BERGER1 in 1931 first noted slow electrical activity as a result of cerebral disease when he reported his electroencephalographic findings from patients with various clinical conditions. His use of only two electrodes, based on his early concept that the activity he recorded was that of the entire brain, did not permit localization of this abnormal activity. Adrian and Matthews2 demonstrated that the recorded electrical changes were not uniform over all of the brain's surface, and localized the Berger or α-rhythm to the posterior areas. Walter3 in 1936 with the use of three electrodes, was able to record focal slow activity around a brain tumor, applying the term "delta" to these slow waves. There is now universal agreement designating any wave forms with a frequency below 4/second as "delta." With the extensive testing of normal adults, notably the study of Gibbs and Gibbs,4 focal δactivity was