Neuronal encoding of sound
Encyclopedia
The neuronal encoding of sound is the representation of auditory
sensation and perception in the nervous system
.
This article explores the basic physiological principles of sound perception, and traces hearing
mechanisms from sound
as pressure waves in air to the transduction of these waves into electrical impulses (action potential
s) along auditory nerve fibers, and further processing in the brain.
are continually redefined. Thus what is known now of the auditory system has changed in the recent times and thus conceivably in the next two years or so, much of this will change.
This article is structured in a format that starts with a small exploration of what sound is followed by the general anatomy of the ear which in turn will finally give way to explaining the encoding mechanism of the engineering marvel that is the ear. This article traces the route that sound waves first take from generation at an unknown source to their integration and perception by the auditory cortex.
s, which consist of propagating regions of high pressure (compression) and corresponding regions of low pressure (rarefaction).
is the size of the pressure variations in a sound wave, and primarily determines the loudness with which the sound is perceived. In a sinusoidal function such as
, C represents the amplitude of the sound wave.
; it is inversely proportional to the wavelength
. The wavelength of a sound is the distance between any two consecutive matching points on the waveform. The audible frequency range for humans is about 20 Hz to 20 000 Hz at infants. Hearing of higher frequencies decreases with age limiting to about 16000 Hz for adults and even down to 3000 Hz for elders.
(visible parts including ear lobes and concha), and the auditory meatus
(the passage way for sound). The fundamental function of this part of the ear is to gather sound energy and deliver it to the eardrum
. Resonances of the external ear selectively boost sound pressure with frequency in the range 2–5 kHz.
The pinna as a result of its asymmetrical structure is able to provide further cues about the elevation from which the sound originated. The vertical asymmetry of the pinna selectively amplifies sounds of higher frequency from high elevation thereby providing spatial information by virtue of it mechanical design.
plays a crucial role in the auditory process, as it essentially converts pressure variations in air to perturbations in the fluids of the inner ear. In other words, it is the mechanical transfer function that allows for efficient transfer of collected sound energy between two different media. The three small bones that are responsible for this complex process are the malleus
, the incus
, and the stapes, collectively known as the ear ossicles
. The impedance matching is done through via lever ratios and the ratio of areas of the tympanic membrane (ear drum) and the footplate of the stapes, creating a transformer
-like mechanism. Furthermore the ossicles are arranged in such a manner as to resonate at 700–800 Hz while at the same time protecting the inner ear from excessive energy. A certain degree of top-down control is present at the middle ear level primarily through two muscles present in this anatomical region: the tensor tympani
and the stapedius
. These two muscles can restrain the ossicles so as to reduce the amount of energy that is transmitted into the inner ear in loud surroundings.
of the inner ear
, a marvel of physiological engineering, acts as both a frequency analyzer and nonlinear acoustic amplifier.
The cochlea has over 32,000 hair cell
s. Outer hair cells primarily provide amplification of traveling waves that are induced by sound energy, while inner hair cells detect the motion of those waves and excite the (Type I) neurons of the auditory nerve. The basal end of the cochlea, where sounds enter from the middle ear, encodes the higher end of the audible frequency range while the apical end of the cochlea encodes the lower end of the frequency range. This tonotopy
plays a crucial role in hearing, as it allows for spectral separation of sounds. A cross section of the cochlea will reveal an anatomical structure with three main chambers (scala vestibuli
, scala media
, and scala tympani
). At the apical end of the cochlea, at an opening known as the helicotrema, the scala vestibuli merges with the scala tympani. The fluid found in these two cochlear chambers is perilymph
, while scala media, or the cochlear duct, is filled with endolymph
.
s in the cochlea are at the core of the auditory system's special functionality (similar hair cells are located in the semicircular canals). Their primary function is mechano-transduction, or conversion between mechanical and neural signals. The relatively small number of the auditory hair cells is surprising when compared to other sensory cells such as the rods and cones of the visual system. Thus the loss of low number (in the order of thousands) of auditory hair cells can be devastating while the loss of a larger number of retinal cells (in the order to hundreds of thousands) will not be as bad from a sensory standpoint.
