Electroencephalogram (EEG)

Test Overview

An electroencephalogram (EEG) is a test that measures and records the electrical activity of your brain by using sensors (electrodes) attached to your scalp with a paste and connected by wires to a computer. The computer records and displays your brain's electrical activity on the screen or on paper as wavy lines. Certain conditions, such as seizures can be detected by observing changes in the normal pattern of the brain's electrical activity.

Why It Is Done

An electroencephalogram (EEG) may be done to:

  • Help confirm the diagnosis of epilepsy and determine the best treatment option based on seizure type.
  • Identify and locate a suspected brain lesion such as: tumor, inflammation and infection.
  • Evaluate periods of unconsciousness or dementia.
  • Help predict a person's chance of recovery after a change in consciousness.
  • Study some sleep disorders, such as narcolepsy.

How to Prepare

Before the day of the electroencephalogram (EEG) test, your doctor will need to know what medications you are taking.

Avoid foods that contain caffeine (such as coffee, tea, cola, and chocolate) for at least 8 hours before the test. Eat a small meal shortly before the test, because low blood sugar (hypoglycemia) may produce an abnormal test.

Since the electrodes are attached to your scalp, it is important that your hair be clean and free of sprays, oils, creams, lotions, and other hair preparations. Shampoo your hair and rinse with clear water the evening before or the morning of the test. Do not apply any hair conditioners or oils after shampooing.

To detect certain types of abnormal electrical activity in the brain, you may have to be asleep during the recording. As a result, you may be asked to reduce your sleep time to 2 to 3 hours the night before the test by going to bed later and getting up earlier than usual. If you know that you are going to have a sleep-deprived EEG, plan to have someone drive you to and from the test.

How It Is Done

An electroencephalogram (EEG) may be done in a hospital or in a doctor's office by an EEG technologist. The EEG record is analyzed by a doctor who is specially trained to diagnose and treat disorders affecting the nervous system (neurologist).

You will be asked to lie on your back on a bed or examining table or relax in a chair with your eyes closed. The EEG technologist will attach 16 to 25 flat metal discs (electrodes) to different places on your head, using a sticky paste to hold the electrodes in place. The electrodes are connected by wires to a machine that amplifies and records the electrical activity inside the brain. The machine records the electrical activity as a series of wavy lines drawn by a row of pens on a moving piece of paper or as an image on the computer screen.

Try to lie still, with your eyes closed during the recording, and do not talk to the technologist unless you need to. The technologist will observe you directly during the test. The usual recording time is 25-45 minutes, if a longer recording is needed, the recording may be stopped from time to time to allow you to stretch and reposition yourself.

In addition to the recording of your brain's electrical activity while you are resting, certain procedures may be done to observe how your brain responds to different forms of stimulation:

  • You may be asked to breathe deeply and rapidly (hyperventilate). Usually you will take 20 breaths a minute for 3 minutes.
  • You may be asked to look at a bright, flashing light called a strobe. This is called photic or stroboscopic stimulation.
  • You may be asked to go to sleep.

After the test, you may resume normal activities. However, if you were sleep-deprived, have someone drive you home after the test.

How It Feels

An electroencephalogram (EEG) is a painless test. You will not feel anything out of the ordinary during the EEG recording.

A paste is used to position the electrodes, and some paste may remain in your hair after the test, so you will have to wash your hair to remove it.

If you are asked to breathe rapidly, you may feel lightheaded or have some numbness in your fingers. This reaction is normal. It will go away within a few minutes after you start breathing normally again.


The EEG will be read by a trained neurologist. A report of the findings will be sent to the physician who ordered the test within a few days after the test.

Developed by DECS | Contact Information | Privacy Statement | Site Accessibility

MSU is an affirmative action, equal opportunity employer

© Michigan State University Board of Trustees, East Lansing, MI 48824