Electroencephalogram (EEG)

An electroencephalogram, usually abbreviated EEG, is a medical test that records electrical activity in the brain. During the test, the brain's spontaneous electrical signals are traced onto paper. The electroencephalograph is the machine that amplifies and records the electrical signals from the brain. The electroencephalogram is the paper strip the machine produces. The EEG changes with disease or brain disorder, such as epilepsy, so it can be a useful diagnostic tool, but usually must be accompanied by other diagnostic tests to be definitive.

To perform an EEG, electrodes, which are wires designed to detect electrical signals, are placed on the cranium either by inserting a needle into the scalp or by attaching the wire with a special adhesive. The electrodes are placed in pairs so that the difference in electric potential between them can be measured. The wires are connected to the electroencephalograph, where the signal is amplified and directed into pens that record the waves on a moving paper chart. The tracing appears as a series of peaks and troughs drawn as lines by the recording pens.

Basic alpha waves, which originate in the cortex, can be recorded if the subject closes his eyes and puts his brain "at rest" as much as possible. Of course, the brain is never still, so some brain activity is going on and is recorded in waves of about six to 12 per second, with an average of about 10 per second. The voltage of these waves is from five to 100 microvolts. A microvolt is oneone millionth of a volt. Thus, a considerable amount of amplification is required to raise the voltage to a discernable level.

The rate of the waves, that is, the number that occur per second, appears to be a better diagnostic indicator than does the amplitude, or strength. Changes in the rate indicating a slowing or speeding up are significant, and unconsciousness occurs at either extreme. Sleep, stupor, and deep anesthesia are associated with slow waves and grand mal seizures cause an elevated rate of brain waves. The only time the EEG line is straight and without any wave indication is at death. A person who is brain dead has a straight, flat EEG line.

The rates of alpha waves are intermediate compared with other waves recorded on the EEG. Faster waves, 14–50 waves per second, that are lower in voltage than alpha waves are called beta waves. Very slow waves, averaging 0.5-5 per second, are delta waves. The slowest brain waves are associated with an area of localized brain damage such as may occur from a stroke or blow on the head.

The individual at rest and generating a fairly steady pattern of alpha waves can be distracted by a sound or touch. The alpha waves then flatten somewhat, that is their voltage is less and their pattern becomes more irregular when the individual's attention is focused. Any difficult mental effort such as multiplying two four-digit numbers will decrease the amplitude of the waves, and any pronounced emotional excitement will flatten the pattern. The brain wave pattern will change to one of very slow waves, about three per second, in deep sleep.

Though the basic EEG pattern remains a standard one from person to person, each individual has his own unique EEG pattern. The same individual given two separate EEG tests weeks or months apart will generate the same alpha wave pattern, assuming the conditions of the tests are the same. Identical twins will both have the same pattern. One twin will virtually match the second twin to the extent that the two tracings appear to be from the same individual on two separate occasions.

Though the EEG is a useful diagnostic tool, its use in brain research is limited. The electrodes detect the activity of only a few neurons in the cortex out of the billions that are present. Electrode placement is standardized so the EEG can be interpreted by any trained neurologist. Also, the electrical activity being measured is from the surface of the cortex and not from the deeper areas of the brain.