electroencephalography

(əlĕk'trōĕnsĕf'əlŏg`rafē), science of recording and analyzing the electrical activity of the brainbrain,
the supervisory center of the nervous system in all vertebrates. It also serves as the site of emotions, memory, self-awareness, and thought. Anatomy and Function
..... Click the link for more information. . 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 epilepsyepilepsy,
a chronic disorder of cerebral function characterized by periodic convulsive seizures. There are many conditions that have epileptic seizures. Sudden discharge of excess electrical activity, which can be either generalized (involving many areas of cells in the brain)
..... Click the link for more information. , cerebral tumor, encephalitisencephalitis
, general term used to describe a diffuse inflammation of the brain and spinal cord, usually of viral origin, often transmitted by mosquitoes, in contrast to a bacterial infection of the meninges (membrane surrounding the brain and spinal cord), known as meningitis.
..... Click the link for more information. , and strokestroke,
destruction of brain tissue as a result of intracerebral hemorrhage or infarction caused by thrombosis (clotting) or embolus (obstruction in a blood vessel caused by clotted blood or other foreign matter circulating in the bloodstream); formerly called apoplexy.
..... Click the link for more information. , is greatly enhanced. Electroencephalograms have also proven valuable in the general study of brain physiology and in the particular study of sleepsleep,
resting state in which an individual becomes relatively quiescent and relatively unaware of the environment. During sleep, which is in part a period of rest and relaxation, most physiological functions such as body temperature, blood pressure, and rate of breathing and
..... Click the link for more information. . Various types of Eastern meditation, e.g., yogayoga
[Skt.,=union], general term for spiritual disciplines in Hinduism, Buddhism, and throughout S Asia that are directed toward attaining higher consciousness and liberation from ignorance, suffering, and rebirth.
..... Click the link for more information. , 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 biofeedbackbiofeedback,
method for learning to increase one's ability to control biological responses, such as blood pressure, muscle tension, and heart rate. Sophisticated instruments are often used to measure physiological responses and make them apparent to the patient, who then tries
..... Click the link for more information. , 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 deathdeath,
cessation of all life (metabolic) processes. Death may involve the organism as a whole (somatic death) or may be confined to cells and tissues within the organism. Causes of death in human beings include injury, acute or chronic disease, and neoplasia (cancer).
..... Click the link for more information. ).

Electroencephalography

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

Electroencephalography

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.

REFERENCES

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.

E. A. ZHIRMUNSKAIA

electroencephalography

[i¦lek·trō·en‚sef·ə′läg·rə·fē] (medicine) The medical specialty concerned with the production and interpretation of electroencephalograms.

electroencephalography

Electroencephalography

Definition

Electroencephalography, or EEG, is a neurological test that uses an electronic monitoring device to measure and record electrical activity in the brain.

Purpose

The EEG is a key tool in the diagnosis and management of epilepsy and other seizure disorders. It is also used to assist in the diagnosis of brain damage and disease (e.g., stroke, tumors, encephalitis), mental retardation, sleep disorders, degenerative diseases such as Alzheimer's disease and Parkinson's disease, and certain mental disorders (e.g., alcoholism, schizophrenia, autism).An EEG may also be used to monitor brain activity during surgery and to determine brain death.

Precautions

Electroencephalography should be administered and interpreted by a trained medical professional only. Data from an EEG is only one element of a complete medical and/or psychological patient assessment, and should never be used alone as the sole basis for a diagnosis.

