Medial olivocochlear efferent reflex inhibition of human cochlear nerve responses

A Guinea Pig Model Suggests That Objective Assessment of Acoustic Hearing Preservation in Human Ears With Cochlear Implants Is Confounded by Shifts in the Spatial Origin of Acoustically Evoked Potential Measurements Along the Cochlear Length

OBJECTIVES

Our recent empirical findings have shown that the auditory nerve compound action potential (CAP) evoked by a low-level tone burst originates from a narrow cochlear region tuned to the tone burst frequency. At moderate to high sound levels, the origins shift to the most sensitive audiometric regions rather than the extended high-frequency regions of the cochlear base. This means that measurements evoked from extended high-frequency sound stimuli can shift toward the apex with increasing level. Here we translate this study to understand the spatial origin of acoustically evoked responses from ears that receive cochlear implants, an emerging area of research and clinical practice that is not completely understood. An essential step is to first understand the influence of the cochlear implant in otherwise naive ears. Our objective was to understand how function of the high-frequency cochlear base, which can be excited by the intense low-frequency sounds that are frequently used for objective intra- and postoperative monitoring, can be influenced by the presence of the cochlear implant.

DESIGN

We acoustically evoked responses and made measurements with an electrode placed near the guinea pig round window. The cochlear implant was not utilized for either electrical stimulation or recording purposes. With the cochlear implant in situ, CAPs were acoustically evoked from 2 to 16 kHz tone bursts of various levels while utilizing the slow perfusion of a kainic acid solution from the cochlear apex to the cochlear aqueduct in the base, which sequentially reduced neural responses from finely spaced cochlear frequency regions. This cochlear perfusion technique reveals the spatial origin of evoked potential measurements and provides insight on what influence the presence of an implant has on acoustical hearing.

RESULTS

Threshold measurements at 3 to 11 kHz were elevated by implantation. In an individual ear, thresholds were elevated and lowered as cochlear implant was respectively inserted and removed, indicative of "conductive hearing loss" induced by the implant. The maximum threshold elevation occurred at most sensitive region of the naive guinea pig ear (33.66 dB at 8 kHz), making 11 kHz the most sensitive region to acoustic sounds for guinea pig ears with cochlear implants. Conversely, the acute implantation did not affect the low-frequency, 500 Hz thresholds and suprathreshold function, as shown by the auditory nerve overlapped waveform. As the sound pressure level of the tone bursts increased, mean data show that the spatial origin of CAPs along the cochlear length shifted toward the most sensitive cochlear region of implanted ears, not the extended high-frequency cochlear regions. However, data from individual ears showed that after implantation, measurements from moderate to high sound pressure levels originate in places that are unique to each ear.

CONCLUSIONS

Alterations to function of the cochlear base from the in situ cochlear implant may influence objective measurements of implanted ears that are frequently made with intense low-frequency sound stimuli. Our results from guinea pigs advance the interpretation of measurements used to understand how and when residual acoustic hearing is lost in human ears receiving a cochlear implant.

Effects of contralateral noise on envelope-following responses, auditory-nerve compound action potentials, and otoacoustic emissions measured simultaneously

This study assessed whether the effects of contralateral acoustic stimulation (CAS) are consistent with eliciting the medial olivocochlear (MOC) reflex for measurements sensitive to outer hair cell (otoacoustic emissions, OAEs), auditory-nerve (AN; compound action potential, CAP), and brainstem/cortical (envelope-following response, EFR) function. The effects of CAS were evaluated for simultaneous measurement of OAEs, CAPs, and EFRs in participants with normal hearing. Clicks were presented at 40 or 98 Hz in three ipsilateral noise conditions (no noise, 45 dB SPL, and 55 dB SPL). For the no noise condition, CAS suppressed or enhanced EFR amplitudes for 40- and 98-Hz clicks, respectively, while CAS had no significant effect on CAP amplitudes. A follow-up experiment using slower rates (4.4-22.2 Hz) assessed whether this insignificant CAS effect on CAPs was from ipsilateral MOC stimulation or AN adaptation; however, CAS effects remained insignificant despite favorable signal-to-noise ratios. CAS-related enhancements of EFR and CAP amplitudes in ipsilateral noise were not observed, contrary to the anti-masking effect of the MOC reflex. EFR and OAE suppression from CAS were not significantly correlated. Thus, the effects of CAS on EFRs may not be solely mediated by the MOC reflex and may be partially mediated by higher auditory centers.

Subcortical auditory model including efferent dynamic gain control with inputs from cochlear nucleus and inferior colliculus

An auditory model has been developed with a time-varying, gain-control signal based on the physiology of the efferent system and subcortical neural pathways. The medial olivocochlear (MOC) efferent stage of the model receives excitatory projections from fluctuation-sensitive model neurons of the inferior colliculus (IC) and wide-dynamic-range model neurons of the cochlear nucleus. The response of the model MOC stage dynamically controls cochlear gain via simulated outer hair cells. In response to amplitude-modulated (AM) noise, firing rates of most IC neurons with band-enhanced modulation transfer functions in awake rabbits increase over a time course consistent with the dynamics of the MOC efferent feedback. These changes in the rates of IC neurons in awake rabbits were employed to adjust the parameters of the efferent stage of the proposed model. Responses of the proposed model to AM noise were able to simulate the increasing IC rate over time, whereas the model without the efferent system did not show this trend. The proposed model with efferent gain control provides a powerful tool for testing hypotheses, shedding insight on mechanisms in hearing, specifically those involving the efferent system.

Auditory brainstem mechanisms likely compensate for self-imposed peripheral inhibition

Feedback networks in the brain regulate downstream auditory function as peripheral as the cochlea. However, the upstream neural consequences of this peripheral regulation are less understood. For instance, the medial olivocochlear reflex (MOCR) in the brainstem causes putative attenuation of responses generated in the cochlea and cortex, but those generated in the brainstem are perplexingly unaffected. Based on known neural circuitry, we hypothesized that the inhibition of peripheral input is compensated for by positive feedback in the brainstem over time. We predicted that the inhibition could be captured at the brainstem with shorter (1.5 s) than previously employed long duration (240 s) stimuli where this inhibition is likely compensated for. Results from 16 normal-hearing human listeners support our hypothesis in that when the MOCR is activated, there is a robust reduction of responses generated at the periphery, brainstem, and cortex for short-duration stimuli. Such inhibition at the brainstem, however, diminishes for long-duration stimuli suggesting some compensatory mechanisms at play. Our findings provide a novel non-invasive window into potential gain compensation mechanisms in the brainstem that may have implications for auditory disorders such as tinnitus. Our methodology will be useful in the evaluation of efferent function in individuals with hearing loss.

