Electromagnetic activity of the brain

The electromagnetic activity of the human brain is governed by the quasi-static theory of Maxwell's equations [229, 284] which, to some extent, decouples the electric from the magnetic behavior. The brain is modelled as a conductive medium with certain conductivity and a magnetic permeability which is equal to the permeability of the air. The brain is surrounded first by the cerebrospinal fluid, then by the skull, and finally by the scalp, all having different conductivities but the same magnetic permeability. In most cases though the brain–head system is assumed to be a single homogeneous medium. A primary neuronal current within the conductive brain tissue excites a secondary induction current and both currents generate an electric potential, which is measured on the surface of the head, and a magnetic flux, which is measured outside, but close to, the head. Recordings of the electric potential concern the theory of electroencephalography (EEG) and recordings of the magnetic flux density concern the theory of magnetoencephalography (MEG). The forward problem of EEG or MEG consists of the calculation of the electric potential or the magnetic induction, respectively, from a complete knowledge of the primary neuronal current. The inverse problem of EEG or MEG consists of the identification of the location and intensity of the neuronal current from the electric potential or the surface of the head, or of the magnetic induction outside the head, respectively.