Olivocochlear Bundle Research Articles

Cholinergic top-down influences on the auditory brainstem

Abstract:Descending connections are present in many sensory systems and support adaptive information processing. This allows the sensory brain to code a wider range of inputs. A well characterized descending system is the olivo-cochlear cholinergic innervation of the inner ear, which mediates a reduction of the sensitivity of the inner ear upon perception of intense sounds. Because this inhibits the response to background noise, the olivo-cochlear system supports detection of transient sound events. Olivo-cochlear neurons also innervate the cochlear nucleus through axon collaterals. Here, acetylcholine increases the excitability of central neurons without reducing their temporal precision. Thus their target neurons in the superior olivary complex can more effectively process binaural temporal cues. We argue that the central effect of the olivo-cochlear system augments the peripheral effect. In addition, olivo-cochlear cholinergic neurons are under top-down control of cortical inputs, providing further adaptability of information processing on the level of the auditory brainstem.

Clinical relationship between auditory neuropathy and nervous system diseases.

Objective:To explore the clinical relationship between auditory neuropathy (AN) and nervous system diseases.Methods:A total of 134 AN patients who were treated in our hospital from December 2011 to April 2016 were selected. Then 120 cases (240 ears) with complete data of pure tone audiometry and acoustic immittance test were selected as an AN1 group, which was compared with 30 patients (49 ears) with general sensorineural hearing loss (SHL) in regard to the results of pure tone audiometry and acoustic immittance test. On the other hand, 79 cases (158 ears) of the 134 patients with complete data of DP otoacoustic emission test were selected as an AN2 group, which was compared with 30 normal subjects (60 ears) regarding the results of DP otoacoustic emission test.Results:Increases in the pure-tone hearing threshold by air conduction of AN1 group significantly exceeded those of SHL group at 0.125 and 0.25 kHz (low frequency) (P<0.05). The former group had significantly lower values at 1.0, 2.0 kHz (moderate frequency) and 4.0, 8.0 kHz (high frequency) (P<0.05). Of 134 patients, 14 (19 ears) had evoked V wave upon auditory brainstem response, whereas no waves after I wave were evoked in other tested ears. Distortion product (DP) otoacoustic emissions could all be evoked. AN2 group had significantly higher amplitudes of DP-gram than those of normal control group at 0.5 and 0.7 kHz (low frequency) (P<0.05). Except for three cases of unsteady walking and 10 of dizziness, others did not suffer from typical symptoms of vertigo attack. As to caloric test-induced electronystagmograms, there were 30 bilaterally normal cases (75.0%), one case of left-side semicircular canal paresis (25%) and nine cases of bilateral semicircular canal paresis (22.5%). Four patients with other nervous system diseases were complicated with AN. Other nervous system disorders included three cases of optic nerve atrophy and 7 of lower limb nerve damage.Conclusion:According to characteristic hearing dysfunction, AN may occur in the afferent pathway of acoustic nerve, probably accompanied by the pathological changes of efferent nerve in the olivocochlear system inside the brainstem.

Tinnitus, Medial Olivocochlear System, and Music Exposure in Adolescents.

Introduction:The most common cause of tinnitus is the exposure to noise; in the case of adolescents, music is the main sound source they are exposed to. Currently, one of the hypotheses about the genesis of tinnitus is related to the deterioration in the functioning of the medial olivocochlear system (MOCS).Aim:The aim of this study was to determine the presence or absence of tinnitus in adolescents with normal hearing and to relate it to: (a) the functioning of the MOCS, by the contralateral suppression of the transient evoked otoacoustic emissions (TEOAEs) and (b) the musical general exposure (MGE).Materials and Methods:A cross-sectional descriptive correlational study was conducted. The sample was composed by adolescents with ages between 14 and 15. Two questionnaires were administered, one in relation to the subjective report of tinnitus and the other in relation to recreational activities to know the MGE.Results:The results showed that the amplitude of frequencies (1000, 1500, 2000, and 3000 Hz) and global amplitude of TEOAEs, with and without acoustic contralateral stimulation, were higher in the group without tinnitus, with a statistically significant difference (P < 0.05). The suppressive effect was higher in the group without tinnitus; however, there was no statistically significant difference. Contrastingly, a significant association (P < 0.05) between exposure to music and tinnitus was observed; 72.41% of the adolescents with high exposure to music had tinnitus.Discussion and Conclusion:The results of the present investigation provide a contribution to the hypothesis of “the participation of the MOCS.” Furthermore, a high MGE can be considered a risk factor for the onset of tinnitus.

