On the effects on source localisation of volume currents in neuroelectric and neuromagnetic signals

By assuming the surfaces of discontinuity in the electrical conductivity of brain tissues and fluids to be many and of complicated geometry, the influence of volume-current effects (due to the surfaces of discontinuity) on evoked electro- and magnetoencephalographic signals is assessed. The supposition that in such complicated geometry volume-current effects tend to modify the electrostatic potential significantly is supported by the properties of the solution of the partial differential equation for the electrostatic potential when the electric conductivity has a short-scaled stochastic component. The potential of volume-current effects to dominate the EEG signal is thereby reaffirmed. Moreover, it is shown that even in the case of the strongest possible volume-current effects they only contribute a term of the order of unity to the neuromagnetic signal, allowing for the possibility to determine the primary source location from the position of the peaks in the neuromagnetic signal. In the same limit any information on source location carried by the neuroelectric signal is obliterated. A set of one-dimensional model equations is used to exemplify the expected differences in the behaviour of the neuroelectric and neuromagnetic signals.