Middle ear muscle and medial olivocochlear activity inferred from individual human ears via cochlear potentials

The peripheral auditory system is influenced by the medial olivocochlear (MOC) and middle ear muscle (MEM) reflexes. When elicited by contralateral acoustic stimulation (CAS), these reflexes reduce cochlear amplification (MOC reflex) and limit low-frequency transmission through the middle ear (MEM reflex). The independent roles of these reflexes on auditory physiology and perception are difficult to distinguish. The amplitude of the cochlear microphonic (CM) is expected to increase or decrease when the MOC and MEM reflexes are elicited by CAS, respectively, which could lead to a straightforward interpretation of what reflex is dominant for a given CAS level. CM and ear canal sound pressure level (SPL) were measured for a 500 Hz, 90 dB SPL probe in the presence of contralateral broadband noise (CBBN) for levels ranging from 45-75 dB SPL. In most subjects, CM amplitude increased for CBBN levels of 45 and 55 dB SPL, while no change in ear canal SPL was observed, consistent with eliciting the MOC reflex. Conversely, CM amplitude decreased, and ear canal SPL increased in the presence of 65 and 75 dB SPL CBBN, consistent with eliciting the MEM reflex. A CM-based test of the MOC reflex may facilitate detection of MEM effects and the assessment of adults with cochlear hearing loss.

Assessing Medial Olivocochlear Reflex Strengths via Auditory Brainstem Response: Measurement and Variability in Normal-Hearing Individuals

PURPOSE

Optimal measurement settings to measure the medial olivocochlear reflex (MOCR) in humans have not yet been defined. The purpose of this study was to advance the representation of the MOCR in auditory brainstem response (ABR) as an addition to the current diagnostic portfolio.

PARTICIPANTS AND METHOD

Twelve female and 14 male normal-hearing adults participated in the study. Potential effects of a contralateral acoustic stimulus (CAS) on amplitude changes were investigated by recording ABR waveform profiles on the left side at click intensities of 50/60/70 dB nHL with and without CAS (60 dB SPL). Secondly, to detect potential chronological order influences, measurement settings were rearranged on the right side and measurements were repeated. Additionally, ABR thresholds were recorded with and without a CAS in 10 patients.

RESULTS

When the effect of contralateral suppression was analyzed on the basis of amplitude changes, there was a change under administration of the CAS signal that was statistically significant. Interestingly, the order of recordings affected the degree of amplitude change. In three out of 10 patients, reproducible suppression effects on ABR thresholds were detectable upon CAS presentation.

CONCLUSIONS

To our knowledge, this is the largest study dealing with the recording of the MOCR elicited by a contralateral noise via ABR in normal-hearing individuals. Effects of MOCR are measurable via amplitude changes upon CAS administration. Chronological orders influence the impact of this effect on amplitude changes. Optimal measurement settings have not yet been defined. However, experiments such as this study may help to further improve measurements, and thus advance the representation of the MOC reflex in ABR as an addition to the current diagnostic portfolio.

Lack of correlation between medial olivocochlear reflex strength and sentence recognition in noise

OBJECTIVE

The medial olivocochlear (MOC) reflex provides unmasking of sounds in noise, but its contribution to speech-in-noise perception remains unclear due to conflicting results. This study determined associations between MOC reflex strength and sentence recognition in noise in individuals with normal hearing.

DESIGN

MOC reflex strength was assessed using contralateral inhibition of transient-evoked otoacoustic emissions (TEOAEs). Scores on the AzBio sentence task were quantified at three signal-to-noise ratios (SNRs). Additionally, slope and threshold of the psychometric function were computed. Associations between MOC reflex strength and speech-in-noise outcomes were assessed using Spearman rank correlations.

STUDY SAMPLE

Nineteen young adults with normal hearing participated, with data from 17 individuals (mean age = 21.8 years) included in the analysis.

RESULTS

Contralateral noise significantly decreased the amplitude of TEOAEs. A range of contralateral inhibition values was exhibited across participants. Scores increased significantly with increasing SNR. Contrary to hypotheses, there were no significant correlations between MOC reflex strength and score, nor were there any significant correlations between MOC reflex strength and measures of the psychometric function.

CONCLUSIONS

Results found no significant monotonic relationship between MOC reflex strength and sentence recognition in noise. Future work is needed to determine the functional role of the MOC reflex.

The role of medial olivocochlear activity in contralateral suppression of auditory steady-state responses

OBJECTIVE

The auditory steady-state response (ASSR) amplitudes fall in the presence of contralateral noise. However, whether and to what extent medial olivocochlear (MOC) activity involves in contralateral suppression of ASSR remain unclear. Therefore, we assess the role of MOC activity in contralateral suppression of ASSR.

METHODS

Mice were treated with strychnine to completely eliminate MOC activity and then measured ASSR amplitudes in the presence of contralateral noise.

RESULTS

The contralateral noise reduces ASSR amplitudes at some stimulus intensity. After treating with the strychnine to eliminate MOC activity, ASSR amplitudes recovered again.

CONCLUSIONS

MOC activity participated in contralateral suppression of ASSR.

Contralateral inhibition of distortion product otoacoustic emissions in young noise-exposed Veterans

Although animal models show a clear link between noise exposure and damage to afferent cochlear synapses, the relationship between noise exposure and efferent function appears to be more complex. Animal studies indicate that high intensity noise exposure reduces efferent medial olivocochlear (MOC) reflex strength, whereas chronic moderate noise exposure is associated with a conditioning effect that enhances the MOC reflex. The MOC reflex is predicted to improve speech-in-noise perception and protects against noise-induced auditory damage by reducing cochlear gain. In humans, MOC reflex strength can be estimated by measuring contralateral inhibition of distortion product otoacoustic emissions (DPOAEs). The objective of this study was to determine the impact of military noise exposure on efferent auditory function by measuring DPOAE contralateral inhibition in young Veterans and non-Veterans with normal audiograms. Compared with non-Veteran controls, Veterans with high levels of reported noise exposure demonstrated a trend of reduced contralateral inhibition across a broad frequency range, suggesting efferent damage. Veterans with moderate noise exposure showed trends of reduced inhibition from 3 to 4 kHz but greater inhibition from 1 to 1.5 kHz, consistent with conditioning. These findings suggest that, in humans, the impact of noise exposure on the MOC reflex differs depending on the noise intensity and duration.

Firing Rate Adaptation of the Human Auditory Nerve Optimizes Neural Signal-to-Noise Ratios

Several physiological mechanisms act on the response of the auditory nerve (AN) during acoustic stimulation, resulting in an adjustment in auditory gain. These mechanisms include-but are not limited to-firing rate adaptation, dynamic range adaptation, the middle ear muscle reflex, and the medial olivocochlear reflex. A potential role of these mechanisms is to improve the neural signal-to-noise ratio (SNR) at the output of the AN in real time. This study tested the hypothesis that neural SNRs, inferred from non-invasive assessment of the human AN, improve over the duration of acoustic stimulation. Cochlear potentials were measured in response to a series of six high-level clicks embedded in a series of six lower-level broadband noise bursts. This paradigm elicited a compound action potential (CAP) in response to each click and to the onset of each noise burst. The ratio of CAP amplitudes elicited by each click and noise burst pair (i.e., neural SNR) was tracked over the six click/noise bursts. The main finding was a rapid (< 24 ms) increase in neural SNR from the first to the second click/noise burst, consistent with a real-time adjustment in the response of the auditory periphery toward improving the SNR of the signal transmitted to the brainstem. Analysis of cochlear microphonic and ear canal sound pressure recordings, as well as the time course for this improvement in neural SNR, supports the conclusion that firing rate adaptation is likely the primary mechanism responsible for improving neural SNR, while dynamic range adaptation, the middle ear muscle reflex, and the medial olivocochlear reflex played a secondary role on the effects observed in this study. Real-time improvements in neural SNR are significant because they may be essential for robust encoding of speech and other relevant stimuli in the presence of background noise.