Assessment of Hearing Impairment in Parkinson's Disease: Implications for Differential Diagnosis and Disease Progression

Background and objective: Non-motor symptoms (NMS) of Parkinson's disease (PD) are still underestimated and causative of disability and poor quality of life. Recently, it has been suggested that hearing impairment could be included into the spectrum of NMS, although both mechanisms and phenomenology are unclear. In this study we investigated the peripheral auditory pathway of PD, in patients with asymmetric rest tremor (ART) without dopaminergic denervation, and comparison with healthy controls (HC), aiming to detect differential alterations of cochlear functioning and medial olivocochlear system (MOC).Methods: 23 PD patients, 9 with ART and 19 HC were assessed for auditory functions with clinical examination and Transient-evoked otoacoustic emissions (TEOAEs). PD and ART groups were also evaluated with Unified Parkinson's Disease Rating Scale (UPDRS) II-III and Hoehn and Yahr scale. One-way ANOVA analysis and Pearson's test were performed to measure differences between groups and correlations.Results: TEOAE responses in PD patients were significantly lower compared to HC at 3 and 4 kHz, bilaterally. PD patients showed statistically significant lower TEOAEs at the same frequencies compared to ARTs. In addition, a MOC dysfunction in PD patients was observed. Conversely, no difference was found between ART and HC in all tests performed.Conclusion: PD patients, differently from both ART patients and HC, show abnormalities of basal TEOAEs at the highest frequencies. Auditory dysfunction correlates to motor disturbances, suggesting an underlying dopaminergic pathogenic mechanism. Early recognition of hearing impairment may represent a tool for patient assessment and help in the differential diagnosis in ART patients.

Research about suppression effect and auditory processing in individuals who stutter.

To verify the auditory processing abilities and occurrence of the suppression effect of Otoacoustic Emissions (OAE) in individuals who stutter. The study sample comprised 15 adult individuals who stutter, aged 18-40 years, with stuttering severity ranging from mild to severe, paired according to gender, age, and schooling with individuals without speech complaint or disorder. All participants underwent conventional clinical evaluation, specific stuttering assessment, and basic (audiometry, imitanciometry, and measurement of acoustic reflexes) and specific (auditory processing evaluation and measurement of suppression effect of OAEs) audiological assessments. Data were statistically analyzed with application of the Fisher's Exact Test and the Mann-Whitney Test. The group of individuals who stutter (Study Group - SG) presented higher incidence of auditory processing disorders. The auditory processing assessments used to differentiate the groups of stutterers and non-stutterers (Control Group - CG) were the Nonverbal Dichotic Test and the Frequency Pattern Test. The SG presented higher incidence of absence of suppression effect of OAEs, indicating abnormal functioning of the efferent medial olivocochlear system. The auditory processing abilities investigated in this study differentiate individuals who stutter from non-stutterers, with greater changes in the first. Functioning of the efferent medial olivocochlear system showed a deficit in stutterers, indicating difficulties in auditory discrimination, especially in the presence of noise.

SLC44A4 mutation causes autosomal dominant hereditary postlingual non-syndromic mid-frequency hearing loss.

Clinical, genetic, and functional investigations were performed to identify the causative mutation in a distinctive Chinese family with postlingual non-syndromic mid-frequency sensorineural hearing loss. Whole-exome sequencing revealed SLC44A4, which encodes the choline transport protein, as the pathogenic gene in this family. In the zebrafish model, downregulation of slc44a4 using morpholinos led to significant abnormalities in the zebrafish inner ear and lateral line neuromasts and contributed, to some extent, to disabilities in hearing and balance. SH-SY5Y cells transfected with SLC44A4 showed higher choline uptake and acetylcholine release than that of cells transfected with mutant SLC44A4. We concluded that mutation of SLC44A4 may cause defects in the Choline- acetylcholine system, which is crucial to the efferent innervation of hair cells in the olivocochlear bundle for the maintenance of physiological function of outer hair cells and the protection of hair cells from acoustic injury, leading to hearing loss.

