EEG Atlas: Normal Awake EEG

Last Updated: September 27, 2006

Synonyms and related keywords: alpha rhythm, mu rhythm, beta activity, electroencephalography

AUTHOR INFORMATION

Author: Selim R Benbadis, MD, Professor of Neurology, Director of Comprehensive Epilepsy Program, Departments of Neurology and Neurosurgery, University of South Florida College of Medicine, Tampa General Hospital

Coauthor(s): Diego Rielo, MD, Staff Physician, Department of Neurology, Memorial Hospital West, Memorial Hospital Pembroke, Memorial Healthcare

Selim R Benbadis, MD, is a member of the following medical societies: American Academy of Neurology, American Academy of Sleep Medicine, American Clinical Neurophysiology Society, American Epilepsy Society, and American Medical Association

Editor(s): Leslie Huszar, MD, Consulting Staff, Department of Neurology, Indian River Memorial Hospital; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; Norberto Alvarez, MD, Assistant Professor, Department of Neurology, Harvard Medical School; Consulting Staff, Department of Neurology, Boston Children's Hospital; Paul E Barkhaus, MD, Professor, Department of Neurology, Medical College of Wisconsin; Director of Neuromuscular Diseases, Milwaukee Veterans Administration Medical Center; and Nicholas Lorenzo, MD, Chief Editor, eMedicine Neurology; Consulting Staff, Neurology Specialists and Consultants

INTRODUCTION

This article describes the most common features of the normal awake EEG. The images at the end of the article show representative examples of the features discussed here.

The alpha rhythm is the most prominent feature of the normal mature EEG. It typically is identified first during the review.

Beta activity refers to a frequency band rather than a distinct (specific) rhythm such as alpha or mu. Beta activity is commonly present in the EEG of healthy people. However, it is often difficult to see because of its low amplitude.

Gastaut initially described the mu rhythm in 1952. This morphologically distinct activity is observed in approximately 17-19% of young adults.

WAVEFORM DESCRIPTION

Alpha rhythm

The normal alpha rhythm has the following characteristics:

Frequency of 8-12 Hz - Lower limit of normal generally accepted in adults and children older than 8 years is 8 Hz

Location - Posterior dominant; occasionally, the maximum may be a little more anterior, and it may be more widespread

Morphology - Rhythmic, regular, and waxing and waning

Amplitude - Generally 20-100 mV

Reactivity - Best seen with eyes closed; attenuates with eye opening

Beta activity

Normal beta activity has the following characteristics:

Frequency (by definition) greater than 13 Hz - Common 18-25 Hz, less common 14-16 Hz, and rare 35-40 Hz

Location - Mostly frontocentral but somewhat variable; some describe various types according to location and reactivity: generalized, precentral, and posterior

Morphology - Usually rhythmic, waxing and waning, and symmetric

Amplitude - Usually range of 5-20 mV

Reactivity - Most common 18- to 25-Hz beta activity enhanced during stages I and II sleep and tends to decrease during deeper sleep stages; central beta activity may be reactive (attenuates) to voluntary movements and proprioceptive stimuli; in infants older than 6 months, onset of sleep marked by increased beta activity in central and postcentral regions

Mu rhythm

Characteristics of the mu rhythms are as follows:

Frequency of 7-11 Hz - Generally in alpha frequency band (8-12 Hz)

Location - Centroparietal area

Morphology - Archlike shape or like an "m"; most often asymmetric and asynchronous between the 2 sides and may be unilateral

Amplitude - Generally low to medium and comparable to that of the alpha rhythm

Reactivity - Most characteristic feature defining the mu rhythm; mu rhythm attenuates with contralateral extremity movement, the thought of a movement, or tactile stimulation; contrary to the alpha rhythm, does not react to eye opening and closing

The mu rhythm has been documented on subdural recording of both sensory and motor cortex and shows the same characteristics as that seen on surface EEG, including distribution, morphology, and reactivity. Furthermore, some correspondence exists between functional mapping of sensorimotor function and somatotopic distribution of mu reactivity.

