Auditory cortex, auditory area of ​​brain location and function (2023)

The auditory cortex is located on the lateral surface of the brain's temporal lobe. The primary auditory cortex corresponds roughly to Brodmann areas 41 and 42. It lies in the posterior half of the superior temporal gyrus and also dips into the lateral sulcus like the transverse temporal gyri, also called Heschl's gyri. The primary auditory cortex is the region of the brain responsible for processing auditory information (sound). Although the auditory cortex has several subdivisions, a rough distinction can be made between a primary auditory cortex, a secondary auditory cortex (peripheral area) and a tertiary auditory cortex (waist area) (Figure 1). These structures are formed concentrically around each other, with the primary auditory cortex in the center and the tertiary auditory cortex on the outside. The primary auditory cortex (A1) is located in the superior temporal gyrus called Heschl's gyrus, a ridge in the temporal lobe, on the lower lip of the deep fissure between the temporal and parietal lobes known as the lateral sulcus (Sylvian fissure).¹. The primary auditory cortex (A1) receives point-to-point information from the ventral division of the medial geniculate complex; therefore contains an accurate tonotopic map². The waist regions of the auditory cortex receive more diffuse information from the waist regions of the medial geniculate complex and are therefore less precise in their tonotopic organization.

Information from the peripheral auditory system reaches the central auditory nucleus through the auditory nerve. The auditory nerve transmits auditory information through a series of nuclei to the cortex, where perception takes place. These nuclei include: 1) cochlear nucleus, 2) superior olivary nuclei, 3) lateral lemniscus, 4) inferior colliculus, and 5) medial geniculate nucleus³. The auditory information that travels up the auditory pathways starts in the auditory nerve. These nerves synapse within the cochlear nucleus. Much of the auditory information is then transmitted via crossed fibers to the superior olivary complex. From there, information travels up the contralateral side of the brainstem and cerebrum to the cortex. It should be noted that a significant number of neurons within the auditory system have cross-fibers at each level of the auditory system. This is probably due to the fact that many aspects of auditory processing require ipsilateral and contralateral information. Therefore, all levels of the central auditory system receive and process information from both the ipsilateral and contralateral sides.

The primary auditory cortex (A1) is tonotopically organized and has a topographic map of the cochlea (Figure 1), which means that specific cells in the auditory cortex are sensitive to specific frequencies, as are the primary visual cortex (V1) and the primary somatic cortex. sensory cortex cortex (S1) have topographic maps of their respective sensory epithelia. However, unlike the visual and somatic sensory systems, the cochlea has already decomposed the acoustic stimulus so that it is organized tonotopically along the length of the basilar membrane. Thus, the primary auditory cortex (A1) is said to comprise a tonotopic map, as is the case with most of the ascending auditory structures between the cochlea and the cortex. Orthogonal to the frequency axis of the tonotopic map is a striped matrix of binaural features. Neurons in one lane are excited in both ears and are therefore called EE cells, while neurons in the next lane are excited in one ear and inhibited in the other ear (EI cells). The EE and EI stripes alternate. The types of sensory processing that occur in the other compartments of the auditory cortex are not well understood, but they are likely to be important for higher-order processing of natural sounds, including those used for communication. It seems that some areas specialize in processing combinations of frequencies, while others specialize in processing amplitude or frequency modulations. This is an intriguing feature that has been retained for most of the radio play circle. This area of ​​the brain is believed to “identify the fundamental elements of music, such as pitch and volume. This makes sense, as this is the area that receives direct input from the medial geniculate nucleus of the thalamus.

The secondary auditory cortex was shown to process "harmonic, melodic and rhythmic patterns". The tertiary auditory cortex supposedly integrates everything into the overall music experience.⁴.

Sounds that are particularly important for intraspecific communication usually have a highly ordered temporal structure. In humans, the best example of such time-varying signals is speech, where different phonetic sequences are perceived as different syllables and words. Behavioral studies in cats and monkeys show that the auditory cortex is particularly important for processing temporal tone sequences. When the auditory cortex is ablated in these animals, they lose the ability to distinguish between two complex sounds that have the same frequency components but differ in time. Therefore, without auditory cortex, monkeys cannot distinguish one conspecific communication sound from another. Studies in human patients with bilateral damage to the auditory cortex also reveal severe problems in processing the temporal order of sounds. Therefore, it seems likely that certain regions of the human auditory cortex are specialized for processing elementary speech sounds, as well as other temporally complex acoustic signals such as music. Wernicke's area, crucial for understanding human language, is located in the secondary auditory cortex (Figure 2).

