The majority of normal adult subjects, during the awake state with the eyes closed, have a dominant rhythm of about 10 Hz. The alpha rhythm, defined as 8- to 13-Hz activity, occurs predominantly in the posterior half of the brain, especially in the occipital region. The International Federation of Societies of Electrophysiology and Clinical Neurophysiology¹ (IFSECN) defines alpha rhythm as follows: Rhythm is at 8 to 13 Hz occurring during wakefulness over the posterior regions of the head, generally maximum amplitude over the occipital areas. Amplitude varies but is mostly below 50 µV in the adult. It is best seen with eyes closed during physical relaxation and relative mental inactivity and blocked or attenuated by attention, especially visual, and mental effort.

The IFSECN emphasizes that the term “alpha rhythm” should be used specifically for those rhythms that fulfill the above criteria. Activity in the alpha frequency band (which differs from the alpha rhythm in respect to topography and/or reactivity) should be referred to as rhythm of alpha frequency or simply alpha activity.

Alpha rhythm is most prominent when the subject is awake but relaxed with eyes closed. As Hans Berger discovered, the alpha rhythm is diminished or abolished by eye opening, described as alpha blocking or desynchronization (Fig. 7-1A and B). Mental concentration or external stimulation also reduces the alpha rhythm. Opening the eyes in total darkness attenuates the alpha rhythm only transiently, and the rhythm soon reappears, although it is again reduced by conscious effort to see in the darkness.

In the normal adult population, the mean frequency of the alpha rhythm is 10.2 Hz, and in less than 5%, it is faster than 11.5 Hz or slower than 8.5 Hz.² Under stable conditions, the frequency generally varies only about ±0.5 Hz in one recording. There are progressive maturational changes in the basic waking background activity from infancy, young childhood, adolescence, and to young adulthood. It is, therefore, important to know the evolution of EEG changes with age. Before the age of 3 months, the occipital rhythm is not well defined and may not react to eyes opening (Fig. 7-2). By 6 months of age, a measurable background activity appears (Fig. 7-3), and by 1 year of age, a more sustained 5- to 6-Hz theta rhythm can be recognized³,⁴ (Fig. 7-4). The activity becomes close to 8 Hz (7.5- to 9.5-Hz range) by the age of 3 years³,⁴ (Fig. 7-5). The frequency of the alpha rhythm progressively increases to 10 Hz until age 15 to 16 years and then plateaus⁵ (Figs. 7-6, 7-7 and 7-8). As age increases, there is a progressive reduction of the theta-delta slow waves (posterior slow of youth; see “Morphology, Amplitude, and Frequency Variation of Background Activity” in this chapter) intervening in the alpha rhythm.

Later in life, the frequency of the occipital alpha rhythm tends to decrease slightly, but the mean frequency is generally maintained at or above 9 Hz⁶,⁷ (Figs. 7-9 and 7-10). An alpha rhythm occurring consistently at or slower than 8 Hz should be considered abnormal even in elderly individuals.⁸,⁹ The frequency of the alpha rhythm has been shown to be closely related to cerebral blood flow. Significant slowing of the alpha rhythm frequency occurs with a decrease of cerebral blood flow, and a faster frequency occurs with an increase of blood flow.¹⁰ A frequency faster than 11 Hz is more often found in women, and there may be a slight increase in the mean frequency during the luteinized phase prior to menstruation.¹¹

For a reliable measurement of the basic waking background activity, it is important to record the EEG during the fully awake state, but relaxed with the eyes closed. Occasionally, the first few seconds after closing the eyes might produce an alpha rhythm that is slightly faster than in other resting and eyes-closed state (“squeak effect”). Alpha rhythm should be measured after this faster rhythm disappears. In the eyes-open state, especially in children, the EEG appears to be slower. In the young child age group, spontaneous eye closure is a sign of drowsiness; therefore, the background activity measured during this state may be slower than that during the fully awake state. It is necessary to record at least a portion (10 to 30 seconds) of the EEG during a passive eyes-closed state when the child is fully awake. This may require the eyes to be held closed by a technologist (Fig. 7-11).

The amplitude of the alpha rhythm in an adult measured by the referential (ear reference) montage is generally 40 to 50 µV. The amplitude measurement differs depending on the electrode derivation; a derivation of short interelectrode distance, such as T6-O2 or P4-O2, shows smaller amplitude than a long interelectrode derivation, such as O2-A2. Using the P4-O2 derivation, 75% of normal adults have alpha rhythm amplitude of 15 to 45 µV.² In about 10% of the normal population, the background activity is very low voltage without a measurable alpha rhythm.¹² In some recordings, the alpha rhythm is hardly recognized shortly after the recording starts, but may soon appear as the subject relaxes as the recording proceeds. In some individuals with low-voltage background activity without appreciable alpha rhythm, hyperventilation may bring out a better defined alpha rhythm. The alpha rhythm in children usually has a larger amplitude than in adults; the average amplitude of an alpha rhythm in the T5-O1 derivation is 50 to 60 µV (age 3 to 15 years), and about 10% of children (age 6 to 9 years) show greater than 100 µV.⁴ Very low-voltage alpha rhythm, less than 30 µV, is extremely rare, especially in children less than 10 years old, and may be considered abnormal.

