Evoked potential: what it is and how it helps to study the brain

In psychological evaluation there are different types of tests: objective, subjective, constructive, projective... Depending on what they intend to evaluate, how they do it, and their theoretical orientation underlying. In this article we will talk about an objective test, the evoked potential test.

It is a neurophysiological test that began to be used in 1947. It allows to evaluate neural activity through brain stimulation. In addition, it is used to determine the presence of alterations, diseases such as multiple sclerosis and tumors. We will know its characteristics, uses, operation and types.

• Related article: "Action potential: what is it and what are its phases?"

The Evoked Potential Test: An Objective Test

In psychological evaluation, the evoked potential test is classified as an objective psychophysiological technique.

Objective tests imply that the administration, recording, scoring and analysis of your data, are carried out from devices. On the other hand, they are tests with a very low ecological validity, since they are used in artificially created situations.

This particular test it is minimally invasive, painless (although it can cause some discomfort in some people) and safe, which was used for the first time in 1947.

The test evaluates a very specific type of psychophysiological response. Specifically, is used to study the electrical activity of the brain in response to sensory stimuli of different types (auditory, visual, somatosensory,...), that is, stimuli that come from any modality, and of short duration. This type of response seems to be related to the intellectual level of the person.

How does it work?

The evoked potential test It is used to detect possible diseases related to the functioning of brain activity (neural connections).

Specifically, what it does is determine the speed of the functioning of brain connections; that is, if it is too slow, it is likely that the myelin sheaths, a layer that covers the neurons of the nervous system, are damaged. Myelin is used for neuronal transmissions to occur quickly and efficiently.

That is, the test Helps determine if there is a myelin lesion. We speak then of a process of demyelination, typical in diseases such as multiple sclerosis. Thus, the evoked potential test would make it possible to detect this type of disease (or confirm its diagnosis).

• You may be interested in: "Myelin: definition, functions and characteristics"

Uses and functions

In addition to detecting possible diseases such as multiple sclerosis, the evoked potential test allows us to find alterations in brain function that have to do with the conductance of neurons, that is to say, with the flow of electricity at the brain level and with how the information is transmitted (with greater or lesser fluidity, speed, etc.).

This technique will be useful when the previous neuropsychological examinations are not clear enough, or do not allow the pathology to be diagnosed accurately or reliably. It can be applied in cases of suspected alteration.

On the other hand, the evoked potential technique also makes it possible to detect alterations in vision (such as some types of blindness), when the optic nerve is damaged. In addition, it can help diagnose tumors.

finally, too used in comatose patients, in order to assess your brain function or activity.

How is it used?

In relation to the aforementioned, what the evoked potential test basically does is detect if the neuronal (electrical) conduction system is damaged. This system has to do with the different senses (hearing, sight, hearing,...), and it may happen that one of them is damaged, and the others are not, always depending on the pathology presented by the patient.

This test is especially useful when a certain disease or injury does not produce obvious symptoms, or produces "silenced" symptoms, since it allows to corroborate a previous suspicion of disease evaluated with other types of neuropsychological tests more of a clinical nature.

On the other hand, in the event that there is a disease that affects the electrical activity brain, the evoked potential test allows us to provide information about the evolution of one's own disease; In the case of a lesion in the Central Nervous System, it helps to determine which area(s) is(are) affected and to what extent (extension of the affected area).

Finally, this test helps to define changes that have occurred in the functioning and/or neuropsychological state of a person (for example in a patient with dementia, since it is progressive).

How does it apply?

The potential test is easy to apply; for this, generally electrodes are placed on the person's scalp (Ideally, she washes her hair the day before, and does not use additional products). Specifically, the electrodes will be placed on the areas to be stimulated.

Once the electrodes are placed in the corresponding areas (which will vary in each case), the researcher or professional who applies the evoked potential test will proceed to stimulate the examined subject, through sensory stimuli, which can be of three types: auditory, visual and somatosensory.

Subsequently, it will collect the results "issued" by the brain, and that allow to determine its activity, as well as possible lesions in the myelin layer that covers the neurons.

Guys

There are three types of evoked potential tests, depending on the type of stimulus that is applied:

1. auditory stimulation

When the stimuli that are applied are auditory, we speak of an auditory evoked potential (AEP) test. Thus, the stimuli will be auditory and different types of tone, intensity, noises may be used, etc.

It is useful for diagnosing hearing difficulties, different types of deafness, etc. In this case, the electrodes will be placed on the scalp and on the earlobe.

2. visual stimulation

Here the stimuli are visual, and the test is a visual evoked potential (VEP) test. This kind of test It will allow us to diagnose vision problems that involve an affectation of the optic nerve. The stimulus or stimuli will consist of a screen with black and white squares, which the examined subject must observe.

3. somatosensory stimulation

Finally, the third type of evoked potential test is the one that is performed from somatosensory stimulation (the test for short is called PESS). Used to diagnose problems that arise in the spinal cord and they can cause different types of symptoms, such as numbness or paralysis in the legs or arms.

The stimuli that are applied are electrical (of mild intensity), and in this case, the electrodes are placed in different areas that can vary, such as the knees or the wrists.

Precautions and Considerations

There are some conditions of the person that can interfere with the results of the evoked potential test. That is why the professional who applies it must take them into account.

Some of the most common factors or variables that can interfere are: that the person examined has an inflamed middle ear (in the case of the hearing test) or that they have some type of hearing disability (this condition can affect all sensory modalities), who presents severe myopia (in the visual test), who presents movement disorders such as muscle spasms in the neck or head, etc.

Bibliographic references:

• Fernndez-Ballesteros, R. (2005). Introduction to Psychological Assessment I and II. Ed. Pyramid. Madrid.
• Moreno, C. (2005). Psychological evaluation. Concept, process and application in the areas of development and intelligence. Ed. Sanz and Torres. Madrid.
• Walsh, P., Kane, N. & Butler, S. (2005). The clinical role of evoked potentials. British Medical Journal, 76(suppl. 2): 16-22.