The influence of the volume conductor on electric source estimation.

Abstract

This tutorial concerns the influence of the head on the estimation of sources from MEG and EEG data. The solution of the forward problem involves the calculation of the potential or magnetic field distribution at or near the outer surface of the volume conductor (i.e., the head), when the sources, the geometry and conductivities of the volume conductor are given. The estimation of electrical sources within the brain from the measured EEGs and /o r MEGs is called the solution of the inverse problem. The solution of the inverse problem is not un ique ly determined, different sources can produce similar electric potentials and magnetic fields. In order to solve the inverse problem, one has to make assumptions about the sources and the volume conductor in accordance with the electrophysiological reality. Since the neuronal response is slow compared with the time scales at which inductive and displacement currents occur, the electric field is conservative and all currents form closed loops. Consequently, part of the current resides outside the neurons. The extra-neuronal current, which is determined by the three-dimensional distribution of the electrical conductivity of the human head, contributes to the magnetic and electric field. However, our knowledge of this parameter is rather limited. Algorithms are available to solve the inverse problem for both EEG and MEG using a head model consisting of an arbitrary number of concentric spheres, where the outer surface is locally fitted to the head. Forward