Objective assessment of subjective tinnitus through contralateral suppression of otoacoustic emissions by white noise; suggested cut-off points

Objective: Normative otoacoustic emission (OAE) suppression values are currently lacking and the role of cochlear efferent innervation in tinnitus is controversial. The aim of this study was to investigate the association between tinnitus and medial olivocochlear bundle (MOCB) malfunction. Potential suppression amplitude cut-off criteria that could differentiate participants with tinnitus from those without were sought. Design: Mean suppression amplitudes of transient evoked OAEs and distortion product OAEs by contralateral white noise (50 dBSL) were recorded. Six mean suppression amplitudes criteria were validated as possible cut-off points. Study sample: The population consisted of normal hearing (n = 78) or presbycusic adults (n = 19) with tinnitus or without (n = 28 and 13, respectively) chronic tinnitus (in total, n = 138 78 females/60males, aged 49 ± 14 years). Results: Participants with mean suppression values lower than 0.5–1 dBSPL seem to present a high probability to report tinnitus (specificity 88–97%). On the other hand, participants with mean suppression values larger than 2–2.5dBSPL seem to present a high probability of the absence of tinnitus (sensitivity 87–99%). Correlations were stronger among participants with bilateral presence or absence of tinnitus. Conclusions: This study seem to confirm an association between tinnitus and low suppression amplitudes (<1 dBSPL), which might evolve into an objective examination tool, supplementary to conventional audiological testing.

Assessing the Effects of Pesticide Exposure on the Auditory System based on the Suppression of Transient-Evoked Otoacoustic Emissions

Introduction: Although noise-induced hearing loss is the most significant occupational hazard affecting the auditory system, studies on hearing conservation have shown that frequent exposure to chemicals in the workplace may also cause hearing disorders. Objective: The aim of this study was to evaluate the medial olivocochlear system in individuals exposed to pesticides. Method: This is a cross-sectional descriptive epidemiological study. After applying the inclusion criteria, we evaluated 205 students from an agricultural school located in the city of Nova Friburgo, state of Rio de Janeiro, Brazil, who were exposed to pesticides. Participants were evaluated based on a questionnaire to assess the degree of pesticide exposure, transient-evoked otoacoustic emissions (TEOAEs), and investigation of the suppressive effect of TEOAEs. Results: The assessment considering the suppression of TEOAEs demonstrated that 38% of participants had absence of suppression in the right ear, 45.9% had absence of suppression in the left ear, 38% had unilateral absence, and 39% had bilateral absence. Our findings suggest that there is an association between absence of suppression and quartiles of exposure. Conclusion: Chronic exposure to pesticides can affect the medial olivocochlear system, regardless of concomitant exposure to noise.

Functioning of medial olivocochlear bundle in right- and left-handed individuals

ABSTRACTFunctional symmetry of medial olivocochlear bundle (MOCB) as a function of handedness remains to be well investigated. The current study aimed to assess the functional symmetry of MOCB through contralateral inhibition of otoacoustic emissions (OAEs) in right- and left-handed individuals. Thirteen left-handed and 13 right-handed individuals in the age range of 19–25 years participated. Behavioural experiment involved measuring speech perception in noise and vocal reaction time. Physiological experiment involved measuring the contralateral inhibition of OAEs in both the ears of participants. Findings of the current study revealed lack of functional asymmetry in right- as well as left-handed individuals. Results of the current study suggest that right- as well as left-handed individuals do not demonstrate functional asymmetry at the level of descending auditory pathways unlike the higher cortical centres.

Morphofunctional alterations in the olivocochlear efferent system of the genetic audiogenic seizure-prone hamster GASH:Sal.

