Beta waves: have a frequency range from 13-15 to 60 Hz and an amplitude of about 30 µV. Beta waves are the ones registered on an EEG when the subject is awake, alert, and actively processing information. Some scientists distinguish the range above 30-35 Hz as gamma waves, which may be related to consciousness–that is, the making of connections among various parts of the brain in order to form coherent concepts.
Alpha waves: have a frequency range from eight to twelve Hz and an amplitude of 30 to 50 µV. Alpha waves are typically found in people who are awake but have their eyes closed and are relaxing or meditating.
Theta waves: have a frequency range from three to eight Hz and an amplitude of 50 to 100 µV. Theta waves are associated with memory, emotions, and activity in the limbic system.
Delta waves: range from 0.5 to three or four Hz in frequency and 100 to 200 µV in amplitude. Delta waves are observed when individuals are in deep sleep or in a coma.
Lastly, when there are no brain waves present, the EEG shows a flat-line trace, which is a clinical sign of brain death.