Cochlear hair cells are organized as inner hair cells and outer hair cells; inner and outer refer to relative position from the axis of the cochlear spiral. The inner hair cells are the primary sensory receptors and a significant amount of the sensory input to the auditory cortex occurs from these hair cells. Outer hair cells on the other hand boost the mechanical signal by using electromechanical feedback.
. These stereocilia are anatomically arranged in order of progressive height. The actin
filaments present in these stereocilia are highly interlinked and even cross linked with fibrin
that makes these pseudo ciliary projections quite stiff. In addition to stereocilia, a true ciliary structure known as the kinocilium
exists and is believed to play a role in hair cell degeneration that is caused by exposure to high frequencies.
The stereocilia are hinged where they attach to the apical membrane. When displaced along a plane parallel to the tallest stereocilium, the tallest stereocilium depolarizes, which in turn causes subsequent depolarizations in the smaller stereocilia in that specific bundle. This serial depolarization is due to interconnected MET (mechano-electrical transduction) channels which open when mechanical perturbations occur in the endolymph that bathes the apical ends of the hair cells. These MET channels are interconnected with filaments known as tip links and fall under the category of cation selective transduction channels. Potassium is the ion that initiates the depolarization cascade by entering the cell through an open MET channel. This depolarization event induces calcium vesicles to fuse with the basal end of the hair cell, which in turn causes the generation of an action potential in the auditory neuron. Hyperpolarization of the hair cell, which occurs when potassium leaves the cell, is equally important as it is what prevents the merging of calcium vesicles with the basal end of the hair cell. Thus, as elsewhere in the body, the transduction is dependent on the concentration/distribution of ions. The perilymph which is found in the scala tympani has a low potassium concentration while the endolymph found in the scala media has a high potassium concentration with an electrical potential of 80mV in comparison to the perilymph. The stereocilia are highly sensitive with the ability to measure perturbations as small as fluid fluctuations of 0.3 nm, and can convert this depolarizing potential into a nerve impulse in about 10 microseconds.
: Type I and Type II. Each type of neuron has specific cell selectivity within the cochlea. The Mechanism that determines the selectivity of each type of neuron for a specific hair cell has been proposed by two diametrically opposed theories in neuroscience known as the peripheral instruction hypothesis and the cell autonomous instruction hypothesis. The peripheral instruction hypothesis states that phenotypic differentiation between the two neurons are not made until after these undifferentiated neurons attach to hair cells which in turn will dictate the differentiation pathway. The cell autonomous instruction hypothesis states that differentiation into Type I and Type II neurons occur following the last phase of mitotic division but preceding innervations. Both types of neuron participate in the encoding of sound for transmission to the brain.
.
.
(A1), in the superior temporal gyrus
of the temporal lobe
. Most areas up to and including A1 are tonotopically mapped (that is, frequencies are kept in an ordered arrangement). Like lower regions, this region of the brain has combination-sensitive neurons that have nonlinear responses to stimuli.
Recent studies conducted in bat
s and other mammals have revealed that the ability to process and interpret modulation in frequencies primarily occurs in the superior and middle temporal gyri
of the temporal lobe. Lateralization of brain function
exists in the cortex, with the processing of speech in the left cerebral hemisphere
and environmental sounds in the right hemisphere of the auditory cortex. Music
, with its influence on emotions, is also processed in the right hemisphere of the auditory cortex. While the reason for such localization is not quite understood, lateralization in this instance does not imply exclusivity as both hemispheres do participate in the processing, but one hemisphere tends to play a more significant role than the other.
Hearing (sense)
Hearing is the ability to perceive sound by detecting vibrations through an organ such as the ear. It is one of the traditional five senses...
sensation and perception in the nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
.
This article explores the basic physiological principles of sound perception, and traces hearing
Hearing (sense)
Hearing is the ability to perceive sound by detecting vibrations through an organ such as the ear. It is one of the traditional five senses...
mechanisms from sound
Sound
Sound is a mechanical wave that 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.-Propagation of...
as pressure waves in air to the transduction of these waves into electrical impulses (action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
s) along auditory nerve fibers, and further processing in the brain.
Introduction
The complexities of contemporary neuroscienceNeuroscience
Neuroscience is the scientific study of the nervous system. Traditionally, neuroscience has been seen as a branch of biology. However, it is currently an interdisciplinary science that collaborates with other fields such as chemistry, computer science, engineering, linguistics, mathematics,...
are continually redefined. Thus what is known now of the auditory system has changed in the recent times and thus conceivably in the next two years or so, much of this will change.
