Although it was demonstrated decades ago that the electrical conductivity of white matter tissue is anisotropic , this information is often neglected in EEG/MEG source localization studies using realistic head models. On the other hand, modeling methods like FEM  are able to include anisotropic conductivity. One reason for omitting this information may be the lack of a non-invasive method, capable to map spatially resolved anisotropic conductivity to incorporate this information in a realistic head model. Another reason may be that the benefit of including anisotropy information is not always obvious. This study aims to evaluate the sensitivity of the EEG/MEG inverse solution by neglecting white matter anisotropy for almost all possible cortical positions in gray matter in a whole-head high-resolution human FEM simulation study.
We employed a high resolution FEM head model (~3.3 × 106 elements) and distributed over 25.000 dipoles almost uniformly spaced over the cortical area (gray matter ribbon) with moments oriented perpendicular to the gray matter surface. For each of these dipole positions, we calculated the EEG and MEG forward solutions while considering the anisotropy of white matter. The latter information was derived from MR-Diffusion Tensor data. With the forward solutions EEG/MEG focal source reconstructions were performed by using a moving dipole fit approach and neglecting the anisotropy information of the white matter segment. The deviations between dipoles used to solve the forward solution and dipoles reconstructed by solving the inverse problem were evaluated by means of dipole shift, magnitude and orientation change.
The results for the dipole shift show that the dipole localization errors are in general smaller with MEG compared to EEG when anisotropy is neglected. The magnitude errors, however, are very similar for both MEG and EEG. For the orientation error EEG was found to be less sensitive than MEG. Overall, we found that the results for EEG and MEG mapped onto the brain surface were very similar, although the maps for the MEG solution appeared sharper. In general, highly affected dipole positions were found mainly in the sulci of the medial part of each hemisphere but also in the main sulci of the temporal lobe. Strong influences were observed with dipole positions located in the posterior part of the lateral sulcus, the superior temporal sulcus, the postcentral sulcus and the transverse occipital sulcus.
Our results are important for EEG/MEG source localization procedures using ECD (equivalent current dipole) models with dipoles located in regions that are highly influenced by anisotropic conductivity. Therefore, localization analyses should consider anisotropy when modeling volume conductors.
Inverse Problems 2004, 20:1099-1116. Publisher Full Text