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Electroencephalogram (EEG)

Test Overview

An electroencephalogram (EEG) is a test that measures and records the electrical activity of your brain. Special sensors called electrodes are attached to your head. They're hooked by wires to a computer. The computer records your brain's electrical activity on the screen. Or it may record the activity on paper as wavy lines. Changes from the normal pattern of electrical activity can show certain conditions, such as seizures.

Why It Is Done

An EEG may be done to:

  • Check for epilepsy and see what type of seizures are occurring. EEG is the most useful and important test for checking if someone has epilepsy.
  • Check for problems with loss of consciousness or dementia.
  • Help find what is causing a change in consciousness.
  • Find out if a person who is in a coma is brain-dead.
  • Study sleep disorders, such as narcolepsy.
  • Watch brain activity while a person is getting general anesthesia for brain surgery.

How To Prepare

  • Tell your doctor ALL the medicines and natural health products you take. Some may increase the risk of problems during your test. Your doctor will tell you if you should stop taking any of them before the test and how soon to do it.
  • Do not eat or drink anything with caffeine in it for 12 hours before the test. This includes cola, energy drinks, and chocolate.
  • Shampoo your hair and rinse with clear water the evening before or the morning of the test. Do not put any hair conditioner or oil on after you wash your hair.
  • Your doctor may ask you not to sleep the night before the test or to sleep for only about 4 or 5 hours. This is because some types of brain activity can only be seen if you are asleep. If your doctor asks you to get less sleep than normal, plan to have someone drive you to and from the test.

How It Is Done

An EEG may be done in a hospital or in a doctor's office. An EEG technologist does the test. The EEG record is read by a doctor who is trained to diagnose and treat problems that affect the nervous system (neurologist).

You will be asked to lie on your back on a bed or table. Or you may sit in a chair with your eyes closed. The EEG technologist will attach several flat metal discs (electrodes) to different places on your head. A sticky paste is used to hold them in place. Instead of separate electrodes, you may wear a cap with several fixed electrodes. In rare cases, the electrodes may be attached to the scalp with tiny needles.

The electrodes are hooked by wires to a computer that records the electrical activity in the brain. A machine can show the activity as a series of wavy lines on a piece of paper. Or the activity may be shown as an image on the computer screen.

You will need to lie still with your eyes closed during the recording. The technologist will watch you directly or through a window during the test. The recording may be stopped from time to time. This allows you to stretch and change your position.

The technologist may ask you to do different things during the test to see what activity your brain does at that time.

  • You may be asked to take deep and rapid breaths (hyperventilate). In most cases, you will take 20 breaths a minute for 3 minutes.
  • You may be asked to look at a bright, flashing light called a strobe.
  • You may be asked to go to sleep. If you can't fall asleep, you may get a sedative to help you sleep. If an EEG is being done to check a sleep problem, your brain's electrical activity may be recorded all night.

How long the test takes

The test will take about 1 to 2 hours.

How It Feels

There is no pain during an EEG.

If needle electrodes are used (which is rare), you will feel a brief, sharp prick when each electrode is put in. It will feel kind of like having a hair pulled out. If electrodes are placed in your nose, they may tickle.

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


An EEG is a very safe test. The electrical activity of your brain is recorded. But no electrical current is put into your body. An EEG is not the same as electroshock (electroconvulsive) therapy.

If you have a seizure disorder such as epilepsy, the flashing lights may trigger a seizure. Or a seizure may happen if you hyperventilate. If it happens, the technologist is trained to take care of you during the seizure.


EEG test results are ready on the same day or the next day.

There are several types of brain waves.

  • Alpha waves are present only when you're awake with your eyes closed but you are mentally alert. Alpha waves go away when your eyes are open or you are concentrating.
  • Beta waves are normally found when you are alert or have taken high doses of certain medicines, such as benzodiazepines.
  • Delta waves are normally found only in young children and in people who are asleep.
  • Theta waves are normally found only in young children and in people who are asleep.
Electroencephalogram (EEG)


In adults who are awake, the EEG shows mostly alpha waves and beta waves.

The two sides of the brain show similar patterns of electrical activity.

There are no abnormal bursts of electrical activity and no slow brain waves on the EEG tracing.

If flashing lights are used during the test, one area of the brain (the occipital region) may have a brief response after each flash of light. But the brain waves are normal.


The two sides of the brain show different patterns of electrical activity. This may mean that there's a problem in one area or side of the brain.

The EEG shows sudden bursts of electrical activity called spikes. Or the test shows sudden slowing of brain waves in the brain. These changes may be caused by a brain tumour, infection, injury, stroke, or epilepsy. When a person has epilepsy, the location and exact pattern of the abnormal brain waves may help show the type of epilepsy or seizures. In many people with epilepsy, the EEG may appear normal between seizures. An EEG by itself does not diagnose or rule out epilepsy or a seizure problem.

The EEG records changes in the brain waves that may not be in just one area of the brain. A problem that affects the whole brain may cause these kinds of changes. This includes drug intoxication, infections (encephalitis), and metabolic disorders (such as diabetic ketoacidosis). These problems change the chemical balance in the body, including the brain.

The EEG shows delta waves or too many theta waves in adults who are awake. This may mean that there is a brain injury or brain illness. Some medicines can also cause this.

The EEG shows no electrical activity in the brain. This is a "flat" or "straight-line" EEG. This means that brain function has stopped. It's usually caused by lack of oxygen or blood flow inside the brain. It may happen when a person has been in a coma. In some cases, severe sedation from drugs can cause a flat EEG.


Current as of: May 1, 2023

Author: Healthwise Staff
Clinical Review Board
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