The topography of basic EEG rhythms brings a new dimension to our understanding of normal and pathologic electrogenesis in the brain. But our experience has shown that it is valuable to make use of the numeric spectral data in order to best decide which rhythms should be mapped. This study demonstrates that quantification of basic EEG rhythms is valuable, not only for beta activity in pharmacology or as an aid for the diagnosis and localization of epilepsy (as has been reported in the literature), but for the other rhythms as well. Our experience to date suggests that EEG rhythms be divided into two major groups: 1) occipital rhythms of extremely variable power which have a peak frequency equal to or greater than 8 Hz and which react to eye opening and 2) parietal-central rhythms which have a frequency equal to or less than 7.5 Hz and which are unreactive to eye opening. Alteration of these characteristics may occur as a result of widely different types of pathology but are usually not visible in the traditional EEG.