An EEG is a picture of brain waves made by the process of electroencephalography. Each brain cell (neuron) in the brain produces a tiny electrical charge; as more neurons become active, the sum of these tiny electrical charges can be detected on the surface of the scalp. The electrical activity is recorded by placing small electrodes on the scalp; activity is magnified 1 million times and recorded as brain waves. These brain waves provide a picture of the activity going on under the surface, inside the brain.

Brain waves occur in various patterns, and so EEG activity can be used to determine whether a person is awake or in one of several sleep stages; EEG activity can also be used to evaluate coma or brain death.

Abnormal patterns of EEG activity can be used to help diagnose such conditions as epilepsy, herpes simplex encephalitis and dementia. Before undergoing brain surgery, patients sometimes undergo an EEG in which the electrodes are inserted under the scalp, on the surface of the brain, to allow more precise recordings.

Further reading: Illustrated Guide to Diagnostic Tests, 2nd edition. Springhouse Corporation, Springhouse PA, 1998