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Electroencephalogram (EEG)

What is an electroencephalogram (EEG)?

Also called electroencephalography, or brain wave test, an electroencephalogram (EEG) detects abnormalities in the brain waves or electrical activity of the brain. During the procedure, electrodes consisting of small metal discs with thin wires are pasted on the scalp. The electrodes detect tiny electrical charges that result from the activity of the brain cells. The charges are amplified and appear as a graph on a computer screen or as a recording that may be printed out on paper. The physician then interprets the reading.

Related procedures that may be performed are evoked potential studies. These studies are used to measure electrical activity in the brain in response to stimulation of sight, sound, or touch.

Different types of normal brain waves:

An EEG records patterns of brain activity. Among the basic waveforms are the alpha, beta, theta, and delta rhythms.

  • Alpha waves occur at a frequency of 8 to 12 cycles per second in a regular rhythm. They are present only when you are awake but have your eyes closed. Usually they disappear when you open your eyes or start mentally concentrating.
  • Beta waves occur at a frequency of 13 to 30 cycles per second. They are usually associated with anxiety, depression, or the use of sedatives.
  • Theta waves occur at a frequency of 4 to 7 cycles per second. They are most common in children and young adults.
  • Delta waves occur at a frequency of 0.5 to 3.5 cycles per second. They generally occur only in young children during sleep.

During an EEG, typically about 100 pages of activity are evaluated. Special attention is paid to the basic waveforms, but brief bursts of energy and responses to stimuli, such as light, are also examined.

What happens during an EEG?

An EEG may be performed on an outpatient basis or as part of your stay in a hospital. Procedures may vary depending on your condition and your physician's practices.

Generally, an EEG procedure follows this process:

  1. You will be asked to relax in a reclining chair or lie on a bed.
  2. Between 16 and 25 electrodes will be attached to your scalp with a special paste, or a cap containing the electrodes will be used.
  3. You will be asked to close your eyes, relax, and be still.
  4. Once the recording begins you will need to remain still throughout the test. You may be monitored through a window in an adjoining room to observe any movements that can cause an inaccurate reading, such as swallowing or blinking. The recording may be stopped periodically to let you rest or reposition yourself.
  5. After the initial recording performed at rest, you may be tested with various stimuli to produce activity that does not show up while you are resting. For example, you may be asked to breathe deeply and rapidly for three minutes, or you may be exposed to a bright light.
  6. This study is generally performed by an EEG technician and may take approximately 45 minutes to two hours.
  7. If you are being evaluated for a sleep disorder, the EEG may be performed while you are asleep.

Procedures which might use an EEG:

Treatment for the following neurological conditions might include an EEG:


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