Evidence of a medial olivocochlear involvement in contralateral suppression of otoacoustic emissions in humans

Effect of Contralateral Acoustic Stimulation on Temporal Processing Abilities in Individuals with Normal Hearing

The aim of the article was to compare the conditions of silent and contralateral noise on the temporal processing parameters. A total of 40 participants (20 males and 20 females) were enrolled in the study with a mean age of 21.7 years, participants with normal hearing thresholds and no history of middle ear pathology were enrolled for the study. The temporal processing tests such as duration discrimination test, gap detection test, and temporal modulation transfer function tests were carried out in these 40 individuals in the two conditions of silent and contralateral noise using MATLAB. Statistical analysis was carried out using the SPSS version 25.0 were descriptive and inferential statistics were carried out. Data was normally distributed on the Shapiro-Wilk's test of normality due to which a paired t test was carried out to establish the nature of significance between the silent and contralateral noise condition. Results reveal the presence of significant difference (p < 0.01) between the groups for all the parameters of temporal resolution with contralateral noise condition performing better than silent condition for the parameters. There is a positive effect of the efferent auditory pathway on the temporal resolution parameters thus implying that speech perception in noise is improved in the presence of background noise for normal hearing individuals due to this effect.

Usefulness of phase gradients of otoacoustic emissions in auditory health screening: An exploration with swept tones

Otoacoustic emissions (OAEs) are low-level sounds generated by the cochlea and widely used as a noninvasive tool to inspect cochlear impairments. However, only the amplitude information of OAE signals is used in current clinical tests, while the OAE phase containing important information about cochlear functions is commonly discarded, due to the insufficient frequency-resolution of existing OAE tests. In this study, swept tones with time-varying frequencies were used to measure stimulus frequency OAEs (SFOAEs) in human subjects, so that high-resolution phase spectra that are not available in existing OAE tests could be obtained and analyzed. The results showed that the phase of swept-tone SFOAEs demonstrated steep gradients as the frequency increased in human subjects with normal hearing. The steep phase gradients were sensitive to auditory functional abnormality caused by cochlear damage and stimulus artifacts introduced by system distortions. At low stimulus levels, the group delays derived from the phase gradients decreased from around 8.5 to 3 ms as the frequency increased from 1 to 10 kHz for subjects with normal hearing, and the pattern of group-delay versus frequency function showed significant difference for subjects with hearing loss. By using the swept-tone technology, the study suggests that the OAE phase gradients could provide highly sensitive information about the cochlear functions and therefore should be integrated into the conventional methods to improve the reliability of auditory health screening.

Audiological Involvement in Patients with Systemic Sclerosis

OBJECTIVE

The aim of the present study was to evaluate hearing loss in patients diagnosed with SSc (systemic sclerosis) and to investigate the relation between hearing loss, subtypes of the disease, its duration and clinical findings, and antibody positivity.

METHODS

The study included 47 patients with SSc and 44 healthy controls. Audiometric, tympanometric and otoacoustic emission measurements were applied to both groups.

RESULTS

The evaluation of the participants medical history showed that among the patients with SSc, 19.1% experienced ear fullness, 27.7% experienced vertigo and 36.2% experienced tinnitus. Hearing loss was detected in 23.4% of the patients with SSc. The corresponding result was 4.3% in the control group with a statistically significant difference (p = 0.001). Transient evoked otoacoustic emission (TEOAE) amplitude values were significantly lower in the patients both ears with SSc than the control group (p < 0.005). Duration of disease was significantly longer and DLCO (diffusing capacity of the lungs for carbon monoxide) values were significantly higher in the patients with SSc and sensorineural hearing loss.

CONCLUSIONS

The present study found that the incidence of hearing loss was significantly higher in the study group than healthy control group. In addition to other organ involvements, cochlear involvement occurs in these patients, and further studies are required.

The relationship between ipsilateral cochlear gain reduction and speech-in-noise recognition at positive and negative signal-to-noise ratios

Active mechanisms that regulate cochlear gain are hypothesized to influence speech-in-noise perception. However, evidence of a relationship between the amount of cochlear gain reduction and speech-in-noise recognition is mixed. Findings may conflict across studies because different signal-to-noise ratios (SNRs) were used to evaluate speech-in-noise recognition. Also, there is evidence that ipsilateral elicitation of cochlear gain reduction may be stronger than contralateral elicitation, yet, most studies have investigated the contralateral descending pathway. The hypothesis that the relationship between ipsilateral cochlear gain reduction and speech-in-noise recognition depends on the SNR was tested. A forward masking technique was used to quantify the ipsilateral cochlear gain reduction in 24 young adult listeners with normal hearing. Speech-in-noise recognition was measured with the PRESTO-R sentence test using speech-shaped noise presented at -3, 0, and +3 dB SNR. Interestingly, greater cochlear gain reduction was associated with lower speech-in-noise recognition, and the strength of this correlation increased as the SNR became more adverse. These findings support the hypothesis that the SNR influences the relationship between ipsilateral cochlear gain reduction and speech-in-noise recognition. Future studies investigating the relationship between cochlear gain reduction and speech-in-noise recognition should consider the SNR and both descending pathways.

Projections from the ventral nucleus of the lateral lemniscus to the cochlea in the mouse

Auditory efferents originate in the central auditory system and project to the cochlea. Although the specific anatomy of the olivocochlear (OC) efferents can vary between species, two types of auditory efferents have been identified based upon the general location of their cell bodies and their distinctly different axon terminations in the organ of Corti. In the mouse, the relatively small somata of the lateral (LOC) efferents reside in the lateral superior olive (LSO), have unmyelinated axons, and terminate around ipsilateral inner hair cells (IHCs), primarily against the afferent processes of type I auditory nerve fibers. In contrast, the larger somata of the medial (MOC) efferents are distributed in the ventral nucleus of the trapezoid body (VNTB), have myelinated axons, and terminate bilaterally against the base of multiple outer hair cells (OHCs). Using in vivo retrograde cell body marking, anterograde axon tracing, immunohistochemistry, and electron microscopy, we have identified a group of efferent neurons in mouse, whose cell bodies reside in the ventral nucleus of the lateral lemniscus (VNLL). By virtue of their location, we call them dorsal efferent (DE) neurons. Labeled DE cells were immuno-negative for tyrosine hydroxylase, glycine, and GABA, but immuno-positive for choline acetyltransferase. Morphologically, DEs resembled LOC efferents by their small somata, unmyelinated axons, and ipsilateral projection to IHCs. These three classes of efferent neurons all project axons directly to the cochlea and exhibit cholinergic staining characteristics. The challenge is to discover the contributions of this new population of neurons to auditory efferent function.

