The auditory cortex is the part of the
temporal lobe
The temporal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain.
The temporal lobe is involved in proc ...
that processes auditory information in humans and many other
vertebrates. It is a part of the
auditory system
The auditory system is the sensory system for the sense of hearing. It includes both the sensory organs (the ears) and the auditory parts of the sensory system.
System overview
The outer ear funnels sound vibrations to the eardrum, increasin ...
, performing basic and higher functions in
hearing
Hearing, or auditory perception, is the ability to perceive sounds through an organ, such as an ear, by detecting vibrations as periodic changes in the pressure of a surrounding medium. The academic field concerned with hearing is audit ...
, such as possible relations to
language switching
In linguistics, code-switching or language alternation occurs when a speaker alternates between two or more languages, or language varieties, in the context of a single conversation or situation. Code-switching is different from plurilingualism ...
.
[Cf. Pickles, James O. (2012). ''An Introduction to the Physiology of Hearing'' (4th ed.). Bingley, UK: Emerald Group Publishing Limited, p. 238.] It is located bilaterally, roughly at the upper sides of the
temporal lobe
The temporal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain.
The temporal lobe is involved in proc ...
s – in humans, curving down and onto the medial surface, on the superior temporal plane, within the
lateral sulcus and comprising parts of the
transverse temporal gyri, and the
superior temporal gyrus, including the planum polare and
planum temporale (roughly
Brodmann areas 41 and 42, and partially
22).
The auditory cortex takes part in the spectrotemporal, meaning involving time and frequency, analysis of the inputs passed on from the ear. The cortex then filters and passes on the information to the dual stream of speech processing. The auditory cortex's function may help explain why particular brain damage leads to particular outcomes. For example, unilateral destruction, in a region of the auditory pathway above the
cochlear nucleus
The cochlear nuclear (CN) complex comprises two cranial nerve nuclei in the human brainstem, the ventral cochlear nucleus (VCN) and the dorsal cochlear nucleus (DCN).
The ventral cochlear nucleus is unlayered whereas the dorsal cochlear nucle ...
, results in slight hearing loss, whereas bilateral destruction results in
cortical deafness.
Structure
The auditory cortex was previously subdivided into primary (A1) and secondary (A2) projection areas and further association areas. The modern divisions of the auditory cortex are the core (which includes primary auditory cortex, A1), the belt (secondary auditory cortex, A2), and the parabelt (tertiary auditory cortex, A3). The belt is the area immediately surrounding the core; the parabelt is adjacent to the lateral side of the belt.
[Cf. Pickles, James O. (2012). ''An Introduction to the Physiology of Hearing'' (4th ed.). Bingley, UK: Emerald Group Publishing Limited, p. 211 f.]
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. Within the core (A1), its structure preserves
tonotopy, the orderly representation of frequency, due to its ability to map low to high frequencies corresponding to the apex and base, respectively, of the
cochlea
The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory org ...
.
Data about the auditory cortex has been obtained through studies in rodents, cats, macaques, and other animals. In humans, the structure and function of the auditory cortex has been studied using
functional magnetic resonance imaging (fMRI),
electroencephalography
Electroencephalography (EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocor ...
(EEG), and
electrocorticography.
Development
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. Sexual dimorphism within the auditory cortex can be seen in humans between males in females through the planum temporale, encompassing Wernicke's region, for the planum temporale within males has been observed to have a larger planum temporale volume on average, reflecting previous studies discussing interactions between sex hormones and asymmetrical brain development.
Function
As with other primary sensory cortical areas, auditory sensations reach
perception
Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous system, ...
only if received and processed by a
cortical area. Evidence for this comes from
lesion
A lesion is any damage or abnormal change in the tissue of an organism, usually caused by disease or trauma. ''Lesion'' is derived from the Latin "injury". Lesions may occur in plants as well as animals.
Types
There is no designated classif ...
studies in human patients who have sustained damage to cortical areas through
tumor
A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...
s or
strokes, or from animal experiments in which cortical areas were deactivated by surgical lesions or other methods. Damage to the auditory cortex in humans leads to a loss of any
awareness
Awareness is the state of being conscious of something. More specifically, it is the ability to directly know and perceive, to feel, or to be cognizant of events. Another definition describes it as a state wherein a subject is aware of some info ...
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
The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal ( alertness), and temperature regulation. The name comes from the Greek ''mesos'', " ...
.
Neurons in the auditory cortex are organized according to the frequency of sound to which they respond best.