The genetic audiogenic seizure hamster (GASH:Sal) is a model of a form of reflex epilepsy that is manifested as generalized tonic-clonic seizures induced by external acoustic stimulation. The morphofunctional alterations in the auditory system of the GASH:Sal that may contribute to seizure susceptibility have not been thoroughly determined. In this study, we analyzed the olivocochlear efferent system of the GASH:Sal from the organ of Corti, including outer and inner hair cells, to the olivocochlear neurons, including shell, lateral, and medial olivocochlear (LOC and MOC) neurons that innervate the cochlear receptor. To achieve this, we carried out a multi-technical approach that combined auditory hearing screenings, scanning electron microscopy, morphometric analysis of labeled LOC and MOC neurons after unilateral Fluoro-Gold injections into the cochlea, and 3D reconstruction of the lateral superior olive (LSO). Our results showed that the GASH:Sal exhibited higher auditory brain response (ABR) thresholds than their controls, as well as absence of distortion-product of otoacoustic emissions (DPOAEs) in a wide range of frequencies. The ABR and DPOAE results also showed differences between the left and right ears, indicating asymmetrical hearing alterations in the GASH:Sal. These alterations in the peripheral auditory activity correlated with morphological alterations. At the cochlear level, the scanning electron microscopy analysis showed marked distortions of the stereocilia from basal to apical cochlear turns in the GASH:Sal, which were not observed in the control hamsters. At the brainstem level, MOC, LOC, and shell neurons had reduced soma areas compared with control animals. This LOC neuron shrinkage contributed to reduction in the LSO volume of the GASH:Sal as shown in the 3D reconstruction analysis. Our study demonstrated that the morphofunctional alterations of the olivocochlear efferent system are innate components of the GASH:Sal, which might contribute to their susceptibility to audiogenic seizures. This article is part of a Special Issue entitled "Genetic and Reflex Epilepsies, Audiogenic Seizures and Strains: From Experimental Models to the Clinic".

Descending projections from the inferior colliculus to medial olivocochlear efferents: Mice with normal hearing, early onset hearing loss, and congenital deafness

Auditory efferent neurons reside in the brain and innervate the sensory hair cells of the cochlea to modulate incoming acoustic signals. Two groups of efferents have been described in mouse and this report will focus on the medial olivocochlear (MOC) system. Electrophysiological data suggest the MOC efferents function in selective listening by differentially attenuating auditory nerve fiber activity in quiet and noisy conditions. Because speech understanding in noise is impaired in age-related hearing loss, we asked whether pathologic changes in input to MOC neurons from higher centers could be involved. The present study investigated the anatomical nature of descending projections from the inferior colliculus (IC) to MOCs in 3-month old mice with normal hearing, and in 6-month old mice with normal hearing (CBA/CaH), early onset progressive hearing loss (DBA/2), and congenital deafness (homozygous Shaker-2). Anterograde tracers were injected into the IC and retrograde tracers into the cochlea. Electron microscopic analysis of double-labelled tissue confirmed direct synaptic contact from the IC onto MOCs in all cohorts. These labelled terminals are indicative of excitatory neurotransmission because they contain round synaptic vesicles, exhibit asymmetric membrane specializations, and are co-labelled with antibodies against VGlut2, a glutamate transporter. 3D reconstructions of the terminal fields indicate that in normal hearing mice, descending projections from the IC are arranged tonotopically with low frequencies projecting laterally and progressively higher frequencies projecting more medially. Along the mediolateral axis, the projections of DBA/2 mice with acquired high frequency hearing loss were shifted medially towards expected higher frequency projecting regions. Shaker-2 mice with congenital deafness had a much broader spatial projection, revealing abnormalities in the topography of connections. These data suggest that loss in precision of IC directed MOC activation could contribute to impaired signal detection in noise.

Selective Attention to Visual Stimuli Using Auditory Distractors Is Altered in Alpha-9 Nicotinic Receptor Subunit Knock-Out Mice.

