One of the most excellent reviews ever written about alpha rhythms is by O. N. Markand. His paper appeared in the Journal of Clinical Neurophysiology, in 1990, and it was titled "Alpha rhythms", forever hijacking the coveted title from authors of future journal articles on this important subject.
"The alpha rhythm usually consists of continuous sinusoidal waves of gradually waxing and waning amplitude, often producing a spindle-like appearance. It is rarely perfectly sinusoidal (monorhythmic alpha). In a single EEG study the alpha rhythm is never strictly monorhythmic, its frequency varies even under stable conditions to the extent of +/- 0.5 Hz above the mean." (O.N. Markand, 1990)
The following selection from Markand discusses the scalp distribution of alpha very well, which is important because there are multiple generators of alpha rhythms in the brain, not just one alpha. If a person simply refers to alpha, then most people will assume he means the occipital alpha.
"Since the classic studies of Adrian and Matthews (1934) and Adrian and Yamagiwa (1935), it is established that alpha rhythm is most prominent over the occipital, parietal, and posterior temporal regions. In referential montages, the occipital electrodes (O1, O2) usually demonstrate the maximum amplitude, with occasional exception when higher amplitudes may be recorded from the parietal or posterior temporal electrodes (P3/P4, T5/T6). The field of distribution more anteriorly is subject to marked interindividual variation. It may extend into central and midtemporal areas and occasionally as far forward as the midfrontal location (F3/F4 electrodes). Furthermore, the spatial distribution over the two hemispheres in a given individual may be quite asymmetric; the alpha rhythm may be restricted over the posterior hemisphere on one side and more widespread in distribution over the other. More anterior rhythms are usually of lower amplitude, show less reactivity, and have slightly different frequency (usually 0.5-1.0 Hz slower) than the dominant occipital rhythm. It can, therefore, be argued whether these rhythms should be truly described as alpha rhythm, since they fail to conform to the accepted definition of the alpha rhythm. There is also some suggestion that the reactivities of the occipital and more anterior alpha rhythms may differ; the former blocking on visual activation and the latter on psychosensory activation." (O.N. Markand, 1990)
The study of the scalp distribution of alpha waves led to a very important discovery for understanding right-left symmetry of the body: the amplitude is usually smaller over the left hemisphere. The right hemisphere shows higher voltage amplitude of alpha waves and also right-sided predominance in humans is six times as frequent as the left. This asymmetry has been related to cerebral dominance and interpreted as the expression of a state of greater activation of the dominant hemisphere, where the amplitude is less.
The degree of interhemispheric asymmetry of the amplitude of the alpha rhythm has functional consequences on personality. Artists, visual designers, and schizophrenia patients are more likely to be left-handed, and to show higher alpha amplitudes over the left hemisphere, than most of the general population. Thus a subgroup of individuals are distinguished from the major percentage of society members based on both their behavior and scalp distribution of alpha.
"Early studies established that a low-voltage EEG with little or no discernible alpha rhythm was not uncommon in normal adults with no neurologic or psychiatric problem. The amount of alpha rhythm varies considerably among individuals; abundance of the alpha rhythm relates to "percentage time alpha" recorded under stable conditions... Some normal subjects may exhibit no or very little posterior dominant rhythm, whereas others may show high-amplitude alpha rhythm, which may occur almost continuously. This was clearly appreciated by Adrian and Matthews (1934) who observed that one of them (Adrian) "gave" prominent alpha rhythm, whereas in the other (Matthews), "the rhythm may not appear at all at the beginning of an examination and seldom persists for long without intermission." " (O.N. Markand, 1990)
While the voltage amplitude and hemispheric distribution of alpha rhythm is not the final word on what makes psychiatric patients psychotic, it is a common finding that is none the less very interesting.
Next, Markand describes the development of alpha from baby to centenarian. It is notable that a human baby is not born with a fully developed alpha rhythm. It takes several years for it to gain in amplitude and frequency.
