Binaural fusion or binaural integration is a

cognitive Cognition refers to "the mental action or process of acquiring knowledge and understanding through thought, experience, and the senses". It encompasses all aspects of intellectual functions and processes such as: perception, attention, thought, ...

process that involves the combination of different auditory information presented binaurally, or to each

ear An ear is the organ that enables hearing and, in mammals, body balance using the vestibular system. In mammals, the ear is usually described as having three parts—the outer ear, the middle ear and the inner ear. The outer ear consists of ...

. In humans, this process is essential in understanding

speech Speech is a human vocal communication using language. Each language uses Phonetics, phonetic combinations of vowel and consonant sounds that form the sound of its words (that is, all English words sound different from all French words, even if ...

as one ear may pick up more information about the speech

stimuli A stimulus is something that causes a physiological response. It may refer to: * Stimulation ** Stimulus (physiology), something external that influences an activity ** Stimulus (psychology), a concept in behaviorism and perception * Stimulus (eco ...

than the other. The process of binaural fusion is important for computing the location of sound sources in the

horizontal plane In astronomy, geography, and related sciences and contexts, a '' direction'' or ''plane'' passing by a given point is said to be vertical if it contains the local gravity direction at that point. Conversely, a direction or plane is said to be hor ...

(

sound localization Sound localization is a listener's ability to identify the location or origin of a detected sound in direction and distance. The sound localization mechanisms of the mammalian auditory system have been extensively studied. The auditory system u ...

), and it is important for sound segregation. Sound segregation refers the ability to identify acoustic components from one or more sound sources. The binaural auditory system is highly dynamic and capable of rapidly adjusting tuning properties depending on the context in which sounds are heard. Each

eardrum 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 The outer ear, external ear, or auris externa is the extern ...

moves one-dimensionally; the auditory

brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...

analyzes and compares movements of both eardrums to extract physical cues and synthesize auditory objects. When stimulation from a

sound In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the ...

reaches the ear, the eardrum deflects in a mechanical fashion, and the three middle ear bones ( ossicles) transmit the mechanical signal to 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 ...

, where

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 transform the mechanical signal into an electrical signal. The auditory nerve, also called 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 ...

, then transmits

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 to the central auditory

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 th ...

. In binaural fusion, inputs from both ears integrate and fuse to create a complete auditory picture at the

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is cont ...

. Therefore, the signals sent to the central auditory nervous system are representative of this complete picture, integrated information from both ears instead of a single ear. The binaural squelch effect is a result of nuclei of the brainstem processing timing,

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 amplit ...

, and spectral differences between the two ears. Sounds are integrated and then separated into auditory objects. For this effect to take place, neural integration from both sides is required.

Anatomy

As sound travels into the inner eardrum of

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

s, it encounters the hair cells that line the

basilar membrane The basilar membrane is a stiff structural element within the cochlea of the inner ear which separates two liquid-filled tubes that run along the coil of the cochlea, the scala media and the scala tympani. The basilar membrane moves up and down in ...

of the cochlea in the

inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...

. The cochlea receives auditory information to be binaurally integrated. At the cochlea, this information is converted into electrical impulses that travel by means of the cochlear nerve, which spans from the cochlea to the

ventral cochlear nucleus In the ventral cochlear nucleus (VCN), auditory nerve fibers enter the brain via the nerve root in the VCN. The ventral cochlear nucleus is divided into the anterior ventral (anteroventral) cochlear nucleus (AVCN) and the posterior ventral (poste ...

, which is located in the

pons The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum. The pons is also called the pons Varolii ("bridge of Va ...

of the brainstem. The

lateral lemniscus The lateral lemniscus is a tract of axons in the brainstem that carries information about sound from the cochlear nucleus to various brainstem nuclei and ultimately the contralateral inferior colliculus of the midbrain. Three distinct, primarily ...

projects from 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 ...

to the superior olivary complex (SOC), a set of brainstem nuclei that consists primarily of two nuclei, the medial superior olive (MSO) and the lateral superior olive (LSO), and is the major site of binaural fusion. The subdivision of the ventral cochlear nucleus that concerns binaural fusion is the anterior ventral cochlear nucleus (AVCN). The AVCN consists of spherical bushy cells and globular bushy cells and can also transmit signals to the medial nucleus of the trapezoid body (MNTB), whose neuron projects to the MSO. Transmissions from the SOC travel to the inferior colliculus (IC) via the lateral lemniscus. At the level of the IC, binaural fusion is complete. The signal ascends to the thalamocortical system, and sensory inputs to 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 directions, ...

are then relayed to 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 ...