Standardization of the TEOAE Contralateral Suppression Test in Terms of Stimulus Intensity and Contralateral Noise Duration in Individuals with Normal Hearing

BACKGROUND

 A standard method and parameter study were performed for the contralateral suppression test.

PURPOSE

 Our study aimed to determine the contralateral transient-evoked otoacoustic emission (TEOAE) suppression test method and stimulus-noise parameters that have a standard procedure and will enable the efferent system to be easily evaluated in clinics.

RESEARCH DESIGN

 This study was conducted in two parts with two different groups of participants as a within-subjects design. In the first part, the signal-to-noise ratio at which maximum suppression obtained was investigated with 29 participants. In the second part, the optimal contralateral noise presentation method (in terms of noise-time or noise-sweep) was examined with 21 participants.

STUDY SAMPLE

 In the first part 29 young adults aged between 18 and 32 (23.03 ± 2.84), 20 females and 9 males, and in the second part 21 young adults aged between 19 and 34 years (mean age: 23.71 ± 3.48 years), 16 females and 5 males, participated in the second part. All participants had normal hearing.

DATA COLLECTION AND ANALYSIS

 To obtain maximum OAE suppression, different parameters were tested with the Otodynamics ILO292-II OAE device at both parts of the study in a double-walled audiometric test booths. Multirepeated analysis of variance, pairwise comparison, Friedman test, and Wilcoxon signed-rank tests were used for statistical analysis.

RESULTS

 In the first part, maximum suppression was achieved at 65 dB peSPL (decibel peak-equivalent sound pressure level) TEOAE stimulus and 65 dB SPL broadband noise. In the second part, maximum suppression was obtained in noise-time method with 30 seconds "duration."

CONCLUSIONS

 To provide standardization in clinics, it can be recommended that the contralateral suppression of OAEs was measured at 65 dB peSPL TEOAE stimulus and 65 dB SPL broadband noise in the linear stimulation mode with Otodynamics ILO 292-II double-probe OAE device. To obtain maximum suppression, the noise-time method with 30 seconds duration can be used.

Contralateral noise effects on otoacoustic emissions and electrophysiologic responses in normal-hearing adults

Contralateral noise inhibits the amplitudes of cochlear and neural responses. These measures may hold potential diagnostic utility. The medial olivocochlear (MOC) reflex underlies the inhibition of cochlear responses but the extent to which it contributes to inhibition of neural responses remains unclear. Mertes and Leek [J. Acoust. Soc. Am. 140, 2027-2038 (2016)] recently examined contralateral inhibition of cochlear responses [transient-evoked otoacoustic emissions (TEOAEs)] and neural responses [auditory steady-state responses (ASSRs)] in humans and found that the two measures were not correlated, but potential confounds of older age and hearing loss were present. The current study controlled for these confounds by examining a group of young, normal-hearing adults. Additionally, measurements of the auditory brainstem response (ABR) were obtained. Responses were elicited using clicks with and without contralateral broadband noise. Changes in TEOAE and ASSR magnitude as well as ABR wave V latency were examined. Results indicated that contralateral inhibition of ASSRs was significantly larger than that of TEOAEs and that the two measures were uncorrelated. Additionally, there was no significant change in wave V latency. Results suggest that further work is needed to understand the mechanism underlying contralateral inhibition of the ASSR.

The effect of broadband elicitor laterality on psychoacoustic gain reduction across signal frequency

There are psychoacoustic methods thought to measure gain reduction, which may be from the medial olivocochlear reflex (MOCR), a bilateral feedback loop that adjusts cochlear gain. Although studies have used ipsilateral and contralateral elicitors and have examined strength at different signal frequencies, these factors have not been examined within a single study. Therefore, basic questions about gain reduction, such as the relative strength of ipsilateral vs contralateral elicitation and the relative strength across signal frequency, are not known. In the current study, gain reduction from ipsilateral, contralateral, and bilateral elicitors was measured at 1-, 2-, and 4-kHz signal frequencies using forward masking paradigms at a range of elicitor levels in a repeated measures design. Ipsilateral and bilateral strengths were similar and significantly larger than contralateral strength across signal frequencies. Growth of gain reduction with precursor level tended to differ with signal frequency, although not significantly. Data from previous studies are considered in light of the results of this study. Behavioral results are also considered relative to anatomical and physiological data on the MOCR. These results indicate that, in humans, cochlear gain reduction is broad across frequencies and is robust for ipsilateral and bilateral elicitation but small for contralateral elicitation.

Reducing Auditory Nerve Excitability by Acute Antagonism of Ca2+-Permeable AMPA Receptors

Hearing depends on glutamatergic synaptic transmission mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs). AMPARs are tetramers, where inclusion of the GluA2 subunit reduces overall channel conductance and Ca²⁺ permeability. Cochlear afferent synapses between inner hair cells (IHCs) and auditory nerve fibers (ANFs) contain the AMPAR subunits GluA2, 3, and 4. However, the tetrameric complement of cochlear AMPAR subunits is not known. It was recently shown in mice that chronic intracochlear delivery of IEM-1460, an antagonist selective for GluA2-lacking AMPARs [also known as Ca²⁺-permeable AMPARs (CP-AMPARs)], before, during, and after acoustic overexposure prevented both the trauma to ANF synapses and the ensuing reduction of cochlear nerve activity in response to sound. Surprisingly, baseline measurements of cochlear function before exposure were unaffected by chronic intracochlear delivery of IEM-1460. This suggested that cochlear afferent synapses contain GluA2-lacking CP-AMPARs alongside GluA2-containing Ca²⁺-impermeable AMPA receptors (CI-AMPARs), and that the former can be antagonized for protection while the latter remain conductive. Here, we investigated hearing function in the guinea pig during acute local or systemic delivery of CP-AMPAR antagonists. Acute intracochlear delivery of IEM-1460 or systemic delivery of IEM-1460 or IEM-1925 reduced the amplitude of the ANF compound action potential (CAP) significantly, for all tone levels and frequencies, by > 50% without affecting CAP thresholds or distortion product otoacoustic emissions (DPOAE). Following systemic dosing, IEM-1460 levels in cochlear perilymph were ~ 30% of blood levels, on average, consistent with pharmacokinetic properties predicting permeation of the compounds into the brain and ear. Both compounds were metabolically stable with half-lives >5 h in vitro, and elimination half-lives in vivo of 118 min (IEM-1460) and 68 min (IEM-1925). Heart rate monitoring and off-target binding assays suggest an enhanced safety profile for IEM-1925 over IEM-1460. Compound potency on CAP reduction (IC₅₀ ~ 73 μM IEM-1460) was consistent with a mixture of GluA2-lacking and GluA2-containing AMPARs. These data strongly imply that cochlear afferent synapses of the guinea pig contain GluA2-lacking CP-AMPARs. We propose these CP-AMPARs may be acutely antagonized with systemic dosing, to protect from glutamate excitotoxicity, while transmission at GluA2-containing AMPARs persists to mediate hearing during the protection.

Comparing Simultaneous Electrocochleography and Auditory Brainstem Response Measurements Using Three Different Extratympanic Electrodes

BACKGROUND

 Various extratympanic recording electrodes have been used to make electrocochleography (ECochG) and auditory brainstem response (ABR) measurements in clinics, translational research, and basic science laboratories. However, differences may exist in ECochG and ABR measurements depending on the different types of extratympanic electrodes that are used.

PURPOSE

 The purpose of this research is to compare simultaneously recorded ECochG and ABR responses using three different extratympanic electrodes. This research helps clinicians and researchers to understand how electrode types and recording sites influence EcochG and ABR results. In addition, our findings could provide more normative data to the ECochG and ABR literature as well as give perspective on a preferred electrode approach when performing simultaneous ECochG and ABR testing.

RESEARCH DESIGN

 Ours was a repeated-measures study with measurements being made from individual participants on two separate sessions.

STUDY SAMPLE

 Twenty young adult females with normal hearing.

PROCEDURE

 A three-channel recording system was used to simultaneously record ECochG and ABR measurements in response to alternating polarity click stimuli. In each session, measurements were simultaneously recorded with a TipTrode electrode and one of the tympanic membrane (TM) electrodes.

DATA COLLECTION AND ANALYSIS

 Suprathreshold summating potential (SP) and action potential (AP) of the ECochG and waves I, III, and V of the ABR, and threshold responses (AP and wave V) were identified.

RESULTS

 Compared with the ear canal TipTrode electrode, TM electrodes yielded suprathreshold amplitudes that were larger than those from the ear canal electrode, smaller SP-AP ratios, lower AP thresholds, and less variability. These findings can help guide choices made by clinicians, translational investigators, and basic science researchers on which type of extra-tympanic electrode to use for their intended purpose.

The role of the medial olivocochlear reflex in psychophysical masking and intensity resolution in humans: a review

This review addresses the putative role of the medial olivocochlear (MOC) reflex in psychophysical masking and intensity resolution in humans. A framework for interpreting psychophysical results in terms of the expected influence of the MOC reflex is introduced. This framework is used to review the effects of a precursor or contralateral acoustic stimulation on 1) simultaneous masking of brief tones, 2) behavioral estimates of cochlear gain and frequency resolution in forward masking, 3) the buildup and decay of forward masking, and 4) measures of intensity resolution. Support, or lack thereof, for a role of the MOC reflex in psychophysical perception is discussed in terms of studies on estimates of MOC strength from otoacoustic emissions and the effects of resection of the olivocochlear bundle in patients with vestibular neurectomy. Novel, innovative approaches are needed to resolve the dissatisfying conclusion that current results are unable to definitively confirm or refute the role of the MOC reflex in masking and intensity resolution.

Medial olivocochlear reflex effects on amplitude growth functions of long- and short-latency components of click-evoked otoacoustic emissions in humans

Functional outcomes of medial olivocochlear reflex (MOCR) activation, such as improved hearing in background noise and protection from noise damage, involve moderate to high sound levels. Previous noninvasive measurements of MOCR in humans focused primarily on otoacoustic emissions (OAEs) evoked at low sound levels. Interpreting MOCR effects on OAEs at higher levels is complicated by the possibility of the middle-ear muscle reflex and by components of OAEs arising from different locations along the length of the cochlear spiral. We overcame these issues by presenting click stimuli at a very slow rate and by time-frequency windowing the resulting click-evoked (CE)OAEs into short-latency (SL) and long-latency (LL) components. We characterized the effects of MOCR on CEOAE components using multiple measures to more comprehensively assess these effects throughout much of the dynamic range of hearing. These measures included CEOAE amplitude attenuation, equivalent input attenuation, phase, and slope of growth functions. Results show that MOCR effects are smaller on SL components than LL components, consistent with SL components being generated slightly basal of the characteristic frequency region. Amplitude attenuation measures showed the largest effects at the lowest stimulus levels, but slope change and equivalent input attenuation measures did not decrease at higher stimulus levels. These latter measures are less commonly reported and may provide insight into the variability in listening performance and noise susceptibility seen across individuals.NEW & NOTEWORTHY The auditory efferent system, operating at moderate to high sound levels, may improve hearing in background noise and provide protection from noise damage. We used otoacoustic emissions to measure these efferent effects across a wide range of sound levels and identified level-dependent and independent effects. Previous reports have focused on level-dependent measures. The level-independent effects identified here may provide new insights into the functional relevance of auditory efferent activity in humans.

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.

Correlation and Reliability of Behavioral and Otoacoustic-Emission Estimates of Contralateral Medial Olivocochlear Reflex Strength in Humans

The roles of the medial olivocochlear reflex (MOCR) in human hearing have been widely investigated but remain controversial. We reason that this may be because the effects of MOCR activation on cochlear mechanical responses can be assessed only indirectly in healthy humans, and the different methods used to assess those effects possibly yield different and/or unreliable estimates. One aim of this study was to investigate the correlation between three methods often employed to assess the strength of MOCR activation by contralateral acoustic stimulation (CAS). We measured tone detection thresholds (N = 28), click-evoked otoacoustic emission (CEOAE) input/output (I/O) curves (N = 18), and distortion-product otoacoustic emission (DPOAE) I/O curves (N = 18) for various test frequencies in the presence and the absence of CAS (broadband noise of 60 dB SPL). As expected, CAS worsened tone detection thresholds, suppressed CEOAEs and DPOAEs, and horizontally shifted CEOAE and DPOAE I/O curves to higher levels. However, the CAS effect on tone detection thresholds was not correlated with the horizontal shift of CEOAE or DPOAE I/O curves, and the CAS-induced CEOAE suppression was not correlated with DPOAE suppression. Only the horizontal shifts of CEOAE and DPOAE I/O functions were correlated with each other at 1.5, 2, and 3 kHz. A second aim was to investigate which of the methods is more reliable. The test–retest variability of the CAS effect was high overall but smallest for tone detection thresholds and CEOAEs, suggesting that their use should be prioritized over the use of DPOAEs. Many factors not related with the MOCR, including the limited parametric space studied, the low resolution of the I/O curves, and the reduced numbers of observations due to data exclusion likely contributed to the weak correlations and the large test–retest variability noted. These findings can help us understand the inconsistencies among past studies and improve our understanding of the functional significance of the MOCR.

