Wikipedia

Binaural fusion

Binaural fusionorbinaural integrationis acognitiveprocess that involves the combination of differentauditoryinformation presentedbinaurally,or to eachear.In humans, this process is essential in understandingspeechin noisy and reverberent environments.

The process of binaural fusion is important for perceiving thelocations of sound sources,especially along the horizontal or azimuth direction, and it is important for sound segregation.[1]Sound segregation refers to the ability to identify acoustic components from one or more sound sources.[2]The binaural auditory system is highly dynamic and capable of rapidly adjusting tuning properties depending on the context in which sounds are heard. Eacheardrummoves one-dimensionally; the auditorybrainanalyzes and compares movements of the two eardrums to extract physical cues and perceive auditory objects.[3]

When stimulation from asoundreaches the ear, the eardrum deflects in a mechanical fashion, and the three middle ear bones (ossicles) transmit the mechanical signal to thecochlea,wherehair cellstransform the mechanical signal into an electrical signal. The auditory nerve, also called thecochlear nerve,then transmitsaction potentialsto the central auditorynervous system.[3]

In binaural fusion, inputs from both ears integrate and fuse to create a complete auditory picture in thebrainstem.Therefore, the signals sent to the higher 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 ofnucleiof the brainstem processing timing,amplitude,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.[4]

Anatomy

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Transmissions from theSOC,in theponsof thebrainstem,travel along thelateral lemniscusto theIC,located in themidbrain.Signals are then relayed to thethalamusand further ascending auditory pathway.

Invertebratemammals,as sound waves travel via the eardrum, through thecochleain theinner ear,they stimulate thehair cellsthat line thebasilar membrane.[5]Using these hair cells, the cochlea converts auditory information at each ear into electrical impulses, which travel by means of the auditory nerve (AN) from the cochlea to thecochlear nucleus(CN), which is located in theponsof the brainstem.[6]From theventral CN(VCN), nerve signals project to thesuperior olivary complex(SOC), a set of brainstem nuclei that consists primarily of two nuclei, themedial superior olive (MSO)and thelateral superior olive (LSO),and is the primary site of binaural fusion. The subdivision of the VCN that concerns binaural fusion is theanteroventral cochlear nucleus (AVCN).[3]The AVCN consists ofspherical bushy cellsandglobular bushy cellsand can also transmit signals to themedial nucleus of the trapezoid body (MNTB),whose neurons project to the MSO. Transmissions from the SOC travel to theinferior colliculus (IC)via thelateral lemniscus.At the level of the IC, binaural fusion is more complete. The signal ascends to themedial geniculate body(MGC) of the thalamocortical system; sensory inputs to the MGB are then relayed to theprimary auditory cortex.[3][7][8][9]

Function

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Binaural fusion is responsible for avoiding the creation of multiple sound images from a sound source and its reflections. The advantages of thisphenomenonare more noticeable in small rooms, decreasing as the reflective surfaces are placed farther from the listener.[10]

Central auditory system

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The central auditory system converges inputs from both ears onto neurons within the brainstem. This system contains many subcortical nuclei that collect, integrate, and analyze afferent signals from the ears, for extraction and analysis of the dimensions of sounds. The outcome is a representation of auditory space and auditory objects.[3][11]

The cells of lowerauditory pathwaysare specialized to analyze physical sound parameters.[3]Summationis 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.[4]

Medial superior olive and lateral superior olive

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The MSO contains cells that function in comparing inputs from the left and right cochlear nuclei.[12]The tuning of neurons in the MSO favors low frequencies, whereas those in the LSO favor high frequencies.[13]

GABABreceptorsin the LSO and MSO are involved in balance of excitatory and inhibitory inputs. The GABABreceptors are coupled toG proteinsand provide a way of regulating synaptic efficacy. Specifically, GABABreceptors modulate excitatory and inhibitory inputs to the LSO.[3]Whether the GABABreceptor functions as excitatory or inhibitory for the postsynaptic neuron, depends on the exact location and action of the receptor.[1]

Sound localization

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Sound localization is the ability to correctly identify the directional location of sounds, typically quantified in terms ofazimuth(angle around thehorizontal plane) and elevation (defined in various ways as an angle from the horizontal plane). The time, intensity, and spectral differences in the sounds arriving at the two ears are used in localization. Lateralization (localization in azimuth) of sounds is accomplished primarily by analyzinginteraural time difference (ITD).Localization of high-frequency sounds is aided by analyzinginteraural level difference (ILD)and spectral cues.[4]

Mechanism

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Auditory nerve and cochlear nucleus

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The key mechanisms of the AN and CN are fastsynapsesthat preserve the detail timings, or temporal fine structure, of sounds as transduced to action potentials, from thehair cellsin the cochlea through to the olivary complex. The mechanisms involved include the largest and fastest synapses in the mammalian body, theendbulbs of Held,wheremyelinatedAN fibers innvervate the AVCN, and thecalyx of Held,where neurons from the AVCN innervate the MNTB. The processing and propagation of action potentials through these large excitatory synapses is rapid and temporally precise, and therefore, information about the timing of sound waves, which is crucial to binaural processing, is precisely preserved.[14]

Superior olivary complex

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Binaural processing occurs through the interaction of excitatory and inhibitory inputs in theLSOandMSO.[1][3][12]The SOC processes and integrates binaural information, usually described asITDandILD.This initial processing of ILD and ITD is regulated by GABABreceptors.[1]The exact mechanisms are still being investigated.[15]

Outputs from the MSO and LSO are sent via thelateral lemniscusto 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.[3]Each IC responds primarily to sounds from thecontralateraldirection.

