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A recording of the electrical activity of the brain by means of electrodes placed on the surface of the head. With the EMG and EOG, the EEG is one of the three basic variables used to score sleep stages and waking. Sleep recording in humans uses surface electrodes to record potential differences between brain regions and a neutral reference point, or simply between brain regions. Either the C3 or C4 (central region) placement according to the International 10-20 System is referentially (referred to an earlobe) recorded as the standard electrode derivation from which state scoring is done. EEG is also used to diagnose epilepsy and other brain diseases.