What is an EEG? A Simple Answer

If you have ever read anything about the brain, you’ve probably heard the phrase EEG and wondered: what is an EEG? Many texts use complex terms that can confuse this question, but in fact it can be answered in a simple and straightforward way. By the time you reach the end of this article, you will know what an EEG is and understand a few of its most important uses in science and medicine. What is an EEG? rnEEG stands for electroencephalography. Although this word looks daunting, the process it describes is actually fairly simple. The electrical activity of the brain can be measured by sensors (or electrodes, to give them their proper name) which are placed on the scalp. The “electro” of electroencephalography refers to this electrical activity, while “encephalo” is the Greek word for the brain. The suffix “graphy” indicates that the word refers to the study of this brain activity. Electroencephalography, or EEG, thus refers to the process of measuring the electrical activity of the brain. What is an EEG read out actually measuring? Brain activity consists of the electrical signals produced by the billions of neurons which make up the brain. EEG measures brainwave frequency – literally, how frequently the neurons of the brain are firing. Because these electrical signals are fairly weak, placing electrodes on the scalp means that it is difficult to detect activity from deep within the brain. Most EEG readings are therefore concerned with signals produced by the cerebral cortex, which covers the top surface of the brain and is thus close to the top of the head. It is possible to take deeper brain readings, but electrodes must be implanted inside the skull for this to be possible. What is an EEG frequency band? Different patterns of brain activity are associated with different states of consciousness. For example, when you are relaxed, the neurons in your cortex fire less frequently than when you are alert. In turn, they fire less frequently still when you are asleep. Because the frequency of brain activity decreases as alertness decreases, we can say that brainwaves are slower in states of relaxation, and slower still during sleep. There are many different brainwave frequencies (too many to go into in this article, in fact). Brainwave frequencies are normally grouped into bands, with each band having an associated mental state. What is the use of an EEG? EEG readings can be used in a number of different ways. Abnormalities in EEG readings can be used to diagnose mental health problems or brain damage, or other conditions such as epilepsy. Abnormalities might consist of unusually low or high activity or unusual patterns in the EEG reading (sudden bursts of activity, for example). In addition to its medical functions, EEG monitoring can be used in scientific experiments in order to assess what changes in brain activity are associated with different tasks or behaviours. Of course, there is a great deal more to be known about electroencephalography, but that is beyond the scope of this article. The information contained within this article consists of a brief introduction to the subject, intended to cover the basic question of what is an EEG.