ElectroencephalographyElectroencephalography 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 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. 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.
Wave typesHistorically 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.
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 REM (rapid eye movement) 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 does of opiates. Entraining the brain to a desired stateComputer 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 simulate the brain effectivly 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 and the right ear a steady tone of 510Hz, 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 electral activity can be useful for lucid dreaming, encouraging alertness, astral projection, or simply in order to relax. HistoryRichard 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 tomography 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. See alsoExternal links
Categories: Neurology | Neuroscience | Medical tests |
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