Rapid assessment of sound-evoked olivocochlear feedback: suppression of compound action potentials by contralateral sound

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.

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.

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.

Cochlear Efferent Innervation Is Sparse in Humans and Decreases with Age

The mammalian cochlea is innervated by two cholinergic feedback systems called the medial olivocochlear (MOC) and lateral olivocochlear (LOC) pathways, which send control signals from the brainstem back to the outer hair cells and auditory-nerve fibers, respectively. Despite countless studies of the cochlear projections of these efferent fibers in animal models, comparable data for humans are almost completely lacking. Here, we immunostained the cochlear sensory epithelium from 23 normal-aging humans (14 males and 9 females), 0-86 years of age, with cholinergic markers to quantify the normal density of MOC and LOC projections, and the degree of age-related degeneration. In younger ears, the MOC density peaks in mid-cochlear regions and falls off both apically and basally, whereas the LOC innervation peaks near the apex. In older ears, MOC density decreases dramatically, whereas the LOC density does not. The loss of MOC feedback may contribute to the age-related decrease in word recognition in noise; however, even at its peak, the MOC density is lower than in other mammals, suggesting the MOC pathway is less important for human hearing.SIGNIFICANCE STATEMENT The cochlear epithelium and its sensory innervation are modulated by the olivocochlear (OC) efferent pathway. Although the medial OC (MOC) reflex has been extensively studied in humans, via contralateral sound suppression, the cochlear projections of these cholinergic neurons have not been described in humans. Here, we use immunostaining to quantify the MOC projections to outer hair cells and lateral OC (LOC) projections to the inner hair cell area in humans 0-89 years of age. We show age-related loss of MOC, but not LOC, innervation, which likely contributes to hearing impairments, and a relative paucity of MOC terminals at all ages, which may account for the relative weakness of the human MOC reflex and the difficulty in demonstrating a robust functional role in human experiments.

JJ. Auditory Attention Reduced Ear-Canal Noise in Humans by Reducing Subject Motion, Not by Medial Olivocochlear Efferent Inhibition: Implications for Measuring Otoacoustic Emissions During a Behavioral Task

Otoacoustic emissions (OAEs) are often measured to non-invasively determine activation of medial olivocochlear (MOC) efferents in humans. Usually these experiments assume that ear-canal noise remains constant. However, changes in ear-canal noise have been reported in some behavioral experiments. We studied the variability of ear-canal noise in eight subjects who performed a two-interval-forced-choice (2IFC) sound-level-discrimination task on monaural tone pips in masker noise. Ear-canal noise was recorded directly from the unstimulated ear opposite the task ear. Recordings were also made with similar sounds presented, but no task done. In task trials, ear-canal noise was reduced at the time the subject did the discrimination, relative to the ear-canal noise level earlier in the trial. In two subjects, there was a decrease in ear-canal noise, primarily at 1-2 kHz, with a time course similar to that expected from inhibition by MOC activity elicited by the task-ear masker noise. These were the only subjects with spontaneous OAEs (SOAEs). We hypothesize that the SOAEs were inhibited by MOC activity elicited by the task-ear masker. Based on the standard rationale in OAE experiments that large bursts of ear-canal noise are artifacts due to subject movement, ear-canal noise bursts above a sound-level criterion were removed. As the criterion was lowered and more high- and moderate-level ear-canal noise bursts were removed, the reduction in ear-canal noise level at the time of the 2IFC discrimination decreased to almost zero, for the six subjects without SOAEs. This pattern is opposite that expected from MOC-induced inhibition (which is greater on lower-level sounds), but can be explained by the hypothesis that subjects move less and create fewer bursts of ear-canal noise when they concentrate on doing the task. In no-task trials for these six subjects, the ear-canal noise level was little changed throughout the trial. Our results show that measurements of MOC effects on OAEs must measure and account for changes in ear-canal noise, especially in behavioral experiments. The results also provide a novel way of showing the time course of the buildup of attention via the time course of the reduction in ear-canal noise.

Adversarial relationship between combined medial olivocochlear (MOC) and middle-ear-muscle (MEM) reflexes and alarm-in-noise detection thresholds under negative signal-to-noise ratios (SNRs)

The role of auditory efferent feedback from the medial olivocochlear system (MOCS) and the middle-ear-muscle (MEM) reflex in tonal detection tasks for humans in the presence of noise is not clearly understood. Past studies have yielded inconsistent results on the relationship between efferent feedback and tonal detection thresholds. This study attempts to address this inconsistency. Fifteen human subjects with normal hearing participated in an experiment where they were asked to identify an alarm signal in the presence of 80 dBA background (pink) noise. Masked detection thresholds were estimated using the method of two-interval forced choice (2IFC). Contralateral suppression of transient-evoked otoacoustic emissions (TEOAEs) was measured to estimate the strength of auditory efferent feedback. Subsequent correlation analysis revealed that the contralateral suppression of TEOAEs was significantly negatively correlated (r = -0.526, n = 15, p = 0.0438) with alarm-in-noise (AIN) detection thresholds under negative signal-to-noise conditions. The result implies that the stronger the auditory efferent feedback, the worse the detection thresholds and thus the poorer the tonal detection performance in the presence of loud noise.