Description

Before the EEG begins, a nurse or technician attaches approximately 16-20 electrodes to the patient's scalp with a conductive, washable paste. Depending on the purpose for the EEG, implantable or invasive electrodes are occasionally used. Implantable electrodes include sphenoidal electrodes, which are fine wires inserted under the zygomatic arch, or cheekbone; and depth electrodes, which are surgically-implanted into the brain. The EEG electrodes are painless, and are used to measure the electrical activity in various regions of the brain.For the test, the patient lies on a bed, padded table, or comfortable chair and is asked to relax and remain still during the EEG testing period. An EEG usually takes no more than one hour. During the test procedure, the patient may be asked to breathe slowly or quickly; visual stimuli such as flashing lights or a patterned board may be used to stimulate certain types of brain activity. Throughout the procedure, the electroencephalograph machine makes a continuous graphic record of the patient's brain activity, or brainwaves, on a long strip of recording paper or on a computer screen. This graphic record is called an electroencephalogram.The sleep EEG uses the same equipment and procedures as a regular EEG. Patients undergoing a sleep EEG are encouraged to fall asleep completely rather than just relax. They are typically provided a bed and a quiet room conducive to sleep. A sleep EEG lasts up to three hours.In an ambulatory EEG, patients are hooked up to a portable cassette recorder. They then go about their normal activities, and take their normal rest and sleep for a period of up to 24 hours. During this period, the patient and patient's family record any symptoms or abnormal behaviors, which can later be correlated with the EEG to see if they represent seizures.Many insurance plans provide reimbursement for EEG testing. Costs for an EEG range from $100 to more than $500, depending on the purpose and type of test (i.e., asleep or awake, and invasive or non-invasive electrodes). Because coverage may be dependent on the disorder or illness the EEG is evaluating, patients should check with their individual insurance plan.

Preparation

Full instructions should be given to EEG patients when they schedule their test. Typically, individuals on medications that affect the central nervous system, such as anticonvulsants, stimulants, or antidepressants, are told to discontinue their prescription for a short time prior to the test (usually one to two days). Patients may be asked to avoid food and beverages that contain caffeine, a central nervous system stimulant. However, any such request should be cleared by the treating physician. Patients may also be asked to arrive for the test with clean hair free of spray or other styling products.Patients undergoing a sleep EEG may be asked to remain awake the night before their test. They may be given a sedative prior to the test to induce sleep.

Aftercare

If the patient has suspended regular medication for the test, the EEG nurse or technician should advise him when he can begin taking it again.

Risks

Being off medication for one-two days may trigger seizures. Certain procedures used during EEG may trigger seizures in patients with epilepsy. Those procedures include flashing lights and deep breathing. If the EEG is being used as a diagnostic for epilepsy (i.e., to determine the type of seizures an individual is suffering from), this may be a desired effect, although the patient needs to be monitored closely so that the seizure can be aborted if necessary. This type of test is known as an ictal EEG.

Normal results

In reading and interpreting brainwave patterns, a neurologist or other physician will evaluate the type of brainwaves and the symmetry, location, and consistency of brainwave patterns. He will also look at the brainwave response to certain stimuli presented during the EEG test (such as flashing lights or noise). There are four basic types of brainwaves: alpha, beta, theta, and delta. "Normal" brainwave patterns vary widely, depending on factors of age and activity. For example, awake and relaxed individuals typically register an alpha wave pattern of eight to 13 cycles per second. Young children and sleeping adults may have a delta wave pattern of under four cycles per second.

Abnormal results

The EEG readings of patients with epilepsy or other seizure disorders display bursts or spikes of electrical activity. In focal epilepsy, spikes are restricted to one hemisphere of the brain. If spikes are generalized to both hemispheres of the brain, multifocal epilepsy may be present.The diagnostic brainwave patterns of other disorders varies widely. The appearance of excess theta waves (four to eight cycles per second) may indicate brain injury. Brain wave patterns in patients with brain disease, mental retardation, and brain injury show overall slowing. A trained medical specialist should interpret EEG results in the context of the patient's medical history, and other pertinent medical test results.

Resources

Books

Restak, Richard M. Brainscapes: An Introduction to What Neuroscience Has Learned About the Structure, Function, and Abilities of the Brain. NewYork: Hyperion, 1995.