The amplitude of alpha rhythm generally diminishes with increasing age. This may be due to changes in the attenuation factors by intervening structures between the brain and scalp, such as density of the bone, increased electrical impedance or increased space due to brain atrophy, rather than a decrease in the electrical activity of the brain itself.

Amplitude asymmetry of the occipital alpha rhythm is seen in 60% of adults and in general, the right side shows higher amplitude and wider distribution than the left side (Fig. 7-12). The right being greater than left amplitude asymmetry is especially true in children; 95% of all children show the asymmetric amplitude, but the difference is generally less than 20%.⁵ This asymmetry has been attributed to the difference in skull thickness,¹³,¹⁴ rather than to handedness or speech dominance. Consistent amplitude asymmetry with depression greater than 50% either in an adult or a child is generally considered clinically significant, and this is especially true if the amplitude of the right-sided alpha rhythm is depressed. If, however, the amplitude asymmetry is the only finding, without slowing or other accompanied abnormality, interpretation for determining an abnormality should be rather conservative and cautious; it is always good practice to look for possible technical reasons such as electrode impedance, placement, interelectrode distance (see Fig. 15-48), burr hole (skull defect) effect causing Breach rhythm (see Fig. 7-20; see also Fig. 12-12), scalp edema, etc. (see “Technical Pitfalls and Errors,” Chapter 15). It is the technologist’s role to find, document, and correct possible technical reasons for the asymmetric amplitude.

In determining an abnormality, asymmetric frequency is more reliable than asymmetric amplitude. The difference in alpha frequency between the two sides is small, and a consistent difference of 0.5 Hz or more should be considered abnormal on the side of the slower frequency (see Fig. 6-10). The phase of alpha rhythm varies from ±2.5 ms to ±20 ms.¹⁵ If the alpha rhythms at the O1 and O2 electrodes have the same amplitude and same morphology, and appear with exact synchrony, the O1-O2 derivation should show a “flat” tracing. In reality, the O1-O2 derivation usually shows abundant alpha rhythm with waxing and waning modulation. This indicates that the O1 and O2 alpha rhythms are neither exactly symmetric nor synchronous. Although a routine sweep speed (10 to 15 s/page) appears to show symmetric alpha activity (Fig. 7-13A), a fast sweep tracing reveals intermittently asynchrony of the alpha rhythm between the O1 and O2 electrodes (Fig. 7-13B). Also, the alpha activity is not synchronous between anterior and posterior head regions (Fig. 7-13B).

The majority of adults and children have a posterior dominant alpha rhythm, but in some individuals, alpha activity is maximal in the central-parietal region. In about one third of adults, alpha activity is widely distributed.¹² In a referential recording, the alpha activity often appears more widely distributed than that in an anterior-posterior bipolar derivation (Fig. 7-14A and B). In fact, the alpha activity is often not visible in the Fp1-F3 or Fp1-F7 bipolar derivation.

The alpha rhythm is rarely a simple sinusoidal waveform. However, the complex EEG waveform can be broken down into a small number of sine waves at different frequencies. A mixture of 9- and 10-Hz sine waves, for example, results in waxing and waning rhythmic waves that resemble a spontaneous alpha rhythm (Fig. 7-15), which in turn implies that the normal alpha rhythm is a composite of at least two different activities of close frequency. From the age of 3 years to the late teen years, the basic waking background activity consists of a mixture of alpha rhythm and theta-delta slow waves (see posterior slow waves of youth in “Delta Activity” in this chapter; see also Fig. 7-25). The posterior slow waves of youth progressively decrease toward the end of the teen years. In adults, the frequency of the occipital alpha rhythm is remarkably consistent and shows little variation throughout the day or for long periods of time.¹¹,¹⁵

The alpha rhythm may abruptly assume a frequency, which is half of the ongoing alpha activity, that is, 5-Hz theta rhythm instead of 10-Hz activity (Fig. 7-16A and B). This theta rhythm, called the alpha variant rhythm, usually has a bifurcated configuration implying a subharmonic alpha rhythm. Alpha variants may appear sporadically, persistently, or asymmetrically. When persistent, it could mistakenly be regarded as an abnormally slow background activity (Fig. 7-16A). When asymmetric, it
may appear as focal slowing (Fig. 7-16B). On some occasions, the frequency becomes “double,” appearing as beta activity, which is referred to as “fast” alpha variants. The fast alpha variant rhythm can be induced by hypnotic or anxiolytic medications such as barbiturates or benzodiazepines (Fig. 7-17). The incidence of alpha variants is less than 1% of the normal adult population¹⁶; and there is no known clinical significance or correlate associated with the alpha variant rhythm.