CLINICAL CORRELATION

Alpha rhythm

Occasionally the alpha rhythm is of very low amplitude or even not identifiable. This is not abnormal. In addition to amplitude, other characteristics can vary somewhat without being abnormal, including morphology (eg, spiky), distribution (eg, widespread), and harmonic frequency (eg, slow or fast alpha variant).

Beta activity

In healthy individuals, beta activity commonly can be mildly different (<35%)>

Mu rhythm

Asymmetry, unilaterality, or asynchrony of the mu rhythm is generally not abnormal unless associated with other abnormalities. Very high-voltage mu activity may be recorded in the central regions over skull defects and may become sharp in configuration, and thus can be mistaken for epileptiform discharges. When mu rhythm is detected in an EEG, it should be verified by testing its reactivity.

PICTURES

Caption: Picture 1. A 10-second segment showing a well-formed and well-regulated alpha rhythm at 9 Hz. Note that it is very regular, rhythmic, waxing and waning, and posterior dominant. The contrast between the first and second halves of the page illustrates the reactivity of a normal alpha rhythm, with attenuation upon eye opening.

Picture Type: Rhythm Strip

Caption: Picture 2. Fleeting alpha. At times, as shown here, the alpha rhythm can be identified only in very brief bursts and often immediately after eye closure. If normal in frequency, this is normal.

Picture Type: Rhythm Strip

Caption: Picture 3. This is an example of an alpha rhythm with a wider distribution than is typical. If frequency and reactivity are normal, this is another variation of normal. A similar EEG pattern can be seen in patients in a coma (ie, alpha coma), but in these situations it is usually unreactive.

Picture Type: Rhythm Strip

Caption: Picture 4. This is an example of "slow alpha variant." The patient's alpha rhythm at 12 Hz is seen in the second half of the sample. The first half shows a subharmonic at half that frequency, and this is the "slow alpha variant."

Picture Type: Rhythm Strip

Caption: Picture 5. A sample of awake EEG showing the normal or usual amount of beta activity. As shown here, beta activity is often easier to identify during relaxed wakefulness or early drowsiness.

Picture Type: Rhythm Strip

Caption: Picture 6. Mu rhythm over the left (greater than right) central region. To be absolutely certain that this is a mu rhythm, reactivity should be tested. However, morphology (not absolutely typical but fairly so), frequency, and distribution strongly suggest that this is a mu rhythm.

Picture Type: Rhythm Strip

Caption: Picture 7. An example of a typical normal alpha rhythm, showing clear attenuation upon eye opening (second half of page).

Picture Type: Rhythm Strip

Caption: Picture 8. This is the normal amount of beta activity, frontally predominant, with waxing and waning amplitude.

Picture Type: Rhythm Strip

Caption: Picture 9. Alpha rhythm with somewhat "spiky" or sharply contoured morphology. When fragmented (eg, in drowsiness), this can be misinterpreted as sharp waves.

Picture Type: Rhythm Strip

BIBLIOGRAPHY

Arroyo S, Lesser RP, Gordon B, et al: Functional significance of the mu rhythm of human cortex: an electrophysiologic study with subdural electrodes. Electroencephalogr Clin Neurophysiol 1993 Sep; 87(3): 76-87[Medline].

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Benbadis SR: Introduction to EEG. In: Lee-Chiong T, ed. Sleep: A Comprehensive Handbook. Hoboken, NJ: Wiley & Sons; 2006: 989-1024.

McFarland DJ, Miner LA, Vaughan TM, et al: Mu and beta rhythm topographies during motor imagery and actual movements. Brain Topogr 2000 Spring; 12(3): 177-86[Medline].

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Pfurtscheller G, Neuper C, Krausz G: Functional dissociation of lower and upper frequency mu rhythms in relation to voluntary limb movement. Clin Neurophysiol 2000 Oct; 111(10): 1873-9[Medline].

Pineda JA, Allison BZ, Vankov A: The effects of self-movement, observation, and imagination on mu rhythms and readiness potentials (RP's): toward a brain-computer interface (BCI). IEEE Trans Rehabil Eng 2000 Jun; 8(2): 219-22[Medline].

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