Function

The ear functions to analyze and encode a sound’s

dimension In physics and mathematics, the dimension of a Space (mathematics), mathematical space (or object) is informally defined as the minimum number of coordinates needed to specify any Point (geometry), point within it. Thus, a Line (geometry), lin ...

s. Binaural fusion is responsible for avoiding the creation of multiple sound images from a sound source and its reflections. The advantages of this

phenomenon A phenomenon ( : phenomena) is an observable event. The term came into its modern philosophical usage through Immanuel Kant, who contrasted it with the noumenon, which ''cannot'' be directly observed. Kant was heavily influenced by Gottfried W ...

are more noticeable in small rooms, decreasing as the reflective surfaces are placed farther from the listener.

Central auditory system

The central auditory system converges inputs from both ears (inputs contain no explicit spatial information) onto single neurons within the brainstem. This system contains many subcortical sites that have integrative functions. The auditory nuclei collect, integrate, and analyze afferent supply, the outcome is a representation of auditory space. The subcortical auditory nuclei are responsible for extraction and analysis of dimensions of sounds. The integration of a sound stimulus is a result of analyzing

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 eq ...

(pitch), intensity, and spatial localization of the sound source. Once a sound source has been identified, the cells of lower auditory pathways are specialized to analyze physical sound parameters.

Summation In mathematics, summation is the addition of a sequence of any kind of numbers, called ''addends'' or ''summands''; the result is their ''sum'' or ''total''. Beside numbers, other types of values can be summed as well: functions, vectors, mat ...

is observed when the loudness of a sound from one stimulus is perceived as having been doubled when heard by both ears instead of only one. This process of summation is called binaural summation and is the result of different acoustics at each ear, depending on where sound is coming from. The cochlear nerve spans from the cochlea of the inner ear to the ventral cochlear nuclei located in the pons of the brainstem, relaying auditory signals to the superior olivary complex where it is to be binaurally integrated.

Medial superior olive and lateral superior olive

The MSO contains cells that function in comparing inputs from the left and right cochlear nuclei. The tuning of neurons in the MSO favors low frequencies, whereas those in the LSO favor high frequencies. GABA_B receptors in the LSO and MSO are involved in balance of excitatory and inhibitory inputs. The GABA_B receptors are coupled to

G protein G proteins, also known as guanine nucleotide-binding proteins, are a family of proteins that act as molecular switches inside cells, and are involved in transmitting signals from a variety of stimuli outside a cell to its interior. Their act ...

s and provide a way of regulating synaptic efficacy. Specifically, GABA_B receptors modulate excitatory and inhibitory inputs to the LSO. Whether the GABA_B receptor functions as excitatory or inhibitory for the postsynaptic neuron, depends on the exact location and action of the receptor.

Sound localization

Sound localization is the ability to correctly identify the directional location of sounds. A sound stimulus localized in the

is called

azimuth An azimuth (; from ar, اَلسُّمُوت, as-sumūt, the directions) is an angular measurement in a spherical coordinate system. More specifically, it is the horizontal angle from a cardinal direction, most commonly north. Mathematicall ...

; in the

vertical plane In astronomy, geography, and related sciences and contexts, a '' direction'' or ''plane'' passing by a given point is said to be vertical if it contains the local gravity direction at that point. Conversely, a direction or plane is said to be hor ...

it is referred to as elevation. The time, intensity, and spectral differences in the sound arriving at the two ears are used in localization. Localization of low frequency sounds is accomplished by analyzing interaural time difference (ITD). Localization of high frequency sounds is accomplished by analyzing interaural level difference (ILD).

Mechanism

Binaural hearing

Action potentials originate in the

hair cells 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. ...

of the

and propagate to the

; both the timing of these

action potentials 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, c ...

and the signal they transmit provide information to the SOC about the orientation of sound in space. The processing and propagation of action potentials is rapid, and therefore, information about the timing of the sounds that were heard, which is crucial to binaural processing, is conserved. Each eardrum moves in one dimension, and the auditory brain analyzes and compares the movements of both eardrums in order to synthesize auditory objects. This integration of information from both ears is the essence of binaural fusion. The binaural system of hearing involves sound localization in the horizontal plane, contrasting with the monaural system of hearing, which involves sound localization in the vertical plane.

Superior olivary complex

The primary stage of binaural fusion, the processing of binaural signals, occurs at the SOC, where

afferent fibers Afferent nerve fibers are the axons (nerve fibers) carried by a sensory nerve that relay sensory information from sensory receptors to regions of the brain. Afferent projections ''arrive'' at a particular brain region. Efferent nerve fibers ...

of the left and right auditory pathways first converge. This processing occurs because of the interaction of excitatory and inhibitory inputs in the LSO and MSO. The SOC processes and integrates binaural information, in the form of ITD and

ILD ILD may refer to: Organizations * Independent Lutheran Diocese a small Confessional Lutheran Association in the United States. * International Liaison Department of the Chinese Communist Party, a minister-level department of the Chinese gover ...