Olivocochlear efferent effects on perception and behavior

The role of the mammalian auditory olivocochlear efferent system in hearing has long been the subject of debate. Its ability to protect against damaging noise exposure is clear, but whether or not this is the primary function of a system that evolved in the absence of industrial noise remains controversial. Here we review the behavioral consequences of olivocochlear activation and diminished olivocochlear function. Attempts to demonstrate a role for hearing in noise have yielded conflicting results in both animal and human studies. A role in selective attention to sounds in the presence of distractors, or attention to visual stimuli in the presence of competing auditory stimuli, has been established in animal models, but again behavioral studies in humans remain equivocal. Auditory processing deficits occur in models of congenital olivocochlear dysfunction, but these deficits likely reflect abnormal central auditory development rather than direct effects of olivocochlear feedback. Additional proposed roles in age-related hearing loss, tinnitus, hyperacusis, and binaural or spatial hearing, are intriguing, but require additional study. These behavioral studies almost exclusively focus on medial olivocochlear effects, and many relied on lesioning techniques that can have unspecific effects. The consequences of lateral olivocochlear and of corticofugal pathway activation for perception remain unknown. As new tools for targeted manipulation of olivocochlear neurons emerge, there is potential for a transformation of our understanding of the role of the olivocochlear system in behavior across species.

Evaluation of Efferent Auditory System and Hearing Quality in Parkinson's Disease: Is the Difficulty in Speech Understanding in Complex Listening Conditions Related to Neural Degeneration or Aging?

Purpose The effects of neurological diseases on the auditory system have been a notable issue for investigators because the auditory pathway is closely associated with neural systems. The purposes of this study are to evaluate the efferent auditory system function and hearing quality in Parkinson's disease (PD) and to compare the findings with age-matched individuals without PD to present a perspective on aging. Method The study included 35 individuals with PD (mean age of 48.50 ± 8.00 years) and 35 normal-hearing peers (mean age of 49 ± 10 years). The following tests were administered for all participants: the first section of the Speech, Spatial and Qualities of Hearing Scale; pure-tone audiometry, speech audiometry, tympanometry, and acoustic reflexes; and distortion product otoacoustic emissions (DPOAEs) and contralateral suppression of DPOAEs. SPSS Version 25 was used for statistical analyses, and values of p < .05 were considered statistically significant. Results There were no statistically significant differences in the pure-tone audiometry thresholds and DPOAE responses between the individuals with PD and their normal-hearing peers (p = .732). However, statistically significant differences were found between the groups in suppression levels of DPOAEs and hearing quality (p < .05). In addition, a statistically significant and positive correlation was found between the amount of suppression at some frequencies and the Speech, Spatial and Qualities of Hearing Scale scores. Conclusions This study indicates that medial olivocochlear efferent system function and the hearing quality of individuals with PD were affected adversely due to the results of PD pathophysiology on the hearing system. For optimal intervention and follow-up, tasks related to hearing quality in daily life can also be added to therapies for PD.

Investigating Influences of Medial Olivocochlear Efferent System on Central Auditory Processing and Listening in Noise: A Behavioral and Event-Related Potential Study

This electrophysiological study investigated the role of the medial olivocochlear (MOC) efferents in listening in noise. Both ears of eleven normal-hearing adult participants were tested. The physiological tests consisted of transient-evoked otoacoustic emission (TEOAE) inhibition and the measurement of cortical event-related potentials (ERPs). The mismatch negativity (MMN) and P300 responses were obtained in passive and active listening tasks, respectively. Behavioral responses for the word recognition in noise test were also analyzed. Consistent with previous findings, the TEOAE data showed significant inhibition in the presence of contralateral acoustic stimulation. However, performance in the word recognition in noise test was comparable for the two conditions (i.e., without contralateral stimulation and with contralateral stimulation). Peak latencies and peak amplitudes of MMN and P300 did not show changes with contralateral stimulation. Behavioral performance was also maintained in the P300 task. Together, the results show that the peripheral auditory efferent effects captured via otoacoustic emission (OAE) inhibition might not necessarily be reflected in measures of central cortical processing and behavioral performance. As the MOC effects may not play a role in all listening situations in adults, the functional significance of the cochlear effects of the medial olivocochlear efferents and the optimal conditions conducive to corresponding effects in behavioral and cortical responses remain to be elucidated.

Evaluation of medial olivocochlear efferent system and hearing loss in patients with primary Sjögren’s syndrome

Background/aim

Autoimmune diseases are a remarkable issue for researchers due to their adverse effects on the auditory system, but for primary Sjögren’s syndrome (pSS) there is little research on the comprehensive audiological findings in literature. The main objective of this study was to investigate the medial olivocochlear efferent functions of subjects with pSS and to examine the audiological findings.

Materials and methods

The study included 36 subjects with pSS and 36 healthy subjects. All the subjects underwent testing including pure tone, speech, and high frequency audiometry; tympanometry and acoustic reflexes; distortion product otoacoustic emissions (DPOAE); and suppression of DPOAE.

Results

The hearing thresholds of the pSS group were higher than those of the control at all frequencies (P < 0.001). Minimal to mild sensorineural hearing loss was observed in 52.77% of all the subjects with pSS. Additionally, all of the subjects had type A curve tympanograms, but the static compliances of the pSS group were lower and the acoustic reflex thresholds were higher than in the control (P < 0.001). In suppression levels of DPOAE, no statistically significant difference was found between the groups (P > 0.05).

Conclusion

The study indicates that because of obtaining normal suppression levels in DPOAE, the medial olivocochlear efferent system is functional in pSS. However, there is a need for more tests, including auditory brainstem response, to evaluate the afferent auditory system in pSS.

Efferent Inhibition of the Cochlea

Cholinergic efferent neurons originating in the brainstem innervate the acoustico-lateralis organs (inner ear, lateral line) of vertebrates. These release acetylcholine (ACh) to inhibit hair cells through activation of calcium-dependent potassium channels. In the mammalian cochlea, ACh shunts and suppresses outer hair cell (OHC) electromotility, reducing the essential amplification of basilar membrane motion. Consequently, medial olivocochlear neurons that inhibit OHCs reduce the sensitivity and frequency selectivity of afferent neurons driven by cochlear vibration of inner hair cells (IHCs). The cholinergic synapse on hair cells involves an unusual ionotropic ACh receptor, and a near-membrane postsynaptic cistern. Lateral olivocochlear (LOC) neurons modulate type I afferents by still-to-be-defined synaptic mechanisms. Olivocochlear neurons can be activated by a reflex arc that includes the auditory nerve and projections from the cochlear nucleus. They are also subject to modulation by higher-order central auditory interneurons. Through its actions on cochlear hair cells, afferent neurons, and higher centers, the olivocochlear system protects against age-related and noise-induced hearing loss, improves signal coding in noise under certain conditions, modulates selective attention to sensory stimuli, and influences sound localization.

