Electroencephalography

Electroencephalography is the neurophysiologic exploration of the electrical activity of the brain by the application of electrodes to the scalp. The resulting traces are known as an electroencephalogram (EEG) and represent so-called brain waves. This device is used to assess brain damage, epilepsy and other problems. Historically four major types of brainwave are recognized (alpha, beta, delta and theta).

Beta is the frequency range above 12 Hz (there's no precise agreement on the frequency ranges for all the frequency bands mentioned here). It's often associated with active, busy or anxious thinking and active concentration.

Alpha is the frequency range from 8 Hz to 12 Hz. It's often associated with a relaxed awake mind, daydreaming, watching TV or the first stage in falling asleep.

Theta is the frequency range from 4 Hz to 8 Hz and is associated with REM sleep, creativity, visual imagery, reverie and recollection.

Delta is the frequency range below 4 Hz and is often associated with deep sleep.

Richard Caton (1842–1926), a physician practicing in Liverpool, presented his findings about electrical phenomena of the exposed cerebral hemispheres of rabbits and monkeys in 1875.

Hans Berger (1873–1941) began his studies of the human EEG in 1920.