|Brain: Primary auditory cortex|
The Primary Auditory Cortex is highlighted in magenta, and has been known to interact with all areas highlighted on this neural map
|Latin||Cortex auditivus primus|
The auditory cortex is the part of the cerebral cortex that processes auditory information in humans and other vertebrates. A part of the auditory system, it performs basic and higher functions in hearing.
It is located bilaterally, roughly at the upper sides of the temporal lobes – in humans on the superior temporal plane, within the lateral fissure and comprising parts of Heschl's gyrus and the superior temporal gyrus, including planum polare and planum temporale (roughly Brodmann areas 41, 42, and partially 22).
Previously subdivided into a primary (AI), secondary (AII) and further association areas, it is more recently considered to consist of a number of areas, including AI, that form the so-called core, a number of surrounding areas, the belt, and a number of additional neighbouring areas, the parabelt.
Besides receiving input from the ears via lower parts of the auditory system, it also transmits signals back to these areas and is interconnected with other parts of the cerebral cortex.
Crucial data about the auditory cortex has been obtained through studies in cats, macaques, and other animals. Thus, some knowledge about the human auditory cortex remains speculative.
As with other primary sensory cortical areas, auditory sensations reach perception only if received and processed by a cortical area. Evidence for this comes from lesion studies in human patients who have sustained damage to cortical areas through tumors or strokes, or from animal experiments in which cortical areas were deactivated by surgical lesions or other methods. Damage to the Primary Auditory Cortex in humans leads to a loss of any awareness of sound, but an ability to react reflexively to sounds remains as there is a great deal of subcortical processing in the auditory brainstem and midbrain.
Neurons in the auditory cortex are organized according to the frequency of sound to which they respond best. Neurons at one end of the auditory cortex respond best to low frequencies; neurons at the other respond best to high frequencies. There are multiple auditory areas (much like the multiple areas in the visual cortex), which can be distinguished anatomically and on the basis that they contain a complete "frequency map." The purpose of this frequency map (known as a tonotopic map) is unknown, and is likely to reflect the fact that the cochlea is arranged according to sound frequency. The auditory cortex is involved in tasks such as identifying and segregating auditory "objects" and identifying the location of a sound in space.
Human brain scans have indicated that a peripheral bit of this brain region is active when trying to identify musical pitch. Individual cells consistently get excited by sounds at specific frequencies, or multiples of that frequency.
The auditory cortex is an important yet ambiguous part of the hearing process. When the sound pulses pass into the cortex, the specifics of what exactly takes place are unclear. Distinguished scientist and musician James Beament puts it into perspective when he writes, “The cortex is so complex that the most we may ever hope for is to understand it in principle, since the evidence we already have suggests that no two cortices work in precisely the same way."
In the hearing process, multiple sounds are absorbed simultaneously. The role of the auditory system is to decide which components form the sound link. Many have surmised that this linkage is based on the location of sounds. However, there are numerous distortions of sound when reflected off of different mediums, which makes this thinking unlikely. Instead, the auditory cortex forms groupings based on more reliable fundamentals; in music, for example, this would include harmony, timing, and pitch.
The primary auditory cortex lies in the temporal lobe. More specifically, it is located in the posterior half of the superior temporal gyrus. It also dives into the lateral sulcus as the transverse temporal gyri (also called Heschl's gyri). There are additional areas of the human cerebral cortex that are involved in processing sound, in the frontal and parietal lobes. Animal studies indicate that auditory fields of the cerebral cortex receive ascending input from the auditory thalamus, and that they are interconnected on the same and on the opposite cerebral hemispheres.
The auditory cortex is composed of fields, which differ from each other in both structure and function. The number of fields varies in different species, from as few as 2 in rodents to as many as 15 in the rhesus monkey. The number, location, and organization of fields in the human auditory cortex are not known at this time. What is known about the human auditory cortex comes from a base of knowledge gained from studies in mammals, including primates, used to interpret electrophysiological tests and functional imaging studies of the brain in humans.
When each instrument of a symphony orchestra or the jazz band plays the same note, the quality of each sound is different — but the musician perceives each note as having the same pitch. The neurons of the auditory cortex of the brain are able to respond to pitch. Studies in the marmoset monkey have shown that pitch-selective neurons are located in a cortical region near the anterolateral border of the primary auditory cortex. This location of a pitch-selective area has also been identified in recent functional imaging studies in humans.
