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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. 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 first EEG recording, obtained by Hans Berger in 1929.
Methods 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 µV 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, typically an earlobe electrode, and all other electrodes are measured relative to this single point.
- 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.
- Theta is the frequency range from 4.5 Hz to 8 Hz and is associated with drowsiness, childhood, adolescence and young adulthood. This EEG frequency can sometimes be produced by hyperventilation. Theta waves can be seen during trances, hypnosis, deep day dreams, lucid dreaming and light sleep and the preconscious state just upon waking, and just before falling asleep. Controlled meditation or yogic meditation and/or breathing also produces theta waves. Mystics and Yogis are said to remain conscious and in control at this and at the Delta state.
Rhythmic slow activity in wakefulness is common in young children, but is abnormal in adults. In addition to the above types of rhythmic activity, individual transient waveforms such as sharp waves, spikes, spike-and-wave complexes occur in epilepsy, and other types of transients occur during sleep.
In the transition from wakefulness, through Stage I sleep (drowsiness), Stage II (light) sleep, to Stage III and IV (deep) sleep, first the alpha becomes intermittent and attenuated, then disappears. Stage II sleep is marked by brief bursts of highly rhythmic beta activity (sleep spindles) and K complexes (transient slow waves associated with spindles, often triggered by an auditory stimulus). Stage III and IV are characterized by slow wave activity. After a period of deep sleep, the sleeper cycles back to stage II sleep and/or rapid eye movement (REM) sleep, associated with dreaming. These cycles may occur many times during the night.
EEG under general anesthesia depends on the type of anesthetic employed. With halogenated anesthetics and intravenous agents such as propofol, a rapid (alpha or low beta), nonreactive EEG pattern is seen over most of the scalp, especially anteriorly; in some older terminology this was known as a WAR (widespread anterior rapid) pattern, contrasted with a WAIS (widespread slow) pattern associated with high doses of opiates.
Training the brain to a desired state Computer programs such as Noromaa Solutions Oy's Brainwave Generator (http://www.bwgen.com) are able to stimulate the brain and alter the brain's frequency via an audio stimulus.
For example, if a person is in beta stage (highly alert) and a stimulus of 10Hz is applied to his/her brain for some time, the brain frequency is likely to change towards the applied stimulus. The effect will be relaxing to the person. This phenomenon is also called frequency following response. Entrainment works more efficiently if the stimulus applied to the brain begins at the brain's current frequency, then sweeps to the desired frequency. In practice, it is difficult to determine your brain state without extra equipment (like EEG devices).
To stimulate the brain effectively with audio, binaural beat frequencies must be used, since the ears cannot hear sounds low enough to be useful for brain stimulation. If the left ear is presented with a steady tone of 500Hz (21 cents sharp of B4) and the right ear a steady tone of 510Hz (44 cents flat of C5), these two tones combine in the brain. The difference, 10Hz, is perceived by the brain and is a very effective stimulus for brainwave entrainment. This 10Hz is formed entirely by the brain. When using stereo headphones, the left and right sounds do not mix together until in your brain. The frequency difference, when perceived by brain this way, is called a binaural beat.
Altering the brain's electrical activity can be useful for lucid dreaming, encouraging alertness, astral projection, or simply in order to relax. However, since everyone's characteristic alpha frequency is different and varies with time and age, more advanced systems use neurofeedback to enhance one's ability to produce alpha waves without forcing them to a specific frequency, as entrainment does.
History Richard Caton (1842–1926), a physician practicing in Liverpool, presented his findings about electrical phenomena of the exposed cerebral hemispheres of rabbits and monkeys in 1875.
German physiologist Hans Berger (1873–1941) began his studies of the human EEG in 1920. He gave the device its name and is sometimes credited with inventing the EEG, though others had performed similar experiments. His work was later expanded by Edgar Douglas Adrian.
In the 1950s, English physician Walter Grey Walter developed an adjunct to EEG called EEG topography which allowed for the mapping of electrical activity across the surface of the brain. This enjoyed a brief period of popularity in the 1980's and seemed especially promising for psychiatry. It was never accepted by neurologists and remains a primarily a research tool up to now.
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