Generation and Characterization of α9 and α10 Nicotinic Acetylcholine Receptor Subunit Knockout Mice on a C57BL/6J Background

We generated constitutive knockout mouse models for the α9 and α10 nicotinic acetylcholine receptor (nAChR) subunits by derivation from conditional knockouts by breeding with CRE deleter mice. We then backcrossed them onto a C57BL/6J genetic background. In this manuscript, we report the generation of the strains and an auditory phenotypic characterization of the constitutive α9 and α10 knockouts and a double α9α10 constitutive knockout. Although the α9 and α10 nAChR subunits are relevant to a number of physiological measures, we chose to characterize the mouse with auditory studies to compare them to existing but different α9 and α10 nAChR knockouts (KOs). Auditory brainstem response (ABR) measurements and distortion product otoacoustic emissions (DPOAEs) showed that all constitutive mouse strains had normal hearing. DPOAEs with contralateral noise (efferent adaptation measurements), however, showed that efferent strength was significantly reduced after deletion of both the α9 and α10 subunits, in comparison to wildtype controls. Animals tested were 3-8 weeks of age and efferent strength was not correlated with age. Confocal studies of single and double constitutive KOs showed that all KOs had abnormal efferent innervation of cochlear hair cells. The morphological results are similar to those obtained in other strains using constitutive deletion of exon 4 of α9 or α10 nAChR. The results of our physiological studies, however, differ from previous auditory studies using a α9 KO generated by deletion of the exon 4 region and backcrossed onto a mixed CBA/CaJ X 129Sv background.

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.

Effects of age and hearing loss on overshoot

The detection of a brief, sinusoidal probe in a long broadband, simultaneous masker improves as the probe is delayed from the masker's onset. This improvement ("overshoot") may be mediated by a reduction in cochlear amplifier gain over the timecourse of the masker via the medial olivocochlear (MOC) reflex. Overshoot was measured in younger adults with normal hearing and in older adults with normal and impaired hearing to test the hypothesis that aging and cochlear hearing loss result in abnormal overshoot, consistent with changes in certain structures along the MOC pathway. Overshoot decreased with increasing quiet probe thresholds and was only minimally influenced by increasing age. Marked individual differences in overshoot were observed due to differences in masking thresholds for probes presented near the masker's onset. Model simulations support the interpretation that reduced overshoot in hearing-impaired listeners is due to limited cochlear amplifier gain and therefore less gain to adjust over the timecourse of the masker. Similar overshoot among younger and older adults with normal hearing suggests that age-related changes to mechanisms underlying overshoot do not result in significant differences in overshoot among younger and older adults with normal hearing.

The Corticofugal Effects of Auditory Cortex Microstimulation on Auditory Nerve and Superior Olivary Complex Responses Are Mediated via Alpha-9 Nicotinic Receptor Subunit

Background and Objective

The auditory efferent system is a complex network of descending pathways, which mainly originate in the primary auditory cortex and are directed to several auditory subcortical nuclei. These descending pathways are connected to olivocochlear neurons, which in turn make synapses with auditory nerve neurons and outer hair cells (OHC) of the cochlea. The olivocochlear function can be studied using contralateral acoustic stimulation, which suppresses auditory nerve and cochlear responses. In the present work, we tested the proposal that the corticofugal effects that modulate the strength of the olivocochlear reflex on auditory nerve responses are produced through cholinergic synapses between medial olivocochlear (MOC) neurons and OHCs via alpha-9/10 nicotinic receptors.

Methods

We used wild type (WT) and alpha-9 nicotinic receptor knock-out (KO) mice, which lack cholinergic transmission between MOC neurons and OHC, to record auditory cortex evoked potentials and to evaluate the consequences of auditory cortex electrical microstimulation in the effects produced by contralateral acoustic stimulation on auditory brainstem responses (ABR).

Results

Auditory cortex evoked potentials at 15 kHz were similar in WT and KO mice. We found that auditory cortex microstimulation produces an enhancement of contralateral noise suppression of ABR waves I and III in WT mice but not in KO mice. On the other hand, corticofugal modulations of wave V amplitudes were significant in both genotypes.

Conclusion

These findings show that the corticofugal modulation of contralateral acoustic suppressions of auditory nerve (ABR wave I) and superior olivary complex (ABR wave III) responses are mediated through MOC synapses.

