brain waves
spontaneous, rhythmic electrical impulses emanating from different areas of the brain. Electroencephalographic brain-wave recordings are used to study sleep stages and cognitive processes. According to their frequencies, brain waves are classified as alpha waves (8–12 Hz), beta waves (13–30 Hz), delta waves (1–3 Hz), gamma waves (31–80 Hz), or theta waves (4–7 Hz). The first substantial account of brain waves was given in 1929 by German neuropsychiatrist Hans Berger (1873–1944).