EEG: Checking your brain's electrical activity

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EEG: Checking your brain's electrical activity

EEG (electroencephalogram) — a common diagnostic test for many brain disorders, especially epilepsy.

An EEG (electroencephalogram) is a painless procedure that uses small flat metal discs (electrodes) attached to your scalp to detect electrical activity in your brain. Your brain cells communicate via electrical impulses and your brain is active all the time, even when you're asleep. This activity shows up as wavy lines on an EEG machine.

Who is it for?

EEGs are useful in diagnosing many types of brain disorders, particularly epilepsy. People with epilepsy often have changes in their normal pattern of brain waves, even when they aren't experiencing a seizure.

An EEG can't measure intelligence or detect mental illness. It may be used to confirm brain death in someone in a persistent coma.

How do you prepare?

Wash your hair the night before or the day of the test, but don't use any conditioners, hair creams, sprays or styling gels. Beginning six hours before the test, avoid anything with caffeine. Take your usual medications unless instructed otherwise.

Some children having an EEG may be asked to refrain from eating solid food, including formula and cow's milk, for four hours before the test and any other liquids for at least two hours before the test. They should be awakened earlier than usual and not allowed to take a nap before the test.

If you are supposed to sleep during your EEG test, your doctor may ask you to sleep less or even avoid sleep entirely the night before your EEG. If you have trouble falling asleep for the test, you might be given a sedative to help you relax.

How is it done?

A technologist will first measure your head and mark your scalp with a special pencil, to indicate where to attach the electrodes. Those spots on your scalp may be scrubbed with a gritty cream to improve the quality of the recording.

Using a special adhesive, a technologist will affix between 16 and 25 flat metal discs (electrodes) to your scalp. The electrodes are connected to the EEG machine with wires. Some people wear an elastic cap fitted with electrodes, instead of having the adhesive applied to their scalps.

A small amount of gel will be placed between each electrode and your scalp, and the technician might use an instrument to rub the skin beneath each electrode to ensure a good connection. Simply getting the electrodes placed properly can take up to an hour. The actual recording takes between 30 and 60 minutes. Including the time needed to place and remove the electrodes, the entire procedure can take as long as 2.5 hours.

During most of the test, you will relax in a comfortable position with your eyes closed. At various times during the test, the technician may ask you to open and close your eyes, perform a few simple calculations, read a paragraph, look at a picture, breathe deeply (hyperventilate) for a few minutes, or look at a flashing light.

Your doctor may want you to undergo a video EEG, which may require you to be admitted to a hospital. During this type of test, your body motions are captured by a video camera while the EEG simultaneously records your brain waves during a seizure. This helps your doctor pinpoint the location in your brain where seizures begin.

What can you expect during the test?

You will feel no discomfort during an EEG. The electrodes don't transmit any sensations. They just record your brain waves.


An EEG can help confirm, rule out or provide information that helps with management of the following disorders:

  • Epilepsy
  • Brain tumor
  • Head injury
  • Encephalopathy (diffuse brain dysfunction)
  • Attention-deficit problems
  • Inflammation of the brain (encephalitis)
  • Stroke
  • Sleep disorders
  • Memory impairment


EEGs are very safe. People with epilepsy may have a seizure triggered during the test, but the appropriate medical care is provided if needed.

Last Updated: 05/21/2007
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