Efferent unmasking of speech-in-noise encoding?

OBJECTIVE

The medial olivocochlear (MOC) reflex provides efferent feedback from the brainstem to cochlear outer hair cells. Physiologic studies have demonstrated that the MOC reflex is involved in "unmasking" of signals-in-noise at the level of the auditory nerve; however, its functional importance in human hearing remains unclear.

DESIGN

This study examined relationships between pre-neural measurements of MOC reflex strength (click-evoked otoacoustic emission inhibition; CEOAE) and neural measurements of speech-in-noise encoding (speech frequency following response; sFFR) in four conditions (Quiet, Contralateral Noise, Ipsilateral Noise, and Ipsilateral + Contralateral Noise). Three measures of CEOAE inhibition (amplitude reduction, effective attenuation, and input-output slope inhibition) were used to quantify pre-neural MOC reflex strength. Correlations between pre-neural MOC reflex strength and sFFR "unmasking" (i.e. response recovery from masking effects with activation of the MOC reflex in time and frequency domains) were assessed.

STUDY SAMPLE

18 young adults with normal hearing.

RESULTS

sFFR unmasking effects were insignificant, and there were no correlations between pre-neural MOC reflex strength and sFFR unmasking in the time or frequency domain.

CONCLUSION

Our results do not support the hypothesis that the MOC reflex is involved in speech-in-noise neural encoding, at least for features that are represented in the sFFR at the SNR tested.

Brainstem Representation of Auditory Overshoot in Guinea Pigs Using Auditory Brainstem Responses

Objective

It is easier for a listener to detect a brief tonal signal presented in a longer masking noise by increasing the delay between the signal and the masker. This phenomenon (overshoot) is influenced by a reduction in cochlear amplification and to date, there is no objective tool to investigate it. Therefore, a different paradigm of the auditory brainstem response (ABR) was utilized to measure auditory overshoot. It was assumed that increasing the delay onset time (DOT) between a signal and a masker reduces the latencies of waves I and III.

Materials & Methods

Sixteen normal young male guinea pigs were tested. A tone burst stimulus (signal: 16 kHz, 5ms in duration) and wide-band noise (masker: 0.1-8.0 kHz, 100ms in duration) at three DOTs were used. To diminish the effect of the noise on waves, waveforms were subtracted from those derived from the noise burst alone. The absolute latency of the waves I and III, inter-peak latency of the waves I-III, and amplitude ratio of the waves III/I were compared for the 0, 30, and 100ms DOTs and five signal-to-noise ratios.

Results

The latencies of increased from the 0 to 30ms DOT and then decreased from the 30 to 100ms DOT (p < 0.001). No significant changes were observed in the latency waves at the 100ms DOT compared to the 0ms DOT (p > 0.005). Moreover, there were no significant differences between the three DOTs regarding the inter-peak latency and amplitude ratio of the waves (p <0.005).

Conclusion

The study results showed an overshoot-like electrophysiological effect using ABR. Therefore, an objective test was used to investigate auditory cochlear gain.

Human Auditory-Frequency Tuning Is Sensitive to Tonal Language Experience

Purpose The aim of this study is to investigate whether human auditory frequency tuning can be influenced by tonal language experience. Method Perceptual tuning measured via psychophysical tuning curves and cochlear tuning derived via stimulus-frequency otoacoustic emission suppression tuning curves in 14 native speakers of a tonal language (Mandarin) were compared to those of 14 native speakers of a nontonal language (English) at 1 and 4 kHz. Results Group comparisons of both psychophysical tuning curves (p = .046) and stimulus-frequency otoacoustic emission suppression tuning curves (p = .007) in the 4-kHz region indicated sharper frequency tuning in the Mandarin-speaking group relative to the English-speaking group. The auditory tuning was better at the higher (4 kHz) than the lower (1 kHz) probe frequencies (p < .001). Conclusions The sharper auditory tuning in the 4-kHz cochlear region is associated with long-term tonal language (i.e., Mandarin) experience. Experience-dependent plasticity of tonal language may occur before the sound signal reaches central neural stages, as peripheral as the cochlea.

Efferent-induced shifts in synchronized-spontaneous-otoacoustic-emission magnitude and frequency

Synchronized-spontaneous otoacoustic emissions (SSOAEs) present as slow-decaying emission energy that persists after the transient-evoked otoacoustic emission (TEOAE). SSOAEs possess high amplitudes and signal-to-noise ratios, making them potentially ideal candidates to assay the medial-olivocochlear reflex (MOCR). The current work quantified MOCR-induced changes to SSOAEs over a 36-dB stimulus level range and compared MOCR effects between TEOAE- and SSOAE-based assays. Otoacoustic emissions were evoked using band limited clicks from 52 to 88 dB peak sound pressure level (pSPL) with and without contralateral-acoustic stimulation (CAS) in 25 normal-hearing, female adults. The CAS was 50-dB sound pressure level (SPL) broadband noise and served to activate the MOCR. The number of SSOAEs increased with the stimulus level through approximately 70 dB pSPL. The presentation of CAS resulted in fewer SSOAEs. SSOAEs exhibited compressive growth and approached saturation for stimulus levels of 70 dB pSPL. The primary effects of CAS were a reduction in the SSOAE magnitude and an upward shift in the SSOAE frequency. These changes were not strongly affected by the stimulus level. Time-domain analysis of the SSOAE revealed an increase in the CAS-induced magnitude shift during the decay portion of the SSOAE. Compared to CAS-induced TEOAE magnitude shifts, SSOAE magnitude shifts were typically 2-3 dB larger. Findings support SSOAEs as a means to assay the MOCR.

The olivocochlear reflex strength in awake chinchillas is relevant for behavioural performance during visual selective attention with auditory distractors

The auditory efferent system comprises descending projections from the cerebral cortex to subcortical nuclei, reaching the cochlear receptor through olivocochlear fibres. One of the functions attributed to this corticofugal system is to suppress irrelevant sounds during selective attention to visual stimuli. Medial olivocochlear neurons can also be activated by sounds through a brainstem reflex circuit. Whether the individual variability of this reflex is related to the cognitive capacity to suppress auditory stimuli is still controversial. Here we propose that the individual strength per animal of the olivocochlear reflex is correlated with the ability to suppress auditory distractors during visual attention in awake chinchillas. The olivocochlear reflex was elicited with a contralateral broad-band noise at ~ 60 dB and ipsilateral distortion product otoacoustic emissions were obtained at different frequencies (1-8 kHz). Fourteen chinchillas were evaluated in a behavioural protocol of visual attention with broad-band noise and chinchilla vocalizations as auditory distractors. Results show that the behavioural performance was affected by both distractors and that the magnitudes of the olivocochlear reflex evaluated at multiple frequencies were relevant for behavioural performance during visual discrimination with auditory distractors. These results stress the ecological relevance of the olivocochlear system for suppressing natural distractors.

