Digital EEG allows one to combine recorded EEG channels into new montages without the need to record new data. Using spherical splines, voltages can be estimated at any point on the head. This allows one to generate various montages with the recorded or virtual electrodes at standardized locations, to interpolate bad electrodes, and to generate topographic maps over the whole head. Simulations of EEG activity originating in various brain regions are used to illustrate the effects of known generators on various montages and on whole-head maps. Some properties of spatial filters are introduced, and it is shown how they can be used to develop source montages with signals that estimate the activity in specific brain regions. The usefulness and validity of a source montage designed to focus on temporal lobe activity is illustrated with simulations and examples of temporal lobe spikes and seizures. Additional tools such as cross-correlation among channels, fast Fourier transform, and phase maps are described. These tools are useful in estimating time lags between source channels and in interpreting propagating spike and seizure activity. In combination, these tools help to analyze and to enhance activities that may be hard to detect from the background scalp EEG in traditional montages.
*Section of Biomagnetism, Department of Neurology, University Hospital Heidelberg; †MEGIS Software GmbH, Grafelfing/Munich; and ‡Department of Psychology, University of Konstanz, Germany
Supported by MEGIS Software GmbH, Grafelfing/Munich, Germany, and Nihon Kohden Corporation, Tokyo, Japan, for the development of the commercial EEGFOCUS and BESA software packages that were used to generate the figures, simulations, and analyses in this article.
Address correspondence and reprint requests to Prof. Dr. Michael Scherg, MEGIS Software GmbH, Bahnhofstrasse 95, D-82166 Grafelfing, Germany.