Auditory cortex: characteristics and functions of this brain region.

A summary of the characteristics of the auditory cortex of the brain, and its parts.

The cortex of the brain includes areas specialized for specific tasks. This is the case, for example, of the auditory cortex.

We will devote the following lines to learning more about the function of this part of the brain, its characteristics and most important regions. Likewise, we will see with which other parts of the nervous system and the human organism it is connected in order to fulfill its functions.

What is the auditory cortex of the brain? Location and characteristics

The auditory cortex of the brain is a part of this organ in charge of processing all the information that we obtain through the auditory system, that is to say, the sounds captured by the ears. Its location is in the temporal lobe and within this area we can find it in the so-called Heschl's area, formed by the transverse gyri.

Another way to find this region is to go to the map of the old Brodmann's areas, since the auditory cortex of the brain would be occupying the 41 the auditory cortex of the brain would be occupying the parts 41, 42 and a part of 22within this map. This region of the cerebral cortex can be found both in the brain of human beings and in the brain of a large number of vertebrate animal species.

Parts and structure

In terms of structure, the auditory cortex of the brain can be subdivided into primary (A1), secondary (A2) and tertiary (A3) auditory cerebral cortex. The primary auditory cortex is approximately 3 millimeters thick. At the macrostructural level, we have already seen that it is located in Heschl's area, occupying half of the whole area.

If we go to the microstructure, we can find several ways to study this part of the encephalon. For example, at the level of neuronal arrangement or cytoarchitecture, the A1 part would be part of the so-called koniocortex, a grouping of neurons with a granular appearance. The auditory cortex of the A1 brain has several layers, showing greater density in the number II and IV. As for III, it is characterized by the existence of pyramidal type cells.

If we focus on the chemical composition, or chemoarchitecture, we will find that the A1 zone is largely composed of CO, cytochrome oxidase and AChE, acetylcholinesterase. Finally, the distribution of myelin, or myeloarchitecture, denotes large concentrations of this substance in the primary partprecisely where most sensory projections occur.

Precisely because of this high myelination, the auditory cortex of the primary type brain (A1) can be easily observed by magnetic resonance imaging.

In the case of primates, and more specifically in humans, we can divide this area, from the most central to the most peripheral, as nucleus, inner belt and outer belt.. The nucleus would house the A1 section and also the rostral or R part. The inner girdle would house the auditory cortex of the secondary brain, i.e., the A2 area. Finally, the outer girdle is where we would find the tertiary section, or A3.

The auditory cortex of the brain is part of the so-called neocortex. This area is characterized by the need for a certain amount of stimulation during development in order to develop all functions correctly. In this sense, for the auditory cortex to perform its tasks normally, it must have been exposed to different auditory frequencies in the early stages of the organism's life.

Functions of the auditory cortex of the brain

The function of the auditory cortex of the brain, as is evident, is to process the data captured by the auditory system. If this part of the brain did not do this job, no matter how well the ears functioned structurally, we would have no way of using the sense of hearing, since there would be no reception and interpretation of the sounds captured by the auditory system.

For this reason, some brain injuries due to trauma, diseases, cerebral infarcts or tumors that damage this area, can cause deafness at a functional level, regardless of the fact that the ears are not affected. However, even if the sounds cannot be interpreted, these subjects still show reflex behaviors to some of them.

The explanation for this phenomenon is that, before reaching the auditory cortex of the brain, there is a first processing of the information that takes place in the brainstem and midbrain.

In addition, each group of neurons in the auditory cortex of the brain each group of neurons in the auditory cortex of the brain is specialized to process sounds belonging to a certain frequency.. Thus, it can be observed that, starting from one end, there are neurons that process low frequencies (from 2 Hz) and as we move towards the other end of this cortex, the nerve cells process higher frequencies, up to 128 Hz.

Due to this phenomenon, there are frequency maps or tonotopic maps that indicate exactly which area of the auditory cortex of the brain is dedicated to particular sound frequencies. This region of the brain, by interpreting the data obtained by the ear, is able to locate where sounds are coming from and where they are coming from, is able to locate where sounds come from and also to identify and classify them..

It is not yet known in depth how this part of the brain is able to perform this activity with such precision, because identifying the continuum of a particular sound, ignoring the rest of the noise that is constantly perceived, is extremely complex. One theory is that the key lies in the spatial localization of the sound source, but when this is constantly varying it is not a problem for the auditory cortex of the brain either, so there must be another explanation.

In turn, the auditory cortex of the brain is able to discern between different tonalities, harmony and note timing. This facet is very well observed in terms of musical interpretation and how we are able to distinguish each sound, coming from a whole range of instruments, and interpret them all together.

We have already seen that the auditory cortex of the brain was divided into three parts (primary, secondary and tertiary) and that it is also neuronally structured by the type of sound frequencies they manage. In addition, zone A1 also has connections with other regions of the nervous system such as the thalamus, and more specifically with the area of the auditory cortex.and more specifically with the area of the medial geniculate nucleus.

It is believed that this part is one of those responsible for the interpretation of the volume of the sound and also of the perceived tones.

Types of dysfunctions in the auditory cortex

There are different pathologies that can be caused by lesions or anomalies in the auditory cortex of the brain.

We have already mentioned cortical deafness, which occurs when the A1 area is damaged and therefore the individual cannot process the sounds that his ears are hearing correctly.

If the lesions, on the other hand, are affecting the secondary or tertiary zone, there are other pathologies that the subject may develop. For example, if the damaged area is in the right hemisphere, this person might have problems in recognizing the pitch of sounds, which is known as amusia.. He or she may have difficulty intoning sentences correctly. In this case, the condition would be called dysprosody.

It could even be affecting other sensory regions, for example those involved in visual memory. If the lesion affects the left hemisphere, there are other possibilities. The best known are aphasias, which have to do with difficulties in understanding or using language. One of them is Wernicke's aphasia, which prevents understanding and repeating the words being heard.

Another common aphasia is the anomic aphasia, whereby the person experiencing it has trouble remembering the name of an item.. There may also be another aphasia known as sensory transcortical aphasia, which also affects language comprehension. The last of the possible aphasias is the acoustic and amnesic conduction aphasia, which would cause problems in repeating a sequence of words.

Likewise, with lesions in the auditory cortex of the brain of the left hemisphere one can also suffer from amnesia for verbal elements, which would also be hindering the comprehension of language.which would also be hindering the person's speech. The amusia that we saw in the other hemisphere can also occur here, also related to auditory agnosia, the inability to process the stimuli received through the ear, in this case.

But it may happen that the lesion or disease has affected the auditory cortex of the brain of both hemispheres of the brain, which would mean a bilateral type of ailment. In this type we can find that auditory agnosia we were talking about and also deafness of the verbal type, that is, being unable to process the words that the ears are hearing.

Bibliographical references:

• Delahay, F., Regulés, S. (2006). The brain and music. Revista de Divulgación de la Ciencia de la UNAM.
• Jara, N., Délano, P.H. (2014). Advances in auditory cortex. Journal of otolaryngology and neck surgery.
• Izquierdo, M.A., Oliver, D.L., Malmierca, M.S. (2009). Mechanisms of plasticity (functional and activity-dependent) in the adult and developing auditory brain. Journal of neurology.
• Terreros, G., Wipe, B., León, A., Délano, P.H. (2013). From the auditory cortex to the cochlea: Progress in the auditory efferent system. Journal of otolaryngology and head and neck surgery.