There are other areas of the human cerebral cortex involved in sound processing in the frontal and parietal lobes. Animal experiments indicate that the auditory fields of the cerebral cortex receive information ascending from the auditory thalamus and that they are interconnected in the same and opposite cerebral hemispheres.⁵.

Figure 1. Auditory cortex

Footnote:The human auditory cortex. (A) Diagram showing the brain in left lateral view, including the depths of the lateral sulcus, where part of the auditory cortex that occupies the superior temporal gyrus is normally hidden. The primary auditory cortex (A1) is shown in blue; the areas surrounding the belt of the auditory cortex are red. (B) The primary auditory cortex has a tonotopic organization as shown in this enlarged diagram of an A1 segment.

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Figure 2. Secondary auditory cortex

Footnote:The human auditory cortical areas are related to the processing of speech sounds. (A) Left-sided brain diagram showing locations in the intact hemisphere. (B) An oblique view (dashed line plane in A) shows the cortical areas on the superior surface of the temporal lobe. Note that Wernicke's area, an important region for understanding language, lies just behind the primary auditory cortex.

Figure 3. Listening circles

primary auditory cortex

The primary auditory cortex is the first region of the cerebral cortex to receive auditory information. In humans and other primates, the primary acoustic region in the cerebral cortex is the superior transverse temporal gyrus, also called Heschl's gyrus, a bulge in the temporal lobe at the lower lip of the deep fissure between the temporal and parietal lobes, known as the lateral sulcus ( sylvian fissure) containing the primary auditory cortex. The primary auditory cortex is responsible for translating and processing all sounds and tones and is only minimally affected by the demands of the task. Task Requirement: A test in which the examiner pronounces some words and asks the participant to categorize them acoustically, phonemically or semantically⁶. In addition to Heschl's gyrus, the superior temporal plane has another important area called Wernicke's area. In the past, this domain was thought to play a significant role in language perception and comprehension, but recent evidence shows that this domain is not involved in this process. The researchers found that this process is not a simple task, but also a complex task distributed throughout the brain. The main function of this domain is phonological representation, a process of interpreting the spoken word based on its tones and sounds and trying to associate it with a previously learned sound.⁷.

The lateral surface of the superior temporal gyrus is believed to be the secondary auditory cortex, which also functions in interpreting sounds, but mainly in activities involving task demands.⁶.

Because about half of the fibers in the auditory pathways cross the midline while others ascend the same side of the brain, each ear is represented in both the right and left primary auditory cortex. Because of this, binaural hearing may be minimally affected even if the auditory cortical area on one side is injured by trauma or stroke.

The perception of sounds is associated with the posterior superior temporal gyrus. The superior temporal gyrus contains several important brain structures, including Brodmann 41 and 42, which mark the location of the primary auditory cortex, the cortical region responsible for perceiving the basic properties of sound, such as pitch and rhythm.

The auditory association area is located within the temporal lobe of the brain, in an area called Wernicke's area or area 22. This area near the lateral sulcus is an important region for processing acoustic signals so that they can be distinguished, such as speech, music or noise.

As is common with thalamocortical connections, nuclei within the medial geniculate body that send fibers to the auditory cortex also receive fibers from the same area of ​​cortex. Hearing impairment due to bilateral cortical damage involving both auditory areas has been reported but is extremely rare. However, bilateral lesions of the temporal lubricant have been shown to have far-reaching effects (cortical deafness, in which multiple behaviors are affected, including speech discrimination, localization of sounds, and recognition of weak and short-lived signals).

secondary auditory cortex

The lateral surface of the superior temporal gyrus is believed to be the secondary auditory cortex, which also functions in interpreting sounds, but mainly in activities involving task demands.⁶.

auditory cortex function

Classically, two main functional regions have been described in the auditory cortex:

1. Primary auditory cortex (AI), composed of neurons involved in decoding the cochleotopic and tonotopic spatial representation of a stimulus.
2. Secondary Auditory Cortex (AII), which does not have a clear tonotopic organization, but plays an important role in sound localization and analysis of complex sounds: particularly for certain animal vocalizations and human language. It also plays a role in auditory memory.
3. The belt region, around AI and AII that help integrate hearing with other sensory systems.

When awake, humans, like other animals, are capable of perceiving the small temporal variations of complex sounds. These variations are essential for understanding human language. Several studies that have examined AI have found that, in awake primates, two distinct populations of synchronous and asynchronous neurons (respectively) encode sequential stimuli differently.