Key terms

Epilepsy — A neurological disorder characterized by recurrent seizures with or without a loss of consciousness.Ictal EEG — Used to measure brain activity during a seizure. May be useful in learning more about patients who aren't responding to conventional treatments.

electroencephalography

[e-lek″tro-en-sef″ah-log´-rah-fe] the recording of changes in electric potentials in various areas of the brain by means of electrodes placed on the scalp, on the brain surface, or within the brain itself, and connected to a vacuum tube radio amplifier, which amplifies the impulses more than a million times. The impulses are of sufficient magnitude to move an electromagnetic pen that records the waves" >brain waves. adj., adj electroencephalograph´ic.
The rate, height, and length of the waves vary in different parts of the brain, and each individual has a unique and characteristic pattern. Age and degree of consciousness also cause the wave patterns to differ. Most of the recorded waves in a normal adult's EEG are the occipital alpha waves, which are best obtained from the back of the head (occipital region) when the subject is resting quietly, but not asleep, with the eyes closed. These waves are blocked by excitement or by opening the eyes.
The beta waves, obtained from the central and front parts of the head, are more closely related to the sensory-motor parts of the brain. These waves are blocked in the same way as are alpha waves, by opening the eyes. In a normal EEG the frequencies are predominately within the range of alpha and beta rhythms at the rate of 8 to 30 hertz (cycles per second). During sleep the brain cells generate higher voltage electrical waves, but the rhythm is slowed down to 2 or 3 hertz, sometimes with short “sleep” spindles of about 15 hertz. One should use the word “normal” with caution when speaking of EEG readings. Some persons with mild deviations from normal may have no evidence of cerebral disease, while others with readings within normal ranges may be suffering from a serious disorder.
Irregular slow waves of 2 to 3 hertz, called delta waves or the delta pattern, are normally found in deep sleep and in infants and young children, but they indicate an abnormality in the awake adult. Rhythmic slow waves of 4 to 7 hertz are called theta waves. “Electrical silence,” or no evidence of brain activity, when demonstrated once and then again in 24 hours, has been taken as one of the criteria of death.
Electroencephalography is widely used in studying brain function and in tracing the connections between the parts of the central nervous system. It is particularly valuable in diagnosing epilepsy, brain tumor, and other diseases of and injury to the brain.Patient Care. The electroencephalograph is an extremely sensitive instrument, and readings can be greatly influenced by the actions and physiologic status of the subject. It is apparent, then, that the cooperation of the patient is needed, and that the patient should be properly prepared physically and psychologically in order to obtain an accurate and useful record of brain activity. Patients are more likely to be cooperative if they have had adequate preparation in the purpose of the test, how the procedure will be carried out, and what will be expected of them during the testing. Their fears about electricity and how it will be used must be allayed. They should know that the electrodes lead minute amounts of electrical charge from the body and that there is no danger of electric shock and no relationship of this procedure to electroshock therapy. In most instances the test is painless because the electrodes are attached to the scalp with collodion. If needle electrodes are to be used, however, patients should be told there will be mild discomfort because the needles are extremely small.
A sleep recording usually is taken when a seizure disorder is suspected. The patient will be expected to go to sleep during the test. Some EEG technicians encourage the patient to stay up later than usual the evening before the test and to awaken early in order to be more likely to fall asleep during testing. Medications to produce sleep are given only as a last resort because these drugs alter brain wave patterns. Infants and small children are not allowed to nap before the test.
Other aspects of physical preparation include withholding all anticonvulsants, tranquilizers, and stimulants for at least 24 to 48 hours prior to testing. This includes coffee, tea, cola drinks, and alcohol. Hypoglycemia affects the brain wave patterns and so the patient is told not to skip any meals.
At the beginning of the test a baseline EEG reading is obtained by having the patient lie quietly with the eyes closed in a dimly lit room. The patient is cautioned to avoid movement of the eyelids, mouth, or tongue because these activities can be particularly disruptive. Provocative or “stressing” techniques are sometimes used during the EEG testing. These are particularly useful in the diagnosis of epilepsy because they can evoke seizure potentials on the EEG. The two techniques most often used are hyperventilation and “photic” stimulation, which employs flickering lights to stimulate the brain. When these or any other techniques are anticipated, patients should be informed so they will not become unduly apprehensive before and during the testing.

