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The resistive or non-resistive nature of the extracellular space in the brainis still debated, and is an important issue for correctly modelingextracellular potentials. Here, we first show theoretically that if the mediumis resistive, the frequency scaling should be the same for electroencephalogram(EEG) and magnetoencephalogram (MEG) signals at low frequencies (<10 Hz). Totest this prediction, we analyzed the spectrum of simultaneous EEG and MEGmeasurements in four human subjects. The frequency scaling of EEG displayscoherent variations across the brain, in general between 1/f and 1/f^2, andtends to be smaller in parietal/temporal regions. In a given region, althoughthe variability of the frequency scaling exponent was higher for MEG comparedto EEG, both signals consistently scale with a different exponent. In somecases, the scaling was similar, but only when the signal-to-noise ratio of theMEG was low. Several methods of noise correction for environmental andinstrumental noise were tested, and they all increased the difference betweenEEG and MEG scaling. In conclusion, there is a significant difference infrequency scaling between EEG and MEG, which can be explained if theextracellular medium (including other layers such as dura matter and skull) isglobally non-resistive.

Comparative power spectral analysis of simultaneous  elecroencephalographic and magnetoencephalographic recordings in humans  suggests non-resistive extracellular media