Synchronous neurons analyze slow temporal changes. They respond precisely to low-frequency stimulation (A1), but fail to maintain activity when the number of stimuli increases. Rapid changes in frequency are perceived by these neurons as a continuous tone. They are involved in frequency and intensity analysis.
Asynchronous neurons analyze rapid changes in time (of many stimuli). They can detect short-term fluctuations and accurately distinguish one stimulus from the next.

The functional division of the auditory cortex allows decoding temporal variations of a stimulus with great precision in relation to other centers of the auditory pathway. It allows you to get more information about complex sounds, as well as the location of a sound source and its movement.

Synchronous and asynchronous neurons

• Synchronous neurons always react to each stimulus (click) if the stimulus sequences are more than 20 ms apart (A1). As the intertraining interval decreases (that is, the repetition rate increases), these neurons begin to desynchronize their firing rate. When the interstimulus interval drops below 10 ms (B1), these neurons fire only at the beginning and end of the stimulus (initial and final responses, respectively).
• Asynchronous neurons do not respond synchronously to stimuli (A2 and B2), but their activity progressively increases up to a very high firing rate (B2).

types of processing

Various aspects of ambient noise (e.g. attenuation: how loud the sound is, location in space, frequency and combined sensitivity) are processed in each of the core listening areas. Most auditory nuclei in the brain are arranged tonotopically. In this way, the auditory signals that ascend to the cortex can preserve the frequency information from the environment.³.

Attenuation (the intensity of a sound) is processed in the auditory system by neurons, which fire action potentials at different rates depending on the intensity of the sound. Most neurons respond by increasing their firing rate in response to increased attenuation. More specialized neurons respond maximally to ambient noise within certain intensity ranges³.

The brain processes the position of a sound in space by comparing differences in attenuation and timing of inputs from both ears within the superior olivary complex. If a sound is right in the midline (that is, in front of or behind the head), it will reach both ears at the same time. If it is to the right or left of the midline, there will be a time gap between inputs to the two ears. Within the superior olivary complex, specialized neurons receive information from both ears and can encode this temporal delay (ie, binaural processing).³.

Combination-sensitive neurons are another subset of neurons within the auditory system that exhibit specifically enhanced or inhibited responses to 2 or more sounds with a specific time delay. Combined sensory neurons are located in the inferior colliculus, lateral lemniscus, medial geniculate, and auditory cortex⁸. Since most environmental sounds are not pure tones, these types of combination-sensitive neurons are believed to facilitate improved processing of combinations of sounds that may be important to the individual (eg, speech, communication sounds).⁹.

down loops

In the past, auditory processing was thought to be a simple relay of environmental signals to the cortex. Scientists now know that there is an important descending system of circuitry within the auditory system that helps to modulate auditory processing at all levels. The auditory cortex has bilateral direct projections back to the inferior colliculus, superior olivary complex, and cochlear nucleus¹⁰. These circuits contact neurons in these nuclei, which project to all levels of the central auditory system and to the cochlea (to modulate outer hair cells) within the peripheral auditory system. Connections between descending, ascending and crossing fibers make the auditory system highly interconnected. These descending loops help modulate auditory attention based on a person's relevance, alertness, learned behaviors, and emotional state. These higher-order functions derive from many brain regions (e.g., prefrontal cortex, hippocampus, Meynert's nucleus basalis, and limbic circuits) that have direct or indirect connections with each other and with the auditory cortex.¹¹.

Function of the primary auditory cortex

In AI, neurons are selective for specific frequencies and organized into isofrequency bands that are tonotopically organized. The exact spatial distribution of isofrequency bands is related to the organization of auditory receptors. Its activity depends on the properties of the stimulus: frequency, intensity and position of the sound source in space. Functionally, this region is strongly influenced by the subject's waking state. Several very specific AI neurons are also involved in analyzing complex sounds.

New techniques for studying the cerebral cortex (functional magnetic resonance imaging: fMRI; positron emission tomography: PET; and magnetoencephalography: MEG) suggest that the frequency distribution observed in animals (using traditional experimental methods) does not exactly match that of humans, although all have isofrequency bands as seen using magnetoencephalography (MEG) below. Human fMRI suggests that low frequencies are encoded in the superficial posterolateral regions of the Sylvian fissure, while high frequencies are located in the deeper, anteromedial regions. However, it is important to note that there is some degree of variation between individuals.

Function of the secondary auditory cortex

The secondary auditory cortex also acts in the interpretation of sounds, but mainly in activities that involve task demands.⁶.

references

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