Electrical potentials arising in the cerebral cortex were recorded by means of an apparatus which included three amplifiers for three separate recordings. Records were made on paper on an 8-foot drum, thus making possible eight hours of continuous recording. The subjects, including one set of identical twins, ranged in age from 17 days to 74 years. An analysis of the records shows that regular rhythms of potential occur which differ in different individuals and are affected by various conditions of mental activity, by external stimuli, by emotional states, sleep, hypnosis, alcohol and blindness. (PsycINFO Database Record (c) 2006 APA, all rights reserved)