The MAndarin spoken word-Picture IDentification test in noise-Adaptive (MAPID-A) measures subtle speech-recognition-in-noise changes and spatial release from masking in very young children

Background

Spatial release of masking (SRM) is a measure of an individual’s ability to perform speech-noise segregation, which is usually quantified by the extent of improvement of the individual’s speech recognition performance when the noise is switched from a spatially co-located position (e.g., speech and noise both presented from the front) to a spatially separated position (e.g., speech presented from the front and noise presented from the right side) with reference to the target speech. SRM is a combined measure of head shadow and binaural unmasking benefits. SRM has only been investigated in young children at group level but not at individual participant level in the international literature due to the lack of reliable speech recognition test materials able to detect subtle statistically significant within-participant changes in speech-recognition-in-noise thresholds.

Method

The performance to signal-to-noise ratio (P-SNR) functions of twenty-four disyllabic words were obtained from 40 native Mandarin-speaking children aged 3.6–6.2 years with reported normal speech, language and hearing. The test items’ difficulty levels were homogenized by adjusting the speech intensity level of each item so that the adjusted signal-to-noise ratio for 50% correct score (SNR-50%) point of each item would overlap at the mean SNR-50% point of all test items. In the MAPID-A, the homogenized test items were randomly presented in an adaptive testing procedure at a fixed noise intensity level, but the speech intensity level of the upcoming test item varied in 2-dB SNR steps depending on the recognition result of the previous test item. The SNR reversal point is marked by a change from a decrease to an increase in the SNR or vice versa. Two successive SNR reversal points marked the boundaries of an excursion. The mid-points from 12 excursions (in dB SNR) were averaged to produce the adaptive SNR-50% measure (aSNR-50%).

Results

The aSNR-50% results were obtained from another 12 children aged 4.8–5.3 years with reported normal speech, language and hearing. The average 99% confidence interval (CI) of all participants’ mean aSNR-50% values was ±1.61 dB SNR; therefore, 3.22 dB SNR was the average critical difference required to confirm a significant difference in the scores obtained from the same participant between two test conditions. Statistically significant within-participant SRM was identified in 95% of the participants; in other words, aSNR-50% obtained from the spatially separated condition outperformed aSNR-50% obtained from the spatially co-located condition. The adaptive testing procedure was highly reliable, with an within-participant test-retest reliability of 90.6%. and significantly limited testing time to an average of 4.2 min. This research study has fulfilled its aim on detecting subtle within-participant SRM in very young children starting from 4 years of age with a reliable statistical procedure. MAPID-A offers a reliable and efficient clinical tool to investigate speech-recognition-in-noise and SRM performances in young Mandarin-speaking children.

Conclusions

The narrow CIs, high test-retest reliability, and short testing time has proven that the MAPID-A is a promising sensitive, reliable and time-efficient clinical tool to detect subtle within-participant speech-recognition-in-noise changes in children as young as 4–5 years. The MAPID-A offers a clinical tool to behaviorally track young children’s development in speech-recognition-in-noise and SRM, and to potentially review the development of the auditory neural pathway and the cerebral dominance for speech-recognition-in-noise in young children.

Insights from the third international conference on hyperacusis: causes, evaluation, diagnosis, and treatment

BACKGROUND

Hyperacusis is intolerance of certain everyday sounds that causes significant distress and impairment in social, occupational, recreational, and other day-to-day activities.

OBJECTIVE

The aim of this report is to summarize the key findings and conclusions from the Third International Conference on Hyperacusis.

TOPICS COVERED

The main topics discussed comprise (1) diagnosis of hyperacusis and audiological evaluations, (2) neurobiological aspect of hyperacusis, (3) misophonia, (4) hyperacusis in autism spectrum disorder, (5) noise sensitivity, (6) hyperacusis-related distress and comorbid psychiatric illness, and (7) audiologist-delivered cognitive behavioral therapy for hyperacusis.

CONCLUSIONS

Implications for research and clinical practice are summarised.

Olivocochlear Efferents in Animals and Humans: From Anatomy to Clinical Relevance

Olivocochlear efferents allow the central auditory system to adjust the functioning of the inner ear during active and passive listening. While many aspects of efferent anatomy, physiology and function are well established, others remain controversial. This article reviews the current knowledge on olivocochlear efferents, with emphasis on human medial efferents. The review covers (1) the anatomy and physiology of olivocochlear efferents in animals; (2) the methods used for investigating this auditory feedback system in humans, their limitations and best practices; (3) the characteristics of medial-olivocochlear efferents in humans, with a critical analysis of some discrepancies across human studies and between animal and human studies; (4) the possible roles of olivocochlear efferents in hearing, discussing the evidence in favor and against their role in facilitating the detection of signals in noise and in protecting the auditory system from excessive acoustic stimulation; and (5) the emerging association between abnormal olivocochlear efferent function and several health conditions. Finally, we summarize some open issues and introduce promising approaches for investigating the roles of efferents in human hearing using cochlear implants.

Minimal Effects of Age and Exposure to a Noisy Environment on Hearing in Alpha9 Nicotinic Receptor Knockout Mice

Studies have suggested a role of weakened medial olivocochlear (OC) efferent feedback in accelerated hearing loss and increased susceptibility to noise. The present study investigated the progression of hearing loss with age and exposure to a noisy environment in medial OC-deficient mice. Alpha9 nicotinic acetylcholine receptor knockout (α9KO) and wild types were screened for hearing loss using auditory brainstem responses. α9KO mice housed in a quiet environment did not show increased hearing loss compared to wild types in young adulthood and middle age. Challenging the medial OC system by housing in a noisy environment did not increase hearing loss in α9KO mice compared to wild types. ABR wave 1 amplitudes also did not show differences between α9KO mice and wild types. These data suggest that deficient medial OC feedback does not result in early onset of hearing loss.

Medial olivocochlear function in children with poor speech-in-noise performance and language disorder

OBJECTIVES

Contralateral masking of transient-evoked otoacoustic emissions is a phenomenon that suggests an inhibitory effect of the olivocochlear efferent auditory pathway. Many studies have been inconclusive in demonstrating a clear connection between this system and a behavioral speech-in-noise listening skill. The purpose of this study was to investigate the activation of a medial olivocochlear (MOC) efferent in children with poor speech-in-noise (PSIN) performance and children with language impairment and PSIN (SLI + PSIN).

METHODS

Transient evoked otoacoustic emissions (TEOAEs) with and without contralateral white noise were tested in 52 children (between 6 and 12 years). These children were arranged in three groups: typical development (TD) (n = 25), PSIN (n = 14) and SLI + PSI (n = 13).

RESULTS

PSIN and SLI + PSI groups presented reduced otoacoustic emission suppression in comparison with the TD group.

CONCLUSION

Our finding suggests differences in MOC function among children with typical development and children with poor SIN and language problems.