, entering the brainstem from the cochleae. This initial processing of ILD and ITD is regulated by GABA_B receptors.

ITD and ILD

The auditory space of binaural hearing is constructed based on the analysis of differences in two different binaural cues in the horizontal plane: sound level, or ILD, and arrival time at the two ears, or ITD, which allow for the comparison of the sound heard at each

. ITD is processed in the MSO and results from sounds arriving earlier at one ear than the other; this occurs when the sound does not arise from directly in front or directly behind the hearer. ILD is processed in the LSO and results from the shadowing effect that is produced at the ear that is farther from the sound source. Outputs from the SOC are targeted to the dorsal nucleus of the lateral lemniscus as well as the IC.

Lateral superior olive

LSO neurons are excited by inputs from one ear and inhibited by inputs from the other, and are therefore referred to as IE neurons. Excitatory inputs are received at the LSO from spherical bushy cells of the

ipsilateral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

cochlear nucleus, which combine inputs coming from several auditory nerve fibers. Inhibitory inputs are received at the LSO from globular bushy cells of the

contralateral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

cochlear nucleus.

Medial superior olive

MSO neurons are excited bilaterally, meaning that they are excited by inputs from both ears, and they are therefore referred to as EE neurons. Fibers from the left cochlear nucleus terminate on the left of MSO neurons, and fibers from the right cochlear nucleus terminate on the right of MSO neurons. Excitatory inputs to the MSO from spherical bushy cells are mediated by

glutamate Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a non-essential nutrient for humans, meaning that the human body can syn ...

, and inhibitory inputs to the MSO from globular bushy cells are mediated by

glycine Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid (carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinogeni ...

. MSO neurons extract ITD information from binaural inputs and resolve small differences in the time of arrival of sounds at each ear. Outputs from the MSO and LSO are sent via the

to the IC, which integrates the spatial localization of sound. In the IC, acoustic cues have been processed and filtered into separate streams, forming the basis of auditory object recognition.

Binaural fusion abnormalities in autism

Current research is being performed on the dysfunction of binaural fusion in individuals with

autism The autism spectrum, often referred to as just autism or in the context of a professional diagnosis autism spectrum disorder (ASD) or autism spectrum condition (ASC), is a neurodevelopmental condition (or conditions) characterized by difficulti ...

. The

neurological disorder A neurological disorder is any disorder of the nervous system. Structural, biochemical or electrical abnormalities in the brain, spinal cord or other nerves can result in a range of symptoms. Examples of symptoms include paralysis, muscle weakn ...

autism is associated with many symptoms of impaired brain function, including the degradation of hearing, both unilateral and bilateral. Individuals with autism who experience hearing loss maintain symptoms such as difficulty listening to background noise and impairments in sound localization. Both the ability to distinguish particular speakers from background noise and the process of sound localization are key products of binaural fusion. They are particularly related to the proper function of the SOC, and there is increasing evidence that morphological abnormalities within the brainstem, namely in the SOC, of autistic individuals are a cause of the hearing difficulties. The neurons of the MSO of individuals with autism display atypical anatomical features, including atypical cell shape and orientation of the cell body as well as stellate and

fusiform Fusiform means having a spindle-like shape that is wide in the middle and tapers at both ends. It is similar to the lemon-shape, but often implies a focal broadening of a structure that continues from one or both ends, such as an aneurysm on a b ...

formations. Data also suggests that neurons of the LSO and MNTB contain distinct

dysmorphology Teratology is the study of abnormalities of physiological development in organisms during their life span. It is a sub-discipline in medical genetics which focuses on the classification of congenital abnormalities in Dysmorphic feature, dysmorph ...

in autistic individuals, such as irregular stellate and fusiform shapes and a smaller than normal size. Moreover, a significant depletion of SOC neurons is seen in the brainstem of autistic individuals. All of these structures play a crucial role in the proper functioning of binaural fusion, so their dysmorphology may be at least partially responsible for the incidence of these auditory symptoms in autistic patients.

References

External links

* * {{cite web, url= https://www.audiologyonline.com/articles/binaural-integration-an-overview-1145 , title=Binaural Integration: An Overview , first=Deborah , last=Moncrieff , publisher=audiologyonline.com , date=2002-12-02 , accessdate=2018-03-11 Cognition Hearing