e·lec·tro·en·ceph·a·log·ra·phy (EEG),

(ē-lek'trō-en-sef'ă-log'ră-fē), Registration of the electrical potentials recorded by an electroencephalograph.

electroencephalography

Neurology A technique in which the brain's electrical activity is measured by electrodes, and sent to a device that records impulses–brain waves on a continuous feed sheet of paper; EEG is used to evaluate seizure disorders, diminished consciousness, intracranial masses or vascular lesions. See Sleep encephalogram.

e·lec·tro·en·ceph·a·log·ra·phy

(EEG) (ĕ-lek'trō-en-sef'ă-log'ră-fē) Registration of the electrical potentials recorded by an electroencephalograph.

electroencephalography

The process of making a multiple tracing, by voltmeter-operated pens, of the electrical activity of the brain. The multiple readings are of the constantly varying voltage differences occurring between pairs of points on the scalp of the subject. The electroencephalograph (EEG) is affected by sleep, HYPERVENTILATION, drugs, concussion, brain injury, brain tumours, bleeding within the brain (cerebral haemorrhage), brain inflammation (ENCEPHALITIS), EPILEPSY and various psychiatric conditions. It also assists in the determination of brain death.

e·lec·tro·en·ceph·a·log·ra·phy

(EEG) (ĕ-lek'trō-en-sef'ă-log'ră-fē) Registration of electrical potentials recorded by an electroencephalograph.