Effect of Contralateral Medial Olivocochlear Feedback on Perceptual Estimates of Cochlear Gain and Compression

The active cochlear mechanism amplifies responses to low-intensity sounds, compresses the range of input sound intensities to a smaller output range, and increases cochlear frequency selectivity. The gain of the active mechanism can be modulated by the medial olivocochlear (MOC) efferent system, creating the possibility of top-down control at the earliest level of auditory processing. In humans, MOC function has mostly been measured by the suppression of otoacoustic emissions (OAEs), typically as a result of MOC activation by a contralateral elicitor sound. The exact relationship between OAE suppression and cochlear gain reduction, however, remains unclear. Here, we measured the effect of a contralateral MOC elicitor on perceptual estimates of cochlear gain and compression, obtained using the established temporal masking curve (TMC) method. The measurements were taken at a signal frequency of 2 kHz and compared with measurements of click-evoked OAE suppression. The elicitor was a broadband noise, set to a sound pressure level of 54 dB to avoid triggering the middle ear muscle reflex. Despite its low level, the elicitor had a significant effect on the TMCs, consistent with a reduction in cochlear gain. The amount of gain reduction was estimated as 4.4 dB on average, corresponding to around 18 % of the without-elicitor gain. As a result, the compression exponent increased from 0.18 to 0.27.

The Hyperactivity of Efferent Auditory System in Patients with Schizophrenia: A Transient Evoked Otoacoustic Emissions Study

OBJECTIVE

Electrophysiological studies, which are mostly focused on afferent pathway, have proven that auditory processing deficits exist in patients with schizophrenia. Nevertheless, reports on the suppressive effect of efferent auditory pathway on cochlear outer hair cells among schizophrenia patients are limited. The present, case-control, study examined the contralateral suppression of transient evoked otoacoustic emissions (TEOAEs) in patients with schizophrenia.

METHODS

Participants were twenty-three healthy controls and sixteen schizophrenia patients with normal hearing, middle ear and cochlear outer hair cells function. Absolute non-linear and linear TEOAEs were measured in both ears by delivering clicks stimuli at 80 dB SPL and 60 dB SPL respectively. Subsequently, contralateral suppression was determined by subtracting the absolute TEOAEs response obtained at 60 dBpe SPL during the absence and presence of contralateral white noise delivered at 65 dB HL. No attention tasks were conducted during measurements.

RESULTS

We found no significant difference in absolute TEOAEs responses at 80 dB SPL, in either diagnosis or ear groups (p>0.05). However, the overall contralateral suppression was significantly larger in schizophrenia patients (p<0.05). Specifically, patients with schizophrenia demonstrated significantly increased right ear contralateral suppression compared to healthy control (p<0.05).

CONCLUSION

The present findings suggest increased inhibitory effect of efferent auditory pathway especially on the right cochlear outer hair cells. Further studies to investigate increased suppressive effects are crucial to expand the current understanding of auditory hallucination mechanisms in schizophrenia patients.

Effects of Presentation Rate and Attention on Auditory Discrimination: A Comparison of Long-Latency Auditory Evoked Potentials in School-Aged Children and Adults

Decoding human speech requires both perception and integration of brief, successive auditory stimuli that enter the central nervous system as well as the allocation of attention to language-relevant signals. This study assesses the role of attention on processing rapid transient stimuli in adults and children. Cortical responses (EEG/ERPs), specifically mismatch negativity (MMN) responses, to paired tones (standard 100-100 Hz; deviant 100-300 Hz) separated by a 300, 70 or 10 ms silent gap (ISI) were recorded under Ignore and Attend conditions in 21 adults and 23 children (6-11 years old). In adults, an attention-related enhancement was found for all rate conditions and laterality effects (L>R) were observed. In children, 2 auditory discrimination-related peaks were identified from the difference wave (deviant-standard): an early peak (eMMN) at about 100-300 ms indexing sensory processing, and a later peak (LDN), at about 400-600 ms, thought to reflect reorientation to the deviant stimuli or "second-look" processing. Results revealed differing patterns of activation and attention modulation for the eMMN in children as compared to the MMN in adults: The eMMN had a more frontal topography as compared to adults and attention played a significantly greater role in childrens' rate processing. The pattern of findings for the LDN was consistent with hypothesized mechanisms related to further processing of complex stimuli. The differences between eMMN and LDN observed here support the premise that separate cognitive processes and mechanisms underlie these ERP peaks. These findings are the first to show that the eMMN and LDN differ under different temporal and attentional conditions, and that a more complete understanding of children's responses to rapid successive auditory stimulation requires an examination of both peaks.

Distortion-product otoacoustic emissions and contralateral suppression findings in children with Asperger's Syndrome

OBJECTIVES

Children with Asperger's Syndrome (AS) often demonstrate auditory behaviors such as hypersensitivity to sounds and poor performance in noisy environments. These auditory behaviors may be related to cochlear dysfunction and abnormal medial olivocochlear bundle (MOCB) activity. The objective of this study was to examine the distortion-product otoacoustic emissions (DPOAEs) with and without contralateral white noise to evaluate outer hair cell activity and MOCB activity in children with AS.

METHODS

A case control study where 18 boys with AS and 18 age-matched control subjects participated in the study. For both groups, DPOAEs were recorded at 4031, 2627, 1969, and 1359 Hz f2 frequencies with and without contralateral white noise at 30 dB SL. DPOAE SNRs and difference scores were analyzed for possible differences between both groups and age subgroups (young and old children).

RESULTS

In the quiet condition, there were no significant group or ear differences in DPOAEs SNR. However, DPOAEs SNR were larger at 4031 Hz than at lower frequencies in both groups, mostly due to negative effect of background noise on low frequency response. Contralateral noise resulted in both suppression and enhancement of the DPOAE SNRs in 93% of the control group and 90% of the AS group. However, there were no significant differences in suppression and enhancement between the two groups or age subgroups. The young controls had right ear advantage and significantly larger suppression at all frequencies except 4031 Hz than old controls. The young children with AS had slight left ear advantage and significantly larger suppression only at 2672 Hz compared to the old children with AS.

CONCLUSIONS

The results, indicating minor differences in DPOAEs and contralateral suppression and enhancement of DPOAEs between both control and AS groups and age subgroups, suggest subtle differences in the function of the outer hair cells and the MOCB activity. Therefore, other central auditory processing in the temporal lobe, limbic system and autonomic nervous system may be involved in the generation of hypersensitivity to sounds and difficulty understanding in noisy environments in children with AS.

