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Electroencephalogram (EEG): what is it and how is it used?

Electroencephalogram (EEG): what is it and how is it used?

January 25, 2022

The word electroencephalogram is not unknown today . Many people, whether for medical or research purposes, have ever submitted to one. And whether this is the case or not the cinema, literature or popular wisdom can make appear in our heads the typical image of a person with a kind of helmet full of electrodes connected to it.

But know what it is, what exactly it measures, what it is used for or how it works An electroencephalogram may not be as well known. That is why in this article we will observe different aspects of this measurement instrument so used in the field of medicine.


What is the electroencephalogram?

The electroencephalogram is a physiological evaluation technique that is used to study the functioning of the nervous system through the recording of the electrical activity of the brain, specifically the cerebral cortex.

To understand the meaning of this technique, we must bear in mind that the activity of the brain is based on the emission and transmission of electrochemical impulses, signals of nervous activity that can be detected by the correct techniques. Thus, through an electroencephalogram it is possible detect the habitual functioning pattern of our brain and the activation of the brain or concrete parts of it before external or internal stimulation.


In this technique an instrument called electroencephalograph is used , which records the electrical activity of that to which it is connected. This instrument receives information from a series of electrodes that would be located in certain areas of the patient's head and with which neuronal activity is recorded.

What does it measure?

The electroencephalogram allows to measure, as we have mentioned, the electrical activity of the brain . Regardless of the purpose of the encephalogram, this activity can occur in the form of various types of waves.

Measurements can be made during wakefulness or during sleep, depending on the purpose of the test. Through the electrodes the measurement system captures the emission of brain waves and their rhythm, shape, duration and frequency of emission.

Types of waves

The captured waves they can be alpha, beta, theta and delta . Each will cause the electroencephalograph to draw one or the other wave frequency pattern.


Alpha waves appear in moments of relaxation or tasks that do not require concentration or effort.

Beta waves are usually reflect the realization of an intense mental effort , appearing generally while we are awake or during REM sleep.

Theta waves are observed just like alpha waves when we are relaxed, but in this case they are more frequent at times when in addition to being relaxed we are sleepy , being the most predominant type of wave during phase two of non-REM sleep.

Finally, the delta waves are those that are linked to deep sleep , being those that have traditionally been linked to rest and repair of nervous tissues.

Through the encephalogram can be measured both the pattern of general functioning of the brain and the differences between some areas with others, through the analysis of voltage differences between different areas.

  • Related article: "Types of brain waves: Delta, Theta, Alpha, Beta and Gamma"

Performance of the test

The basic operation of this technique does not have great complexity. The test is based on the placement of a series of electrodes in strategic points of the head , fixing a small cloth helmet previously placed on the scalp of the patient or subject of studies or directly on the scalp.

The employee record measures a voltage difference between two electrodes , being these placed in pairs to make measurements.

Phases of the use of the encephalograph

In the first place, the test is prepared, placing the subject to be evaluated and fixing in it the elements that allow the recording of brain activity. For it a type of capillary gel is applied that allows improving the conduction of electricity and more precisely fix the electrodes, whose collation is done next. Generally about twenty electrodes are placed, creating a montage that allows to obtain a correct activity of the nervous system.

In this assembly, it is usual to use the 10/20 system, placing the electrodes in an equidistant manner in which 10-20% of the axes of the brain are separated. In addition, the assembly can be bipolar, if it is intended to record brain activity and the difference between two points, or monopolar if a specific point is compared with one without brain activity.

Once the electrodes are placed , we proceed to make the measurement, registering first the basal rhythm of the individual with both closed and open eyes, and then provoking a light stimulation in it to observe the reaction of brain activity. Some common stimuli are a slight photostimulation or hyperventilation of the patient. The subject can also be asked to do some type of physical or mental activity.

As the test is performed, a series of results are obtained that indicate how the nervous system acts and how it reacts to stimulation.

The results obtained by the measurement can be registered and either printed or directly reflected on a monitor . But the recording of the waves does not have a significance by itself, having to perform an analysis of the implications of basal functioning and / or any alteration detected over the time that the registration has taken place.

Uses and applications of the electroencephalogram

Considering all the above, we must bear in mind that the use of the electroencephalogram is not done by mere caprice. It is used only with specific objectives and when there is suspicion of certain diseases or an investigation is being carried out.

As far as research is concerned, the electroencephalogram is used in those experiments in which it is required to know the brain activity in a certain state or while carrying out concrete actions. Thus, it serves to explore how our brain works and how it reacts to stimuli or specific activities. It also allows to evaluate if there are large differences between the activation of a specific area and others.

Regarding its use in medicine, It can be used to detect if the brain has normal functioning , control the state of consciousness during a surgical intervention or if there are alterations in the wave emission pattern.

In this aspect tends to use this type of technique when it is suspected of the presence of disorders such as epilepsy (coming to voluntarily cause the crisis to record how and what happens), dementias, encephalopathies, typical outbreaks of some mental disorders and even differentiate between coma and brain death (while in the first there is brain activity the second would show a flat electroencephalogram). It is also widely used to analyze sleep problems and disorders.

Contraindications and adverse effects

The application of an electroencephalogram does not usually cause problems in those in which it is performed, being a non-invasive technique that does not present contraindications in the majority of the population, not even in pregnant women.

One of the few exceptions are cases of epilepsy in which it could cause the appearance of a crisis during the performance of the test, which in many cases is sought to identify hyper-activated areas. However, in serious cases, the risk of provoking a new crisis should be assessed.

Bibliographic references:

  • Gil-Nagel, A .; Parra, J .; Iriarte, J.M. & Kanner, A. (2002). Manual of electroencephalography. Madrid: McGraw-Hill, S.A.U.
  • Niedermeyer, E. & da Silva, F. L .. (2005). Electroencephalography: basic principles, clinical applications, and related fields. Lippincott Williams & Wilkins.
  • Ramos-Argüelles, F .; Alonso, M.T .; Egozcue, S .; Pabón, R.M. and Morales, G. (2009). Basic electroencephalography techniques: clinical principles and applications. Annals of the Sis. Saint. Navarra, vol. 32 (Suppl 3), Pamplona.

Electroencephalogram (EEG) (January 2022).


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