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electroencephalography (əlĕkˌtrōĕnsĕfˌəlŏgˈrafē), science of recording and analyzing the electrical activity of the brain. Electrodes, placed on or just under the scalp, are linked to an electroencephalograph, which is an amplifier connected to a mechanism that converts electrical impulses into the vertical movement of a pen over a sheet of paper. The recording traced by the pen is called an electroencephalogram (EEG). Readings may be obtained for a particular brain site by coupling a single electrode with an indifferent, or neutral, lead (monopolar technique) or between two areas of the brain through two independent electrodes (bipolar technique). The combination of impulses that are being recorded at any one time is called a montage.

Brainwave Patterns

The electrical activity of the brain was first demonstrated in 1929 by the German psychiatrist Hans Berger. The scientific professions were slow in giving proper attention to Berger's discovery of the brain rhythms he named alpha waves, but since then at least three other standard brainwave patterns have been isolated and identified. Alpha waves are fast, medium-amplitude oscillations, now known to represent the background activity of the brain in the physically and psychologically healthy adult. They are most characteristically visible during dream-sleep or when a subject is relaxing with eyes closed. Delta waves are large, slow-moving, regular waves, typically associated with the deepest levels of sleep. In children up to the age of puberty the appearance of high-amplitude theta waves, having a velocity between those of alpha and delta rhythms, usually signals the onset of emotional stimulation. The presence of theta waves in adults may be a sign of brain damage or of an immature personality. Beta rhythms are small, very fast wave patterns that indicate intense physiological stress, such as that resulting from barbiturate intoxification.

Uses of EEGs

By observing abnormalities in recordings and determining the area of the brain from which they originate, the physician's ability to diagnose and treat such conditions as epilepsy, cerebral tumor, encephalitis, and stroke, is greatly enhanced. Electroencephalograms have also proved valuable in the general study of brain physiology and in the particular study of sleep. Various types of Eastern meditation, e.g., yoga, use techniques that increase alpha and theta wave activity. Because of concomitant physiological changes during meditation, e.g., lessened anxiety, the techniques have recently become popular in the West. Using EEGs to enhance biofeedback, a subject can be taught to monitor and regulate his or her own brain waves; the technique has been used experimentally in control of epilepsy. EEGs are also used to determine brain death (see death).
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The biomedical technology and science of recording the minute electric currents produced by the brains of human beings and other animals. Electroencephalography (EEG) has important clinical significance for the diagnosis of brain disease. The interpretation of EEG records has become a clinical specialty for neurological diagnosis.

The recording machine, the electroencephalograph, usually produces a 16-channel ink-written record of brain waves, the electroencephalogram. It is interpreted by an electroencephalographer. The placement of about 20 equally spaced electrodes pasted to the surface of the scalp is in accordance with the standard positions adopted by the International Federation of EEG, and is called the 10/20 system. Electrode positions are carefully measured so that subsequent EEGs from the same person can be compared. About 10 patterns or montages of combinations of electrode pairs are selected for transforming the spatial location from the scalp to the channels which are traced on the EEG pen writer.

The aggregate of synchronized neuronal activity from hundreds of thousands or millions of neurons acting together form the electrical patterns on the surface of the brain (brain waves). The cellular basis of the EEG depends on the spontaneous fluctuations of postsynaptic membrane potentials between the inside and the outside of the dendritic processes of postsynaptic cells. See Synaptic transmission

Electrical voltage is transduced from the scalp by differential input amplifiers and amplified about a million times in order to drive the pens for the paper record. The recording usually takes 30–60 min during a relaxed waking state, and also during sleep when possible. Often, activating procedures are used, such as a flickering light stimulator and hyperventilation or overbreathing for about 3 min.

EEG waves are defined by form and frequency. Various frequencies are given Greek letter designations. Alpha rhythm is defined as 8–12-Hz sinusoidal rhythmical waves. Alpha waves are normally present during the waking and relaxed state and enhanced by closing the eyes. They are suppressed or desynchronized when the eyes are open, or when the individual is emotionally aroused or doing mental work. They may be synchronized by bright light flashes and driven over a wide range of frequencies by repetitive visual stimulation (alpha driving). They are of highest amplitude in the posterior regions of the brain. The alpha rhythm develops with age, reaching maturity by about 12 years, stabilizes, and then declines in frequency and amplitude in old age (over 65).

Beta rhythms are faster, low-voltage sinusoidal waves, usually about 14–30 Hz. They are more prominent in the frontal areas. They are often synchronized and prevalent during sedation with phenobarbital or with the use of tranquilizers and some sedative drugs.

Slower rhythms are theta and delta waves. Theta waves of 4–7 Hz usually replace the alpha rhythm during drowsiness and light sleep. Delta waves of 0.5–4 Hz are present during deep sleep in normal people of all ages and they are the primary waves present in the records of normal infants. Delta waves are almost always pathological in the waking records of adults.

