Are Sources of EEG and MEG rhythmic activity the same? An analysis based on BC-VARETA
In the resting state (closed or open eyes) the electroencephalogram (EEG) and the magnetoencephalogram (MEG) exhibit rhythmic brain activity is typically the 10 Hz alpha rhythm. It has a topographic frequency spectral distribution that is, quite similar for both modalities--something not surprising since both EEG and MEG are generated by the same basic oscillations in thalamocortical circuitry. However, different physical aspects underpin the two types of signals. Does this difference lead to a different distribution of reconstructed sources for EEG and MEG rhythms? This question is important for the transferal of results from one modality to the other but has surprisingly received scant attention till now. We address this issue by comparing eyes open EEG source spectra recorded from 70 subjects from the Cuban Human Brain Mapping project with the MEG of 70 subjects from the Human Connectome Project. Source spectra for each voxel and frequencies between 0-50Hz with 100 frequency points were obtained via a novel sparse-covariance inverse method (BC-VARETA) based on individualized BEM head models with subject-specific regularization parameters (noise to signal ratio). We performed a univariate permutation-based rank test among sources of both modalities and found out no differences. To carry out an unbiased comparison we computed sources from eLORETA and LCMV, performed the same permutation-based comparison, and found the same results we got with BC-VARETA.