As the mass responses from all activated neural tissues, EEG/MEG signals can be regarded as a weighted sum of instantaneous neuronal currents throughout the entire brain volume. In other words, EEG/MEG reflects the synchronized electrical behavior of an assembly of neurons within a certain brain region or even across regions. Asynchronized electrical activities have little contribution to EEG/MEG, because their random consequences are virtually cancelled out when summed over locations. The large-scale synchrony serves as the major factor to identify which specific aspects of brain electrophysiology contribute to EEG/MEG.
