Characteristics of Normal EEG

Characteristics of Normal EEG

Thoru Yamada

Elizabeth Meng

Normal Awake EEG

EEG shows considerable interindividual variability and also changes significantly depending on the level of consciousness. Further, there are progressive maturational changes from infancy, childhood, adolescence, young adulthood, and to the elderly. Despite a great deal of variability, there are general characteristics of EEG patterns according to age, level of consciousness, and state of brain function, which enable us to determine normality and abnormality.


Definition of Alpha Rhythm

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.

Frequency of Alpha Rhythm

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).

FIGURE 7-1 | Typical normal alpha rhythm during the eyes-closed awake state in a 35-year-old man. Note attenuation of alpha rhythm by eyes opening. (A) is a referential montage and (B) is a longitudinal bipolar montage. (A and B are the same EEG samples.)

FIGURE 7-2 | EEG of a 1-month-old baby in the awake state. The basic background activity consists of a various mixture of 2- to 3-Hz delta and 4- to 5-Hz theta waves. There is little difference between eyes-open and eyes-closed states.

FIGURE 7-3 | EEG of a 5-month-old baby in the awake state. Note the more sustained theta rhythm (4 to 5 Hz) as the basic background activity, as compared to the EEG of a 1-month-old baby.

FIGURE 7-4 | EEG of a 14-month-old child in awake state. Waking background activity mostly consists of 5- to 7-Hz theta rhythm.

FIGURE 7-5 | EEG of a 3-year-old child in the awake state. Dominant rhythm becomes close to 8-Hz alpha rhythm with interspersed delta slow waves, that is, posterior slow waves of youth (a few examples are indicated by asterisk marks).

FIGURE 7-6 | EEG of a 5-year-old child in the awake state. Background activity consists of 9- to 10-Hz alpha rhythm with interspersed delta waves, that is, posterior slow waves of youth (a few examples are indicated by asterisk mark).

FIGURE 7-7 | EEG of a 10-year-old child in the awake state. Background activity consists of 9- to 10-Hz alpha rhythm with fewer posterior slow waves of youth.

FIGURE 7-8 | EEG of a 15-year-old adolescent in the awake state. Background activity consists of a predominant alpha rhythm.

FIGURE 7-9 | EEG of a 50-year-old in the awake state. The amplitude is generally lower than the EEG of youth.

FIGURE 7-10 | EEG of an 80-year-old in the awake state. In general, there may be slight reduction of alpha rhythm frequency in an elderly individual, but the frequency should not be at or slower than 8 Hz.

FIGURE 7-11 | EEG of a 5-year-old child during the eyes-open state. This is the same individual as that in Figure 7-6 (eyes-closed state). Note irregular theta-delta slow waves as background with eyes-open and well-modulated 9- to 10-Hz alpha rhythm after eyes closed.

Amplitude of Alpha Rhythm

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.

Symmetry of Alpha Rhythm

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.

FIGURE 7-12 | Distribution of alpha activity expressed by an amplitude spectrum. The frequency of alpha activity (alpha rhythm) is 10 Hz (left column). The topographic map shows maximum alpha amplitude at the occipital region and asymmetrical spread more to the right side (darker area indicates greater amplitude of alpha power shown in the right column).

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).

FIGURE 7-13 | A,B: The routine sweep speed (10 s/page) for an EEG shows more or less symmetric alpha activities and rhythm between homologous electrodes (A). When a faster sweep speed (2 s/page) is used (B), considerable asymmetry and asynchrony become evident between homologous electrodes (most asynchronous alpha rhythm is shown by rectangular box). Also, note the asynchrony of alpha activity from an anterior to poster plane (1, 2, 3, and 4 markers in A and B correspond to each other).

Distribution of Alpha Activity

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.

FIGURE 7-14 | Comparison of alpha distribution between a referential (A) and anterior-posterior bipolar derivation (B). (A and B are the same EEG sample.) Note the relatively widespread alpha activity shown in the referential recording (A) becomes posterior dominant with little alpha activity in anterior head region (Fp1-F3, Fp2-F4, Fp1-F7, and Fp2-F8) in longitudinal bipolar derivation (B).

Morphology, Amplitude, and Frequency Variation of Background Activity

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

Alpha Variants

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.

FIGURE 7-15 | Model for decompression of the background alpha rhythm. Mixture of 9-Hz (C) and 10-Hz (D) sine waves results in waxing and waning sinusoidal morphology (B), resembling the actual normal alpha rhythm (A). (Modified from Kiloh LG, McComas AJ, Osselton JW, et al. Technology and methodology. In: Clinical Electroencephalography. 4th Ed. London: Butterworth & Co., 1981, with permission.)