This article is structured in a format that starts with a small exploration of what sound is followed by the general anatomy of the ear which in turn will finally give way to explaining the encoding mechanism of the engineering marvel that is the ear. This article traces the route that sound waves first take from generation at an unknown source to their integration and perception by the auditory cortex.
Basic physics of sound
Sound waves are what physicists call longitudinal waveLongitudinal wave
Longitudinal waves, as known as "l-waves", are waves that have the same direction of vibration as their direction of travel, which means that the movement of the medium is in the same direction as or the opposite direction to the motion of the wave. Mechanical longitudinal waves have been also...
s, which consist of propagating regions of high pressure (compression) and corresponding regions of low pressure (rarefaction).
Waveform
Waveform is a description of the general shape of the sound wave. Waveforms are sometimes described by the sum of sinusoids, via Fourier analysis.Amplitude
AmplitudeAmplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
is the size of the pressure variations in a sound wave, and primarily determines the loudness with which the sound is perceived. In a sinusoidal function such as
, C represents the amplitude of the sound wave.Frequency and wavelength
The frequency of a sound is defined as the number of repetitions of its waveform per second, and is measured in hertzHertz
The 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 the sine wave, particularly those used in radio and audio applications....
; it is inversely proportional to the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
. The wavelength of a sound is the distance between any two consecutive matching points on the waveform. The audible frequency range for humans is about 20 Hz to 20 000 Hz at infants. Hearing of higher frequencies decreases with age limiting to about 16000 Hz for adults and even down to 3000 Hz for elders.
Anatomy of the ear
Given the simple physics of sound, the anatomy and physiology of hearing can be studied in greater detail.External ear
The external ear consists of the pinna or auricleEar
The ear is the organ that detects sound. It not only receives sound, but also aids in balance and body position. The ear is part of the auditory system....
(visible parts including ear lobes and concha), and the auditory meatus
Auditory meatus
Auditory meatus can refer to:* external auditory meatus* internal auditory meatus...
(the passage way for sound). The fundamental function of this part of the ear is to gather sound energy and deliver it to the eardrum
Eardrum
The eardrum, or tympanic membrane, is a thin membrane that separates the external ear from the middle ear in humans and other tetrapods. Its function is to transmit sound from the air to the ossicles inside the middle ear. The malleus bone bridges the gap between the eardrum and the other ossicles...
. Resonances of the external ear selectively boost sound pressure with frequency in the range 2–5 kHz.
The pinna as a result of its asymmetrical structure is able to provide further cues about the elevation from which the sound originated. The vertical asymmetry of the pinna selectively amplifies sounds of higher frequency from high elevation thereby providing spatial information by virtue of it mechanical design.
Middle ear
The middle earMiddle ear
The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The mammalian middle ear contains three ossicles, which couple vibration of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear has...
plays a crucial role in the auditory process, as it essentially converts pressure variations in air to perturbations in the fluids of the inner ear. In other words, it is the mechanical transfer function that allows for efficient transfer of collected sound energy between two different media. The three small bones that are responsible for this complex process are the malleus
Malleus
The malleus or hammer is a hammer-shaped small bone or ossicle of the middle ear which connects with the incus and is attached to the inner surface of the eardrum...
, the incus
Incus
The incus or anvil is the anvil-shaped small bone or ossicle in themiddle ear. It connects the malleus to the stapes. It was first described by Alessandro Achillini of Bologna.The incus transmits sound vibrations from the malleus to the stapes....
, and the stapes, collectively known as the ear ossicles
Ossicles
The ossicles are the three smallest bones in the human body. They are contained within the middle ear space and serve to transmit sounds from the air to the fluid-filled labyrinth . The absence of the auditory ossicles would constitute a moderate-to-severe hearing loss...
