Inverse sensitivity to approximate dipolar source models

Abstract

Abstract The current dipole is a widely used source model in forward and inverse EEG and MEG applications. Analytic solutions to the governing field equations have been developed for several approximations of the human head using ideal dipoles as the source model. Numeric approaches such as the finite element method have become popular because they allow the use of anatomically detailed head models. Although numeric methods can represent more realistic domains, the sources in such models are often an approximation of the ideal dipole. We previously compared the electromagnetic fields produced by alternative models of dipolar sources in finite element models of the head to the fields produced by infinitesimal dipolar sources in analytic models of the head. That study found that the practice of projecting current sources to the nearest nodes of a finite element mesh was inaccurate. The “subtraction approach” was found to be very accurate, but can be difficult to apply, especially with commercial solver codes. That work also presented an approximate formulation that provided reasonably good results, based on the Laplace equivalence between voltage and current sources. In this article we verify the applicability of the Laplace dipole approximation to inverse problems using a realistic finite element model of the head.