单词	electroencephalography
释义	electroencephalography e·lec·tro·en·ceph·a·lo·graph E0075400 (ĭ-lĕk′trō-ĕn-sĕf′ə-lə-grăf′)n. An instrument that measures electrical potentials on the scalp and generates a record of the electrical activity of the brain. Also called encephalograph. e·lec′tro·en·ceph′a·lo·graph′ic adj.e·lec′tro·en·ceph′a·log′ra·phy (-lŏg′rə-fē) n. electroencephalography A diagnostic method of examining the electrical impulses of the brain using electrodes attached to the head and to a recording device to make an electroencephalogram. Translations électroencéphalographie electroencephalography electroencephalography (əlĕk'trōĕnsĕf'əlŏg`rafē), science of recording and analyzing the electrical activity of the brainbrain, the supervisory center of the nervous system in all vertebrates. It also serves as the site of emotions, memory, self-awareness, and thought. Anatomy and Function ..... Click the link for more information. . 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 epilepsyepilepsy, a chronic disorder of cerebral function characterized by periodic convulsive seizures. There are many conditions that have epileptic seizures. Sudden discharge of excess electrical activity, which can be either generalized (involving many areas of cells in the brain) ..... Click the link for more information. , cerebral tumor, encephalitisencephalitis , general term used to describe a diffuse inflammation of the brain and spinal cord, usually of viral origin, often transmitted by mosquitoes, in contrast to a bacterial infection of the meninges (membrane surrounding the brain and spinal cord), known as meningitis. ..... Click the link for more information. , and strokestroke, destruction of brain tissue as a result of intracerebral hemorrhage or infarction caused by thrombosis (clotting) or embolus (obstruction in a blood vessel caused by clotted blood or other foreign matter circulating in the bloodstream); formerly called apoplexy. ..... Click the link for more information. , is greatly enhanced. Electroencephalograms have also proven valuable in the general study of brain physiology and in the particular study of sleepsleep, resting state in which an individual becomes relatively quiescent and relatively unaware of the environment. During sleep, which is in part a period of rest and relaxation, most physiological functions such as body temperature, blood pressure, and rate of breathing and ..... Click the link for more information. . Various types of Eastern meditation, e.g., yogayoga [Skt.,=union], general term for spiritual disciplines in Hinduism, Buddhism, and throughout S Asia that are directed toward attaining higher consciousness and liberation from ignorance, suffering, and rebirth. ..... Click the link for more information. , 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 biofeedbackbiofeedback, method for learning to increase one's ability to control biological responses, such as blood pressure, muscle tension, and heart rate. Sophisticated instruments are often used to measure physiological responses and make them apparent to the patient, who then tries ..... Click the link for more information. , 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 deathdeath, cessation of all life (metabolic) processes. Death may involve the organism as a whole (somatic death) or may be confined to cells and tissues within the organism. Causes of death in human beings include injury, acute or chronic disease, and neoplasia (cancer). ..... Click the link for more information. ). Electroencephalography 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 Electroencephalography 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. REFERENCES 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. E. A. ZHIRMUNSKAIA electroencephalography [i¦lek·trō·en‚sef·ə′läg·rə·fē] (medicine) The medical specialty concerned with the production and interpretation of electroencephalograms. electroencephalography Electroencephalography Definition Electroencephalography, or EEG, is a neurological test that uses an electronic monitoring device to measure and record electrical activity in the brain. Purpose The EEG is a key tool in the diagnosis and management of epilepsy and other seizure disorders. It is also used to assist in the diagnosis of brain damage and disease (e.g., stroke, tumors, encephalitis), mental retardation, sleep disorders, degenerative diseases such as Alzheimer's disease and Parkinson's disease, and certain mental disorders (e.g., alcoholism, schizophrenia, autism).An EEG may also be used to monitor brain activity during surgery and to determine brain death. Precautions Electroencephalography should be administered and interpreted by a trained medical professional only. Data from an EEG is only one element of a complete medical and/or psychological patient assessment, and should never be used alone as the sole basis for a diagnosis. Description Before the EEG begins, a nurse or technician attaches approximately 16-20 electrodes to the patient's scalp with a conductive, washable paste. Depending on the purpose for the EEG, implantable or invasive electrodes are occasionally used. Implantable electrodes include sphenoidal electrodes, which are fine wires inserted under the zygomatic arch, or cheekbone; and depth electrodes, which are surgically-implanted into the brain. The EEG electrodes are painless, and are used to measure the electrical activity in various regions of the brain.For the test, the patient lies on a bed, padded table, or comfortable chair and is asked to relax and remain still during the EEG testing period. An EEG usually takes no more than one hour. During the test procedure, the patient may be asked to breathe slowly or quickly; visual stimuli such as flashing lights or a patterned board may be used to stimulate certain types of brain activity. Throughout