The human brain generates electrical potentials strong enough to be detected through the intact skull. A record of these potentials is analogous to the familiar electrocardiogram and, like the latter, furnishes a convenient external index of the activity of a hidden organ. The fact that the brain of a living animal is the seat of electrical activity was discovered by Caton1in 1875, but, although the observation was repeatedly confirmed and considerably elaborated by other workers, it was completely ignored by most neurophysiologists until the last decade. Then, with the aid of the new and powerful electron tube amplifiers developed by radio communication engineers, the field was reopened by Fischer2and his pupil Kornmüller in Germany, Adrian3in England and Bartley and Newman,4Bartley and Bishop5and others in this country. Particular credit from the medical point of view is due to Berger,6a