Central masking with bilateral cochlear implants

Across bilateral cochlear implants, contralateral threshold shift has been investigated as a function of electrode difference between the masking and probe electrodes. For contralateral electric masking, maximum threshold elevations occurred when the position of the masker and probe electrode was approximately place-matched across ears. The amount of masking diminished with increasing masker-probe electrode separation. Place-dependent masking occurred in both sequentially implanted ears, and was not affected by the masker intensity or the time delay from the masker onset. When compared to previous contralateral masking results in normal hearing, the similarities between place-dependent central masking patterns suggest comparable mechanisms of overlapping excitation in the central auditory nervous system.

Wavelet analysis demonstrates no abnormality in contralateral suppression of otoacoustic emissions in tinnitus patients

The efferent auditory system is thought to play a role in the origin of tinnitus. Part of this system can be tested in humans with contralateral suppression of otoacoustic emissions. Stimulation of the medial olivocochlear efferent system is responsible for this reduction of otoacoustic emissions after contralateral acoustic stimulation. Previous research on patients with tinnitus showed inconclusive results. With wavelet analysis both time and frequency information of the emission can be analysed and compared. Contralateral suppression of otoacoustic emissions was therefore measured in tinnitus patients (n = 26) and normal subjects (n = 37) and analysed using wavelets. No significant difference in suppression was found between the tinnitus patients and the control group.

The MOC reflex during active listening to speech

PURPOSE

The purpose of this study was to test the hypothesis that active listening to speech would increase medial olivocochlear (MOC) efferent activity for the right vs. the left ear.

METHOD

Click-evoked otoacoustic emissions (CEOAEs) were evoked by 60-dB p.e. SPL clicks in 13 normally hearing adults in 4 test conditions for each ear: (a) in quiet; (b) with 60-dB SPL contralateral broadband noise; (c) with words embedded (at -3-dB signal-to-noise ratio [SNR]) in 60-dB SPL contralateral noise during which listeners directed attention to the words; and (d) for the same SNR as in the 3rd condition, with words played backwards.

RESULTS

There was greater suppression during active listening compared with passive listening that was apparent in the latency range of 6- to 18-ms poststimulus onset. Ear differences in CEOAE amplitude were observed in all conditions, with right-ear amplitudes larger than those for the left. The absolute difference between CEOAE amplitude in quiet and with contralateral noise, a metric of suppression, was equivalent for right and left ears. When the amplitude differences were normalized, suppression was greater for noise presented to the right and the effect measured for a probe in the left ear.

CONCLUSION

The findings support the theory that cortical mechanisms involved in listening to speech affect cochlear function through the MOC efferent system.

Contralateral acoustic stimulation alters the magnitude and phase of distortion product otoacoustic emissions

Activation of medial olivocochlear efferents through contralateral acoustic stimulation (CAS) has been shown to modulate distortion product otoacoustic emission (DPOAE) level in various ways (enhancement, reduction, or no change). The goal of this study was to investigate the effect of a range of CAS levels on DPOAE fine structure. The 2f(1)-f(2) DPOAE was recorded (f(2)/f(1)=1.22, L(1)=55 dB, and L(2)=40 dB) from eight normal-hearing subjects, using both a frequency-sweep paradigm and a fixed frequency paradigm. Contamination due to the middle ear muscle reflex was avoided by monitoring the magnitude and phase of a probe in the test ear and by monitoring DPOAE stimulus levels throughout testing. Results show modulations in both level and frequency of DPOAE fine structure patterns. Frequency shifts observed at DPOAE level minima could explain reports of enhancement in DPOAE level due to efferent activation. CAS affected the magnitude and phase of the DPOAE component from the characteristic frequency region to a greater extent than the component from the overlap region between the stimulus tones. This differential effect explains the occasional enhancement observed in DPOAE level as well as the frequency shift in fine structure patterns.

The effect of contralateral acoustic stimulation on spontaneous otoacoustic emissions

Evoked otoacoustic emissions are often used to study the medial olivocochlear (MOC) efferents in humans. There has been concern that the emission-evoking stimulus may itself elicit efferent activity and alter the evoked otoacoustic emission. Spontaneous otoacoustic emissions (SOAEs) are hence advantageous as no external stimulation is necessary to record the response in the test ear. Contralateral acoustic stimulation (CAS) has been shown to suppress SOAE level and elevate SOAE frequency, but the time course of these effects is largely unknown. By utilizing the Choi-Williams distribution, here we report a gradual adaptation during the presence of CAS and an overshoot following CAS offset in both SOAE magnitude and frequency from six normal-hearing female human subjects. Furthermore, we have quantified the time constants of both magnitude and frequency shifts at the onset, presence, and offset of four levels of CAS. Most studies using contralateral elicitors do not stringently control the middle-ear muscle (MEM) reflex, leaving the results difficult to interpret. In addition to clinically available measures of the MEM reflex, we have incorporated a sensitive laboratory technique to monitor the MEM reflex in our subjects, allowing us to interpret the results with greater confidence.

Maturation of the auditory system: 2. Transient otoacoustic emission suppression as an index of the medial olivocochlear bundle maturation

Contralateral suppression of transient otoacoustic emissions in 42 premature babies (84 ears; post-conceptional age [PCA] 30-36 weeks) was compared to that of 39 full-term babies (78 ears; PCA: 37-45 weeks). Eighteen healthy adults and ten young children (5-14 years old) were studied as controls. Risk factors for hearing loss were registered in both preterm and full-term groups. An ILO-92 otoacoustic emission recording system was used to deliver linear clicks to the ear examined and broadband noise to the contralateral ear in an alternating on and off mode. Suppression in full-term babies was statistically higher than in preterms, whereas no differences existed between children and adults and children and full-terms. Peripheral auditory lateralization was evident in adults but was observed only as a trend in newborns. Only prematurity at the time of examination and aminoglycoside treatment for more than seven days had a negative impact on suppression. The results support the conclusion that maturation of the efferent system takes place from 30 to 45 weeks PCA. The exact age at which this maturation is accomplished has not yet been clearly determined.

TEOAE suppression in adults with learning disabilities

The presentation of contralateral noise during the recording of transient evoked otoacoustic emissions (TEOAEs) reduces the amplitude of the TEOAE in normally-hearing adults. This is known as TEOAE suppression. The present study investigated TEOAE suppression in 18 adults with learning disabilities (LDs) compared to 18 adults without LDs. TEOAEs were elicited by 60 dB p.e. SPL clicks and were suppressed by the presentation of 60 dB SPL contralateral broadband noise. Suppression was measured as a change in the overall TEOAE response amplitude, and also analysed in 2-ms epochs representing different TEOAE frequency-response bands. A significant interaction was evident between group type and ear tested. Participants in the control group had right ear dominance for the suppression effect, whereas the left ear was found to be dominant for the LD group. These findings suggest a mechanism of the medial olivary cochlear bundle and efferent auditory pathway that differs in those with LD compared to those with typical learning abilities.