. The impedance matching is done through via lever ratios and the ratio of areas of the tympanic membrane (ear drum) and the footplate of the stapes, creating a transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
-like mechanism. Furthermore the ossicles are arranged in such a manner as to resonate at 700–800 Hz while at the same time protecting the inner ear from excessive energy. A certain degree of top-down control is present at the middle ear level primarily through two muscles present in this anatomical region: the tensor tympani
Tensor tympani
The tensor tympani, the larger of the two muscles of the tympanic cavity, is contained in the bony canal above the osseous portion of the auditory tube...
and the stapedius
Stapedius
The stapedius is the smallest skeletal muscle in the human body. At just over one millimeter in length, its purpose is to stabilize the smallest bone in the body, the stapes....
. These two muscles can restrain the ossicles so as to reduce the amount of energy that is transmitted into the inner ear in loud surroundings.
Inner ear
The cochleaCochlea
The cochlea is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth, making 2.5 turns around its axis, the modiolus....
of the inner ear
Inner ear
The inner ear is the innermost part of the vertebrate ear. In mammals, it consists of the bony labyrinth, a hollow cavity in the temporal bone of the skull with a system of passages comprising two main functional parts:...
, a marvel of physiological engineering, acts as both a frequency analyzer and nonlinear acoustic amplifier.
The cochlea has over 32,000 hair cell
Hair cell
Hair cells are the sensory receptors of both the auditory system and the vestibular system in all vertebrates. In mammals, the auditory hair cells are located within the organ of Corti on a thin basilar membrane in the cochlea of the inner ear...
s. Outer hair cells primarily provide amplification of traveling waves that are induced by sound energy, while inner hair cells detect the motion of those waves and excite the (Type I) neurons of the auditory nerve. The basal end of the cochlea, where sounds enter from the middle ear, encodes the higher end of the audible frequency range while the apical end of the cochlea encodes the lower end of the frequency range. This tonotopy
Tonotopy
In physiology, tonotopy is 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...
plays a crucial role in hearing, as it allows for spectral separation of sounds. A cross section of the cochlea will reveal an anatomical structure with three main chambers (scala vestibuli
Scala vestibuli
Scala vestibuli is a perilymph-filled cavity inside the cochlea of the inner ear that conducts sound vibrations to the scala media.It is separated from the scala media by Reissner's membrane and extends from the vestibule of the ear to the helicotrema where it joins scala tympani.-External links:* ...
, scala media
Scala media
The cochlear duct is an endolymph filled cavity inside the cochlea, located in between the scala tympani and the scala vestibuli, separated by the basilar membrane and Reissner's membrane respectively....
, and scala tympani
Scala tympani
Scala tympani is one of the perilymph-filled cavities in the cochlear labyrinth of the human ear. It is separated from the scala media by the basilar membrane, and it extends from the round window to the helicotrema, where it continues as scala vestibuli....
). At the apical end of the cochlea, at an opening known as the helicotrema, the scala vestibuli merges with the scala tympani. The fluid found in these two cochlear chambers is perilymph
Perilymph
Perilymph is an extracellular fluid located within the cochlea in two of its three compartments: the scala tympani and scala vestibuli. The ionic composition of perilymph is comparable to that of plasma and cerebrospinal fluid...
, while scala media, or the cochlear duct, is filled with endolymph
Endolymph
Endolymph is the fluid contained in the membranous labyrinth of the inner ear. It is also called Scarpa's fluid, after Antonio Scarpa.-Composition:...
.
Auditory hair cells
The auditory hair cellHair cell
Hair cells are the sensory receptors of both the auditory system and the vestibular system in all vertebrates. In mammals, the auditory hair cells are located within the organ of Corti on a thin basilar membrane in the cochlea of the inner ear...
s in the cochlea are at the core of the auditory system's special functionality (similar hair cells are located in the semicircular canals). Their primary function is mechano-transduction, or conversion between mechanical and neural signals. The relatively small number of the auditory hair cells is surprising when compared to other sensory cells such as the rods and cones of the visual system. Thus the loss of low number (in the order of thousands) of auditory hair cells can be devastating while the loss of a larger number of retinal cells (in the order to hundreds of thousands) will not be as bad from a sensory standpoint.
Cochlear hair cells are organized as inner hair cells and outer hair cells; inner and outer refer to relative position from the axis of the cochlear spiral. The inner hair cells are the primary sensory receptors and a significant amount of the sensory input to the auditory cortex occurs from these hair cells. Outer hair cells on the other hand boost the mechanical signal by using electromechanical feedback.
