Referenced EEG and head volume conductor model: geometry and parametrical setting

Brain electrical activity effects spread (spatially) over the whole head volume conductor. Electric scalp potentials (EEG) are the measurable evidences of such activity. EEG forward problem solution involves computing the scalp potentials at a finite set of sensor locations for a source configuration in a specified volume conductor model of the head or of part of it (reduced model). The use of reduced models is appealing for computational reasons. The skull could be, according to its conductivity, the natural bound for bioelectrical currents flow. However there are huge uncertainties on actual skull conductivity (i.e., 1/80 or 1/15 of brain conductivity). We show here the limits besides which model reduction is possible preserving EEG simulation accuracy according to the two competitive definitions for skull conductivity. The identified limits involve a proper choice for the EEG reference (Cz) as well as dipole equivalent source characteristics (position and orientation). To this end we adopted realistic test head models extended to different percentages of two reference models (extended to the chin) which differ for skull conductivity set either to 1/80 or 1/15 of brain conductivity.