"The alpha frequency is 5-6 Hz by age 1, 6-8 Hz by end of 2 years, and 7-9 Hz by the end of 3 years. Further increase in the frequency of the alpha rhythm after this period is more gradual and mean alpha frequency of 9 Hz is reached by 7-9 years, and 10 Hz by the middle of the second decade; from a practical standpoint, if the occipital dominant rhythm fails to attain 8 Hz in a child of 4 years or more, it is usually considered to be an abnormality. An EEG of a very low voltage with virtual lack of alpha rhythm is exceedingly rare in normal children. Although adult frequency of 9-10 Hz is reached by approximately the end of the first decade, there is considerable admixture of theta-delta components ("slow waves of youth") interspersed between trains of alpha waves. During the second decade these slow transients gradually disappear. The frequency of the alpha rhythm changes markedly during maturation through childhood but after the age of 20 years it becomes quite stable. In an adult the alpha frequency is probably the most stable feature of the EEG rhythms. For any one particular person, alpha frequency remains constant throughout adult life, varying little until old age, when a slight decline in the frequency occurs. The mean alpha frequency does drop slightly after the age of 60 years by approximately 1 Hz, but most healthy elderly will continue to maintain alpha frequency of 9-10 Hz. The average posterior rhythm in seven centenarians (100-105 years) was 8.6 Hz." (O.N. Markand, 1990)
The alpha rhythm is one of the most noticeable features of human brain, but it has also been observed in dog, monkey, and cat. The alpha rhythm is about 10 Hz in primates and the amplitude is small even when recorded from dural electrodes; the animal that most strikingly resembles man with regard to alpha rhythm is dog. Commonalities between species suggest that alpha rhythms could be an organizing feature for neuronal ensembles.
When alpha waves are blocked or suddenly disappear, the EEG signal left behind is described as a low-voltage EEG signal. Numerous people have observed the low-voltage EEG signal so it has been called many different things, including "alpha block". Markand writes,
"Different terms, such as "low-voltage fast", "low-voltage random frequency", "low voltage", "flat posterior tracing", and "flat EEG" have been applied to such tracings in the past. "Low-voltage record" is the term proposed by the IFSECN and is defined as follows: "A waking record characterized by activity of amplitudes not greater than 20 uV over all head regions. With appropriate instrumental sensitivities this activity can be shown to be composed primarily of beta, theta, and to some degree, delta waves with or without alpha activity over the posterior areas." " (O.N. Markand, 1990)
The subject of alpha block relates to the concept of whether the brain is "doing something" or "not doing something" and we will return to the subject of alpha block at a later time.
"Despite six decades of extensive work, the biological significance of alpha rhythm remains a mystery. The very special condition of relaxation, darkness, and quiet that are main requisites to the alpha rhythm are hardly representative of much of our normal waking life. Instead, such conditions are really precursors of sleep, and the slightest effort, noise, or other stimuli will quickly break up this apparent optimal stage of rhythmic oscillation during wakefulness. Whether alpha rhythm has any function or is simply an interesting epiphenomenon is a question often raised... Schmitt et al. (1976) demonstrated that slow electrical potentials taking place in the dendrites of one neuron may influence, by electrical conductivity, the electrical activity of dendrites of neighboring neurons. It is likely, therefore, that the conduction of the slow electrical phenomena (associated with alpha rhythm) over comparatively large distances due to partial synchrony may exert an influence on dendrites of a large number of neurons, an influence that must have some functional significance." (O.N. Markand, 1990)
Markand does not attribute the alpha rhythm to the firing of synapses. Rather, it is proposed that alpha rhythms reflect the movement of slow electrical phenomena across the dendrites of a large number of neurons. Moving the electrical phenomena across large distances in the brain is thought to require the cooperative efforts of many neurons.
Markand, O. N. 1990. Alpha rhythms. Journal of Clinical Neurophysiology, 7, 163-189.