What happens during an electroencephalogram (EEG)?

One reason doctors use an EEG is if they think someone has a neurological (nerve-related) disorder such as epilepsy or brain damage. Sometimes it is used during surgery to monitor the effects of an anesthetic. An EEG can also provide information about how the brain is functioning in patients who are in intensive care or doing a sleep study. It can confirm brain death as well.

EEGs used to play an important role in diagnosing strokes or detecting brain tumors. But nowadays imaging techniques such as CT or MRI scans are used instead.

A total of 21 electrodes are typically used in an EEG. To make it easier to put the electrodes in the right place on your head, the electrodes and cables are usually attached to a cap that is pulled over your head. Before the test, the electrodes are coated in a special gel that makes it easier to measure the electrical activity.

There is no need to shave your head. Your hair should be clean and free of residues from products such as styling foam, gel or hair spray.

During the measurement, you lie or sit in a relaxed position and stay as still as possible. An assistant will give you directions, such as to open your eyes or to take deep breaths. Sometimes, certain stimuli (such as a flickering light) may be used to trigger brain activity. The recording takes about 20 to 30 minutes.

Unlike an ECG, which has a pattern with spikes in it, an EEG shows multiple waves. The shape of these waves mainly depends on how active the brain currently is. This is different when you’re awake or asleep, or when you’re alert or tired. Each wave provides information about the activity of the nerve cells in one specific area of the brain.

Brain waves are divided into the following main types:

• Alpha waves (frequency 8 to 13 cycles per second (Hz, hertz)): These describe the activity of the brain when you're resting with your eyes closed but you’re awake. This is your baseline.
• Beta waves (14 to 30 Hz): When your eyes are open, your senses are stimulated and you’re mentally active, these higher, irregular frequencies are recorded.
• Gamma waves (over 30 Hz): These occur when you're very alert and when you're learning something.
• Theta waves (4 to 7 Hz): Lower frequencies arise when you are falling asleep or very exhausted, for example.
• Delta waves (0.5 to 3.5 Hz): The slowest, usually synchronized waves can be observed during deep sleep.

Normal EEG waves (left) and the spike-and-wave pattern in epilepsy (right)
Illustration: Normal EEG waves (left) and the spike-and-wave pattern in epilepsy (right)

Everybody’s baseline EEG pattern is different. This means that “normal” EEG recordings can vary greatly from person to person. This variation is even greater in children, and their wave patterns are much slower and less regular than in adults.

If an EEG is done for diagnostic purposes, the focus is on how often waves occur (frequency) and how high they are (amplitude). Deviations from typical wave patterns can be signs of an illness or disorder. During epileptic seizures, for example, some of the waves are particularly high and steep (spike-and-wave pattern). Many medications that affect the brain also change the EEG pattern.