Influence of contralateral acoustic stimulation on the quadratic distortion product f2-f1 in humans

Contralateral acoustic stimulation is known to activate the medial olivocochlear system which is capable of modulating the amplification process in the outer hair cells of the inner ear. We investigated the influence of different levels of contralateral broadband noise on distortion product otoacoustic emissions in humans, with a particular focus on the quadratic distortion product at f2-f1. The primary stimulus frequency ratio was optimized to yield maximum f2-f1 level. While the cubic distortion product at 2f1-f2 was not significantly affected during contralateral noise stimulation, the level of f2-f1 was reduced by up to 4.8dB on average (maximum: 10.1dB), with significant suppression occurring for noise levels as low as 40dB SPL. In addition, a significant phase lead was observed. Quadratic distortions are minimal at a symmetrical position of the transfer function of the cochlear amplifier. The observed sensitivity of f2-f1 to contralateral noise stimulation could hence be resulting from a shift of the operating state and/or a change in the gain of the cochlear amplification due to contralateral induced efferent modulation of the outer hair cell properties.

Neural correlates of perceptual learning in the auditory brainstem: efferent activity predicts and reflects improvement at a speech-in-noise discrimination task

An extensive corticofugal system extends from the auditory cortex toward subcortical nuclei along the auditory pathway. Corticofugal influences reach even into the inner ear via the efferents of the olivocochlear bundle, the medial branch of which modulates preneural sound amplification gain. This corticofugal system is thought to contribute to neuroplasticity underlying auditory perceptual learning. In the present study, we investigated the involvement of the medial olivocochlear bundle (MOCB) in perceptual learning as a result of auditory training. MOCB activity was monitored in normal-hearing adult listeners during a 5 d training regimen on a consonant-vowel phoneme-in-noise discrimination task. The results show significant group learning, with great inter-individual variability in initial performance and improvement. As observed in previous auditory training studies, poor initial performers tended to show greater learning. Strikingly, MOCB activity measured on the first training day strongly predicted the subsequent amount of improvement, such that weaker initial MOCB activity was associated with greater improvement. Moreover, in listeners that improved significantly, an increase in MOCB activity was observed after training. Thus, as discrimination thresholds of listeners converged over the course of training, differences in MOCB activity between listeners decreased. Additional analysis showed that MOCB activity did not explain variation in performance between listeners on any training day but rather reflected an individual listener's performance relative to their personal optimal range. The findings suggest an MOCB-mediated listening strategy that facilitates speech-in-noise perception. The operation of this strategy is flexible and susceptible to training, presumably because of task-related adaptation of descending control from the cortex.

Contralateral suppression of linear and nonlinear transient evoked otoacoustic emissions in neonates at risk for hearing loss

To investigate the transient evoked otoacoustic emissions (TEOAE) contralateral suppression in neonates at risk for hearing loss, 55 neonates at risk for hearing loss (risk group) and 72 full-term neonates not at such risk (control group) were bilaterally tested. In all neonates, the TEOAE were recorded in two stimulation modes (linear and nonlinear clicks), with and without contralateral acoustic stimulation. Findings revealed significant contralateral suppression of otoacoustic emissions in both groups, but the amount of TEOAE contralateral suppression was reduced for at risk group (p=0.001), supporting the hypothesis that medial olivocochlear bundle function is reduced in neonates at risk for hearing loss. The combination of contralateral acoustic stimulation and TEOAE enables easy and noninvasive study of auditory efferent function. However it should be emphasized that the reduction in TEOAE contralateral suppression in the risk group, statistically identified as a group effect, might not be detectable in individual cases. Further studies are needed in order to determine whether the lower amount of TEOAE contralateral suppression in neonates at risk for hearing loss represents a risk for developing auditory processing disorders.

LEARNING OUTCOMES

The reader will be introduced to the study using auditory efferent pathway activation by contralateral acoustic stimulation (CAS), resulting in the TEOAE suppression effect. The characteristics of TEOAE suppression in the neonatal population, in which it provides evidence of the reduced medial olivocochlear system function in those at risk for hearing loss, will also be addressed.

Contralateral suppression of distortion product otoacoustic emissions and the middle-ear muscle reflex in human ears

Distortion product otoacoustic emissions (DPOAEs) were measured in the absence and presence of contralateral noise at five levels--below, equal to, and above the middle-ear muscle (MEM) reflex threshold. The resultant changes in DPOAE level and phase were dependent on stimulus frequency and noise level. Both low-level noise, believed to elicit the medial olivocochlear (MOC) reflex, and high-level noise, thought to activate both MOC and MEM reflexes, significantly decreased the DPOAE level. However, the shift from sole MOC effect to mixed MOC and MEM effects was not as dramatic as we thought. While low-level noise resulted in a minimum DPOAE phase change, high-level noise caused a substantial phase lead for 1 and 2kHz. With increasing frequency, phase lag became more notable. The present study suggests the following: (1) DPOAE contralateral suppression by low-level sound most likely does not involve the effect of the MEM reflex and signal crossover; and (2) combined analysis of DPOAE level and phase changes warrants further investigations to overcome the difficulty in separating the effects of MOC efferents and MEM contraction. The results also imply that OAE measurement has the potential for being used to investigate the effect of the MEM reflex on sound transmission.

Auditory Efferent Function Is Affected in Multiple Sclerosis

OBJECTIVE

This study evaluated efferent medial olivocochlear (MOC) function in patients with multiple sclerosis (MS). Various afferent auditory abnormalities have been described in MS, but there is a paucity of data on efferent function. The brain stem is a site of predilection for MS plaques and the efferent MOC pathway may be affected at this level.

METHODS

The study included 30 patients who had normal hearing. According to MRI findings, they were divided into two groups: those with an identifiable brain stem lesion (n = 10) and those with MS lesions in other parts of the central nervous system but without demonstrable MS plaques in the brain stem (n = 20). MOC function was evaluated by the olivocochlear suppression test, using transient evoked otoacoustic emissions. All subjects underwent standard auditory tests, including pure-tone audiometry and recording of auditory brain stem evoked responses. Twenty-two healthy subjects with normal hearing, matched for age and gender, served as a reference group for the auditory data.

RESULTS

The results showed that 66.6% of all patients had reduced MOC function, particularly those (90%) with identified lesions of the brain stem on MRI. Furthermore, abnormal MOC function was found in 55% patients without evidence of a brain stem lesion on MRI.

CONCLUSIONS

This study provides the evidence for a deficit of efferent auditory function in the majority of patients with MS. Taking into consideration the possible roles of the MOC system in processing of auditory information, abnormal MOC suppression in patients with MS may explain a variety of auditory presentations that are currently largely overlooked. This study also highlights the diagnostic value of the MOC suppression test as a site-of-lesion diagnostic test in MS and in identifying subtle brain stem lesions undetected by MRI, suggesting that subtle brain stem lesions may exist and that the MOC suppression test is sufficiently sensitive to detect them. Accordingly, the MOC suppression test may provide a tool for an early diagnosis of MS.

