Electroencephalography (EEG) - Brainwaves
Delta waves (up to 4Hz)
These have the lowest frequency (cycles, or times per second; measured in Hertz, Hz) and the highest amplitude (the height from baseline to the top of the peak). It is associated with slow-wave sleep in adults. It is this sleep that is typically disrupted with alcohol so reductions in delta activity are often seen in alcoholics
Mu waves (8-13Hz)
These are of a similar frequency range to Alpha waves but arise in a distinctly different region of the brain prompting neurophysiologists to give them a separate category. Mu waves arise in the sensorimotor cortex (sides) of the brain whereas alpha waves originate in the posterior regions of the brain. These brainwaves are seen when the hands and arms are idle.
Alpha waves (7-13Hz)
This is the brainwave of the normal relaxed waking state and also present when the subject is relaxed with their eyes closed.
Theta waves (4-7Hz)
These are the brainwaves of interest. This is the brainwave of focussed attention and deep meditation. It is thought that the frequency of tribal music and of binaural beats use this frequency to allow the tribesmen or meditators to respectively enter meditative states.
Beta waves (14-30Hz)
Increases in beta activity are seen in busy or anxious thinking and in active concentration.
Gamma waves (30-100Hz)
As in most areas in this almost esoteric topic, little is known about gamma waves. They are the highest frequency brain wave and may be implicated in the unit of conscious perception. That is to say give continuity to our outward perceptions. This of course is rigorously disputed by those who claim conscious cannot be objectively measured, myself included.