A model of brain electrical activity (in short “source model”) is composed of bioelectric units distributed within the brain volume or over the brain surface, or confined to few locations in the brain. A single source unit is often defined as a current dipole, which well-approximates the synchronized synaptic currents at a columnar level [9, 12]. Any brain electrical activity can be modeled by a distribution of current dipoles within the entire brain volume. One may further choose to confine the dipole locations to the cortical gray matter and constrain the dipole orientations to be perpendicular to the cortical surface [26]. Alternatively but less frequently, a bioelectric source unit can also be defined as a current monopole [48], current multipole [49], or extracellular potential [50–58]. Moreover, when brain electrical activity is confined to a few focal regions, one may prefer to use a source model consisting of only a few discrete dipoles, namely the equivalent current dipole (ECD) model [59–62]. As opposed to the general applicability of the distributed source models, the ECD model assumes small extents of the neural