Neuron
A neuron, neurone, or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous ...
s 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
The visual cortex of the brain is the area of the cerebral cortex that processes visual information. It is located in the occipital lobe. Sensory input originating from the eyes travels through the lateral geniculate nucleus in the thalamus and ...
), 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) likely reflects the fact that the
cochlea
The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory org ...
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. For example, it has been shown that A1 encodes complex and abstract aspects of auditory stimuli without encoding their "raw" aspects like frequency content, presence of a distinct sound or its echoes.
Human
brain scans indicated that a peripheral part 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
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from '' angular frequency''. Frequency is measured in hertz (Hz) which is ...
.
The auditory cortex plays an important yet ambiguous role in hearing. When the auditory information passes into the cortex, the specifics of what exactly takes place are unclear. There is a large degree of individual variation in the auditory cortex, as noted by English biologist
James Beament, who wrote, "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 transduced 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 different media, which makes this thinking unlikely. The auditory cortex forms groupings based on fundamentals; in music, for example, this would include
harmony
In music, harmony is the process by which individual sounds are joined together or composed into whole units or compositions. Often, the term harmony refers to simultaneously occurring frequencies, pitches ( tones, notes), or chords. Howev ...
,
timing
Timing is the tracking or planning of the spacing of events in time. It may refer to:
* Timekeeping, the process of measuring the passage of time
* Synchronization, controlling the timing of a process relative to another process
* Time metrolog ...
, and
pitch.
The primary auditory cortex lies in the
superior temporal gyrus of the temporal lobe and extends into the
lateral sulcus and the
transverse temporal gyri (also called ''Heschl's gyri''). Final sound processing is then performed by the
parietal and
frontal lobes of the human
cerebral cortex
The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consisting o ...
. 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 hemisphere
The vertebrate cerebrum (brain) is formed by two cerebral hemispheres that are separated by a groove, the longitudinal fissure. The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres ...
s.
The auditory cortex is composed of fields that differ from each other in both structure and function. The number of fields varies in different species, from as few as 2 in
rodent
Rodents (from Latin , 'to gnaw') are mammals of the Order (biology), order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are roden ...
s 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
Functional imaging (or physiological imaging) is a medical imaging technique of detecting or measuring changes in metabolism, blood flow, regional chemical composition, and absorption.
As opposed to structural imaging, functional imaging centers ...
studies of the brain in humans.
When each instrument of a
symphony orchestra
An orchestra (; ) is a large instrumental ensemble typical of classical music, which combines instruments from different families.
There are typically four main sections of instruments:
* bowed string instruments, such as the violin, viola, ...
or
jazz band
A jazz band (jazz ensemble or jazz combo) is a musical ensemble that plays jazz music. Jazz bands vary in the quantity of its members and the style of jazz that they play but it is common to find a jazz band made up of a rhythm section and a ...
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
Anterolateral may refer to:
* Anterolateral central arteries
* Anterolateral ligament
* Anterolateral ganglionic branches
* Anterolateral sulcus of medulla
* Anterolateral sulcus of spinal cord
* Anterolateral system
The spinothalamic tract ...
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
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the '' carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informat ...
by numerous
neurotransmitter
A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell.
Neur ...
s, including
norepinephrine
Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic chemical in the catecholamine family that functions in the brain and body as both a hormone and neurotransmitter. The name "noradrenaline" (from Latin '' ad ...
, which has been shown to decrease
cellular excitability in all layers of the
temporal cortex
The temporal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain.
The temporal lobe is involved in p ...
.
alpha-1 adrenergic receptor
alpha-1 (α1) adrenergic receptors are G protein-coupled receptors (GPCRs) associated with the Gq heterotrimeric G protein. α1-adrenergic receptors are subdivided into three highly homologous subtypes, i.e., α1A-, α1B-, and α1D-adrenerg ...
activation, by norepinephrine, decreases
glutamatergic excitatory postsynaptic potential
In neuroscience, an excitatory postsynaptic potential (EPSP) is a postsynaptic potential that makes the postsynaptic neuron more likely to fire an action potential. This temporary depolarization of postsynaptic membrane potential, caused by the ...
s at
AMPA receptor
The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (also known as AMPA receptor, AMPAR, or quisqualate receptor) is an ionotropic transmembrane receptor for glutamate (iGluR) that mediates fast synaptic transmission in the ce ...
s.
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. Tonotopic mapping is preserved throughout most of the audition circuit. The primary auditory cortex receives direct input from the
medial geniculate nucleus
The medial geniculate nucleus (MGN) or medial geniculate body (MGB) is part of the auditory thalamus and represents the thalamic relay between the inferior colliculus (IC) and the auditory cortex (AC). It is made up of a number of sub-nuclei tha ...
of the
thalamus
The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all direction ...
and thus is thought to identify the fundamental elements of music, such as
pitch and
loudness.