FIGURE 7-16 | Bilateral (A) and unilateral dominant alpha variant rhythm (B) indicated by underlines. Note the notched wave configuration in the alpha variant waveform indicating a subcomponent corresponding to the subharmonic alpha rhythm.


The mu rhythm is alpha activity (8 to 10 Hz) in the central region. Some may be slightly slower than alpha frequency. Due to its characteristic arch-shaped waveform, the mu rhythm has been named “comb” or “wicket rhythm,” or “rhythme rolandique en arceau” (“arch” rhythm in French) (Fig. 7-18; see also Fig. 6-1). Mu rhythm is more commonly seen in adoles-cents
and young adults (17% to 19%) and less commonly in the elderly and in children less than 4 years old.2,17 The incidence could be as high as 60% in some studies.18,19,20 It is twice as common in girls as in boys.4,5 Before the first description of mu rhythm by Gastaut et al.20 in 1952, in which the mu rhythm was blocked by contralateral limb movement, Jasper and Penfield21 found that the central beta rhythm recorded by electrocorticogram was attenuated by contralateral limb movement. In fact, the mu rhythm waveform consists of a mixture of alpha activity and the second harmonic beta rhythm. Indeed, beta rhythm always accompanies mu rhythm. The mu rhythm is also suppressed by intention of movement, sensory stimulation, and, to some extent, mental activity.22,23,24

FIGURE 7-17 | Fast alpha variant consisting of beta activity, which is attenuated by eyes opening, acting like an ordinary alpha rhythm.

FIGURE 7-18 | Asymmetric mu rhythm. With eyes opening, alpha rhythm (shown by oval circle) disappears, while mu rhythm at right central electrodes persists (shown by a rectangular box).

In contrast to the more or less symmetric appearance of the occipital alpha rhythm, mu rhythm is often asynchronous and asymmetric (see Fig. 7-18). In some normal subjects, it may appear exclusively in only one side throughout the recording. Eye opening affects the occipital alpha rhythm independently from the mu rhythm so that alpha is blocked, but mu rhythm persists (see Fig. 7-18). Although the mu rhythm is primarily a waking pattern, in some individuals, mu may be seen in stage I or even stage II sleep without concomitant appearance of alpha rhythm (Fig. 7-19A and B).25 This also supports the independent functions of mu and alpha rhythms. Mu rhythm accentuation is common in patients with a history of a craniotomy where there is a burr hole near the C3 or C4 electrode

(Fig. 7-20). This is due to enhancement of alpha and beta amplitudes with a greater degree of beta rhythm secondary to bone (skull) defect as Breach rhythm (see next chapter).

FIGURE 7-19 | Mu rhythm in awake (A) and in light sleep (B), shown by oval circles. While the alpha rhythm disappears, mu rhythm still appears intermittently in light sleep (B) indicating dissociated function between alpha and mu rhythm. Sleep spindles and vertex sharp wave shown by rectangular box indicate stage 2 sleep. (A and B are from the same subject.)

FIGURE 7-20 | Breach rhythm at C3 electrode secondary to the skull defect near C3 electrode from previous craniotomy. Note the large mu rhythm at C3 electrode. Also, irregular delta waves are present at T3 electrode.


Activity with a frequency higher than 14 Hz but less than 30 Hz is defined as beta activity. Beta activity may appear diffusely or more prominently in the frontal regions. The amplitude is usually less than 20 µV. There is considerable interindividual variability and no particular age preference in the incidence of beta activity. Overall, however, beta activity is more common in infants and young children less than 1½ years old and then diminishes in both amplitude and incidence with increasing age.26 Whenever beta activity is abundant, other EEG activities, namely, alpha or theta waves, assume a “spiky” appearance.

Beta activity is enhanced by sedative, hypnotic, or anxiolytic drugs (barbiturates, benzodiazepines) (Fig. 7-21A; see Fig. 8-2). It usually becomes more prominent and slower in frequency appearing waveform of sleep “spindle-like” activity in sleep (Fig. 7-21B; see also Figs. 8-3 and 8-4; see also “Anxiolytic or Hypnotic Drugs,” Chapter 11). Beta enhancement by medication is not dose-dependent but depends more on the individual’s sensitivity.

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Nov 14, 2018 | Posted by in NEUROLOGY | Comments Off on Characteristics of Normal EEG
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