Mechano-transduction
A hair bundle is found on the apical surface of each hair cell. Each hair bundle has about 300 projections of actin cytoskeleton known as stereociliaStereocilia
In the inner ear, stereocilia are the mechanosensing organelles of hair cells, which respond to fluid motion in numerous types of animals for various functions, including hearing and balance. They are about 10–50 micrometers in length and share some similar features of microvilli...
. These stereocilia are anatomically arranged in order of progressive height. The actin
Actin
Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...
filaments present in these stereocilia are highly interlinked and even cross linked with fibrin
Fibrin
Fibrin is a fibrous, non-globular protein involved in the clotting of blood. It is a fibrillar protein that is polymerised to form a "mesh" that forms a hemostatic plug or clot over a wound site....
that makes these pseudo ciliary projections quite stiff. In addition to stereocilia, a true ciliary structure known as the kinocilium
Kinocilium
A kinocilium is a special type of cilium on the apex of hair cells located in the sensory epithelium of the vertebrate inner ear.-Anatomy in humans:...
exists and is believed to play a role in hair cell degeneration that is caused by exposure to high frequencies.
The stereocilia are hinged where they attach to the apical membrane. When displaced along a plane parallel to the tallest stereocilium, the tallest stereocilium depolarizes, which in turn causes subsequent depolarizations in the smaller stereocilia in that specific bundle. This serial depolarization is due to interconnected MET (mechano-electrical transduction) channels which open when mechanical perturbations occur in the endolymph that bathes the apical ends of the hair cells. These MET channels are interconnected with filaments known as tip links and fall under the category of cation selective transduction channels. Potassium is the ion that initiates the depolarization cascade by entering the cell through an open MET channel. This depolarization event induces calcium vesicles to fuse with the basal end of the hair cell, which in turn causes the generation of an action potential in the auditory neuron. Hyperpolarization of the hair cell, which occurs when potassium leaves the cell, is equally important as it is what prevents the merging of calcium vesicles with the basal end of the hair cell. Thus, as elsewhere in the body, the transduction is dependent on the concentration/distribution of ions. The perilymph which is found in the scala tympani has a low potassium concentration while the endolymph found in the scala media has a high potassium concentration with an electrical potential of 80mV in comparison to the perilymph. The stereocilia are highly sensitive with the ability to measure perturbations as small as fluid fluctuations of 0.3 nm, and can convert this depolarizing potential into a nerve impulse in about 10 microseconds.
Nerve fibers from the cochlea
There are two types of afferent neurons found in the cochlear nerveCochlear nerve
The cochlear nerve is a nerve in the head that carries signals from the cochlea of the inner ear to the brain...
: Type I and Type II. Each type of neuron has specific cell selectivity within the cochlea. The Mechanism that determines the selectivity of each type of neuron for a specific hair cell has been proposed by two diametrically opposed theories in neuroscience known as the peripheral instruction hypothesis and the cell autonomous instruction hypothesis. The peripheral instruction hypothesis states that phenotypic differentiation between the two neurons are not made until after these undifferentiated neurons attach to hair cells which in turn will dictate the differentiation pathway. The cell autonomous instruction hypothesis states that differentiation into Type I and Type II neurons occur following the last phase of mitotic division but preceding innervations. Both types of neuron participate in the encoding of sound for transmission to the brain.
Type I neurons
Type I neurons innervate inner hair cells. There is significantly greater convergence of this type of neuron towards the basal end in comparison with the apical end. A radial fiber bundle acts as an intermediary between Type I neurons and inner hair cells. The ratio of innervation that is seen between Type I neurons and inner hair cells is 1:1 which results in high signal transmission fidelity and resolution.Type II neurons
Type II neurons on the other hand innervate outer hair cells. However, there is significantly greater convergence of this type of neuron towards the apex end in comparison with the basal end. A 1:30-60 ratio of innervation is seen between Type II neurons and outer hair cells which in turn make these neurons ideal for electromechanical feedback. Type II neurons can be physiologically manipulated to innervate inner hair cells provided outer hair cells have been destroyed either through mechanical damage or by chemical damage induced by drugs such as gentamicinGentamicin
Gentamicin is an aminoglycoside antibiotic, used to treat many types of bacterial infections, particularly those caused by Gram-negative organisms. However, gentamicin is not used for Neisseria gonorrhoeae, Neisseria meningitidis or Legionella pneumophila...
