EEG and MEG measure external electrical potentials and magnetic fluxes, respectively, which arise collectively from mass neuronal responses within the brain. Such electromagnetic signals propagate (virtually) instantaneously from the activated neuronal tissues via volume conduction to the recording sites on/above the scalp surface [8–11]. The instantaneous nature of EEG/MEG indicates an intrinsically high temporal resolution and precision, which make them well suited for studying brain functions on the neuronal time scale. The collective nature suggests low spatial resolution and specificity, which impede mapping brain functions in great regional details [12]. This is regardless of recent advancements in electromagnetic source imaging (ESI) [13–15], which has led to great strides in improving the EEG/MEG spatial resolution to a centimeter scale or even smaller.
