
of noise correction for environmental and instrumental noise were tested, and they
all increased the difference between EEG and MEG scaling. In conclusion, there is
a significant difference in frequency scaling between EEG and MEG, which can be explained
if the extracellular medium (including other layers such as dura matter and skull)
is globally non-resistive.