Head model extension for the study of bioelectric phenomena.

Bioelectrical phenomena spread within the whole body (the conductor medium) independently of electrical source position within the body. However, under certain circumstances, it is possible to limit the volume within which the study can be done. Given its high resistivity, the skull limits the spread of bioelectrical currents due to brain sources and it leaves only few holes for current flow, namely the occipital hole and the openings for the optic nerves. This is a simulation study performed adopting realistic head models extended to different percentages of the whole head volume conductor to determine the possibility of limiting the model volume for bioelectric field analysis. A realistic head model extended to the chin was used as reference model to analyze three reduced model extensions: 80% of the volume of the reference model (including the neck upper part), 70% (including all the skull but not the neck) and 60% (cutting the head at cerebellum level). The lower limit of the reduced model was named "cut-plane". We simulated the head electrical potential generated by various dipole current sources within the brain, either far from or near the cut-plane and either orthogonal or parallel to it. The scalp potential distributions were compared between each reduced model and the reference model by means of relative-difference measure (RDM). The larger differences were found for sources near the cut-plane and for sources orthogonal to it. The differences increased non linearly with model volume reduction, dramatically augmenting as the skull was intercepted by the cut-plane. The same model performed differently according to source position relative to the particular head structure.