The influence of conductivity changes in boundary element compartments on the forward and inverse problem in electroencephalography and magnetoencephalography

Source localization based on magnetoencephalographic and electroencephalogr aphic data requires knowledge of the conductivity values of the head. The a im of this paper is to examine the influence of compartment conductivity ch anges on the neuromagnetic field and the electric scalp potential for the w idely used three compartment boundary element models. Both the analysis of measurement data and the simulations with dipoles distributed in the brain produced two significant results. First, we found the electric potentials t o be approximately one order of magnitude more sensitive to conductivity ch anges than the magnetic fields. This was valid for the field and potential topology (and hence dipole localization), and for the amplitude (and hence dipole strength). Second, changes in brain compartment conductivity yield t he lowest change in the electric potentials topology (and hence dipole loca lization), but a very strong change in the amplitude (and hence in the dipo le strength). We conclude that for the magnetic fields the influence of com partment conductivity changes is not important in terms of dipole localizat ion and strength estimation. For the electric potentials however, both dipo le localization and strength estimation are significantly influenced by the compartment conductivity.