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

TL;DR

This study compares EEG and MEG spectral scaling in humans, providing evidence that the extracellular medium in the brain is likely non-resistive, based on differences in frequency scaling behaviors.

Contribution

It offers a theoretical prediction and empirical evidence that EEG and MEG frequency scaling differences indicate a non-resistive extracellular medium in the brain.

Findings

EEG and MEG show different frequency scaling exponents.

Noise correction methods increase EEG-MEG scaling differences.

Spectral scaling differences support non-resistive extracellular media hypothesis.

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.