The neural encoding of sound is the representation of
auditory sensation
Sensation (psychology) refers to the processing of the senses by the sensory system.
Sensation or sensations may also refer to:
In arts and entertainment In literature
* Sensation (fiction), a fiction writing mode
* Sensation novel, a Britis ...
and
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 ...
in the
nervous system
In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes ...
.
This article explores the basic
physiological principles of sound perception, and traces
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 ...
mechanisms from sound as pressure waves in air to the transduction of these waves into electrical impulses (
action potential
An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...
s) along
auditory nerve
The cochlear nerve (also auditory nerve or acoustic nerve) is one of two parts of the vestibulocochlear nerve, a cranial nerve present in amniotes, the other part being the vestibular nerve. The cochlear nerve carries auditory sensory information ...
fibers, and further processing in the brain.
Introduction
The complexities of contemporary
neuroscience
Neuroscience is the scientific study of the nervous system (the brain, spinal cord, and peripheral nervous system), its functions and disorders. It is a multidisciplinary science that combines physiology, anatomy, molecular biology, developme ...
are continually redefined. Thus what is known of the auditory system has been continually changing.
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 wave
Longitudinal waves are waves in which the vibration of the medium is parallel ("along") to the direction the wave travels and displacement of the medium is in the same (or opposite) direction of the wave propagation. Mechanical longitudinal waves ...
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
Amplitude
The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...
is the size (magnitude) 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
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 o ...
; frequency is inversely proportional to
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...
(in a medium of uniform propagation velocity, such as sound in air). The wavelength of a sound is the distance between any two consecutive matching points on the waveform. The
audible frequency range for young humans is about 20 Hz to 20 kHz. Hearing of higher frequencies decreases with age, limiting to about 16 kHz for adults, and even down to 3 kHz for elders.
Anatomy of the ear
Given the simple physics of sound, the anatomy and physiology of hearing can be studied in greater detail.
Outer ear
The Outer ear consists of the
pinna or
auricle (visible parts including ear lobes and concha), and the
auditory meatus (the passageway 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 its mechanical design.
Middle ear
![Middle Ear](https://upload.wikimedia.org/wikipedia/commons/9/93/Middle_Ear.jpg)
The middle ear 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 malleus, or hammer, is a hammer-shaped small bone or ossicle of the middle ear. It connects with the incus, and is attached to the inner surface of the eardrum. The word is Latin for 'hammer' or 'mallet'. It transmits the sound vibrations f ...
, the
incus
The ''incus'' (plural incudes) or anvil is a bone in the middle ear. The anvil-shaped small bone is one of three ossicles in the middle ear. The ''incus'' receives vibrations from the ''malleus'', to which it is connected laterally, and transmit ...
, and the
stapes
The ''stapes'' or stirrup is a bone in the middle ear of humans and other animals which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its annular ligament, which allows the foo ...
, collectively known as the ear
ossicles.
The impedance matching is done through via lever ratios and the ratio of areas of the
tympanic membrane
In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped membrane that separates the external ear from the middle ear. Its function is to transmit sound from the a ...
and the footplate of the stapes, creating a
transformer
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the 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
The tensor tympani is a muscle within the middle ear, located in the bony canal above the bony part of the auditory tube, and connects to the malleus bone. Its role is to dampen loud sounds, such as those produced from chewing, shouting, or th ...
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.
Inner ear
![Inner Ear](https://upload.wikimedia.org/wikipedia/commons/d/d6/Inner_Ear.jpg)
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 or ...
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
Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment. ...
s.
Outer hair cell
Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment. ...
s primarily provide amplification of traveling waves that are induced by sound energy, while
inner hair cell
Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment. ...
s detect the motion of those waves and excite the (Type I) neurons of the
auditory nerve
The cochlear nerve (also auditory nerve or acoustic nerve) is one of two parts of the vestibulocochlear nerve, a cranial nerve present in amniotes, the other part being the vestibular nerve. The cochlear nerve carries auditory sensory information ...
.
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
In physiology, tonotopy (from Greek tono = frequency and topos = place) 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 ...