During selective attention, subjects voluntarily focus their cognitive resources on a specific stimulus while ignoring others. Top-down filtering of peripheral sensory responses by higher structures of the brain has been proposed as one of the mechanisms responsible for selective attention. A prerequisite to accomplish top-down modulation of the activity of peripheral structures is the presence of corticofugal pathways. The mammalian auditory efferent system is a unique neural network that originates in the auditory cortex and projects to the cochlear receptor through the olivocochlear bundle, and it has been proposed to function as a top-down filter of peripheral auditory responses during attention to cross-modal stimuli. However, to date, there is no conclusive evidence of the involvement of olivocochlear neurons in selective attention paradigms. Here, we trained wild-type and α-9 nicotinic receptor subunit knock-out (KO) mice, which lack cholinergic transmission between medial olivocochlear neurons and outer hair cells, in a two-choice visual discrimination task and studied the behavioral consequences of adding different types of auditory distractors. In addition, we evaluated the effects of contralateral noise on auditory nerve responses as a measure of the individual strength of the olivocochlear reflex. We demonstrate that KO mice have a reduced olivocochlear reflex strength and perform poorly in a visual selective attention paradigm. These results confirm that an intact medial olivocochlear transmission aids in ignoring auditory distraction during selective attention to visual stimuli. The auditory efferent system is a neural network that originates in the auditory cortex and projects to the cochlear receptor through the olivocochlear system. It has been proposed to function as a top-down filter of peripheral auditory responses during attention to cross-modal stimuli. However, to date, there is no conclusive evidence of the involvement of olivocochlear neurons in selective attention paradigms. Here, we studied the behavioral consequences of adding different types of auditory distractors in a visual selective attention task in wild-type and α-9 nicotinic receptor knock-out (KO) mice. We demonstrate that KO mice perform poorly in the selective attention paradigm and that an intact medial olivocochlear transmission aids in ignoring auditory distractors during attention.

Editorial: Auditory Efferent System: New Insights from Cortex to Cochlea.

Editorial: Auditory Efferent System: New Insights from Cortex to Cochlea.

Loss of glycine receptors containing the α3 subunit compromises auditory nerve activity, but not outer hair cell function

Inhibitory glycine receptors containing the α3 subunit (GlyRα3) regulate sensory information processing in the CNS and retina. In previous work, we demonstrated the presence of postsynaptic GlyRα3 immunoreactivity at efferent synapses of the medial and lateral olivocochlear bundle in the organ of Corti; however, the role of these α3-GlyRs in auditory signalling has remained elusive. The present study analyzes distortion-product otoacoustic emissions (DPOAEs) and auditory brainstem responses (ABRs) of knockout mice with a targeted inactivation of the Glra3 gene (Glra3−/−) and their wildtype littermates (Glra3+/+) before and seven days after acoustic trauma (AT; 4–16 kHz, 120 dB SPL, 1 h).Before AT, DPOAE thresholds were slightly, but significantly lower, and DPOAE amplitudes were slightly larger in Glra3−/− as compared to Glra3+/+ mice. While click- and f-ABR thresholds were similar in both genotypes before AT, threshold-normalized click-ABR wave I amplitudes were smaller in Glra3−/− mice as compared to their wildtype littermates. Following AT, both the decrement of ABR wave I amplitudes and the delay of wave I latencies were more pronounced in Glra3−/− than Glra3+/+ mice. Accordingly, correlation between early click-evoked ABR signals (0–2.5 ms from stimulus onset) before and after AT was significantly reduced for Glra3−/− as compared to Glra3+/+ mice. In summary, these results show that loss of α3-GlyRs compromises suprathreshold auditory nerve activity, but not outer hair cell function.

