Effect of source location on the scalp potential asymmetry in a numerical model of the head

The correlation between source asymmetry in the brain and the potential amplitude asymmetry on the scalp was studied by a two-dimensional (2-D) numerical model of the head. The model employed computerized tomography (CT) images to define the different compartments of the head. The source was modeled by a dipole layer in the occiput for an occipital source (visual evoked potential generators) or a dipole layer around the cortex representing spontaneous activity generators. The volume conductor equation for the potential distribution was solved numerically using a finite volume method for two CT images; one had relatively symmetric left-right anatomy while the other had a fair deviation of 6° between the occiput and the nasion-inion line. By examining several arrangements of sources, it has been demonstrated that source asymmetry can cause nonnegligible asymmetry in the potential amplitude at the homotopic points on the scalp. This asymmetry, that is not related to real physiologic or psychological origin, should be taken into consideration in any EEG potential distribution analysis.