Interelectrode distance and amplitude of the scalp EEG.

The relationship between interelectrode distance and amplitude of the scalp EEG was investigated quantitatively, using differential recording arrays extending anteriorly from Oz and coronally from T3. Amplitude was taken as the square root of the power spectrum. For total signal amplitude and for individual frequency bands, measured data correspond well to an exponential function of the form Y = A(1 - e-BX). The inverse of the exponential coefficient B may be taken as a space constant, and the asymptote A as an estimate of absolute EEG amplitude. The concept of the space constant is consistent with current models of neuronal synchrony in the scalp EEG. The coefficients A and B may be useful indices in EEG analysis.