What Is EEG? - Definition & Test

Instructor: Meghan Greenwood

Meghan has taught undergraduate and graduate level science courses and has a PhD in Immunology.

This lesson will define electroencephalogram, or EEG, as well as describe what it is used for. The actual test procedure will also be summarized, including any risks and benefits.

What Is an EEG?

It's ALIVE! Think of the last time you took a test or had a very puzzling dream. Our brain has a way of recalling information, as well as creating its own stories based on our thoughts and experiences. Although it may not actively pump blood like the heart or churn food like the stomach, the brain is a very dynamic organ. Every time we have a thought or a dream, every time we touch something hot or see a flying bird, chemical and electrical reactions occur in the brain.

The brain is made up of billions of cells called neurons. Neurons are the messengers in the brain, sending a chemical signal to other neurons and generating a small electric signal each time they fire. Such signals can be measured to determine brain function.

Brain lobes
brain lobes

An electroencephalogram, or EEG, is a method for measuring your brain's electrical activity. What looks like quite the freaky Halloween costume, an EEG cap typically contains 21 electrodes. The electrodes are strategically placed on certain lobes of the brain, including the frontal, temporal, parietal, and occipital lobes. This placement is part of the international 10-20 system, a well-adopted method for electrode positioning.

An EEG recording cap contains many different electrodes strategically placed on specific lobes of the brain.
EEG cap

The EEG Test

Despite the intimidating look of the EEG cap, the test itself is painless. EEGs are typically conducted in a hospital, where the technician will place the cap on your scalp or, alternative to the cap, attach several electrodes onto your scalp using a paste. The electrodes are then hooked to a computer for recording brain activity.

Once you are lying back in a relaxed, comfortable position, you may be asked to do several things. For example, the technician may ask you to breathe rapidly or look at a bright light. If sleep is part of the test, you will be asked to rest. The entire procedure can be done over 60 minutes to several hours, depending on if sleep is a factor.

Benefits and Risks

Overall, EEGs are very safe and are useful in evaluating changes in your brain activity that can be used to diagnose brain disorders. If a person has a seizure disorder, he or she may experience a seizure from the flashing lights that occur during the test, but medical care will be provided if needed.

EEGs are not only good for monitoring typical brain activity, but they can also help diagnose certain health conditions. An EEG can be used to observe seizures, epilepsy, stroke, sleep problems, head injuries, tumors, and diseases, such as Alzheimer's disease and dementia.

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