Estrogen Blockade Reduces Auditory Feedback in CBA Mice

OBJECTIVE: To examine the effects of estrogen suppression on age-related changes in distortion product otoacoustic emissions (DPOAEs) and contralateral suppression (CS) of DPOAEs in CBA mice.

STUDY DESIGN AND SETTING: Young CBA mice received a slow-release shoulder implantation of either tamoxifen or placebo. Serial DPOAEs and CS of DPOAEs were obtained at 3-week intervals over a period of 9 weeks.

RESULTS: Although DPOAEs were maintained over the study interval, CS of DPOAEs decreased significantly with age in the experimental group. No such declines were observed in either the control animals or the untreated male mice.

CONCLUSIONS: Estrogen suppression negatively affects the MOC efferent feedback system.

SIGNIFICANCE: Our results support the hypothesis that estrogen plays an important role in the maintenance of auditory integrity. Additionally, our findings raise intriguing questions about auditory effects of hormonal shifts in humans resulting from menopause, hormone supplements such as oral contraceptives, and hormone replacement therapy as well as antiestrogens.

Corticofugal modulation of peripheral auditory responses

The auditory efferent system originates in the auditory cortex and projects to the medial geniculate body (MGB), inferior colliculus (IC), cochlear nucleus (CN) and superior olivary complex (SOC) reaching the cochlea through olivocochlear (OC) fibers. This unique neuronal network is organized in several afferent-efferent feedback loops including: the (i) colliculo-thalamic-cortico-collicular; (ii) cortico-(collicular)-OC; and (iii) cortico-(collicular)-CN pathways. Recent experiments demonstrate that blocking ongoing auditory-cortex activity with pharmacological and physical methods modulates the amplitude of cochlear potentials. In addition, auditory-cortex microstimulation independently modulates cochlear sensitivity and the strength of the OC reflex. In this mini-review, anatomical and physiological evidence supporting the presence of a functional efferent network from the auditory cortex to the cochlear receptor is presented. Special emphasis is given to the corticofugal effects on initial auditory processing, that is, on CN, auditory nerve and cochlear responses. A working model of three parallel pathways from the auditory cortex to the cochlea and auditory nerve is proposed.

Stronger efferent suppression of cochlear neural potentials by contralateral acoustic stimulation in awake than in anesthetized chinchilla

There are two types of sensory cells in the mammalian cochlea, inner hair cells, which make synaptic contact with auditory-nerve afferent fibers, and outer hair cells that are innervated by crossed and uncrossed medial olivocochlear (MOC) efferent fibers. Contralateral acoustic stimulation activates the uncrossed efferent MOC fibers reducing cochlear neural responses, thus modifying the input to the central auditory system. The chinchilla, among all studied mammals, displays the lowest percentage of uncrossed MOC fibers raising questions about the strength and frequency distribution of the contralateral-sound effect in this species. On the other hand, MOC effects on cochlear sensitivity have been mainly studied in anesthetized animals and since the MOC-neuron activity depends on the level of anesthesia, it is important to assess the influence of anesthesia in the strength of efferent effects. Seven adult chinchillas (Chinchilla laniger) were chronically implanted with round-window electrodes in both cochleae. We compared the effect of contralateral sound in awake and anesthetized condition. Compound action potentials (CAP) and cochlear microphonics (CM) were measured in the ipsilateral cochlea in response to tones in absence and presence of contralateral sound. Control measurements performed after middle-ear muscles section in one animal discarded any possible middle-ear reflex activation. Contralateral sound produced CAP amplitude reductions in all chinchillas, with suppression effects greater by about 1–3 dB in awake than in anesthetized animals. In contrast, CM amplitude increases of up to 1.9 dB were found in only three awake chinchillas. In both conditions the strongest efferent effects were produced by contralateral tones at frequencies equal or close to those of ipsilateral tones. Contralateral CAP suppressions for 1–6 kHz ipsilateral tones corresponded to a span of uncrossed MOC fiber innervation reaching at least the central third of the chinchilla cochlea.