Hábitos e queixas auditivas de adolescentes usuários de estéreos pessoais

RESUMO Objetivo: caracterizar os hábitos e queixas auditivas de adolescentes normoouvintes, usuários de estéreos pessoais e relacioná-los com a atividade do sistema olivococlear medial. Métodos: foram avaliados 21 indivíduos, com idades entre 12 e 19 anos, com audição normal. Todos responderam a um questionário fechado referente ao comportamento auditivo e a queixa de zumbido e realizaram avaliação audiológica: Audiometria, Imitanciometria, Emissões Otoacústicas Evocadas Transientes e Efeito de Supressão. Resultados: todos os participantes do estudo ficam expostos à música, a maioria, menos de duas horas diárias. Observou-se a presença do efeito de supressão na orelha direita em 66,67% e na esquerda em 61,90% dos adolescentes. Não foi encontrada associação significante entre a presença do efeito de supressão entre as orelhas, os gêneros e entre tempos de exposição sonora diária e ausência do efeito de supressão. Parte da amostra dos adolescentes referiu ter zumbido, destes, a maioria referiu zumbido em ambas as orelhas. Houve associação significante entre a exposição sonora diária e a presença de zumbido e entre a ausência do efeito de supressão da orelha direita e a presença de zumbido. Conclusão: os adolescentes pesquisados referiram ouvir música diariamente, menos de duas horas diárias. A maioria apresentou efeito de supressão. Houve associação entre queixa de zumbido e ausência do efeito de supressão na orelha direita. Além disso, quanto maior o tempo de exposição sonora maior também a presença de queixa de zumbido nestes adolescentes.

Contralateral Noise Stimulation Delays P300 Latency in School-Aged Children.

Background and ObjectiveThe auditory cortex modulates auditory afferents through the olivocochlear system, which innervates the outer hair cells and the afferent neurons under the inner hair cells in the cochlea. Most of the studies that investigated the efferent activity in humans focused on evaluating the suppression of the otoacoustic emissions by stimulating the contralateral ear with noise, which assesses the activation of the medial olivocochlear bundle. The neurophysiology and the mechanisms involving efferent activity on higher regions of the auditory pathway, however, are still unknown. Also, the lack of studies investigating the effects of noise on human auditory cortex, especially in peadiatric population, points to the need for recording the late auditory potentials in noise conditions. Assessing the auditory efferents in schoolaged children is highly important due to some of its attributed functions such as selective attention and signal detection in noise, which are important abilities related to the development of language and academic skills. For this reason, the aim of the present study was to evaluate the effects of noise on P300 responses of children with normal hearing.MethodsP300 was recorded in 27 children aged from 8 to 14 years with normal hearing in two conditions: with and whitout contralateral white noise stimulation.ResultsP300 latencies were significantly longer at the presence of contralateral noise. No significant changes were observed for the amplitude values.ConclusionContralateral white noise stimulation delayed P300 latency in a group of school-aged children with normal hearing. These results suggest a possible influence of the medial olivocochlear activation on P300 responses under noise condition.

Frequency Tuning of the Efferent Effect on Cochlear Gain in Humans.

Cochlear gain reduction via efferent feedback from the medial olivocochlear bundle is frequency specific (Guinan, Curr Opin Otolaryngol Head Neck Surg 18:447-453, 2010). The present study with humans used the Fixed Duration Masking Curve psychoacoustical method (Yasin et al., J Acoust Soc Am 133:4145-4155, 2013a; Yasin et al., Basic aspects of hearing: physiology and perception, pp 39-46, 2013b; Yasin et al., J Neurosci 34:15319-15326, 2014) to estimate the frequency specificity of the efferent effect at the cochlear level. The combined duration of the masker-plus-signal stimulus was 25 ms, within the efferent onset delay of about 31-43 ms (James et al., Clin Otolaryngol 27:106-112, 2002). Masker level (4.0 or 1.8 kHz) at threshold was obtained for a 4-kHz signal in the absence or presence of an ipsilateral 60 dB SPL, 160-ms precursor (200-Hz bandwidth) centred at frequencies between 2.5 and 5.5 kHz. Efferent-mediated cochlear gain reduction was greatest for precursors with frequencies the same as, or close to that of, the signal (gain was reduced by about 20 dB), and least for precursors with frequencies well removed from that of the signal (gain remained at around 40 dB). The tuning of the efferent effect filter (tuning extending 0.5-0.7 octaves above and below the signal frequency) is within the range obtained in humans using otoacoustic emissions (Lilaonitkul and Guinan, J Assoc Res Otolaryngol 10:459-470, 2009; Zhao and Dhar, J Neurophysiol 108:25-30, 2012). The 10 dB bandwidth of the efferent-effect filter at 4000 Hz was about 1300 Hz (Q(10) of 3.1). The FDMC method can be used to provide an unbiased measure of the bandwidth of the efferent effect filter using ipsilateral efferent stimulation.

