A linkage from the source model to the electromagnetic signals at the sensor locations on the scalp can be established by constructing a volume conductor model that resembles the human head in both geometry and conductivity. Historically, the head volume conductor is modeled as concentric three-shell [57, 64, 65] or four-shell [66] spherical models. More accurate forward solutions become possible by using numerical algorithms, such as the boundary element method (BEM) [61, 67, 68], finite element method (FEM) [69, 70, 211], and finite difference method (FDM) [71]. Such numeric models allow us to incorporate the realistic geometries of the head compartments segmented from anatomical images such as MRI. The conductivities of different head tissues, especially for the skull in EEG, may be specified according to empirical values [64], in vitro measurements [72], in vivo estimates [73, 74], or potentially the results of electrical impedance tomography (EIT) [75] or MR electrical impedance tomography (MREIT) [76]. In addition, diffusion tensor magnetic resonance imaging (DT-MRI) [77] provides a new means to obtain the anisotropic conductivity of the cerebral white matter [78–80].
