Alpha wave
Alpha waves, or the alpha rhythm, are neural oscillations in the frequency range of 8–12 Hz[1] likely originating from the synchronous and coherent (in phase or constructive) electrical activity of thalamic pacemaker cells in humans. Historically, they are also called "Berger's waves" after Hans Berger, who first described them when he invented the EEG in 1924.[2]
Not to be confused with Alpha particle.
Alpha waves are one type of brain waves detected by electrophysiological and closely related methods, such as by electroencephalography (EEG) or magnetoencephalography (MEG), and can be quantified using quantitative electroencephalography (qEEG). They can be predominantly recorded from the occipital lobes during wakeful relaxation with closed eyes and were the earliest brain rhythm recorded in humans.[3] Alpha waves are reduced with open eyes and sleep, while they are enhanced during drowsiness. Occipital alpha waves during periods of eyes closed are the strongest EEG brain signals.[4]
Historically, alpha waves were thought to represent the activity of the visual cortex in an idle state. More recently, research suggests that they inhibit areas of the cortex not in use, or alternatively that they play an active role in network coordination and communication.[5] Whether they are inhibitory or play an active role in attention links to their direction of propagation, with top-down rearward waves being inhibitory, and forward bottom-up ones aiding visual attentional processes.[6]
An alpha-like variant called a mu wave can be found over the primary motor cortex.
Research[edit]
Possible types and origins[edit]
Some researchers posit that there are at least two forms of alpha waves, which may have different functions in the wake-sleep cycle.
Alpha waves are present at different stages of the wake-sleep cycle.[7] 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,[8][9] although there has been speculation that it has a thalamic origin.[10] Several analyses suggest that cortical alpha leads pulvinar (thalamic) alpha, complicating prevailing theories of a thalamic pacemaker. Halgren, M et al. discovered that alpha acts within the nervous system by propagating from cortex to thalamus and higher-order to lower-order cortex.[11] The experimental and computational models explored by Traub RD et al. suggested cortical- a lamina- and principal neuron subtype specific origin for the visual alpha rhythm.[12] On the basis of examination of patients with congenital visual defects, it was established that the existence of an efficient and complete visual pathway is necessary for the development of a correct EEG activity pattern.[13] 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.[14]
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.
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.[15] However, it is possible that these explanations may be misleading, as they only focus on alpha waves being generated from the occipital lobe.
Meditation[edit]
Mindfulness meditation has been shown to increase alpha wave power in both healthy subjects and patients.[16] Practitioners of Transcendental Meditation have demonstrated a one-Hertz reduction in alpha wave frequency relative to controls.[17]
Alpha wave intrusion[edit]
Alpha wave intrusion occurs when the alpha waves appear with non-REM sleep when delta activity is expected. It is hypothesized to be associated with fibromyalgia with increased phasic alpha sleep activity correlated with clinical manifestations of fibromyalgia, such as longer pain duration.[18]
Despite this, alpha wave intrusion has not been significantly linked to any major sleep disorder, including chronic fatigue syndrome, and major depression. However, it is common in chronic fatigued patients, and may amplify the effects of other sleep disorders.[19]
Mistake prediction[edit]
Following this lapse-of-attention line of thought, a recent study indicates that alpha waves may be used to predict mistakes. In it, MEGs measured increases of up to 25% in alpha brain wave activity before mistakes occurred. This study used common sense: alpha waves indicate idleness, and mistakes are often made when a person is doing something automatically, or "on auto-pilot", and not paying attention to the task they are performing. After the mistake was noticed by the subject, there was a decrease in alpha waves as the subject began paying more attention. This study hopes to promote the use of wireless EEG technology on employees in high-risk fields, such as air traffic controlling, to monitor alpha wave activity and gauge the attention level of the employee.[20]
Processing of visual information in memory[edit]
A study has shown that the appearance of an alpha rhythm with open eyes can be a predictor of visual information processing in working memory.[21] It was shown that the moment of appearance of alpha activity depends on the type of stimulus in memory and the number of visual characteristics (color, shape, etc.) that it needs to keep in memory. The authors suggest that the appearance of the alpha rhythm with open eyes may indicate a temporary shutdown of visual information processing in the primary visual cortex at the moments when the subject analyzes the image in visual memory. At these moments, information is processed in the association areas of the visual cortex (hV4, V3v, VO1, VO2 areas).[22]
Visual learning[edit]
One study suggests that a "visual flicker paradigm to entrain individuals at their own brain rhythm (i.e. peak alpha frequency)" can result in substantially faster perceptual visual learning, maintained the day following training.
In particular, the entrainment substantially accelerated learning in a discrimination task to detect targets embedded in background clutter or to identify radial vs. concentric Glass patterns embedded in noise compared to entrainment that does not match an individual's alpha frequency.[23]
Measurement[edit]
EEG artefacts[edit]
As demonstrated by Dr. Adrian R. M. Upton, it is possible for extraneous sources (ambient fluctuations detected with a mound of Jell-O in Upton's experiments) to cause signals to appear on an EEG readout, causing false signals to be interpreted as healthy alpha waves. This finding suggests that it is possible that a non-flat EEG could lead to the interpretation that a patient is still living when in fact he or she is long dead.[24]
Cecil Adams from The Straight Dope discusses this scenario: