The
primary auditory cortex is the region of the
brainThe brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all...
that is responsible for processing of
auditory- Outer ear :The folds of cartilage surrounding the ear canal are called the pinna. Sound waves are reflected and attenuated when they hit the pinna, and these changes provide additional information that will help the brain determine the direction from which the sounds came.The sound waves enter...
(
soundSound is a travelling wave which is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.- Perception of sound...
) information. It is located on the temporal lobe, and performs the basics of hearing; pitch and volume.
Function
As with other primary sensory cortical areas, auditory sensations reach
perceptionIn philosophy, psychology, and cognitive science, perception is the process of attaining awareness or understanding of sensory information. The word "perception" comes from the Latin words perceptio, percipio, and means "receiving, collecting, action of taking possession, apprehension with the mind...
only if received and processed by a
corticalThe cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
area. Evidence for this comes from lesion studies in human patients who have sustained damage to cortical areas through
tumorA tumor or tumour is the name for a neoplasm or a solid lesion formed by an abnormal growth of cells which looks like a swelling. Tumor is not synonymous with cancer...
s or
strokeA stroke is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage or due to a hemorrhage...
s, or from animal experiments in which cortical areas were deactivated by cooling or locally applied drug treatment. Damage to the Primary Auditory Cortex in humans leads to a loss of any '
awarenessAwareness is the state or ability to perceive, to feel, or to be conscious of events, objects or sensory patterns. In this level of consciousness, sense data can be confirmed by an observer without necessarily implying understanding. More broadly, it is the state or quality of being aware of...
' 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.
NeuronA neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks...
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 cortexThe term visual cortex refers to the primary visual cortex and extrastriate visual cortical areas such as V2, V3, V4, and V5. The primary visual cortex is anatomically equivalent to Brodmann area 17, or BA17. The extrastriate cortical areas consist of Brodmann area 18 and Brodmann area 19...
), 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 mapTonotopy refers to the spatial arrangement of where sounds of different frequency are processed in the brain. Tones close to each other in terms of frequency are represented in topologically neighbouring regions in the brain...
) is unknown and is likely to reflect the fact that the
cochleaThe cochlea is the auditory portion of the inner ear. Its core component is the Organ of Corti, the sensory organ of hearing, which is distributed along the partition separating fluid chambers in the coiled tapered tube of 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
frequencyFrequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the 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 hearing process, multiple sounds are being absorbed simultaneously. The role of the auditory system is to decide which components from the sound link. Many have surmised that this linking is based on location of sounds; however, there are numerous distortions of sound when reflected off different mediums, which makes this thinking unlikely. Instead, the auditory cortex forms groupings based on other more of the reliable, fundamentals. In music for example, this would include
harmonyIn music, harmony is the use of simultaneous pitches , or chords. The study of harmony involves chords and their construction and chord progressions and the principles of connection that govern them. Harmony is often said to refer to the "vertical" aspect of music, as distinguished from melodic...
,
timingTiming is the spacing of events in time. Some typical uses are:* The act of measuring the elapsed time of something or someone, often at athletic events such as swimming or running, where participants are timed with a device such as a stopwatch....
, and
pitchPitch is a resin, a viscous substance produced by plants or formed from petroleum.Pitch may also refer to:-Music and sound:* pitch , the perceived frequency of sound including "definite pitch" and "indefinite pitch"...
.
The primary auditory cortex is about the same as
Brodmann areaA Brodmann area is a region of the cortex defined based on its cytoarchitecture, or organization of cells.Brodmann areas were originally defined and numbered by the German neurologist Korbinian Brodmann based on the cortical cytoarchitectonic organisation of neurons he observed in the cerebral...
s 41 and 42. It lies in the posterior half of the
superior temporal gyrusThe superior temporal gyrus is one of three gyri in the temporal lobe of the human brain, which is located laterally to the head, situated somewhat above the external ear.The superior temporal gyrus is bounded by:* the lateral sulcus above;...
and also dives into the
lateral sulcusThe lateral sulcus is one of the most prominent structures of the human brain.-Anatomy:...
as the transverse temporal gyri (also called
Heschl's gyri).
The primary auditory cortex is located in the
temporal lobeThe temporal lobe is a region of the cerebral cortex that is located beneath the Sylvian fissure on both cerebral hemispheres of the mammalian brain....
. There are additional areas of the human
cerebral cortexThe cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
that are involved in processing sound, in the
frontalThe frontal lobe is an area in the brain of humans and other mammals, located at the front of each cerebral hemisphere and positioned anterior to the parietal lobes and above and anterior to the temporal lobes...
and
parietal lobeThe parietal lobe is a lobe in the brain. It is positioned above the occipital lobe and behind the frontal lobe....
s.
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 hemisphereA cerebral hemisphere is one of the two regions of the eutherian brain that are delineated by the median plane, . The brain can thus be described as being divided into left and right cerebral hemispheres...
s.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
rodentRodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....
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
mammalMammals are a class of vertebrate, air-breathing animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair and/or fur, three middle ear bones used in hearing, and a neocortex region in the brain.Mammals are...
s, including primates, used to interpret electrophysiologic tests and
functional imagingFunctional imaging , is a method of detecting or measuring changes in metabolism, blood flow, regional chemical composition, and absorption....
studies of the brain in humans.
When each instrument of the symphony orchestra or the
jazz bandA jazz band is a musical ensemble that plays jazz music usually without a conductor. Jazz bands usually consist of a rhythm section and a horn section...
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 auditory cortex does not just receive input from lower centers and the ear, but also provides it.
Brodmann area 41
This area is also known as
anterior transverse temporal area 41 (H). It is a subdivision of the
cytoarchitecturallyCytoarchitecture or Cytoarchitectonics is the study of the cellular composition of the body's tissues under the microscope...
-defined
temporalThe temporal lobe is a region of the cerebral cortex that is located beneath the Sylvian fissure on both cerebral hemispheres of the mammalian brain....
region of
cerebral cortexThe cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
, occupying the anterior transverse temporal gyrus (H) in the bank of the
lateral sulcusThe lateral sulcus is one of the most prominent structures of the human brain.-Anatomy:...
on the dorsal surface of the
temporal lobeThe temporal lobe is a region of the cerebral cortex that is located beneath the Sylvian fissure on both cerebral hemispheres of the mammalian brain....
. Brodmann area 41 is bounded medially by the parainsular area 52 (H) and laterally by the posterior transverse temporal area 42 (H) (Brodmann-1909).
Brodmann area 42
This area is also known as
posterior transverse temporal area 42 (H). It is a subdivision of the cytoarchitecturally-defined temporal region of cerebral cortex, located in the bank of the lateral sulcus on the dorsal surface of the temporal lobe. Brodmann area 42 is bounded medially by the anterior transverse temporal area 41 (H) and laterally by the superior temporal area 22 (Brodmann-1909).
Relationship to 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 AC in the middle and the tertiary AC on the outside.
The primary auditory cortex is tonotopically organized, which means that certain cells in the auditory cortex are sensitive to specific 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
pitchPitch represents the perceived fundamental frequency of a sound. It is one of the four major auditory attributes of sounds along with loudness, timbre and sound source location. When the actual fundamental frequency can be precisely determined through physical measurement, it may differ from the...
and loudness. This makes sense as this is the area which receives direct input from the
medial geniculate nucleusThe Medial Geniculate Nucleus or Medial Geniculate Body is part of the auditory thalamus and represents the thalamic relay between the inferior colliculus and the auditory cortex...
of the
thalamusThe thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...
. The secondary auditory cortex has been indicated in the processing of “harmonic, melodic and rhythmic patterns.” The tertiary auditory cortex supposedly integrates everything into the overall experience of music.
An evoked response study of congenitally deaf kittens by Klinke et al. utilized field potentials to measure cortical plasticity in the auditory cortex. These kittens were stimulated and measured against a control or un-stimulated congenitally deaf cat (CDC) and normal hearing cats. The field
potential*The mathematical study of potentials is known as potential theory; it is the study of harmonic functions on manifolds. This mathematical formulation arises from the fact that, in physics, the scalar potential is irrotational, and thus has a vanishing Laplacian — the very definition of a harmonic...
s measured for artificially stimulated CDC was 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 waveA gamma wave is a pattern of brain waves in humanswith a frequency between 25 to 100 Hz, though 40 Hz is prototypical.According to a popular 20-year-old theory, gamma waves may be implicated in creating the unity of conscious perception...
frequency. When subjects are exposed to three or four cycles of a 40
hertzThe hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of sine wave, particularly those used in radio and audio applications....
click, an abnormal
spikŠpik is a mountain in Slovenian Julian Alps. Its summit is at 2,472 m above sea level. The mountain has been named after its pointed shape....
e appears in the
EEGElectroencephalography is the recording of electrical activity along the scalp produced by the firing of neurons within the brain. In clinical contexts, EEG refers to the recording of the brain's spontaneous electrical activity over a short period of time, usually 20–40 minutes, as recorded from...
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 (20 to 40 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"Yankee Doodle" is a well-known Anglo-American song, the origin of which dates back to the Seven Years' War. It is often sung patriotically in the United States today and is the state anthem of Connecticut...
and
Frere Jacques"Frère Jacques" is a French nursery melody. The song is traditionally sung in a round. When the first person gets to the star the next person starts at the beginning...
. 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 hemisphereA cerebral hemisphere is one of the two regions of the eutherian brain that are delineated by the median plane, . The brain can thus be described as being divided into left and right cerebral hemispheres...
. The right auditory cortex has long been shown to be more sensitive to
tonalityTonality is a system of music in which specific hierarchical pitch relationships are based on a key "center", or tonic. The term tonalité originated with Alexandre-Étienne Choron and was borrowed by François-Joseph Fétis in 1840...
, while the left auditory cortex has been shown to be more sensitive to minute sequential differences in sound specifically speech.
Hallucinations have been shown to produce
oscillationOscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power...
s which are parallel (although not exactly the same as) the gamma frequency range. Sperling showed in his 2004 study that auditory hallucinations produce band wavelengths in the range of 12.5-30 Hz. The bands occurred in the left auditory cortex of a
schizophrenicSchizophrenia is a severe mental illness characterized by a disintegration of the process of thinking, of contact with reality, and of emotional responsiveness. It most commonly manifests as auditory hallucinations, paranoid or bizarre delusions, or disorganized speech and thinking with...
and were controlled against 13 controls (18) . This aligns with the studies of people remembering a song in their minds; they do not perceive any sound, but experience the
melodyA melody , also tune, voice, or line, is a linear succession of musical tones which is perceived as a single entity...
,
rhythmRhythm is a "movement marked by the regulated succession of strong and weak elements, or of opposite or different conditions." While rhythm most commonly applies to sound, such as music and spoken language, it may also refer to visual presentation, as "timed movement through space."-Rhythm in...
and overall experience of
soundSound is a travelling wave which is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.- Perception of sound...
. When schizophrenics experience hallucinations, it is the primary auditory cortex which becomes active. This is characteristically different from remembering a sound stimulus, which only faintly activates the tertiary auditory cortex. By deduction, an artificial stimulation of the primary auditory cortex should elicit an incredibly real auditory hallucination. The termination of all audition and
musicMusic is an art form whose medium is sound. Common elements of music are pitch , rhythm , dynamics, and the sonic qualities of timbre and texture...
into the tertiary auditory cortex creates a fascinating nexus of aural information. If this theory is true, it would be interesting to study a subject with a damaged, TAC or one with artificially suppressed function. This would be very difficult to do as the tertiary cortex is simply a ring around the secondary, which is a ring around the primary AC.
TonePitch represents the perceived fundamental frequency of a sound. It is one of the four major auditory attributes of sounds along with loudness, timbre and sound source location. When the actual fundamental frequency can be precisely determined through physical measurement, it may differ from the...
is perceived in more places than just the auditory cortex; one specifically fascinating area is the rostromedial
prefrontal cortexThe prefrontal cortex is the anterior part of the frontal lobes of the brain, lying in front of the motor and premotor areas.This brain region has been implicated in planning complex cognitive behaviors, personality expression, decision making and moderating correct social behavior...
. 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 result of which displayed several areas which are not normally considered to be part of the audition process. The rostromedial prefrontal cortex is a subsection of the medial prefrontal cortex, which projects to the
amygdalaThe ' are almond-shaped groups of nuclei located deep within the medial temporal lobes of the brain in complex vertebrates, including humans...
, and is thought to aid in the inhibition of negative
emotionEmotion is associated with mood, temperament, personality and disposition, and motivation. The English word 'emotion' is derived from the French word émouvoir. This is based on the Latin emovere, where e- means 'out' and movere means 'move'...
. The medial prefrontal cortex is thought to be the core developmental difference between the impulsive teenager and the calm adult. The rostromedial prefrontal cortex is tonality sensitive, meaning it is activated by the tones and frequencies of resonant sounds and music.