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

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

But know what it is, what it measures exactly, what it is used for or how it works an EEG may not be as well known. That is why in this article we are going to observe different aspects of this measuring instrument so widely used in the field of medicine.

What is the electroencephalogram?

The EEG is a physiological assessment technique 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, it must be borne in mind that the activity of the brain is based on the emission and transmission of electrochemical pulses, some signs of nervous activity that can be detected through the correct techniques. Thus, through an electroencephalogram it is possible

detect the normal functioning pattern of our brain and the activation of the brain or specific parts of it in the face of external or internal stimulation.

In this technique an instrument called an EEG is used, which records the electrical activity of what it is connected to. This instrument receives the 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 us to measure, as we have mentioned, the electrical activity of the brain. Regardless of the objective of the encephalogram, this activity can occur in the form of various types of waves.

Measurements can be made while awake or during sleep, depending on the purpose for which the test is performed. 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 waves captured can be alpha, beta, theta and delta. Each will cause the EEG to draw one or another wave frequency pattern.

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

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

Theta waves are observed just like the 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, 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, it is possible to measure both the general functioning pattern of the brain and the differences between some areas with others, through the analysis of the voltage differences between different areas.

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

Test performance

The basic operation of this technique is not very complex. The test is based on the placing a series of electrodes at strategic points on the head, attaching to 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 be able to carry out measurements.

Phases of the use of the encephalograph

In the first place, the test is prepared, sitting the subject to evaluate and fixing in him the elements that allow registering brain activity. For it a type of hair gel is applied to improve the conduction of electricity and more precisely fix the electrodes, the collation of which is made below. Generally, around twenty electrodes are placed, creating a setup that allows obtaining a correct activity of the nervous system.

In this setup it is common to use the 10/20 system, placing the electrodes in an equidistant way in which they are separated between 10 and 20% of the axes of the brain. Furthermore, the set-up 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 attached, we proceed to make the measurement, registering first the basal rhythm of the individual both with the eyes closed as open, and then gently stimulating it to observe the reaction of the activity cerebral. Some common stimuli are light photostimulation or hyperventilation of the patient. The subject can also be asked to do some kind 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 recorded and either printed or directly reflected on a monitor. But the record of the waves does not have a significance by itself, having to carry out an analysis of the implications of baseline functioning and / or any alteration detected over the time that the record.

Uses and applications of the EEG

Considering all of the above, it must be borne in mind that the use of the EEG is not done on a whim. It is used only for specific purposes and when either certain ailments are suspected or an investigation is underway.

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

Regarding its use in medicine, can be used to detect whether the brain is functioning normally, monitor the state of consciousness during a surgical intervention or if there are alterations in the wave emission pattern.

In this regard, this type of technique tends to be used when the presence of disorders such as epilepsy (getting to voluntarily provoke crises 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 EEG). It is also widely used to analyze problems and sleep 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 It 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 seizure during the test, which in many cases is sought to be able to identify the areas hyperactivated. However, in serious cases, the risk of causing a new crisis should be assessed.

Bibliographic references:

• Gil-Nagel, A.; Parra, J.; Iriarte, J.M. & Kanner, A. (2002). Electroencephalography Manual. 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 techniques of electroencephalography: principles and clinical applications. Annals of the Sis. Saint. Navarra, vol. 32 (Suppl. 3), Pamplona.