An electroencephalogram, usually abbreviated EEG, is a medical test that records electrical activity in the brain. During the test, the brain's spontaneous electrical signals are traced onto paper. The electroencephalograph is the machine that amplifies and records the electrical signals from the brain. The electroencephalogram is the paper strip the machine produces. The EEG changes with disease or brain disorder, such as epilepsy, so it can be a useful diagnostic tool, but usually must be accompanied by other diagnostic tests to be definitive.

To perform an EEG, electrodes, which are wires designed to detect electrical signals, are placed on the cranium by attaching the wire with a special adhesive. The electrodes are placed in pairs so that the difference in electric potential between them can be measured. The wires are connected to the electroencephalograph, where the signal is amplified and directed into pens that record the...