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
The vestibular duct or scala vestibuli is a perilymph-filled cavity inside the cochlea of the inner ear that conducts sound vibrations to the cochlear duct.
It is separated from the cochlear duct by Reissner's membrane and extends from the vesti ...
,
scala media
Scala or SCALA may refer to:
Automobiles
* Renault Scala, multiple automobile models
* Škoda Scala, a Czech compact hatchback
Music
* Scala (band), an English electronic music group
* Escala (group), an electronic string quartet formerly known ...
, and
scala tympani
The tympanic duct or scala tympani is one of the perilymph-filled cavities in the inner ear of humans. It is separated from the cochlear duct by the basilar membrane, and it extends from the round window to the helicotrema, where it continues as ...
).
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 is an extracellular fluid located within the inner ear. It is found within the scala tympani and scala vestibuli of the cochlea. The ionic composition of perilymph is comparable to that of plasma and cerebrospinal fluid. The major ...
, while scala media, or the
cochlear duct
The cochlear duct (bounded by the scala media) is an endolymph filled cavity inside the cochlea, located between the tympanic duct and the vestibular duct, separated by the basilar membrane and the vestibular membrane (Reissner's membrane) re ...
, is filled with
endolymph
Endolymph is the fluid contained in the membranous labyrinth of the inner ear. The major cation in endolymph is potassium, with the values of sodium and potassium concentration in the endolymph being 0.91 mM and 154 mM, respectively. I ...
.
Transduction
Auditory hair cells
The auditory
hair cell
Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment. ...
s in the cochlea are at the core of the auditory system's special functionality (similar hair cells are located in the
semicircular canal
In mathematics (and more specifically geometry), a semicircle is a one-dimensional locus of points that forms half of a circle. The full arc of a semicircle always measures 180° (equivalently, radians, or a half-turn). It has only one line o ...
s). Their primary function is
mechanotransduction
In cellular biology, mechanotransduction ('' mechano'' + '' transduction'') is any of various mechanisms by which cells convert mechanical stimulus into electrochemical activity. This form of sensory transduction is responsible for a number of ...
, 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
A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiati ...
of the
visual system
The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight (th ...
. Thus the loss of a lower 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.
Mechanotransduction
The apical surface of each cochlear hair cell contains a
hair bundle. Each hair bundle contains approximately 300 fine projections known as
stereocilia
Stereocilia (or stereovilli or villi) are non-motile apical cell modifications. They are distinct from cilia and microvilli, but are closely related to microvilli. They form single "finger-like" projections that may be branched, with normal cell ...
, formed by actin cytoskeletal elements.
The stereocilia in a hair bundle are arranged in multiple rows of different heights. In addition to the stereocilia, a true
ciliary structure known as the
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
Kinocilia are found on the apical surface of hair cells and are involved in both the morphogenesis ...
exists and is believed to play a role in hair cell degeneration that is caused by exposure to high frequencies.
A stereocilium is able to bend at its point of attachment to the apical surface of the hair cell. The
actin
Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ov ...
filaments that form the core of a stereocilium are highly interlinked and cross linked with
fibrin, and are therefore stiff and inflexible at positions other than the base. When stereocilia in the tallest row are deflected in the positive-stimulus direction, the shorter rows of stereocilia are also deflected.
These simultaneous deflections occur due to filaments called tip links that attach the side of each taller stereocilium to the top of the shorter stereocilium in the adjacent row. When the tallest stereocilia are deflected, tension is produced in the tip links and causes the stereocilia in the other rows to deflect as well. At the lower end of each tip link is one or more mechano-electrical transduction (MET) channels, which are opened by tension in the tip links.
These MET channels are cation-selective transduction channels that allow potassium and calcium ions to enter the hair cell from the endolymph that bathes its apical end.
The influx of cations, particularly potassium, through the open MET channels causes the membrane potential of the hair cell to depolarize. This depolarization opens voltage-gated calcium channels to allow the further influx of calcium. This results in an increase in the calcium concentration, which triggers the exocytosis of neurotransmitter vesicles at
ribbon synapses at the basolateral surface of the hair cell. The release of neurotransmitter at a ribbon synapse, in turn, generates an action potential in the connected auditory-nerve fiber.