Localization-in-noise and binaural medial olivocochlear functioning in children and young adults.

Children as young as 5 yr old localize sounds as accurately as adults in quiet in the frontal hemifield. However, children's ability to localize in noise and in the front/back (F/B) dimension are scantily studied. To address this, the first part of this study investigated localization-in-noise ability of children vs young adults in two maskers: broadband noise (BBN) and speech-babble (SB) at three signal-to-noise ratios: -12, -6, and 0 dB. In the second part, relationship between binaural medial olivocochlear system (MOC) function and localization-in-noise was investigated. In both studies, 21 children and 21 young adults participated. Results indicate, while children are able to differentiate sounds arriving in the F/B dimension on par with adults in quiet and in BBN, larger differences were found for SB. Accuracy of children's localization in noise (for both maskers) in the lateral plane was also poorer than adults'. Significant differences in binaural MOC interaction (mBIC; the difference between the sum of two monaural- and binaural-MOC strength) between adults and children were also found. For reasons which are not clear, adult F/B localization in BBN correlates better with mBIC while children's F/B localization in SB correlated better with binaural MOC strength.

The brainstem efferent acoustic chiasm in pigmented and albino rats

The present study examined whether structural peculiarities in the brain-efferent pathway to the organ of Corti may underlie functional differences in hearing between pigmented and albino individuals of the same mammalian species. Pigmented Brown-Norway rats and albino Wistar rats received unilateral injections of an aqueous solution of the retrograde neuronal tracer Fluorogold (FG) into the scala tympani of the cochlea to identify olivocochlear neurons (OCN) in the brainstem superior olivary complex. After five days, brains were perfusion-fixed and brainstem sections were cut and analyzed with respect to retrogradely labeled neurons. Intrinsic neurons of the lateral system were located exclusively in the ipsilateral lateral superior olive (LSO) in both groups. Shell neurons surrounding the LSO and in periolivary regions, which made up only 5–8% of all OCN, were more often contralaterally located in albino than in pigmented animals. A striking difference was observed in the laterality of neurons of the medial olivocochlear (MOC) system, which provided more than one third of all OCN. These neurons, located in the rostral periolivary region and in the ventral nucleus of the trapezoid body, were observed contralateral to 45% in pigmented and to 68% in albino animals. Our study, the first to compare the origin of the olivocochlear bundle in pigmented and albino rats, provides evidence for differences in the crossing pattern of the olivocochlear pathway. These were found predominantly in the MOC system providing the direct efferent innervation of cochlear outer hair cells. Our findings may account for the alterations in auditory perception observed in albino mammals including man.

Chronic Conductive Hearing Loss Leads to Cochlear Degeneration

Synapses between cochlear nerve terminals and hair cells are the most vulnerable elements in the inner ear in both noise-induced and age-related hearing loss, and this neuropathy is exacerbated in the absence of efferent feedback from the olivocochlear bundle. If age-related loss is dominated by a lifetime of exposure to environmental sounds, reduction of acoustic drive to the inner ear might improve cochlear preservation throughout life. To test this, we removed the tympanic membrane unilaterally in one group of young adult mice, removed the olivocochlear bundle in another group and compared their cochlear function and innervation to age-matched controls one year later. Results showed that tympanic membrane removal, and the associated threshold elevation, was counterproductive: cochlear efferent innervation was dramatically reduced, especially the lateral olivocochlear terminals to the inner hair cell area, and there was a corresponding reduction in the number of cochlear nerve synapses. This loss led to a decrease in the amplitude of the suprathreshold cochlear neural responses. Similar results were seen in two cases with conductive hearing loss due to chronic otitis media. Outer hair cell death was increased only in ears lacking medial olivocochlear innervation following olivocochlear bundle cuts. Results suggest the novel ideas that 1) the olivocochlear efferent pathway has a dramatic use-dependent plasticity even in the adult ear and 2) a component of the lingering auditory processing disorder seen in humans after persistent middle-ear infections is cochlear in origin.