An
evoked response
An evoked potential or evoked response is an electrical potential in a specific pattern recorded from a specific part of the nervous system, especially the brain, of a human or other animals following presentation of a stimulus such as a light fla ...
study of congenitally deaf kittens used
local field potential
Local field potentials (LFP) are transient electrical signals generated in nervous and other tissues by the summed and synchronous electrical activity of the individual cells (e.g. neurons) in that tissue. LFP are "extracellular" signals, meaning ...
s 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 finding accords with a study by Eckart Altenmuller, in which it was observed that students who received musical instruction had greater cortical activation than those who did not.
The auditory cortex has distinct responses to sounds in the
gamma band. When subjects are exposed to three or four cycles of a 40
hertz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one her ...
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 gamma frequencies are
resonant frequencies of certain areas of the brain and appear 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. In a 2000 study by Kneif and colleagues, subjects were presented 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
Magnetoencephalography (MEG) is a functional neuroimaging technique for mapping brain activity by recording magnetic fields produced by electrical currents occurring naturally in the brain, using very sensitive magnetometers. Arrays of SQUIDs ( ...
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
omitted stimulus response (OSR) 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 OSR 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
Tonality is the arrangement of pitches and/or chords of a musical work in a hierarchy of perceived relations, stabilities, attractions and directionality. In this hierarchy, the single pitch or triadic chord with the greatest stability is cal ...
(high spectral resolution), while the left auditory cortex has been shown to be more sensitive to minute sequential differences (rapid temporal changes) 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
In mammalian brain anatomy, the prefrontal cortex (PFC) covers the front part of the frontal lobe of the cerebral cortex. The PFC contains the Brodmann areas BA8, BA9, BA10, BA11, BA12, BA13, BA14, BA24, BA25, BA32, BA44, BA45, BA ...
(RMPFC). A study explored the areas of the brain which were active during tonality processing, using
fMRI
Functional magnetic resonance imaging or functional MRI (fMRI) measures brain activity by detecting changes associated with blood flow. This technique relies on the fact that cerebral blood flow and neuronal activation are coupled. When an area o ...
. The results of this experiment showed preferential
blood-oxygen-level-dependent
Blood-oxygen-level-dependent imaging, or BOLD-contrast imaging, is a method used in functional magnetic resonance imaging (fMRI) to observe different areas of the brain or other organs, which are found to be active at any given time.
Theory
Ne ...
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
The amygdala (; plural: amygdalae or amygdalas; also '; Latin from Greek, , ', 'almond', 'tonsil') is one of two almond-shaped clusters of nuclei located deep and medially within the temporal lobes of the brain's cerebrum in complex v ...
, and is thought to aid in the inhibition of negative
emotion
Emotions are mental states brought on by neurophysiological changes, variously associated with thoughts, feelings, behavioral responses, and a degree of pleasure or displeasure. There is currently no scientific consensus on a definition. ...
.
Another study has suggested that people who experience 'chills' while listening to music have a higher volume of fibres connecting their auditory cortex to areas associated with emotional processing.
In a study involving
dichotic listening
Dichotic listening is a psychological test commonly used to investigate selective attention and the lateralization of brain function within the auditory system. It is used within the fields of cognitive psychology and neuroscience.
In a standard ...
to speech, in which one message is presented to the right ear and another to the left, it was found that the participants chose letters with stops (e.g. 'p', 't', 'k', 'b') far more often when presented to the right ear than the left. However, when presented with phonemic sounds of longer duration, such as vowels, the participants did not favor any particular ear.
Due to the contralateral nature of the auditory system, the right ear is connected to Wernicke's area, located within the posterior section of the superior temporal gyrus in the left cerebral hemisphere.
Sounds entering the auditory cortex are treated differently depending on whether or not they register as speech. When people listen to speech, according to the strong and weak
speech mode hypotheses, they, respectively, engage perceptual mechanisms unique to speech or engage their knowledge of language as a whole.
See also
*
Auditory system
The auditory system is the sensory system for the sense of hearing. It includes both the sensory organs (the ears) and the auditory parts of the sensory system.
System overview
The outer ear funnels sound vibrations to the eardrum, increasin ...
*
Neuronal encoding of sound
*
Noise health effects
References
External links
* : area 41
* : area 42
{{Auditory and vestibular pathways
Auditory system
Cerebral cortex
Audiology