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An electroencephalogram (EEG) is a (non-intrusive) recording of the electrical activity of the brain.

The recording is obtained by placing electrodes on the scalp, usually after preparing the scalp area by light abrasion and application of a conductive gel to reduce impedance. Neuroscientists and biological psychiatrists use EEGs to study the function of the brain by recording brain waves during controlled behavior of human volunteers and animals in lab experiments. Theories to explain sleep often rely on EEG patterns recorded during sleep sessions. In addition, the procedure is used clinically to assist in the diagnosis of epilepsy.

EEG was discovered by British neurologist Richard Caton in 1885, in research animals, and later developed by German neurologist Hans Berger for use in humans in 1929. A more evolved type of EEG, called EEG topography, which makes an electrical map of the surface of the brain, was developed in the 1950s by British physician and engineeer Walter Grey Walter

See also electroencephalography.