The olivocochlear reflex strength and cochlear sensitivity are independently modulated by auditory cortex microstimulation

In mammals, efferent projections to the cochlear receptor are constituted by olivocochlear (OC) fibers that originate in the superior olivary complex. Medial and lateral OC neurons make synapses with outer hair cells and with auditory nerve fibers, respectively. In addition to the OC system, there are also descending projections from the auditory cortex that are directed towards the thalamus, inferior colliculus, cochlear nucleus, and superior olivary complex. Olivocochlear function can be assessed by measuring a brainstem reflex mediated by auditory nerve fibers, cochlear nucleus neurons, and OC fibers. Although it is known that the OC reflex is activated by contralateral acoustic stimulation and produces a suppression of cochlear responses, the influence of cortical descending pathways in the OC reflex is largely unknown. Here, we used auditory cortex electrical microstimulation in chinchillas to study a possible cortical modulation of cochlear and auditory nerve responses to tones in the absence and presence of contralateral noise. We found that cortical microstimulation produces two different peripheral modulations: (i) changes in cochlear sensitivity evidenced by amplitude modulation of cochlear microphonics and auditory nerve compound action potentials and (ii) enhancement or suppression of the OC reflex strength as measured by auditory nerve responses, which depended on the intersubject variability of the OC reflex. Moreover, both corticofugal effects were not correlated, suggesting the presence of two functionally different efferent pathways. These results demonstrate that auditory cortex electrical microstimulation independently modulates the OC reflex strength and cochlear sensitivity.

Exploring the role of feedback-based auditory reflexes in forward masking by schroeder-phase complexes

Several studies have postulated that psychoacoustic measures of auditory perception are influenced by efferent-induced changes in cochlear responses, but these postulations have generally remained untested. This study measured the effect of stimulus phase curvature and temporal envelope modulation on the medial olivocochlear reflex (MOCR) and on the middle-ear muscle reflex (MEMR). The role of the MOCR was tested by measuring changes in the ear-canal pressure at 6 kHz in the presence and absence of a band-limited harmonic complex tone with various phase curvatures, centered either at (on-frequency) or well below (off-frequency) the 6-kHz probe frequency. The influence of possible MEMR effects was examined by measuring phase-gradient functions for the elicitor effects and by measuring changes in the ear-canal pressure with a continuous suppressor of the 6-kHz probe. Both on- and off-frequency complex tone elicitors produced significant changes in ear canal sound pressure. However, the pattern of results was not consistent with the earlier hypotheses postulating that efferent effects produce the psychoacoustic dependence of forward-masked thresholds on masker phase curvature. The results also reveal unexpectedly long time constants associated with some efferent effects, the source of which remains unknown.

Time course of cochlear injury discharge (excitotoxicity) determined by ABR monitoring of contralateral cochlear events

The dynamics of cochlear excitotoxicity can be monitored from effects on the contralateral ear. After unilateral mechanical ablation of the cochlea (in a mouse model) we observed immediate elevations in auditory brainstem evoked response (ABR) thresholds in the contralateral ear. Threshold elevations peaked at 2-3 h post ablation, and returned to baseline levels after 5-6 h. These contralateral effects are initiated by cochlear afferent injury discharges most likely activating the olivocochlear efferent system. Six hours after cochlear injury, ABR thresholds were fully returned to pre-lesion baseline levels and remained normal for up to 10 days of monitoring. We have confirmed that our cochlear ablation procedure increases short-term activity levels in the auditory brainstem and midbrain using c-fos labelling. The study provides insight into the dynamics of glutamate excitotoxicity, a pathological process directly related to acute tinnitus after acoustic trauma, and more generally implicated in many types of brain injury and neuro-degenerative disease.

Contralateral ear occlusion for improving the reliability of otoacoustic emission screening tests

Newborn hearing screening is an established healthcare standard in many countries and testing is feasible using otoacoustic emission (OAE) recording. It is well documented that OAEs can be suppressed by acoustic stimulation of the ear contralateral to the test ear. In clinical otoacoustic emission testing carried out in a sound attenuating booth, ambient noise levels are low such that the efferent system is not activated. However in newborn hearing screening, OAEs are often recorded in hospital or clinic environments, where ambient noise levels can be 60–70 dB SPL. Thus, results in the test ear can be influenced by ambient noise stimulating the opposite ear. Surprisingly, in hearing screening protocols there are no recommendations for avoiding contralateral suppression, that is, protecting the opposite ear from noise by blocking the ear canal. In the present study we have compared transient evoked and distortion product OAEs measured with and without contralateral ear plugging, in environmental settings with ambient noise levels <25 dB SPL, 45 dB SPL, and 55 dB SPL. We found out that without contralateral ear occlusion, ambient noise levels above 55 dB SPL can significantly attenuate OAE signals. We strongly suggest contralateral ear occlusion in OAE based hearing screening in noisy environments.