Contralateral suppression of distortion product otoacoustic emissions: effect of the primary frequency in Dpgrams

The amplitude of the 2f1-f2 distortion product otoacoustic emission (DPOAE) can be suppressed by presenting contralateral acoustic stimulation. To test the hypothesis that DPOAE contralateral suppression is influenced by the primary frequency in DPgrams, DPgrams were recorded at resolutions of 1, 8, and 17 pts/octave, in the absence and presence of contralateral broadband noise (BBN). Participants were 20 normal-hearing human adults. In DPgrams with higher frequency resolutions, DPOAE suppression at amplitude peaks in DPgrams (8 pts/octave: Mean = - 0.92 dB, SD = 0.71 for BBN at 60 dB SPL; 17 pts/octave: Mean = - 0.25 to -1.44 dB, SD = 0.51 to 0.86 for BBN at 40 to 70 dB SPL, respectively) was larger than the suppression at the dips in DPgrams (8 pts/octave: Mean = - 0.13 dB, SD = 1.00; 17 pts/octave: Mean = - 0.03 to -0.73 dB, SD = 0.55 to 0.91). A larger intersubject variability in DPOAE contralateral suppression was observed at the dips. The results suggest that measuring DPOAE contralateral suppression at the primary frequencies corresponding to the peaks in DPgrams with higher frequency resolutions may improve the assessment of the efferent system function.

Olivocochlear efferents: anatomy, physiology, function, and the measurement of efferent effects in humans

This review covers the basic anatomy and physiology of the olivocochlear reflexes and the use of otoacoustic emissions (OAEs) in humans to monitor the effects of one group, the medial olivocochlear (MOC) efferents. MOC fibers synapse on outer hair cells (OHCs), and activation of these fibers inhibits basilar membrane responses to low-level sounds. This MOC-induced decrease in the gain of the cochlear amplifier is reflected in changes in OAEs. Any OAE can be used to monitor MOC effects on the cochlear amplifier. Each OAE type has its own advantages and disadvantages. The most straightforward technique for monitoring MOC effects is to elicit MOC activity with an elicitor sound contralateral to the OAE test ear. MOC effects can also be monitored using an ipsilateral elicitor of MOC activity, but the ipsilateral elicitor brings additional problems caused by suppression and cochlear slow intrinsic effects. To measure MOC effects accurately, one must ensure that there are no middle-ear-muscle contractions. Although standard clinical middle-ear-muscle tests are not adequate for this, adequate tests can usually be done with OAE-measuring instruments. An additional complication is that most probe sounds also elicit MOC activity, although this does not prevent the probe from showing MOC effects elicited by contralateral sound. A variety of data indicate that MOC efferents help to reduce acoustic trauma and lessen the masking of transients by background noise; for instance, they aid in speech comprehension in noise. However, much remains to be learned about the role of efferents in auditory function. Monitoring MOC effects in humans using OAEs should continue to provide valuable insights into the role of MOC efferents and may also provide clinical benefits.

A Clinical Study of the Efferent Auditory System in Patients With Normal Hearing Who Have Acute Tinnitus

OBJECTIVE

Etiological diagnosis and treatment of tinnitus still remain challenging in clinical practice. The aim of this study was to determine the potential contribution of a defective cochlear efferent innervation to the onset of tinnitus in patients with normal hearing.

STUDY DESIGN

Prospective randomized controlled study.

SETTING

Otorhinolaryngology department of a general hospital.

PATIENTS

The patient group consisted of 18 normal-hearing adults (7 men, 11 women) with acute tinnitus (bilateral in 3 patients).

INTERVENTIONS

Tympanogram, stapedial muscle reflex, pure tone audiometry, tinnitus pitch matching, spontaneous otoacoustic emissions, and distortion product otoacoustic emissions (DPOAEs) in the absence and presence of contralateral suppression by white noise.

MAIN OUTCOME MEASURE

DPOAEs suppression amplitudes recorded from tinnitus and nontinnitus ears of the patients' group were compared with each other and with a control group.

RESULTS

The contralateral application of white noise induced the enhancement of DPOAE amplitudes in some patients. The suppression of DPOAE amplitudes by contralateral white noise did not reach statistically significant levels in either ear (with or without tinnitus). On the contrary, under the same conditions, our control group demonstrated statistically significant reduction of DPOAE amplitudes at all frequencies.

CONCLUSION

Patients with normal hearing acuity who have acute tinnitus seem to have a less effective functioning of the cochlear efferent system because the application of contralateral noise enhanced the DPOAEs or suppressed them less intensely than it did in a control group. Further studies may establish the clinical applications for the diagnosis of changes in efferent function, in the subjective evaluation, patient etiological grouping, treatment, or prognosis of tinnitus.

Contralateral Suppression of Otoacoustic Emissions in Children with Specific Language Impairment

OBJECTIVE

The purpose of the investigation was to determine whether a group of children with specific language impairments (SLI) have reduced peripheral auditory processes thought to be associated with speech-in-noise intelligibility.

DESIGN

Transient evoked otoacoustic emissions (TEOAE) and their suppression by the efferent activity of the medial olivocochlear system (MOCS) in response to contralateral acoustic stimulation were used to compare these processes in 18 children with SLI and 21 controls.

RESULTS

The results revealed no group difference in TEOAE suppression effect or left/right asymmetry of TEOAE suppression effect.

CONCLUSIONS

These results suggest that children with SLI do not have auditory processing problems at this peripheral level casting doubt on a hypothesized relationship between strength of MOCS activity and language impairment.

Gentamicin abolishes all cochlear effects of electrical stimulation of the inferior colliculus

Electrical stimulation of the inferior colliculus (IC) has been shown to result in suppression of cochlear output, due to activation of the medial olivocochlear system. This auditory efferent system originates in the brainstem and terminates on the outer hair cells in the cochlea. Recently, excitatory effects of IC stimulation have also been reported, both on cochlear gross potentials and on primary auditory afferents. It has been hypothesized that this excitation is due to co-activation of the lateral olivocochlear system, which synapses on the primary auditory afferent fibres contacting the inner hair cells. If stimulation of the IC leads to the activation of both the medial and lateral olivocochlear system, resulting in a mixture of inhibitory and excitatory effects in the cochlea, then removal of the inhibitory effects, by blocking the medial system, should lead to more pronounced excitatory effects out in the periphery. To investigate this hypothesis, we recorded the effect of IC stimulation on cochlear gross potentials as well as on single auditory primary afferents in guinea pigs following block of the medial olivocochlear system with gentamicin. We found that administration of gentamicin, whether intraperitoneally or by intracochlear perfusion, blocked all effects of IC stimulation, whether inhibitory or excitatory. These data strongly suggest that all effects observed after IC stimulation, both inhibitory as well as excitatory, are due to the activation of the medial olivocochlear system.

