Correlation between source asymmetry and scalp potential asymmetry in a prolate spheroid model of the head

The correlation between electric source asymmetry in the brain and potential amplitude asymmetry developed on the scalp was investigated using a computerized analytical prolate spheroid model of the head. The source was modeled by a single current dipole located in the occipital region of the brain. The potential created by the dipole was calculated using Laplace's equation with boundary conditions while allowing a quasi-static formulation and linear media. The dipole was located in the inner part of a four-compartment medium representing the scalp, the skull (with non-isotropic conduction), the cerebrospinal fluid (CSF), and the cortex. The asymmetry was modeled by a dipole located with an angle to the major axis connecting the nasion and the inion. The present study shows that source asymmetry can cause non-negligible asymmetries in the potential amplitude measured on the scalp above homotopic points of the two hemispheres. A potential asymmetry of up to 15% in the O1-O2 pair of electrodes was found when the dipole is rotated from the symmetric major axis at an angle of 10 degrees. The source asymmetry in the occipital region can be related to falx deviation, which is an asymmetry present in the majority of the population. Since this asymmetry is not related to real physiologic and psychologic sources of potential amplitude asymmetries, it should be taken under consideration when potential distribution analysis is performed.