Measuring the cortical correlation structure of spontaneous oscillatory activity with EEG and MEG.

Power correlations of orthogonalized signals have recently been introduced for MEG as a powerful tool to non-invasively investigate functional connectivity in the human brain. Little is known about the applicability of this approach to EEG, and how compatible the results are between EEG and MEG. To address this, we systematically compared power correlations of simultaneously recorded and source co-registered 64-channel EEG and 275-channel MEG in resting human subjects. For both modalities, connectivity peaked at around 16 Hz. For this frequency range, seed-based correlation maps showed comparable patterns across modalities, with generally more distinct patterns for MEG. A brain-wide pattern correlation analysis also revealed maximum similarity around 16 Hz. Correcting for different signal-to-noise ratio (SNR) across frequencies and modalities revealed pattern correlation between modalities close to one across a broad frequency range from 1 to 32 Hz and only slightly smaller for higher frequencies. The decrease above 32 Hz likely reflected higher susceptibility to muscle artifacts for EEG than for MEG. Our results show that power correlation of orthogonalized signals is feasible for studying functional connectivity with 64-channel EEG. Furthermore, besides differences in SNR, for frequencies from about 8 to 32 Hz, EEG and MEG measure the same correlation patterns across the entire brain.