Inadequacy of the average reference for the topographic mapping of focal enhancements of brain potentials.

The main reason for doing topographic mapping of EEG or evoked potentials is to assess regional changes in brain potentials. The use of an average reference is shown to have perverse effects in this relation, namely because it imposes on the recorded data a zero-centering effect which can reduce, eliminate or even reverse the focal changes of bit-mapped brain potentials. Concurrent studies on a true 3-shell head model suggest that such distortions of human EEG data occur because the average reference is computed from a set of (scalp) recording electrodes which do not survey the bottom half of the head volume so that the integral of scalp-recorded potentials frequently differs from zero. The results also raise the question whether the actual incidence of radial or near-radial (versus tangential) generators has been underestimated in the published data using average reference mapping.