Volume currents in forward and inverse magnetoencephalographic simulations using realistic head models.

Volume currents are important for the accurate calculation of magnetoencephalographic (MEG) forward or inverse simulations in realistic head models. We verify the accuracy of our finite element method implementation for MEG simulations by comparing its results for spheres containing dipoles to those obtained from the analytic solution. We then use this finite element method to show that, in an inhomogeneous, nonspherical realistic head model, the magnetic field normal to the MEG detector due to volume currents often has a magnitude on the same order or greater than the magnitude of the normal component of the primary magnetic field from the dipole. We also demonstrate the disparity in forward solutions between a model that employs spheres, one that uses the realistic head and primary currents alone, and a realistic head model that incorporates both primary and volume currents. In forward and inverse MEG simulations using the inhomogeneous realistic model, the results obtained from calculations containing volume currents are more accurate than those derived without considering volume currents.