Physiological Bases of Magnetoencephalography and Electroencephalography

Understanding the physiological bases of magnetoencephalography (MEG) and electroencephalography (EEG) provides the foundation for developing these techniques as tools for studying human brain functions because this information can serve as a guide for planning experimental studies and for interpreting the data. During the past 50 years, the concept of electrophysiology of neurons has been profoundly modified as new types of active conductance have been discovered in the dendrites and soma. The biophysical models of individual neurons and neuronal networks developed within the framework of modern electrophysiology have provided quantitatively accurate accounts of evoked magnetic fields, extracellular potentials, and intracellular potentials in principal neurons in the tissues within a single theoretical framework. These results are consistent with the conclusion that intracellular currents in active tissues produce both MEG and EEG signals in the cerebellum, hippocampus, and cerebral cortex. We now know that the calcium and potassium currents are the major currents shaping the waveforms of MEG and EEG and that the sodium and potassium currents generate the spikes and high-frequency signals detectable outside the brain. The current dipole moment density, defined as current dipole moment per unit surface area of the active cortex, is governed by the intracellular volume fraction and basic kinetics of the active conductances. This quantity, which is conserved across the evolutionary scale ranging from reptiles to humans, may serve as a useful physiological constraint in interpreting MEG and EEG signals. It is hoped that this foundation will help advance the research on human brain functions.