Effects of electrode properties on EEG measurements and a related inverse problem

A trend in EEG measurements is to increase the number of measurement electr odes in order to improve the spatial resolution of the recorded voltage dis tribution at the scalp. It is assumed that this would implicate better accu racy in the EEG inverse estimates. However, this does not necessarily hold. The reason for this is that the electrodes create a well conducting shunti ng "layer" on the scalp which affects the voltage distribution. This may de crease the information obtained and may therefore worsen the inverse estima tes. Electrodes in EEG inverse problems are commonly modeled as point elect rodes. This model cannot take into account the possible shunting effect of the electrodes. In this study the measurement electrodes are modeled using the so-called complete electrode model which takes into account the actual size of the electrode, the contact impedance between the skin and the elect rode and also the shunting effect of the electrodes. In this paper the effe cts of the electrode size and the contact impedance on the voltage distribu tion are studied by simulations. It is shown that, depending on the size an d the contact impedance of the electrodes, increasing the number of electro des does not necessarily improve the accuracy of the inverse estimates. We also conclude that the use of the point electrode model is quite adequate i n normal EEG studies. The use of a complete electrode model is necessary if electrodes cover more than 50% of the surface area. (C) 2001 IPEM. Publish ed by Elsevier Science Ltd. All rights reserved.