The polar average reference effect: a bias in estimating the head surface integral in EEG recording

A reference-independent measure of potential is helpful for studying the mu ltichannel EEG. The potentials integrated over the surface of the body is a constant, i.e, inactive across time, regardless of the activity and distri bution of brain electric sources. Therefore, the average reference, the mea n of all recording channels at each time point, may be used to approximate an inactive reference. However, this approximation is valid only with accur ate spatial sampling of the scalp fields. Accurate sampling requires a suff icient electrode density and full coverage of the head's surface. If electr odes are concentrated in one region of the surface, such as just on the sca lp, then the average is biased toward that region. Differences from the ave rage will then be smaller in the center of the region, e.g. the vertex, tha n at the periphery. In this paper, we illustrate how this polar average ref erence effect (PARE) may be created by both the inadequate density and the uneven distribution of EEG electrodes. The greater the coverage of the surf ace of the volume conductor, the more the average reference approaches the ideal inactive reference. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.