The EEG consists of the activity of an ensemble of oscillators generating rhythmic activity in several frequency ranges. Oscillator activity is usually random. However, sensory stimuli can be used to couple these oscillators so that they act together in a coherent way. This synchronization and enhancement of EEG activity gives rise to evoked or induced rhythms. Evoked potentials (EP), representing ensembles of neural population responses, are considered the result of a transition from a disordered to an ordered state. Compound ERP manifest a superposition of evoked oscillations in the EEG frequencies ranging from delta to gamma. Natural brain frequencies are denoted as alpha: 8-13 Hz; beta, 18-25; theta: 3.5-7 Hz; delta: 0.5-3.5 Hz, and gamma: 30-70 Hz.15-20