The dissimilar time course of temporary threshold shifts and reduction of inhibition in the inferior colliculus following intense sound exposure

Excessive noise exposure is known to produce an auditory threshold shift, which can be permanent or transient in nature. Recent studies showed that noise-induced temporary threshold shifts are associated with loss of synaptic connections to the inner hair cells and with cochlear nerve degeneration, which is reflected in a decreased amplitude of wave I of the auditory brainstem response (ABR). This suggests that, despite normal auditory thresholds, central auditory processing may be abnormal. We recorded changes in central auditory processing following a sound-induced temporary threshold shift. Anesthetized guinea pigs were exposed for 1 h to a pure tone of 11 kHz (124 dB sound pressure level). Hearing thresholds, amplitudes of ABR waves I and IV, and spontaneous and tone-evoked firing rates in the inferior colliculus (IC) were assessed immediately, one week, two weeks, and four weeks post exposure. Hearing thresholds were elevated immediately following overexposure, but recovered within one week. The amplitude of the ABR wave I was decreased in all sound-exposed animals for all test periods. In contrast, the ABR wave IV amplitude was only decreased immediately after overexposure and recovered within a week. The proportion of IC units that show inhibitory responses to pure tones decreased substantially up to two weeks after overexposure, especially when stimulated with high frequencies. The proportion of excitatory responses to low frequencies was increased. Spontaneous activity was unaffected by the overexposure. Despite rapid normalization of auditory thresholds, our results suggest an increased central gain following sound exposure and an abnormal balance between excitatory and inhibitory responses in the midbrain up to two weeks after overexposure. These findings may be associated with hyperacusis after a sound-induced temporary threshold shift.

Accounting for nonmonotonic precursor duration effects with gain reduction in the temporal window model

The mechanisms of forward masking are not clearly understood. The temporal window model (TWM) proposes that masking occurs via a neural mechanism that integrates within a temporal window. The medial olivocochlear reflex (MOCR), a sound-evoked reflex that reduces cochlear amplifier gain, may also contribute to forward masking if the preceding sound reduces gain for the signal. Psychophysical evidence of gain reduction can be observed using a growth of masking (GOM) paradigm with an off-frequency forward masker and a precursor. The basilar membrane input/output (I/O) function is estimated from the GOM function, and the I/O function gain is reduced by the precursor. In this study, the effect of precursor duration on this gain reduction effect was examined for on- and off-frequency precursors. With on-frequency precursors, thresholds increased with increasing precursor duration, then decreased (rolled over) for longer durations. Thresholds with off-frequency precursors continued to increase with increasing precursor duration. These results are not consistent with solely neural masking, but may reflect gain reduction that selectively affects on-frequency stimuli. The TWM was modified to include history-dependent gain reduction to simulate the MOCR, called the temporal window model-gain reduction (TWM-GR). The TWM-GR predicted rollover and the differences with on- and off-frequency precursors whereas the TWM did not.

Separating the contributions of olivocochlear and middle ear muscle reflexes in modulation of distortion product otoacoustic emission levels

OBJECTIVES

Mediated by the medial olivocochlear system (MOCS), distortion product otoacoustic emission (DPOAE) levels are reduced by presentation of contralateral acoustic stimuli. Such acoustic signals can also evoke a middle ear muscle reflex (MEMR) that also attenuates recorded DPOAE levels. Our aim is to clearly differentiate these two inhibitory mechanisms and to analyze each separately, perhaps allowing the development of novel tests of hearing function.

METHODS

DPOAE were recorded in real time from chinchillas with normal auditory brainstem response thresholds and middle ear function. Amplitude reduction and its onset latency caused by contralateral presentation of intermittent narrow-band noise (NBN) were measured. Stapedius and tensor tympani muscle tendons were divided without disturbing the ossicular chain, and DPOAE testing was repeated.

RESULTS

Peak reduction of (2f1 - f2) DPOAE levels occurred when the center frequency of contralateral NBN approximated the primary tone f2, indicating an f2-frequency-specific response. For a 4.5-kHz centered NBN, DPOAE (f2 = 4.4 kHz) inhibition was 0.1 dB (p < 0.001). This response remained present after tendon division, consistent with an MOCS origin. Low-frequency NBN (center frequency: 0.5 kHz) reduced otoacoustic emission levels (0.1 dB, p < 0.001) across a wide range of DPOAE frequencies. This low-frequency response was abolished by division of the middle ear muscle tendons, clearly indicating MEMR involvement.

CONCLUSIONS

Following middle ear muscle tendon division, DPOAE inhibition by contralateral stimuli approximating the primary tone f2 persists, whereas responses evoked by lower contralateral frequencies are abolished. This distinguishes the different roles of the MOCS (f2 frequency specific) and MEMR (low frequency only) in contralateral modulation of DPOAE. This analysis helps clarify the pathways involved in an objective test that might have clinical benefit in the testing of neonates.

Postnatal maturation of contralateral DPOAE suppression in a precocious animal model (chinchilla) of the human neonate

CONCLUSION

In the neonatal chinchilla, the degree of contralateral distortion product otoacoustic emission (DPOAE) suppression and the latency and time constants of suppression are immature for 40-60 days. This suggests that olivocochlear efferent innervation of outer hair cells is not fully mature at birth in this animal model, and this may also be the case for human neonates.