Jittering stimulus onset attenuates short-latency, synchronized-spontaneous otoacoustic emission energy

Synchronized-spontaneous otoacoustic emissions (SSOAEs) are slow-decaying otoacoustic emissions (OAEs) that persist up to several hundred milliseconds following presentation of a transient stimulus. If the inter-stimulus interval is sufficiently short, SSOAEs will contaminate the stimulus window of the adjacent epoch. In medial-olivocochlear reflex (MOCR) assays, SSOAE contamination can present as a change in the stimulus between quiet and noise conditions, since SSOAEs are sensitive to MOCR activation. Traditionally, a change in the stimulus between MOCR conditions implicates acoustic reflex activation by the contralateral noise; however, this interpretation is potentially confounded by SSOAEs. This study examined the utility of jittering stimulus onset to desynchronize and cancel short-latency SSOAE energy. Transient-evoked (TE) OAEs and SSOAEs were measured from 39 subjects in contralateral-quiet and -noise conditions. Clicks were presented at fixed and quasi-random intervals (by introducing up to 8 ms of jitter). For the fixed-interval condition, spectral differences in the stimulus window between quiet and noise conditions mirrored those in the SSOAE analysis window, consistent with SSOAE contamination. In contrast, spectral differences stemming from SSOAEs were attenuated and/or absent in the stimulus window for the jitter conditions. The use of jitter did not have a statistically significant effect on either TEOAE level or the estimated MOCR.

Efeito da levodopa na mecânica coclear e no sistema auditivo eferente de indivíduos com doença de Parkinson

RESUMO Objetivo Analisar o efeito da levodopa na dinâmica coclear, bem como na via eferente olivococlear medial de indivíduos com doença de Parkinson idiopática (DP). Método Indivíduos com e sem DP, acompanhados em um hospital universitário, realizaram a pesquisa das emissões otoacústicas por produto de distorção (EOAPD) e do efeito inibitório das EOAPD (EIEOA) na presença de ruído contralateral. Foram estabelecidas as medidas de correlação entre os resultados das EOAPD e do EIEOA com estágio Hoehn&Yahr (H&Y), dose diária de levodopa e tempo de diagnóstico da DP. Além disso, as medidas eletroacústicas foram comparadas entre os indivíduos sem DP e com DP, estratificados de acordo com a dose de levodopa administrada diariamente. Resultados Foi identificada correlação fraca e negativa entre a amplitude das EOAPD com a dose diária de levodopa e correlações positivas, de força moderada e fraca, entre o EIEOA com a dose diária de levodopa e o tempo de diagnóstico da DP, respectivamente. A amplitude das EOAPD foi maior nos indivíduos com DP em uso de levodopa ≤ 600 miligramas quando comparada à de indivíduos sem DP e com DP, em uso de dose superior. Já o EIEOA foi menor nos indivíduos em uso de doses ≤ 600 miligramas, quando comparado aos demais grupos. Conclusão Doses diárias de levodopa iguais ou inferiores a 600 mg/dia aumentam as respostas mecanotransdutoras cocleares nas frequências de 2 e 3 kHz, enquanto que a ação dos sistemas eferentes olivococleares é reduzida nesta região.

Psychoacoustic measurements of ipsilateral cochlear gain reduction as a function of signal frequency

Forward masking experiments at 4 kHz have demonstrated that preceding sound can elicit changes in masking patterns consistent with a change in cochlear gain. However, the acoustic environment is filled with complex sounds, often dominated by lower frequencies, and ipsilateral cochlear gain reduction at frequencies below 4 kHz is largely unstudied in the forward masking literature. In this experiment, the magnitude of ipsilateral cochlear gain reduction was explored at 1, 2, and 4 kHz using forward masking techniques in an effort to evaluate a range of frequencies in listeners with normal hearing. Gain reduction estimates were not significantly different at 2 and 4 kHz using two forward masking measurements. Although the frequency was a significant factor in the analysis, post hoc testing supported the interpretation that gain reduction estimates measured without a masker were not significantly different at 1, 2, and 4 kHz. A second experiment provided evidence that forward masking in this paradigm at 1 kHz cannot be explained by excitation alone. This study provides evidence of ipsilateral cochlear gain reduction in humans at frequencies below the 4 kHz region.

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.

Behavioral Pure-Tone Threshold Shifts Caused by Tympanic Membrane Electrodes

OBJECTIVE

To determine whether tympanic membrane (TM) electrodes induce behavioral pure-tone threshold shifts.

DESIGN

Pure-tone thresholds (250 to 8000 Hz) were measured twice in test (n = 18) and control (n = 10) groups. TM electrodes were placed between first and second threshold measurements in the test group, whereas the control group did not receive electrodes. Pure-tone threshold shifts were compared between groups. The effect of TM electrode contact location on threshold shifts was evaluated in the test group.

RESULTS

TM electrodes significantly increased average low-frequency thresholds, 7.5 dB at 250 Hz and 4.2 dB at 500 Hz, and shifts were as large as 25 dB in individual ears. Also, threshold shifts did not appear to vary at any frequency with TM electrode contact location.

CONCLUSIONS

Low-frequency threshold shifts occur when using TM electrodes and insert earphones. These findings are relevant to interpreting electrocochleographic responses to low-frequency stimuli.

Olivocochlear efferents: Their action, effects, measurement and uses, and the impact of the new conception of cochlear mechanical responses

The anatomy and physiology of olivocochlear (OC) efferents are reviewed. To help interpret these, recent advances in cochlear mechanics are also reviewed. Lateral OC (LOC) efferents innervate primary auditory-nerve (AN) fiber dendrites. The most important LOC function may be to reduce auditory neuropathy. Medial OC (MOC) efferents innervate the outer hair cells (OHCs) and act to turn down the gain of cochlear amplification. Cochlear amplification had been thought to act only through basilar membrane (BM) motion, but recent reports show that motion near the reticular lamina (RL) is amplified more than BM motion, and that RL-motion amplification extends to several octaves below the local characteristic frequency. Data on efferent effects on AN-fiber responses, otoacoustic emissions (OAEs) and human psychophysics are reviewed and reinterpreted in the light of the new cochlear-mechanical data. The possible origin of OAEs in RL motion is considered. MOC-effect measuring methods and MOC-induced changes in human responses are also reviewed, including that ipsilateral and contralateral sound can produce MOC effects with different patterns across frequency. MOC efferents help to reduce damage due to acoustic trauma. Many, but not all, reports show that subjects with stronger contralaterally-evoked MOC effects have better ability to detect signals (e.g. speech) in noise, and that MOC effects can be modulated by attention.