.
Brainstem and midbrain
The auditory nervous system includes many stages of information processing between the ear and cortexCerebral cortex
The 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...
.
Auditory cortex
Primary auditory neurons carry action potentials from the cochlea into the transmission pathway shown in the image to the right. Multiple relay stations act as integration and processing centers. The signals reach the first level of cortical processing at the primary auditory cortexPrimary auditory cortex
The primary auditory cortex is the region of the brain that is responsible for the processing of auditory information. Corresponding roughly with Brodmann areas 41 and 42, it is located on the temporal lobe, and performs the basics of hearing—pitch and volume...
(A1), in the superior temporal gyrus
Superior temporal gyrus
The 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;...
of the temporal lobe
Temporal lobe
The temporal lobe is a region of the cerebral cortex that is located beneath the Sylvian fissure on both cerebral hemispheres of the mammalian brain....
. Most areas up to and including A1 are tonotopically mapped (that is, frequencies are kept in an ordered arrangement). Like lower regions, this region of the brain has combination-sensitive neurons that have nonlinear responses to stimuli.
Recent studies conducted in bat
Bat
Bats are mammals of the order Chiroptera "hand" and pteron "wing") whose forelimbs form webbed wings, making them the only mammals naturally capable of true and sustained flight. By contrast, other mammals said to fly, such as flying squirrels, gliding possums, and colugos, glide rather than fly,...
s and other mammals have revealed that the ability to process and interpret modulation in frequencies primarily occurs in the superior and middle temporal gyri
Middle temporal gyrus
Middle temporal gyrus is a gyrus in the brain on the Temporal lobe. It is located between the superior temporal gyrus and inferior temporal gyrus. Its exact function is unknown, but it has been connected with processes as different as contemplating distance, recognition of known faces, and...
of the temporal lobe. Lateralization of brain function
Lateralization of brain function
A longitudinal fissure separates the human brain into two distinct cerebral hemispheres, connected by the corpus callosum. The sides resemble each other and each hemisphere's structure is generally mirrored by the other side. Yet despite the strong anatomical similarities, the functions of each...
exists in the cortex, with the processing of speech in the left cerebral hemisphere
Cerebral hemisphere
A 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. Each of these hemispheres has an outer layer of grey matter called the cerebral cortex that is...
and environmental sounds in the right hemisphere of the auditory cortex. Music
Music
Music is an art form whose medium is sound and silence. Its common elements are pitch , rhythm , dynamics, and the sonic qualities of timbre and texture...
, with its influence on emotions, is also processed in the right hemisphere of the auditory cortex. While the reason for such localization is not quite understood, lateralization in this instance does not imply exclusivity as both hemispheres do participate in the processing, but one hemisphere tends to play a more significant role than the other.
Recent ideas
- Alternation in encoding mechanisms have been noticed as one progresses through the auditory cortex. Encoding shifts from synchronous responses in the cochlear nucleus and later becomes dependent on rate encoding in the inferior colliculusInferior colliculusThe inferior colliculus is the principal midbrain nucleus of the auditory pathway and receives input from several more peripheral brainstem nuclei in the auditory pathway, as well as inputs from the auditory cortex...
. - Despite advances in gene therapy that allows for the alteration of the expression of genes that affect audition, such as ATOH1ATOH1Protein atonal homolog 1 is a protein that in humans is encoded by the ATOH1 gene.- External links :*...
, and the use of viral vectors for such end, the micro-mechanical and neuronal complexities that surrounds the inner ear hair cells, artificial regeneration in vitro remains a distant reality. - Recent studies suggest that the auditory cortex may not be as involved in top down processing as was previous thought. In studies conducted on primates for tasks that required the discrimination of acoustic flutter, Lemus found that the auditory cortex played only a sensory role and had nothing to do with the cognition of the task at hand.
- Due to the presence of the tonotopic maps in the auditory cortex at an early age, it has been assumed that cortical reorganization had little to do with the establishment of these maps. However, recent work by Kandler et al. has shown that these maps are formed as a result of plastic reorganization on a sub-cellular and circuit level.