OBJECTIVES

To track postnatal changes in the dynamics of the olivocochlear efferent system in an animal model with cochlear development at birth similar to that in humans.

METHODS

Real-time measurements of contralateral DPOAE suppression were made in 79 ears of anaesthetized chinchillas, ranging in age from 1 day to 70 days. An adult control group (13 ears) was also tested. DPOAE (2f1-f2; f2 = 4.4 kHz; f2/f1 = 1.22) input/output functions were measured. Dynamics of contralateral broadband noise suppression were measured, including latency and suppression time constants.

RESULTS

DPOAE amplitude input/output functions are immature until 20-30 days postnatally. The maturation period for contralateral suppression amplitude is about 30 days. Latency of onset suppression was 40 ms at birth reducing to adult values (23 ms) at 40 days. The DPOAE suppression time constant was about 350 ms at birth and mature (230 ms) at 60 days.

Test/retest repeatability of effect contralateral acoustic stimulation on the magnitudes of distortion product ototacoustic emissions

OBJECTIVES/HYPOTHESIS

Contralateral inhibition of distortion product otoacoustic emissions (DPOAE) has become an important tool to assess the functioning of the medial olivocochlear efferents in humans. However, before this measurement can be applied clinically, the test/retest repeatability needs to be established. Therefore, the current study aimed at evaluating intra- and intersession test/retest repeatability of contralateral inhibition of DPOAE at 2f(1) -f(2) .

STUDY DESIGN

Prospective Test/Retest Reliability Study.

METHODS

Contralateral inhibition of DPOAE magnitudes were measured in the right ear of 24 adult male participants. To assess the intrasession repeatability, measurements were repeated without altering the position of the DPOAE probe (single-probe-fit). To assess intersession reliability, measurements were repeated on 8 different days (multiple-probe-fit). Repeatability of inhibition of DPOAE magnitudes was evaluated by Cronbach's alpha, interclass correlations, standard error of measurement, and its 95% confidence interval and smallest detectable difference.

RESULTS

DPOAE magnitudes were highly stable and repeatable across different recording sessions. However, test/retest reliability coefficients of DPOAE inhibition magnitudes were less than satisfactory for all the frequencies, in both single-probe-fit and multiple-probe-fit modes.

CONCLUSIONS

Results of the present study showed that contralateral inhibition magnitudes of DPOAEs varied considerably, even though DPOAEs magnitudes remained essentially the same across different recording sessions. As reliability is an essential aspect of any clinical procedure, it is suggested that at present contralateral inhibition of DPOAEs should not be used clinically to evaluate the medial efferent system. Laryngoscope, 2012.

Frequency tuning of the contralateral medial olivocochlear reflex in humans

Activation of the medial olivocochlear (MOC) efferents attenuates cochlear gain and reduces the amplitudes of mechanical, electrical, and neural cochlear outputs. The functional roles of the MOC efferents are not fully understood, especially in humans, despite postulations that they are involved in protection against acoustic trauma, facilitation of transient-sound perception, etc. Delineating the frequency tuning properties of the MOC efferents would provide critical evidence to support or refute these postulated functional roles. By utilizing spontaneous otoacoustic emissions (SOAEs), a cochlear measure sensitive to MOC modulation, we systematically demonstrate in humans that the contralateral MOC reflex is tuned to a fixed frequency band between 500 and 1,000 Hz independent of SOAE frequency. Our results question the role of the MOC reflex in protection against acoustic trauma or facilitation of transient-sound perception.

Influence of acute hyperglycemia on otoacoustic emissions and the medial olivocochlear reflex

Stimulus-frequency (SF) otoacoustic emission (OAE) amplitude and the amplitude of medial olivocochlear (MOC) inhibition of SF OAEs for ipsilateral, contralateral and bilateral MOC reflex elicitors were recorded in six subjects with type 2 diabetes during a glucose tolerance test (GTT). Five of the six subjects were tested twice for a total of 11 trials and three subjects were tested in a control experiment. During the GTT experiment, the subjects' blood glucose was elevated from a euglycemic level below 150 mg/dL to a hyperglycemic level above 160 mg/dL following the consumption of a bolus of 80 g of sugar. A subset of three subjects were tested in a control experiment during which SF OAE and MOC reflex measurements were made while blood sugar levels remained constant within the euglycemic region. Mean SF OAE amplitudes were elevated following glucose consumption. A statistically significant increase in MOC inhibition amplitude was observed during elevated sugar levels for the 11 GTT trials. Maximum inhibition occurred about an hour after glucose consumption when blood glucose levels peaked. Results indicate that acute hyperglycemia influences efferent control of the cochlea in people with type 2 diabetes.

