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

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

What is the electroencephalogram?

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

To understand the meaning of this technique, we must take into account that the activity of the brain is based on the emission and transmission of electrochemical impulses, signals of nerve activity that can be detected by means of the correct techniques. Thus, through an electroencephalogram it is possible to detect the usual pattern of functioning 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 electroencephalograph is used, which records the electrical activity of that to which it is connected. This instrument receives the information from a series of electrodes that would be placed in certain areas of the patient’s head and with which the 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 EEG, this activity can take 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, form, duration and frequency of emission.

Types of waves

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

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

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

Theta waves are observed as well as alpha waves when we are relaxed, but in this case they are more frequent in moments when we are not only relaxed but also sleepy , being the most predominant type of wave during phase two of non-REM sleep.

Finally, delta waves are those linked to deep sleep , which have traditionally been linked to rest and repair of nerve tissues.

Through the encephalogram, both the general pattern of brain functioning and the differences between areas can be measured by analyzing the voltage differences between different areas.

How the test works

The basic operation of this technique is not very complex. The test is based on the placement of a series of electrodes in strategic points of the head , fixed to a small fabric helmet previously placed on the scalp of the patient or subject of studies or directly on the scalp.

The recording used measures a voltage difference between two electrodes , which are placed in pairs to allow measurements to be made.

Phases of Encephalograph Use

First, the test is prepared, sitting the subject to be evaluated and fixing the elements that allow recording the brain activity. For this purpose , a type of capillary gel is applied which makes it possible to improve the conduction of electricity and to fix the electrodes with greater precision. Generally, about twenty electrodes are placed, creating a montage which allows the correct activity of the nervous system to be obtained.

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

Once the electrodes have been placed , the measurement is made, first recording the basal rhythm of the individual with both eyes closed and open, and then causing a slight stimulation in him to observe the reaction of the brain activity. Some common stimuli are mild photostimulation or hyperventilation of the patient. The subject may 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 recorded and either printed out or directly reflected on a monitor . But the recording of the waves does not have a meaning in itself, and an analysis of the implications of the basal operation and/or any alteration detected over the time that the recording has taken place must be carried out.

Uses and applications of the electroencephalogram

Considering all of the above, it should be noted that the use of the electroencephalogram is not done on a mere whim. It is only used for specific purposes and when either certain ailments are suspected 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 specific actions. Thus, it serves to explore how our brain works and how it reacts to specific stimuli or activities. It also allows us to assess whether there are major differences between the activation of a particular area and others.

With regard to its use in medicine, can be used to detect whether the brain is functioning normally , monitor consciousness during surgery or whether there are alterations in the pattern of wave emission.

In this aspect, this type of technique tends to be used when the presence of disorders such as epilepsy is suspected (even provoking the seizures voluntarily in order to record how and what occurs), 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 analyse sleep problems and disorders.

Contraindications and adverse effects

The application of an electroencephalogram does not usually cause problems in those who undergo it, 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 is cases of epilepsy in which a seizure could occur during the test, which in many cases is sought in order to identify the hyperactive areas. However, in serious cases, the risk of provoking a new seizure 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 techniques of electroencephalography: principles and clinical applications. Annals of the Sis. San. Navarra, vol. 32 (Suppl. 3), Pamplona.