An EEG is a test that detects brain waves, or in the electrical activity of your brain. During the procedure electrodes, consisting of small metal discs with thin wires, are placed onto your scalp. The electrodes detect tiny electrical charges that result from the activity of your brain cells. The billions of nerve cells in your brain produce very small electrical signals that form patterns called brain waves, known as Alpha, Beta, Theta and Delta waves. The charges are amplified and appear as a graph on a computer screen.
Each of the four EEG waves is associated with a different level of arousal of the cerebral cortex. Cortical arousal refers to the firing patterns of the neurons of the cerebral cortex. As the frequency of the EEG pattern gets lower, the level of cortical arousal diminishes. As the level of arousal diminishes, the EEG pattern gets higher in amplitude. Thus, frequency and amplitude are inversely related in the EEG. An EEG with a large amplitude and a low frequency indicates a more synchronized brain wave pattern (groups of cells are acting in concert), whereas an EEG with a low amplitude and a high frequency generally corresponds with a desynchronized brain wave pattern (groups of cells are involved in separate activities). The level of cortical arousal is correlated with various psychological and behavioral states.