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Electroencephalography is the neurophysiologic exploration of the electrical activity of the brain by the application of electrodes to the scalp. The resulting traces are known as an electroencephalogram (EEG) and represent so-called brain waves. This device is used to assess brain damage, epilepsy and other problems, such as brain death. EEG can also be used in conjunction with other types of brain imaging.

Neuroscientists and biological psychiatrists use EEGs to study the function of the brain by recording brain waves during controlled behavior of human volunteers and animals in lab experiments. Theories to explain sleep often rely on EEG patterns recorded during sleep sessions. In addition, the procedure is used clinically to assist in the diagnosis of epilepsy.



The recording is obtained by placing electrodes on the scalp, usually after preparing the scalp area by light abrasion and application of a conductive gel to reduce impedance. Each electrode is connected to an input of a differential amplifier (one amplifier per pair of electrodes), which amplifies the voltage between them (typically 1,000–100,000 times, or 60–100 dB of voltage gain), and then displays it on a screen or inputs it to a computer. The amplitude of the EEG is about 100 ÁV when measured on the scalp, and about 1-2 mV when measured on the surface of the brain.

The electrode-amplifier relationships are typically arranged in one of three ways:

Common reference derivation 
One terminal of each amplifier is connected to the same electrode, and all other electrodes are measured relative to this single point. It is typical to use a reference electrode placed somewhere along the scalp midline, or a reference that links both earlobe electrodes.
Average reference derivation 
The outputs of all of the amplifiers are summed and averaged, and this averaged signal is used as the common reference for each amplifier.
Bipolar derivation 
The electrodes are connected in series to an equal number of amplifiers. For example, amplifier 1 measures the difference between electrodes A and B, amplifier 2 measures the difference between B and C, and so on.

Wave types

Historically four major types of continuous rhythmic sinusoidal EEG waves are recognized (alpha, beta, delta and theta). There is no precise agreement on the frequency ranges for each type.

  • Alpha (Berger's wave) is the frequency range from 8.5 Hz to 12 Hz. It is characteristic of a relaxed, alert state of consciousness and is present by the age of two years. Alpha rhythms are best detected with the eyes closed. Alpha attenuates with drowsiness and open eyes, and is best seen over the occipital (visual) cortex. An alpha-like normal variant called mu is sometimes seen over the motor cortex (central scalp) and attenuates with movement, or rather with the intention to move.
  • Beta is the frequency range above 12 Hz. Disorganized, low amplitude beta is often associated with active, busy or anxious thinking and active concentration. Rhythmic beta is associated with various pathologies and drug effects.
  • Delta is the frequency range up to 4 Hz and is often associated with the very young and certain encephalopathies and underlying lesions. It is seen in deep sleep.
  • Gamma is the frequency range above 40 Hz (approximately 30-80 Hz to be precise). Gamma rhythms appear to be involved in higher mental activity, including perception and consciousness. It seems to be associated with consciousness, e.g. it disappears with general anesthesia. Brain Waves ("40Hz") Research.
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See also

External links

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