The EEG reveals functional abnormalities of the brain, whether caused by localized structural lesions, essential paroxysmal states such as epilepsy, or toxic and abnormal metabolic conditions. The three major classes of abnormalities are asymmetries between the hemispheres, slow rhythms, and very sharp waves or spikes. Slow waves represent a depression of cerebral cortical activity or injury in the projection pathways beneath the recording electrodes. Sharp waves or spikes often indicate a hyperexcitable or irritable state of the cortex. During a full epileptic seizure attack, spikes become repetitive and synchronized over the whole surface of the brain.

The EEG is frequently used for the evaluation of comatose states. The record is slowed in all areas in coma, with delta waves predominating. If the EEG becomes isoelectric or flat for several hours, brain function is not recoverable and the coma may be considered terminal. “Brain death” is indicated by a flat EEG, recorded at the highest gain with widely spaced electrode positions and the absence of cerebral reflexes and spontaneous respiration.

Computer advances in the analysis of EEG signals that are emitted by the brain during sensory stimulation and motor responses have led to the discovery and measurement of electrical waves known as event-related potentials or evoked potentials. These responses are averaged by a computer to enhance the small signals and increase the signal-to-noise ratio, so that they may be graphed and seen.

The complexity of evoked potential and EEG analysis makes interpretation difficult in relation to where various components originate and their pattern of spread through time along the neural transmission pathways. In the 1980s, with the development of minicomputers and color graphics screens, the presentation of topographic information could be analyzed in sophisticated statistical ways for research and clinical purposes by electroencephalographers and neurophysiologists. This method is best known as brain electrical activity mapping (BEAM) and is used in many research investigations of brain activity patterns in learning and language dysfunctions, psychiatric disorders, aging changes and dementia, and studies of normal and impaired child development. Difficult neurological diagnostic problems that do not show anatomical deformities by brain scan methods may often be clarified by these new electrographic procedures. See Brain, Neurobiology

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



a method of examining brain activity in animals and humans by recording the total bioelectric activity of the various zones, regions, and lobes of the brain. The procedure is used in modern neurophysiology, neuropathology, and psychiatry.

The active brain, like many other tissues and organs, is a source of electromotive force. However, its electric activity is low, measured in millionths of a volt, and can be recorded only with highly sensitive instruments and amplifiers called electroencephalographs. Electrodes connected by wire to a recording device are attached to the scalp. The recording device produces a graphic representation, or electroencephalogram (EEG), of the variations in the difference in bioelectric potentials of the brain. The EEG reflects both the morphological features of the complex brain structures and the dynamics of their functioning, that is, the synaptic processes that develop on the body and dendrites of neurons of the cerebral cortex. An EEG is a complex curve consisting of waves of different frequencies (periods) with changing phase relations and different amplitudes. The waves are designated according to amplitude and frequency by the Greek letters alpha, beta, delta, and so on. The EEG of healthy persons can be distinguished by their physiological state (sleep and wakefulness, perception of visual or auditory signals, emotions). The EEG of a healthy adult at relative rest shows two main types of rhythms: alpha rhythms with a frequency of 8–13 hertz (Hz) and an amplitude of 25–55 microvolts (mv) and a beta rhythm with a frequency of 14–30 Hz and an amplitude of 15–20 mv. Disease can cause disturbances of the normal EEG pattern, from which the severity and location of a lesion can be determined, for example, the site of a tumor or hemorrhage. Recording an EEG during an operation is useful in monitoring the patient’s condition and controlling the depth of anesthesia.

Of increasing importance for clinical medicine is electrosubcorticography—the recording of the electric activity of the deep divisions of the brain. Electrosubcorticography can be done during a neurosurgical operation or over an extended period of time (electrodes are implanted in the brain). Telectroencephalography is used to record the electric activity of the brain from a distance.

The accuracy of EEG’s and the amount of information derived from them have increased through the supplementation of simple visual evaluation with quantitative methods. Spectral, correlation, and other methods of statistical analysis are now used, and topographical maps of the potential fields of the brain are compiled. Accurate automatic computer-assisted analysis is increasing the usefulness of electroencephalography.


Kratin, Iu. G., and V. I. Gusel’nikov. Tekhnika i metodiki elektroentsefalografii, 2nd ed. Leningrad, 1971.
Zhirmunskaia, E. A. “Bioelektricheskaia aktivnost’ zdorovogo i bol-’nogo mozga cheloveka.” In Klinicheskaia neirofiziologiia. Leningrad, 1972.
Egorova, I. S. Elektroentsefalografiia. Moscow, 1973.
Klinicheskaia elektroentsefalografiia. Moscow, 1973.
Melody klinicheskoi neirofiziologii. Leningrad, 1977.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


The medical specialty concerned with the production and interpretation of electroencephalograms.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
We used electro-encephalography (EEG) to study visual event-related potentials, which in this case means that we are examining the amplitude of brain activity that happens in the period immediately following a stimulus, such as the onset of the word in the Stroop task.
The intra-operative neurophysiological monitoring technique involves usage of electro-physiological methods such as electro-encephalography (EEG), electro-myography (EMG) and evoked potentials to monitor the functional integrity of certain neural structures like nerves, the spinal cord and parts of the brain during surgery.
At T1 and T2, cortical activity was measured at baseline and immediately after stress with cognitive tasks using electro-encephalography (EEG).

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