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The common types of Brain Waves are summarised in the table below.
Electroencephalography (EEG) is a means of measuring brain waves, which has been found to be linked to different states of consciousness as shown in the table above. It is still not fully understood where all the brain waves originate from and what keeps them synchronised (ie all neurons in the brain fire at the same time). All types of brain waves are present at the same time (whilst we are awake), but only one predominates. It is well known that delta waves are associated with deep sleep, theta waves with REM sleep, alpha waves when we are awake and relaxed, and beta waves for normal activity.
Interest in the gamma waves and particularly around 40 Hz arose when Francis Crick hypothesized that this is the brain wave frequency when we are truly conscious. Meditating monks show high gamma wave activity around 40 Hz whilst meditating. The focus in meditation is often on the slower alpha-theta-delta waves, but higher consciousness may be associated with high-gamma waves.
It isn’t known what the maximum brainwave frequency is. High gamma type waves are recorded during lucid dreaming, and extremely agitated mental states.
There are a number of sound and frequency products on the market that attempt to generate these different brain wave states. The sound tapes work by generating stereo tones in both ears with a beat frequency for the desired brain wave, eg 200 Hz in one ear and 210 Hz in the other ear will generate a 10 Hz beat frequency which simulates alpha wavelength. You can make your own brainwave recordings using CoolEdit, Audacity, gnaural, or similar audio editing software. I tried a tape that gradually descended from alpha through theta to delta waves, and it put me to sleep. Other people use gamma recordings to try and improve creativity.