  This paper explores advanced electrode modeling in the context of separate
and parallel transcranial electrical stimulation (tES) and
electroencephalography (EEG) measurements. We focus on boundary condition based
approaches that do not necessitate adding auxiliary elements, e.g. sponges, to
the computational domain. In particular, we investigate the complete electrode
model (CEM) which incorporates a detailed description of the skin-electrode
interface including its contact surface, impedance and normal current
distribution. The CEM can be applied for both tES and EEG electrodes which is
advantageous when a parallel system is used. In comparison to the CEM, we test
two important reduced approaches: the gap model (GAP) and the point electrode
model (PEM). We aim to find out the differences of these approaches for a
realistic numerical setting based on the stimulation of the auditory cortex.
The results obtained suggest, among other things, that GAP and GAP/PEM are
sufficiently accurate for the practical application of tES and parallel
tES/EEG, respectively. Differences between CEM and GAP were observed mainly in
the skin compartment, where only CEM explains the heating effects
characteristic to tES.