Lateral superior olive

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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 fromspherical bushy cellsof theipsilateralcochlear nucleus, which combine inputs coming from several auditory nerve fibers. Precisely timed inhibitory inputs are received at the LSO from the MNTB, relayed fromglobular bushy cellsof thecontralateralcochlear nucleus.[3]

Medial superior olive

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MSO neurons are excited bilaterally, meaning that they are excited by inputs from both ears, and they are therefore referred to as EE neurons.[3]MSO neurons extract ITD information from binaural inputs and resolve small differences in the time of arrival of sounds at each ear.[3]

Binaural fusion abnormalities in autism

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Current research is being performed on the dysfunction of binaural fusion in individuals withautism.Theneurological disorderautism is associated with many symptoms of impaired brain function, including the degradation of hearing, both unilateral and bilateral.[16]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 thatmorphologicalabnormalities within the brainstem, namely in the SOC, of autistic individuals are a cause of the hearing difficulties.[17]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 asstellateandfusiformformations.[18]Data also suggests that neurons of the LSO andMNTBcontain distinctdysmorphologyin 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.[17]

References

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  1. ^abcdGrothe, Benedikt; Koch, Ursula (2011). "Dynamics of binaural processing in the mammalian sound localization pathway--the role of GABA(B) receptors".Hearing Research.279(1–2): 43–50.doi:10.1016/j.heares.2011.03.013.PMID21447375.S2CID7196476.
  2. ^Schwartz, Andrew; McDermott, Josh (2012)."Spatial cues alone produce inaccurate sound segregation: The effect of inter aural time differences".Journal of the Acoustical Society of America.132(1): 357–368.Bibcode:2012ASAJ..132..357S.doi:10.1121/1.4718637.PMC3407160.PMID22779483.
  3. ^abcdefghijklGrothe, Benedikt; Pecka, Michael; McAlpine, David (2010). "Mechanisms of sound localization in mammals".Physiol Rev.90(3): 983–1012.doi:10.1152/physrev.00026.2009.PMID20664077.
  4. ^abcTyler, R.S.; Dunn, C.C.; Witt, S.A.; Preece, J.P. (2003). "Update on bilateral cochlear implantation".Current Opinion in Otolaryngology & Head and Neck Surgery.11(5): 388–393.doi:10.1097/00020840-200310000-00014.PMID14502072.S2CID7209119.
  5. ^Lim, DJ (1980). "Cochlear anatomy related to cochlear micromechanics. A review".J. Acoust. Soc. Am.67(5): 1686–1695.Bibcode:1980ASAJ...67.1686L.doi:10.1121/1.384295.PMID6768784.
  6. ^Moore, JK (2000)."Organization of the human superior olivary complex".Microsc Res Tech.51(4): 403–412.doi:10.1002/1097-0029(20001115)51:4<403::AID-JEMT8>3.0.CO;2-Q.PMID11071722.S2CID10151612.
  7. ^Cant, Nell B; Benson, Christina G (2003). "Parallel auditory pathways: projection patterns of the different neuronal populations in the dorsal and ventral cochlear nuclei".Brain Research Bulletin.60(5–6): 457–474.doi:10.1016/s0361-9230(03)00050-9.PMID12787867.S2CID42563918.
  8. ^Herrero, Maria-Trinidad; Barcia, Carlos; Navarro, Juana Mari (2002). "Functional anatomy of thalamus and basal ganglia".Child's Nerv Syst.18(8): 386–404.doi:10.1007/s00381-002-0604-1.PMID12192499.S2CID8237423.
  9. ^Twefik, Ted L (2019-10-19)."Auditory System Anatomy".
  10. ^Litovsky, R.; Colburn, H.; Yost, W. (1999). "The Precedence Effect".Journal of the Acoustical Society of America.106(4 Pt 1): 1633–1654.Bibcode:1999ASAJ..106.1633L.doi:10.1121/1.427914.PMID10530009.
  11. ^Masterton, R.B. (1992). "Role of the central auditory system in hearing: the new direction".Trends in Neurosciences.15(8): 280–285.doi:10.1016/0166-2236(92)90077-l.PMID1384196.S2CID4024835.
  12. ^abEldredge, D.H.; Miller, J.D. (1971). "Physiology of hearing".Annu. Rev. Physiol.33:281–310.doi:10.1146/annurev.ph.33.030171.001433.PMID4951051.
  13. ^Guinan, JJ; Norris, BE; Guinan, SS (1972). "Single auditory units in the superior olivary complex II: Locations of unit categories and tonotopic organization".Int J Neurosci.4(4): 147–166.doi:10.3109/00207457209164756.
  14. ^Forsythe, Ian D."Excitatory and inhibitory transmission in the superior olivary complex"(PDF).
  15. ^Yin, Tom CT, Phil H. Smith, and Philip X. Joris (2019)."Neural mechanisms of binaural processing in the auditory brainstem".Comprehensive Physiology.9(4): 1503–1575.doi:10.1002/cphy.c180036.PMID31688966.Retrieved7 August2024.{{cite journal}}:CS1 maint: multiple names: authors list (link)
  16. ^Rosenhall, U; Nordin, V; Sandstrom, M (1999). "Autism and hearing loss".J Autism Dev Disord.29(5): 349–357.doi:10.1023/A:1023022709710.PMID10587881.S2CID18700224.
  17. ^abKulesza Jr., Randy J.; Lukose, Richard; Stevens, Lisa Veith (2011). "Malformation of the human superior olive in autism spectrum disorders".Brain Research.1367:360–371.doi:10.1016/j.brainres.2010.10.015.PMID20946889.S2CID39753895.
  18. ^Kulesza, RJ; Mangunay, K (2008). "Morphological features of the medial superior olive in autism".Brain Res.1200:132–137.doi:10.1016/j.brainres.2008.01.009.PMID18291353.S2CID7388703.
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