the procedure, the electroencephalograph machine makes a continuous graphic record of the patient's brain activity, or brainwaves, on a long strip of recording paper or on a computer screen. This graphic record is called an electroencephalogram.The sleep EEG uses the same equipment and procedures as a regular EEG. Patients undergoing a sleep EEG are encouraged to fall asleep completely rather than just relax. They are typically provided a bed and a quiet room conducive to sleep. A sleep EEG lasts up to three hours.In an ambulatory EEG, patients are hooked up to a portable cassette recorder. They then go about their normal activities, and take their normal rest and sleep for a period of up to 24 hours. During this period, the patient and patient's family record any symptoms or abnormal behaviors, which can later be correlated with the EEG to see if they represent seizures.Many insurance plans provide reimbursement for EEG testing. Costs for an EEG range from $100 to more than $500, depending on the purpose and type of test (i.e., asleep or awake, and invasive or non-invasive electrodes). Because coverage may be dependent on the disorder or illness the EEG is evaluating, patients should check with their individual insurance plan. Preparation Full instructions should be given to EEG patients when they schedule their test. Typically, individuals on medications that affect the central nervous system, such as anticonvulsants, stimulants, or antidepressants, are told to discontinue their prescription for a short time prior to the test (usually one to two days). Patients may be asked to avoid food and beverages that contain caffeine, a central nervous system stimulant. However, any such request should be cleared by the treating physician. Patients may also be asked to arrive for the test with clean hair free of spray or other styling products.Patients undergoing a sleep EEG may be asked to remain awake the night before their test. They may be given a sedative prior to the test to induce sleep. Aftercare If the patient has suspended regular medication for the test, the EEG nurse or technician should advise him when he can begin taking it again. Risks Being off medication for one-two days may trigger seizures. Certain procedures used during EEG may trigger seizures in patients with epilepsy. Those procedures include flashing lights and deep breathing. If the EEG is being used as a diagnostic for epilepsy (i.e., to determine the type of seizures an individual is suffering from), this may be a desired effect, although the patient needs to be monitored closely so that the seizure can be aborted if necessary. This type of test is known as an ictal EEG. Normal results In reading and interpreting brainwave patterns, a neurologist or other physician will evaluate the type of brainwaves and the symmetry, location, and consistency of brainwave patterns. He will also look at the brainwave response to certain stimuli presented during the EEG test (such as flashing lights or noise). There are four basic types of brainwaves: alpha, beta, theta, and delta. "Normal" brainwave patterns vary widely, depending on factors of age and activity. For example, awake and relaxed individuals typically register an alpha wave pattern of eight to 13 cycles per second. Young children and sleeping adults may have a delta wave pattern of under four cycles per second. Abnormal results The EEG readings of patients with epilepsy or other seizure disorders display bursts or spikes of electrical activity. In focal epilepsy, spikes are restricted to one hemisphere of the brain. If spikes are generalized to both hemispheres of the brain, multifocal epilepsy may be present.The diagnostic brainwave patterns of other disorders varies widely. The appearance of excess theta waves (four to eight cycles per second) may indicate brain injury. Brain wave patterns in patients with brain disease, mental retardation, and brain injury show overall slowing. A trained medical specialist should interpret EEG results in the context of the patient's medical history, and other pertinent medical test results. Resources Books Restak, Richard M. Brainscapes: An Introduction to What Neuroscience Has Learned About the Structure, Function, and Abilities of the Brain. NewYork: Hyperion, 1995. Key terms Epilepsy — A neurological disorder characterized by recurrent seizures with or without a loss of consciousness.Ictal EEG — Used to measure brain activity during a seizure. May be useful in learning more about patients who aren't responding to conventional treatments. electroencephalography [e-lek″tro-en-sef″ah-log´-rah-fe] the recording of changes in electric potentials in various areas of the brain by means of electrodes placed on the scalp, on the brain surface, or within the brain itself, and connected to a vacuum tube radio amplifier, which amplifies the impulses more than a million times. The impulses are of sufficient magnitude to move an electromagnetic pen that records the waves" >brain waves. adj., adj electroencephalograph´ic. The rate, height, and length of the waves vary in different parts of the brain, and each individual has a unique and characteristic pattern. Age and degree of consciousness also cause the wave patterns to differ. Most of the recorded waves in a normal adult's EEG are the occipital alpha waves, which are best obtained from the back of the head (occipital region) when the subject is resting quietly, but not asleep, with the eyes closed. These waves are blocked by excitement or by opening the eyes. The beta waves, obtained from the central and front parts of the head, are more closely related to the sensory-motor parts of the brain. These waves are blocked in the same way as are alpha waves, by opening the eyes. In a normal EEG the frequencies are predominately within the range of alpha and beta rhythms at the