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 underlying theoretical orientation. In this article we will talk about an objective test, the evoked potential test .

This is a neurophysiological test that was first used in 1947. It allows the evaluation of neuronal activity through brain stimulation. In addition, it is used to determine the presence of disorders, diseases such as multiple sclerosis and tumors. We will know its characteristics, uses, functioning and types.

The Evoked Potential Test: An Objective Test

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

Objective testing means that the administration, recording, scoring and analysis of your data are performed from devices. In contrast, they are tests with very low ecological validity, as they are used in artificially created situations.

This particular test is non-invasive, painless (although it may cause some discomfort in some people) and safe, which was first used in 1947.

The test evaluates a very specific type of psychophysiological response. Specifically, serves 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 person’s intellectual level.

How does it work?

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

Specifically, what it does is determine the speed at which the brain connections are functioning; that is, if they are too slow, it is likely that the myelin sheaths, a layer that covers the neurons of the nervous system, are damaged. The myelin serves to ensure that neural transmissions occur quickly and efficiently.

In other words, the test helps to determine if there is a myelin lesion . We speak then of a demyelination process, typical in diseases such as multiple sclerosis. Thus, the evoked potential test would allow the detection of this type of disease (or confirm its diagnosis).

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, with the flow of electricity at a cerebral level and with how information is transmitted (with greater or lesser fluidity, speed, etc.).

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

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

Finally, also is used in patients in a state of coma , in order to evaluate their brain function or activity.

How do you use it?

In relation to the above, the evoked potential test basically detects whether the neuronal (electrical) conduction system is damaged. This system has to do with the different senses (hearing, sight, hearing,…), and it can 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 evident symptoms, or produces “silenced” symptoms, since allows to corroborate a previous suspicion of disease evaluated with other types of neuropsychological tests more of a clinical type.

On the other hand, in the case that there is a disease that affects the brain’s electrical activity, the evoked potential test provides information on the evolution of the disease itself; in the case that there is a lesion in the Central Nervous System, it helps to determine which area(s) is/are affected and to what extent (extent 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 the dementia is progressive).

How does it apply?

The potential test is easy to apply; for this, generally the electrodes are placed on the person’s scalp (ideally, the person should wash his hair the day before, and not use any additional products). Specifically, the electrodes will be placed on the areas to be stimulated.

Once the electrodes have been 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 subject under examination , through sensory stimuli, which can be of three types: auditory, visual and somatosensory.

Later, it will collect the results that “emit” the brain, and that allow to determine the activity of the same one, as well as possible lesions in the layer of myelin that covers the neurons.

Types

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

1. Auditory stimulation

When the stimuli 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 , etc. can be used.

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

2. Visual stimulation

Here the stimuli are visual, and the test is a visual evoked potential (VEP) test. This type of test will allow us to diagnose vision problems involving an impairment of the optical nerve . The stimulus(s) will consist of a screen with black and white squares, which the subject under examination will have to observe.

3. Somatosensory stimulation

Finally, the third type of evoked potential test is that which is performed based on somatosensory stimulation (the test is called PESS for short). It is used to diagnose problems that appear in the spinal cord and that can cause different types of symptoms, such as numbness or paralysis in the legs or arms.

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

Precautions and considerations

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

Some of the most common factors or variables that can interfere are : that the person examined has a swollen middle ear (in the case of the hearing test) or has some kind of hearing impairment (this condition can affect all sensory modalities), that he/she presents severe myopia (in the case of the visual test), that he/she presents movement alterations such as muscle spasms in the neck or head, etc.

Bibliographic references:

  • Fernández-Ballesteros, R. (2005). Introduction to Psychological Evaluation I and II. Pirámide Publishing House. 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.