Toggle: English / Spanish
An electroencephalogram (EEG) is a test to measure the electrical activity of the brain.
Electroencephalogram; Brain wave test
How the test is performed
Brain cells communicate with each other by producing tiny electrical signals, called impulses.
An EEG measures this activity. The test is done by a EEG specialist in your doctor's office or at a hospital or laboratory.
You will be asked to lie on your back on a bed or in a reclining chair.
Flat metal disks called electrodes are placed all over your scalp. The disks are held in place with a sticky paste. The electrodes are connected by wires to a speaker and recording machine.
The recording machine changes the electrical signals into patterns that can be seen on a computer. It looks like wavy lines.
You will need to lie still during the test with your eyes closed because movement can change the results. But you may be asked to do certain things during the test, such as breathe fast and deeply for several minutes or look at a bright flashing light.
How to prepare for the test
Wash your hair the night before the test. Do not use any oils, sprays, or conditioner on your hair before this test. If you have a hair weave, ask your doctor or nurse for special instructions.
Your health care provider may want you to stop taking certain medicines before the test. Do not change or stop taking any medicines without first talking to your health care provider. Bring a list of your medicines with you.
Avoid all food and drinks containing caffeine for 8 hours before the test.
You may need to sleep during the test. If so, you may be asked to reduce your sleep time the night before. If you are asked to sleep as little as possible before the test, do not eat or drink any caffeine, energy drinks, or other products that help you stay awake.
How the test will feel
The electrodes may feel sticky and strange on your scalp but should not cause any other discomfort. You should not feel any discomfort during the test.
Why the test is performed
EEG is used to look at your brain activity. It can help diagnose seizures. It may also be used to diagnose or monitor the following health conditions:
EEG is also used to:
- Evaluate problems with sleep ( sleep disorders)
- Investigate periods of unconsciousness
- Monitor the brain during brain surgery
An EEG may be done to show that the brain has no activity, in the case of someone who is in a deep coma. It can be helpful when trying to decide if someone is brain dead.
EEG cannot be used to measure intelligence.
Brain electrical activity has a certain number of waves per second (frequencies) that are normal for different levels of alertness. For example, brain waves are faster when you are awake, and slower when you are sleeping.
There are also normal patterns to these waves.
What abnormal results mean
Abnormal results on an EEG test may be due to:
- Abnormal bleeding (hemorrhage)
- An abnormal structure in the brain (such as a brain tumor)
- Attention problems
- Tissue death due to a blockage in blood flow (cerebral infarction)
- Drug or alcohol abuse
- Head injury
- Migraines (in some cases)
- Seizure disorder (such as epilepsy or convulsions)
- Sleep disorder (such as narcolepsy)
- Swelling of the brain (encephalitis)
Note: A normal EEG does not mean that a seizure did not occur.
What the risks are
The procedure is very safe. The flashing lights or fast breathing (hyperventilation) required during the test may trigger seizures in those with seizure disorders. The health care provider performing the EEG is trained to take care of you if this happens.
Emerson RG, Pedley TA. Clinical neurophysiology: electroencephalography and evoked potentials. In: Daroff RB, Fenichel GM, Jankovic J, Mazziotta JC, eds. Bradley’s Neurology in Clinical Practice. 6th ed. Philadelphia, PA: Elsevier Saunders; 2012:chap 32A.
- Last reviewed on 2/27/2013
- Luc Jasmin, MD, PhD, Department of Neurosurgery, Cedars Sinai Medical Center, Los Angeles and Department of Anatomy, University of California, San Francisco, CA. Review provided by VeriMed Healthcare Network. Also reviewed by A.D.A.M. Health Solutions, Ebix, Inc., Editorial Team: David Zieve, MD, MHA, Bethanne Black, Stephanie Slon, and Nissi Wang.
The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed medical professional should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. © 1997- 2013 A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.