Electroencephalogram: Definition, Procedures & Tests

Instructor: Paul Jeffries

Paul has taught psychology for over 10 years.

In this article, you will learn what an electroencephalogram (EEG) is and how it is recorded. You will also learn how is used by researchers to study sleep and by clinicians to study seizures.

What Is an Electroencephalogram?

An electroencephalogram is a recording of the electrical activity of the brain. The human brain is made up of approximately 100 billion cells. These cells, called neurons, use electrical and chemical signals to communicate with each other. Although the electrical signal generated by a single neuron is extremely small, the signals of large groups of neurons near the surface of the brain can be detected through the skull.

This picture shows a man from behind with EEG electrodes attached to his scalp.

The word electroencephalogram is built from three roots: electr (from electric); encephalon (from the Greek enkephalos meaning brain), and gram (from the Greek graphein meaning to write). Since the word electroencephalogram can be a mouthful to say and tedious to write, it is usually just abbreviated to EEG.

The equipment that detects and records the electrical activity of the brain is called an electroencephalograph. The printed record of the brain's activity is an electroencephalogram. It might help you to remember which is which if you think of telegrams. Remember that in old movies good news often came in the form of a telegram. The equipment that sent the joyous news was the teleGRAPH. The actual written message was the teleGRAM.

How EEGs Are Recorded

Hans Berger (1873-1941), a German psychiatrist, is credited with recording the first EEGs in humans. Although Berger's work was dismissed when it was published in 1929, the EEG was eventually accepted after other scientists confirmed Berger's claims. Now EEGs are routinely used for research and medical diagnosis.

Hans Berger
Picture of Hans Berger

The electroencephalograph is a non-invasive technique; that means, it does not require the skull to be punctured or anything to be inserted into the brain. Metal electrodes are placed at specific location across the scalp and held in place by a special conducting gel. In a standard EEG, nineteen electrodes are used, but more or fewer may be used depending on the purpose of the test. When a person has a EEG taken, they usually sit comfortably or even sleep. But occasionally EEGs are recorded while the person walks or engages in some activity.

How EEGs Are Used in Sleep Research

EEGs are performed by researchers to study the brain and by clinicians to diagnosis brain dysfunction. One area of research in which EEGs have been widely used is in the study of sleep. We often think of sleep as an all-or-none phenomenon. Either you are asleep or you are awake. However, EEGs taken while people sleep show that sleep is not just one state.

In fact, there are five stages of sleep. Each stage of sleep is characterized by a different pattern of electrical activity. The first four stages of sleep represent different levels of sleep ranging from light sleep to deep sleep. The fifth stage of sleep is REM sleep, the stage in which most of our vivid, fantastic dreams occur.

Below is an EEG taken while the person was awake and mentally alert. The type of brain waves shown here are called beta waves. Beta waves are high frequency (the peaks and valleys are close together) and low amplitude (the distance between peaks and valleys is relatively small).

An EEG showing beta brain waves.

Compare the beta waves with the delta waves shown below. Delta waves occur when a person is in a deep sleep; the kind of sleep that it is difficult for a sleeper to wake from. Compared to beta waves, delta waves are low frequency (the peaks and valley are farther apart) and high amplitude (the distance between peaks and valleys is relatively large).

An EEG showing delta brain waves.

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