rate of 8 to 30 hertz (cycles per second). During sleep the brain cells generate higher voltage electrical waves, but the rhythm is slowed down to 2 or 3 hertz, sometimes with short “sleep” spindles of about 15 hertz. One should use the word “normal” with caution when speaking of EEG readings. Some persons with mild deviations from normal may have no evidence of cerebral disease, while others with readings within normal ranges may be suffering from a serious disorder. Irregular slow waves of 2 to 3 hertz, called delta waves or the delta pattern, are normally found in deep sleep and in infants and young children, but they indicate an abnormality in the awake adult. Rhythmic slow waves of 4 to 7 hertz are called theta waves. “Electrical silence,” or no evidence of brain activity, when demonstrated once and then again in 24 hours, has been taken as one of the criteria of death. Electroencephalography is widely used in studying brain function and in tracing the connections between the parts of the central nervous system. It is particularly valuable in diagnosing epilepsy, brain tumor, and other diseases of and injury to the brain.Patient Care. The electroencephalograph is an extremely sensitive instrument, and readings can be greatly influenced by the actions and physiologic status of the subject. It is apparent, then, that the cooperation of the patient is needed, and that the patient should be properly prepared physically and psychologically in order to obtain an accurate and useful record of brain activity. Patients are more likely to be cooperative if they have had adequate preparation in the purpose of the test, how the procedure will be carried out, and what will be expected of them during the testing. Their fears about electricity and how it will be used must be allayed. They should know that the electrodes lead minute amounts of electrical charge from the body and that there is no danger of electric shock and no relationship of this procedure to electroshock therapy. In most instances the test is painless because the electrodes are attached to the scalp with collodion. If needle electrodes are to be used, however, patients should be told there will be mild discomfort because the needles are extremely small. A sleep recording usually is taken when a seizure disorder is suspected. The patient will be expected to go to sleep during the test. Some EEG technicians encourage the patient to stay up later than usual the evening before the test and to awaken early in order to be more likely to fall asleep during testing. Medications to produce sleep are given only as a last resort because these drugs alter brain wave patterns. Infants and small children are not allowed to nap before the test. Other aspects of physical preparation include withholding all anticonvulsants, tranquilizers, and stimulants for at least 24 to 48 hours prior to testing. This includes coffee, tea, cola drinks, and alcohol. Hypoglycemia affects the brain wave patterns and so the patient is told not to skip any meals. At the beginning of the test a baseline EEG reading is obtained by having the patient lie quietly with the eyes closed in a dimly lit room. The patient is cautioned to avoid movement of the eyelids, mouth, or tongue because these activities can be particularly disruptive. Provocative or “stressing” techniques are sometimes used during the EEG testing. These are particularly useful in the diagnosis of epilepsy because they can evoke seizure potentials on the EEG. The two techniques most often used are hyperventilation and “photic” stimulation, which employs flickering lights to stimulate the brain. When these or any other techniques are anticipated, patients should be informed so they will not become unduly apprehensive before and during the testing. e·lec·tro·en·ceph·a·log·ra·phy (EEG), (ē-lek'trō-en-sef'ă-log'ră-fē), Registration of the electrical potentials recorded by an electroencephalograph. electroencephalography Neurology A technique in which the brain's electrical activity is measured by electrodes, and sent to a device that records impulses–brain waves on a continuous feed sheet of paper; EEG is used to evaluate seizure disorders, diminished consciousness, intracranial masses or vascular lesions. See Sleep encephalogram. e·lec·tro·en·ceph·a·log·ra·phy (EEG) (ĕ-lek'trō-en-sef'ă-log'ră-fē) Registration of the electrical potentials recorded by an electroencephalograph. electroencephalography The process of making a multiple tracing, by voltmeter-operated pens, of the electrical activity of the brain. The multiple readings are of the constantly varying voltage differences occurring between pairs of points on the scalp of the subject. The electroencephalograph (EEG) is affected by sleep, HYPERVENTILATION, drugs, concussion, brain injury, brain tumours, bleeding within the brain (cerebral haemorrhage), brain inflammation (ENCEPHALITIS), EPILEPSY and various psychiatric conditions. It also assists in the determination of brain death. e·lec·tro·en·ceph·a·log·ra·phy (EEG) (ĕ-lek'trō-en-sef'ă-log'ră-fē) Registration of electrical potentials recorded by an electroencephalograph. ThesaurusSeeelectroencephalograph
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electroencephalography

e·lec·tro·en·ceph·a·lo·graph

electroencephalography

electroencephalography

electroencephalography

Brainwave Patterns

Uses of EEGs

Electroencephalography

Electroencephalography

REFERENCES

electroencephalography

electroencephalography

Electroencephalography

Definition

Purpose

Precautions

Description

Preparation

Aftercare

Risks

Normal results

Abnormal results

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Key terms

electroencephalography

e·lec·tro·en·ceph·a·log·ra·phy (EEG),

electroencephalography

e·lec·tro·en·ceph·a·log·ra·phy

electroencephalography

e·lec·tro·en·ceph·a·log·ra·phy