Efferent modulation of pre-neural and neural distortion products

Distortion product otoacoustic emissions (DPOAEs) and distortion product frequency following responses (DPFFRs) are respectively pre-neural and neural measurements associated with cochlear nonlinearity. Because cochlear nonlinearity is putatively linked to outer hair cell electromotility, DPOAEs and DPFFRs may provide complementary measurements of the human medial olivocochlear (MOC) reflex, which directly modulates outer hair cell function. In this study, we first quantified MOC reflex-induced DPOAE inhibition at spectral fine structure peaks in 22 young human adults with normal hearing. The f1 and f2 tone pairs producing the largest DPOAE fine structure peak for each subject were then used to evoke DPFFRs with and without MOC reflex activation to provide a related neural measure of efferent inhibition. We observed significant positive relationships between DPOAE fine structure peak inhibition and inhibition of DPFFR components representing neural phase locking to f2 and 2f1-f2, but not f1. These findings may support previous observations that the MOC reflex inhibits DPOAE sources differentially. That these effects are maintained and represented in the auditory brainstem suggests that the MOC reflex may exert a potent influence on subsequent subcortical neural representation of sound.

Reflex Modification Audiometry Reveals Dual Roles for Olivocochlear Neurotransmission

Approximately 15% of American adults report some degree of difficulty hearing in a noisy environment or have auditory filtering difficulties. There are objective clinical tests of auditory filtering, yet few tests exist for mouse models that do not rely on extensive training. We have used reflex modification audiometry (RMA) and developed exclusion criteria for the mouse model. This RMA based test makes use of the acoustic startle response (ASR) and the ability of prepulses to inhibit the ASR [i.e., prepulse inhibition (PPI)] to assess the mouse's ability to detect prepulse signals presented in quiet or embedded in masking noise. We have studied PPI behavior across four inbred mouse strains with normal cochlear function and developed pre-testing exclusion criteria and test/retest reliability measures. Moreover, because both the medial (MOC) and the lateral (LOC) olivocochlear efferent feedback systems have been proposed to improve auditory behavior performance, especially in noisy backgrounds, we have examined PPI abilities in mice (with their littermate controls) either lacking the MOC receptor subunit α9 nicotinic acetylcholine receptor [α9 nAChR (–/–)] or expressing an overactive receptor [Ld'T mutation in α9 nAChR KI], or lacking an LOC efferent neuropeptide, alpha calcitonin gene-related peptide [αCGRP (–/–)] only in the CNS. Because CGRP receptor formation has been shown to mature from juvenile to adult ages, we also studied if this maturation would be reflected in PPI behavioral responses in juvenile and adult (+/+) controls and in adult αCGRP (–/–) animals. We show that 50% PPI response thresholds (sound level with 50% correct responses) in quiet are decreased in the (–/–) α9 nAChR animals, and 50% PPI responses are increased for mice with an overactive receptor (α9 nAChR KI) and are increased in adult mice lacking αCGRP (–/–). However, in background noise, only mice lacking αCGRP exhibited increased 50% PPI response thresholds, as there were no significant differences between α9 nAChR adult mouse lines and their littermate controls. These findings suggest that MOC and LOC olivocochlear neurotransmission work in tandem to improve behavioral responses to sound. These experiments further pave the way for rapid behavioral hearing assessments in other mouse models.

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.

SIGNIFICANCE STATEMENT

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.

Assessment of Ipsilateral Efferent Effects in Human via ECochG

Development of electrophysiological means to assess the medial olivocochlear (MOC) system in humans is important to further our understanding of the function of that system and for the refinement and validation of psychoacoustical and otoacoustic emission methods which are thought to probe the MOC. Based on measurements in anesthetized animals it has been hypothesized that the MOC-reflex (MOCR) can enhance the response to signals in noise, and several lines of evidence support such a role in humans. A difficulty in these studies is the isolation of efferent effects. Efferent activation can be triggered by acoustic stimulation of the contralateral or ipsilateral ear, but ipsilateral stimulation is thought to be more effective. However, ipsilateral stimulation complicates interpretation of effects since these sounds can affect the perception of other ipsilateral sounds by mechanisms not involving olivocochlear efferents. We assessed the ipsilaterally evoked MOCR in human using a transtympanic procedure to record mass-potentials from the cochlear promontory or the niche of the round window. Averaged compound action potential (CAP) responses to masked probe tones of 4 kHz with and without a precursor (designed to activate the MOCR but not the stapedius reflex) were extracted with a polarity alternating paradigm. The masker was either a simultaneous narrow band noise masker or a short (20-ms) tonal ON- or OFF-frequency forward masker. The subjects were screened for normal hearing (audiogram, tympanogram, threshold stapedius reflex) and psychoacoustically tested for the presence of a precursor effect. We observed a clear reduction of CAP amplitude by the precursor, for different masking conditions. Even without an MOCR, this is expected because the precursor will affect the response to subsequent stimuli via neural adaptation. To determine whether the precursor also activated the efferent system, we measured the CAP over a range of masker levels, with or without precursor, and for different types of masker. The results show CAP reduction consistent with the type of gain reduction caused by the MOCR. These results generally support psychoacoustical paradigms designed to probe the efferent system as indeed activating the MOCR system, but not all observations are consistent with this mechanism.

Efferent inhibition strength is a physiological correlate of hyperacusis in children with autism spectrum disorder

Autism spectrum disorder (ASD) is a developmental disability that is poorly understood. ASD can influence communication, social interaction, and behavior. Children with ASD often have sensory hypersensitivities, including auditory hypersensitivity (hyperacusis). In adults with hyperacusis who are otherwise neurotypical, the medial olivocochlear (MOC) efferent reflex is stronger than usual. In children with ASD, the MOC reflex has been measured, but without also assessing hyperacusis. We assessed the MOC reflex in children with ASD by measuring the strength of MOC-induced inhibition of transient-evoked otoacoustic emissions (TEOAEs), a noninvasive physiological measure that reflects cochlear amplification. MOC activity was evoked by contralateral noise. Hyperacusis was assessed subjectively on the basis of the children's symptoms. We found a significant correlation between hyperacusis scores and MOC strength in children with ASD. When children were divided into ASD-with-severe-hyperacusis (ASDs), ASD-with-not-severe-hyperacusis (ASDns), and neurotypical (NT) groups, the last two groups had similar hyperacusis and MOC reflexes, whereas the ASDs group, on average, had hyperacusis and MOC reflexes that were approximately twice as strong. The MOC inhibition of TEOAEs averaged larger at all frequencies in the ASDs compared with ASDns and NT groups. The results suggest that the MOC reflex can be used to estimate hyperacusis in children with ASD and might be used to validate future questionnaires to assess hyperacusis. Our results also provide evidence that strong MOC reflexes in children with ASD are associated with hyperacusis and that hyperacusis is a comorbid condition and is not a necessary, integral part of the abnormal neural processing associated with ASD.NEW & NOTEWORTHY Children with autism spectrum disorder (ASD) are a heterogeneous group, some with hyperacusis and some without. Our research shows that hyperacusis can be estimated in children with ASD by using medial olivocochlear (MOC) reflex measurements. By establishing that an objective measure correlates with attributes of hyperacusis, our results enable future work to enable subtyping of children with ASD to provide improved individualized treatments to at-risk children and those without adequate language to describe their hyperacusis symptoms.