Evidence for Corticofugal Modulation of Peripheral Auditory Activity in Humans

Active cochlear micromechanisms, involved in auditory sensitivity, are modulated by the medial olivocochlear efferent system, which projects directly onto the organ of Corti. Both processes can be assessed non-invasively by means of evoked otoacoustic emissions. Animal experiments have revealed top-down control from the auditory cortex to peripheral auditory receptor, supported by anatomical descriptions of descending auditory pathways from auditory areas to the medial olivocochlear efferent system and organ of Corti. Through recording of evoked otoacoustic emissions during presurgical functional brain mapping for refractory epilepsy, we showed that corticofugal modulation of peripheral auditory activity also exists in humans. In 10 epileptic patients, electrical stimulation of the contralateral auditory cortex led to a significant decrease in evoked otoacoustic emission amplitude, whereas no change occurred under stimulation of non-auditory contralateral areas. These findings provide evidence of a cortico-olivocochlear pathway, originating in the auditory cortex and modulating contralateral active cochlear micromechanisms via the medial olivocochlear efferent system, in humans.

Human efferent adaptation of DPOAEs in the L1,L2 space

The adaptive properties of distortion product otoacoustic emissions (DPOAEs) at 2f(1)-f2 were investigated in 12 ears of normally hearing adults aged 18-30 years using long-lasting 1-s primary-tone on-times. In this manner, DPOAE adaptation at a single f2 of 1.55 kHz (f2/f1=1.21) was evaluated as a function of the levels of the primary tones in a matrix of L1, L2 settings, which varied from 45 to 80 dB SPL, in 5-dB steps. DPOAEs were elicited under both monaural and binaural stimulus-presentation conditions. Adaptation was defined as the difference in DPOAE levels between the initial 92-ms baseline measure using a standard protocol and one obtained during the final 92 ms of the prolonged 1-s primary-tones. These differences were averaged across subjects to create contour plots of mean adaptation in the L1,L2 space. The 2f(1)-f2 DPOAE revealed consistent regions of suppression (-0.5 dB difference) or enhancement (+0.5 dB difference) with respect to baseline measures within the L(1),L(2) matrix for both acoustic-stimulation conditions. Specifically, 2f(1)-f2 DPOAE suppressions of 1-2 dB occurred for both monaural and binaural presentations, typically at level combinations in which L1>L2. In contrast, larger 2f(1)-f2 DPOAE enhancements of 3-4 dB occurred for only the binaural condition, at primary-tone level combinations where L1<L2. Although adaptation activity was also evaluated for the DPOAEs at f(2)-f1, 2f(2)-f1, and 3f(1)-2f2, these emissions were either immeasurable (e.g., f(2)-f1) or only present in a subset of subjects over a narrow range of primary-tone frequencies and levels that did not support a systematic analysis. In summary, the 2f(1)-f2 results suggest that a potentially important area for adaptation measures exists in the L1,L2 space, when L1 is lower than L2. This combination of primary-tone levels can lead to large DPOAE adaptation effects that may be related to a notch in the DPOAE response/growth or input/output (I/O) function.

An anatomical study of the vestibulocochlear anastomosis (anastomosis of Oort) in humans: preliminary results

Von Oort first described the vestibulocochlear anastomosis in 1918, and there have been very few anatomical studies of it. It is located at the bottom of the internal auditory meatus and links the saccular nerve to the cochlear nerve. The aim of this study was to investigate the micro-anatomical characteristics of this anastomosis. We dissected 10 human temporal bones from five heads. We found the vestibulocochlear anastomosis in seven of the specimens, of which six were clearly visible. We were unable to uncover it in three specimens due to dissection problems. Its length was evaluated to be between 0.5 and 1 mm, with a diameter of 0.5 mm. The vestibulocochlear anastomosis could be the pathway for the nerve fibers of the cochlear efferent system, whose description remains incomplete. However, no anatomist, histologist or physiologist has demonstrated this function. From the results of this study, it seems likely that the vestibulocochlear anastomosis exists. Other studies are necessary to link it to the efferent system.

Noradrenergic modulation of brainstem nuclei alters cochlear neural output

The peripheral auditory sense organ, the cochlea, receives innervation from lateral and medial olivocochlear neurons in the brainstem. These neurons are able to modulate cochlear neural output. Anatomical studies have shown that one of the neurotransmitters which is present in varicosities surrounding the olivocochlear neurons in the brainstem is noradrenaline and previous work on brainstem slices has demonstrated a generally excitatory effect of noradrenaline on medial olivocochlear neurons. In order to assess in vivo the function of the noradrenergic inputs to olivocochlear neurons, we injected noradrenaline in the brainstem of anaesthetised guinea pigs and recorded ipsilateral cochlear electrical activity. Injections of noradrenaline close to the lateral olivocochlear neurons evoked increases in the sound-driven neural activity from the cochlea, measured as compound action potential (CAP) amplitude, as well as in the spontaneous activity, measured as amplitude of the 900 Hz peak of the spectrum of the neural noise in the cochlear fluids. In contrast, noradrenaline in the vicinity of the medial olivocochlear neurons evoked inhibitory effects on both the CAP amplitude and 900 Hz peak. These results indicate most likely an excitatory action of noradrenaline on both the lateral and medial olivocochlear neurons in the brainstem, and show that such noradrenergic inputs can modulate cochlear function.

Etude anatomo-histologique de l’anastomose vestibulo-cochléaire de Von Oort

OBJECTIVES

The vestibulocochlear anastomosis was first described in 1918 by von Oort. It is situated deeply at the bottom of the internal acoustic meatus, and spreads from the saccular nerve before its terminal ramifications, to the cochlear nerve before its penetration into the cochlea. Nerve fibers of the cochlear efferent system are thought to pass through it. The aim of our study was to investigate the anatomy of the vestibulocochlear anastomosis and characterize its histological features.

METHOD

[corrected] Ten human temporal bones were dissected. Serial sections were obtained for histological evaluation.

RESULTS

The vestibulocochlear anastomosis was found in seven of the specimens, perfectly visualized in six. Average diameter was 0.5 mm with lengths varying from 0.5 to 1 mm. Serial histological sections demonstrated the nervous nature of the anastomosis and its relations with the saccular and cochlear nerves. The epinevrium of the saccular nerve was continuous with the supposed anastomosis in five of the specimens, demonstrating the distinct nature of the anastomosis from the saccular and cochlear nerves. We did not find any evidence linking these fibers to the cochlear efferent system.

DISCUSSION

The vestibulocochlear anastomosis was found in seven of our ten dissections. The anastomosis is probably an anatomic reality composed of nerve fibers. The efferent function of these fibers remains to be demonstrated.