The primary auditory cortex is subject to modulation by numerous neurotransmitters, including norepinephrine, which has been shown to decrease cellular excitability in all layers of the temporal cortex. Norepinephrine decreases glutamatergic excitatory postsynaptic potentials at AMPA receptors by the activation of alpha-1 adrenergic receptors.
Relationship to the auditory system
The auditory cortex is the most highly organized processing unit of sound in the brain. This cortex area is the neural crux of hearing, and—in humans—language and music. The auditory cortex is divided into three separate parts: the primary, secondary, and tertiary auditory cortex. These structures are formed concentrically around one another, with the primary cortex in the middle and the tertiary cortex on the outside.
The primary auditory cortex is tonotopically organized, which means that neighboring cells in the cortex respond to neighboring frequencies. This is a fascinating function which has been preserved throughout most of the audition circuit. This area of the brain is thought to identify the fundamental elements of music, such as pitch and loudness. This makes sense, as this is the area which receives direct input from the medial geniculate nucleus of the thalamus.
An evoked response study of congenitally deaf kittens by Klinke et al. utilized local field potentials to measure cortical plasticity in the auditory cortex. These kittens were stimulated and measured against a control (an un-stimulated congenitally deaf cat (CDC)) and normal hearing cats. The field potentials measured for artificially stimulated CDC were eventually much stronger than that of a normal hearing cat. This is in concordance with Eckart Altenmuller’s study where it was observed that students who received musical instruction had greater cortical activation than those who did not.
The auditory cortex exhibits some strange behavior pertaining to the Gamma wave frequency. When subjects are exposed to three or four cycles of a 40 hertz click, an abnormal spike appears in the EEG data, which is not present for other stimuli. The spike in neuronal activity correlating to this frequency is not restrained to the tonotopic organization of the auditory cortex. It has been theorized that this is a resonant frequency of certain areas of the brain, and appears to affect the visual cortex as well. Gamma band activation (25 to 100 Hz) has been shown to be present during the perception of sensory events and the process of recognition. Kneif et al., in their 2000 study, presented subjects with eight musical notes to well known tunes, such as Yankee Doodle and Frère Jacques. Randomly, the sixth and seventh notes were omitted and an electroencephalogram, as well as a magnetoencephalogram were each employed to measure the neural results. Specifically, the presence of gamma waves, induced by the auditory task at hand, were measured from the temples of the subjects. The OSP response, or omitted stimulus response, was located in a slightly different position; 7 mm more anterior, 13 mm more medial and 13 mm more superior in respect to the complete sets. The OSP recordings were also characteristically lower in gamma waves, as compared to the complete musical set. The evoked responses during the sixth and seventh omitted notes are assumed to be imagined, and were characteristically different, especially in the right hemisphere. The right auditory cortex has long been shown to be more sensitive to tonality, while the left auditory cortex has been shown to be more sensitive to minute sequential differences in sound, such as in speech.
Tonality is represented in more places than just the auditory cortex; one other specific area is the rostromedial prefrontal cortex (RMPFC). Janata et al., in their 2002 study, explored the areas of the brain which were active during tonality processing, by means of the fMRI technique. The results of this experiment showed preferential blood-oxygen-level dependent activation of specific voxels in RMPFC for specific tonal arrangements. Though these collections of voxels do not represent the same tonal arrangements between subjects or within subjects over multiple trials, it is interesting and informative that RMPFC, an area not usually associated with audition, seems to code for immediate tonal arrangements in this respect. RMPFC is a subsection of the medial prefrontal cortex, which projects to many diverse areas including the amygdala, and is thought to aid in the inhibition of negative emotion.
Like many areas in the neocortex, the functional properties of the adult primary auditory cortex (A1) are highly dependent on the sounds encountered early in life. This has been best studied using animal models, especially cats and rats. In the rat, exposure to a single frequency during postnatal day (P) 11 to 13 can cause a 2-fold expansion in the representation of that frequency in A1. Importantly, the change is persistent, in that it lasts throughout the animal's life, and specific, in that the same exposure outside of that period causes no lasting change in the tonotopy of A1.
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