Contralateral suppression of otoacoustic emissions: input-output functions in neonates

Background

The literature suggests that contralateral acoustic stimulation (CAS) alters the amplitude of the distortion product otoacoustic emissions (DPOAEs), but it is still unknown whether the DPOAE Input/Output (I/O) functions are also affected. To elucidate this aspect of the DPOAEs, the present study assessed the effects of CAS on DPOAE I/O functions at the frequencies of 2 kHz and 4 kHz, in a sample of term neonatal subjects.

Material/Methods

Sixty randomly selected neonates were included in the study. The DPOAE I/O functions were obtained at 2 kHz and 4 kHz, in the presence of a 60 dB SPL broad band-contralateral white noise, using the TDH39 headphones contralaterally. DPOAEs were recorded up to a stimulus level of L₂=35 dB peSPL.

Results

Significant DPOAE amplitude suppression effects were observed at various L₂ stimulus levels for both tested frequencies at 2 and 4 kHz. In contrast, the corresponding DPOAE slopes showed various alterations that were not statistically significant.

Conclusions

The data from the present study show that contralateral acoustic stimulation significantly affects only the amplitude of the DPOAE I/O functions; the slope is affected, but not significantly. This observation can shed light on the nature of CAS, suggesting that the latter is primarily a linear phenomenon without the cochlear compression and non-linear components seen in the healthy cochlea. From the available data it is not possible to infer whether the sample size has influenced the obtained results and the study should be repeated with a larger sample size and assessing more frequencies.

Effects of electrical stimulation of olivocochlear fibers in cochlear potentials in the chinchilla

The mammalian cochlea has two types of sensory cells; inner hair cells, which receive auditory-nerve afferent innervation, and outer hair cells, innervated by efferent axons of the medial olivocochlear (MOC) system. The role of the MOC system in hearing is still controversial. Recently, by recording cochlear potentials in behaving chinchillas, we suggested that one of the possible functions of the efferent system is to reduce cochlear sensitivity during attention to other sensory modalities (Delano et al. in J Neurosci 27:4146-4153, 2007). However, in spite of these compelling results, the physiological effects of electrical MOC activation on cochlear potentials have not been described in detail in chinchillas. The main objective of the present work was to describe these efferent effects in the chinchilla, comparing them with those in other species and in behavioral experiments. We activated the MOC efferent axons in chinchillas with sectioned middle-ear muscles by applying current pulses at the fourth-ventricle floor. Auditory-nerve compound action potentials (CAP) and cochlear microphonics (CM) were acquired in response to clicks and tones of several frequencies, using a round-window electrode. Electrical efferent stimulation produced CAP amplitude suppressions reaching up to 11 dB. They were higher for low to moderate sound levels. Additionally, CM amplitude increments were found, the largest (≤ 2.5 dB) for low intensity tones. CAP suppression was present at all stimulus frequencies, but was greatest for 2 kHz. CM increments were highest for low-frequency tones, and almost absent at high frequencies. We conclude that the effect obtained in chinchilla is similar to but smaller than that observed in cats, and that the effects seen in awake chinchillas, albeit different in magnitude, are consistent with the activation of efferent fibers.

Age-related synaptic loss of the medial olivocochlear efferent innervation

Age-related functional decline of the nervous system is consistently observed, though cellular and molecular events responsible for this decline remain largely unknown. One of the most prevalent age-related functional declines is age-related hearing loss (presbycusis), a major cause of which is the loss of outer hair cells (OHCs) and spiral ganglion neurons. Previous studies have also identified an age-related functional decline in the medial olivocochlear (MOC) efferent system prior to age-related loss of OHCs. The present study evaluated the hypothesis that this functional decline of the MOC efferent system is due to age-related synaptic loss of the efferent innervation of the OHCs. To this end, we used a recently-identified transgenic mouse line in which the expression of yellow fluorescent protein (YFP), under the control of neuron-specific elements from the thy1 gene, permits the visualization of the synaptic connections between MOC efferent fibers and OHCs. In this model, there was a dramatic synaptic loss between the MOC efferent fibers and the OHCs in older mice. However, age-related loss of efferent synapses was independent of OHC status. These data demonstrate for the first time that age-related loss of efferent synapses may contribute to the functional decline of the MOC efferent system and that this synaptic loss is not necessary for age-related loss of OHCs.

