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Uses of EEGs
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
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.
REFERENCESKratin, 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.
E. A. ZHIRMUNSKAIA