Electrophysiological Measurements of Peripheral Vestibular Function-A Review of Electrovestibulography

Electrocochleography (EcochG), incorporating the Cochlear Microphonic (CM), the Summating Potential (SP), and the cochlear Compound Action Potential (CAP), has been used to study cochlear function in humans and experimental animals since the 1930s, providing a simple objective tool to assess both hair cell (HC) and nerve sensitivity. The vestibular equivalent of ECochG, termed here Electrovestibulography (EVestG), incorporates responses of the vestibular HCs and nerve. Few research groups have utilized EVestG to study vestibular function. Arguably, this is because stimulating the cochlea in isolation with sound is a trivial matter, whereas stimulating the vestibular system in isolation requires significantly more technical effort. That is, the vestibular system is sensitive to both high-level sound and bone-conducted vibrations, but so is the cochlea, and gross electrical responses of the inner ear to such stimuli can be difficult to interpret. Fortunately, several simple techniques can be employed to isolate vestibular electrical responses. Here, we review the literature underpinning gross vestibular nerve and HC responses, and we discuss the nomenclature used in this field. We also discuss techniques for recording EVestG in experimental animals and humans and highlight how EVestG is furthering our understanding of the vestibular system.

Contralateral Inhibition of Click- and Chirp-Evoked Human Compound Action Potentials

Cochlear outer hair cells (OHC) receive direct efferent feedback from the caudal auditory brainstem via the medial olivocochlear (MOC) bundle. This circuit provides the neural substrate for the MOC reflex, which inhibits cochlear amplifier gain and is believed to play a role in listening in noise and protection from acoustic overexposure. The human MOC reflex has been studied extensively using otoacoustic emissions (OAE) paradigms; however, these measurements are insensitive to subsequent "downstream" efferent effects on the neural ensembles that mediate hearing. In this experiment, click- and chirp-evoked auditory nerve compound action potential (CAP) amplitudes were measured electrocochleographically from the human eardrum without and with MOC reflex activation elicited by contralateral broadband noise. We hypothesized that the chirp would be a more optimal stimulus for measuring neural MOC effects because it synchronizes excitation along the entire length of the basilar membrane and thus evokes a more robust CAP than a click at low to moderate stimulus levels. Chirps produced larger CAPs than clicks at all stimulus intensities (50-80 dB ppeSPL). MOC reflex inhibition of CAPs was larger for chirps than clicks at low stimulus levels when quantified both in terms of amplitude reduction and effective attenuation. Effective attenuation was larger for chirp- and click-evoked CAPs than for click-evoked OAEs measured from the same subjects. Our results suggest that the chirp is an optimal stimulus for evoking CAPs at low stimulus intensities and for assessing MOC reflex effects on the auditory nerve. Further, our work supports previous findings that MOC reflex effects at the level of the auditory nerve are underestimated by measures of OAE inhibition.

Electrically Evoked Medial Olivocochlear Efferent Effects on Stimulus Frequency Otoacoustic Emissions in Guinea Pigs

Stimulus frequency otoacoustic emissions (SFOAEs) are produced by cochlear irregularities reflecting energy from the peak region of the traveling wave (TW). Activation of medial olivocochlear (MOC) efferents reduces cochlear amplification and otoacoustic emissions (OAEs). In other OAEs, MOC activation can produce enhancements. The extent of MOC enhancements of SFOAEs has not been previously studied. In anesthetized guinea pigs, we electrically stimulated MOC fibers and recorded their effects on SFOAEs. MOC stimulation mostly inhibited SFOAEs but sometimes enhanced them. Some enhancements were not near response dips and therefore cannot be explained by a reduction of wavelet cancelations. MOC stimulation always inhibited auditory-nerve compound action potentials showing that cochlear-amplifier gain was not increased. We propose that some SFOAE enhancements arise because shocks excite only a small number of MOC fibers that inhibit a few scattered outer hair cells thereby changing (perhaps increasing) cochlear irregularities and SFOAE amplitudes. Contralateral sound activation is expected to excite approximately one third of MOC efferents and may also change cochlear irregularities. Some papers suggest that large SFOAE components originate far basal of the TW peak, basal of the region that receives cochlear amplification. Using a time-frequency analysis, we separated SFOAEs into components with different latencies. At all SFOAE latencies, most SFOAE components were inhibited by MOC stimulation, but some were enhanced. The MOC inhibition of short-latency SFOAE components is consistent with these components being produced in the cochlear-amplified region near the TW peak.

Concurrent measures of contralateral suppression of transient-evoked otoacoustic emissions and of auditory steady-state responses

Contralateral suppression of otoacoustic emissions (OAEs) is frequently used to assess the medial olivocochlear (MOC) efferent system, and may have clinical utility. However, OAEs are weak or absent in hearing-impaired ears, so little is known about MOC function in the presence of hearing loss. A potential alternative measure is contralateral suppression of the auditory steady-state response (ASSR) because ASSRs are measurable in many hearing-impaired ears. This study compared contralateral suppression of both transient-evoked otoacoustic emissions (TEOAEs) and ASSRs in a group of ten primarily older adults with either normal hearing or mild sensorineural hearing loss. Responses were elicited using 75-dB peak sound pressure level clicks. The MOC was activated using contralateral broadband noise at 60 dB sound pressure level. Measurements were made concurrently to ensure a consistent attentional state between the two measures. The magnitude of contralateral suppression of ASSRs was significantly larger than contralateral suppression of TEOAEs. Both measures usually exhibited high test-retest reliability within a session. However, there was no significant correlation between the magnitude of contralateral suppression of TEOAEs and of ASSRs. Further work is needed to understand the role of the MOC in contralateral suppression of ASSRs.

Influence of medial olivocochlear efferents on the sharpness of cochlear tuning estimates in children

The present study objectively quantified the efferent-induced changes in the sharpness of cochlear tuning estimates and compared these alterations in cochlear tuning between adults and children. Click evoked otoacoustic emissions with and without contralateral broadband noise were recorded from 15 young adults and 14 children aged between 5 and 10 yrs. Time-frequency distributions of click evoked otoacoustic emissions were obtained via the S-transform, and the otoacoustic emission latencies were used to estimate the sharpness of cochlear tuning. Contralateral acoustic stimulation caused a significant reduction in the sharpness of cochlear tuning estimates in the low to mid frequency region, but had no effect in the higher frequencies (3175 and 4000 Hz). The magnitude of efferent-induced changes in cochlear tuning estimates was similar between adults and children. The current evidence suggests that the stimulation of the medial olivocochlear efferent neurons causes similar alterations in cochlear frequency selectivity in adults and children.