Hyperpolarization of the hair cell, which occurs when potassium leaves the cell, is also important, as it stops the influx of calcium and therefore stops the fusion of vesicles at the ribbon synapses. Thus, as elsewhere in the body, the transduction is dependent on the concentration and distribution of ions.
The perilymph that is found in the scala tympani has a low potassium concentration, whereas the endolymph found in the scala media has a high potassium concentration and an electrical potential of about 80 millivolts compared to the perilymph.
Mechanotransduction by stereocilia is highly sensitive and able to detect perturbations as small as fluid fluctuations of 0.3 nanometers, and can convert this mechanical stimulation into an electrical nerve impulse in about 10 microseconds.
Nerve fibers from the cochlea
There are two types of
afferent neurons
Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The cel ...
found in the
cochlear nerve
The cochlear nerve (also auditory nerve or acoustic nerve) is one of two parts of the vestibulocochlear nerve, a cranial nerve present in amniotes, the other part being the vestibular nerve. The cochlear nerve carries auditory sensory informatio ...
: 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
gentamicin.
Brainstem and midbrain
![Transmission of Nerve Impulse](https://upload.wikimedia.org/wikipedia/commons/c/ca/Transmission_of_Nerve_Impulse.jpg)
The
auditory nervous system includes many stages of information processing between the ear and
cortex
Cortex or cortical may refer to:
Biology
* Cortex (anatomy), the outermost layer of an organ
** Cerebral cortex, the outer layer of the vertebrate cerebrum, part of which is the ''forebrain''
*** Motor cortex, the regions of the cerebral cortex i ...
.
Auditory cortex
Primary auditory neurons carry action potentials from the cochlea into the transmission pathway shown in the adjacent image. Multiple relay stations act as integration and processing centers. The signals reach the first level of cortical processing at the
primary auditory cortex
The auditory cortex is the part of the temporal lobe that processes auditory information in humans and many other vertebrates. It is a part of the auditory system, performing basic and higher functions in hearing, such as possible relations to ...
(A1), in the
superior temporal gyrus 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 pro ...
.
Most areas up to and including A1 are tonotopically mapped (that is, frequencies are kept in an ordered arrangement). However, A1 participates in coding more complex and abstract aspects of auditory stimuli without coding well the frequency content, including the presence of a distinct sound or its echoes.
Like lower regions, this region of the brain has combination-sensitive neurons that have
nonlinear
In mathematics and science, a nonlinear system is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathematicians, and many othe ...
responses to stimuli.
Recent studies conducted in
bat
Bats are mammals of the order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most ...
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 is a gyrus in the brain on the temporal lobe. It is located between the superior temporal gyrus and inferior temporal gyrus. It corresponds largely to Brodmann area 21.
The middle temporal gyrus is bounded by:
* the super ...
of the temporal lobe.
Lateralization of brain function
The lateralization of brain function is the tendency for some neural functions or cognitive processes to be specialized to one side of the brain or the other. The median longitudinal fissure separates the human brain into two distinct cerebr ...
exists in the cortex, with the processing of speech in the left
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 ...
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.
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 colliculus
The inferior colliculus (IC) (Latin for ''lower hill'') is the principal midbrain nucleus of the auditory pathway and receives input from several peripheral brainstem nuclei in the auditory pathway, as well as inputs from the auditory cortex. T ...
.
* Despite advances in gene therapy that allow for the alteration of the expression of genes that affect audition, such as
ATOH1
Protein atonal homolog 1 is a protein that in humans is encoded by the ''ATOH1'' gene.
Function
This protein belongs to the basic helix-loop-helix (BHLH) family of transcription factors. It activates E-box dependent transcription along with TC ...
, and the use of viral vectors for such end, the micro-mechanical and neural 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, but these maps are subject to plasticity.
The cortex seems to perform a more complex processing than spectral analysis or even spectro-temporal analysis.
References
{{reflist, 30em
Auditory system
Ear
Psychoacoustics