Types of Alpha Waves
Some researchers posit that there are at least three forms of alpha waves, which may all have different functions in the wake-sleep cycle.
Alpha waves are present at different stages of the wake-sleep cycle. The most widely-researched is during the relaxed mental state, where the subject is at rest with eyes closed, but is not tired or asleep. This alpha activity is centered in the occipital lobe, and is presumed to originate there, although there has been recent speculation that it instead has a thalamic origin. This wave begins appearing at around four months, and is initially a frequency of 4 waves per second. The mature alpha wave, at 10 waves per second, is firmly established by age 3.
The second occurrence of alpha wave activity is during REM sleep. As opposed to the awake form of alpha activity, this form is located in a frontal-central location in the brain. The purpose of alpha activity during REM sleep has yet to be fully understood. Currently, there are arguments that alpha patterns are a normal part of REM sleep, and for the notion that it indicates a semi-arousal period. It has been suggested that this alpha activity is inversely related to REM sleep pressure.
The third occurrence of alpha wave activity is the alpha-delta or slow-wave (SWS) state. This activity spreads across the brain in an anterior-posterior gradient.
It has long been believed that alpha waves indicate a wakeful period during sleep. This has been attributed to studies where subjects report non-refreshing sleep and have EEG records reporting high levels of alpha intrusion into sleep. This occurrence is known as alpha wave intrusion. However, it is possible that these explanations may be misleading, as they only focus on alpha waves being generated from the occipital lobe.
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