Spike-waves, recorded by an electroencephalograph, during an epileptic seizure.

An electroencephalograph is an instrument for recording electrical activity of the brain using several small electrodes on the scalp. Its results are produced in the form of a multiple tracing called an electroencephalogram (EEG). The "brain waves" recorded have certain normal patterns in the alert and sleeping individual. Localized brain diseases and metabolic disturbances cause abnormal wave forms either in particular areas or as a generalized disturbance. The abnormal brain activity is epilepsy, both during convulsions and when the patient appears normal, usually allows diagnosis. The interpretation of EEGs requires skill and experience.