These cases fulfill the conditions of the traditional source analysis. It is based on the assumption that the EEG can be described by one electrical point dipole source at a certain moment or period of time. In principle it would be possible to deal with even more realistic models, which have anisotropic and complex impedances. Further, the minimum size of each compartment in the model could be the dimension of the voxels obtained from the MRI scan. However, all these features demand lot of computing power and are therefore not included in present head models.
Spherical models work fast and are thus optimal for patient studies consisting of large materials.
Anatomical models are time-consuming, but give the individual geometry of the subject and suit for special cases. Although MRI is not needed for the computation in the former case, the MRI scans are often used also in spherical modeling. The sphere can be positioned either in the center of the head or asymmetrically so as to produce an optimal source localization. 2.4 Evaluation of the electrical field in the brain As the electrical sources in the brain have been estimated, it is possible to use the forward approach to solve the electrical field everywhere in the brain. However, the intracranial electrical field distribution seems not to have direct relevance in clinical or neuropsychological applications.