Temporary reduction of distortion product otoacoustic emissions (DPOAEs) immediately following auditory brainstem response (ABR)

The hearing status of an experimental animal is typically assessed in the laboratory setting by the combined use of auditory brainstem response (ABR) and distortion product otoacoustic emissions (DPOAEs), carried out in succession, with the former assay preceding the latter. This study reports a cautionary finding that the use of this accepted regimen yields a reduced DPOAE response. When the DPOAEs were performed after ABR testing, transient reduction of the DPOAE amplitudes was observed at all frequencies in both the inbred, C57/B6 and FVB/N, and the outbred, SW mouse strains. DPOAEs were reduced post-ABR in multiple mouse strains which suggests that this finding is not strain-specific but a general consequence of the preceding ABR analysis. The reduction in DPOAE was temporary: when re-tested at one hour, DPOAE amplitudes recovered to pre-ABR levels. In contrast to the ABR's impact on DPOAE response, ABR thresholds were not altered or reduced when preceded immediately by DPOAE measurements. The molecular alterations underlying the ABR-induced transient reduction of DPOAE remain to be determined. To investigate the potential role of reactive oxygen species in post-ABR DPOAE reduction, transgenic mice over-expressing SOD1, the cytoplasmic enzyme critical for removal of superoxide radicals were subjected to the same auditory testing regimen. Similar to their wild type littermates, the SOD1 transgenic mice also demonstrated post-ABR DPOAE reduction, and thus do not support a role for superoxide radicals in transient reduction of DPOAE. While toxic noise exposure is known to negatively impact OAE, transient decrease in DPOAE levels following standard ABR assay has not been previously described. A practical outcome from this study is a recommendation for reversal of the traditional order for carrying out auditory tests, with the OAE measurements preceding ABR assessment, thus ensuring that the DPOAE response is unaffected.

Concentrated pitch discrimination modulates auditory brainstem responses during contralateral noise exposure

This study examined a notion that auditory discrimination is a requisite for attention-related modulation of the auditory brainstem response (ABR) during contralateral noise exposure. Given that the right ear was exposed continuously with white noise at an intensity of 60-80 dB sound pressure level, tone pips at 80 dB sound pressure level were delivered to the left ear through either single-stimulus or oddball procedures. Participants conducted reading (ignoring task) and counting target tones (attentive task) during stimulation. The oddball but not the single-stimulus procedures elicited task-related modulations in both early (ABR) and late (processing negativity) event-related potentials simultaneously. The elicitation of the attention-related ABR modulation during contralateral noise exposure is thus considered to require auditory discrimination and have the corticofugal nature evidently.

F1 (CBA×C57) mice show superior hearing in old age relative to their parental strains: hybrid vigor or a new animal model for "golden ears"?

Age-related hearing loss - presbycusis - is the most common communication problem and third most prevalent chronic medical disorder of the aged. The CBA and C57BL/6 mouse strains are useful for studying features of presbycusis. The CBA loses its hearing slowly, like most humans. Because the C57 develops a rapid, high frequency hearing loss by middle age, it has an "old" ear but a relatively young brain, a model that helps separate peripheral (cochlear) from central (brain) etiologies. This field of sensory neuroscience lacks a good mouse model for the 5-10% of aged humans with normal cochlear sensitivity, but who have trouble perceiving speech in background noise. We hypothesized that F1 (CBA×C57) hybrids would have better hearing than either parental strain. Measurements of peripheral auditory sensitivity supported this hypothesis, however, a rapid decline in the auditory efferent feedback system, did not. Therefore, F1s might be an optimal model for studying cases where the peripheral hearing is quite good in old age; thereby allowing isolation of central auditory changes due to brain neurodegeneration.

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.

Age-related hearing loss: ear and brain mechanisms

Loss of sensory function in the aged has serious consequences for economic productivity, quality of life, and healthcare costs in the billions each year. Understanding the neural and molecular bases will pave the way for biomedical interventions to prevent, slow, or reverse these conditions. This chapter summarizes new information regarding age changes in the auditory system involving both the ear (peripheral) and brain (central). A goal is to provide findings that have implications for understanding some common biological underpinnings that affect sensory systems, providing a basis for eventual interventions to improve overall sensory functioning, including the chemical senses.

[Distortion product otoacoustic emission levels and input/output-growth functions in normal-hearing individuals with tinnitus and/or hyperacusis]

The purpose of this study was to determine whether distortion product otoacoustic emissions (DPOAEs) can be used to distinguish among four groups with audiometrically normal-hearing sensitivity: (1) control adults without tinnitus or hyperacusis, (2) patients with tinnitus alone, (3) patients with hyperacusis alone, and (4) patients with both tinnitus and hyperacusis. Two types of DPOAE measures were evaluated: (1) the distortion product (DP-)gram measured with fixed primary levels as a function of frequency, and (2) DPOAE input/output (I/O) functions for a range of primary levels between 45 and 70 dB sound pressure level (SPL) at 1000, 2000, and 4000 Hz. DP-grams did not clearly distinguish between the control and patient groups. There was, however, a consistent trend for the three patient groups to have decreased average DP levels at 4000 and 6000 Hz; this notch in the DP-gram was not observed in the response configuration for the control group. In the three patient groups, 51 to 74% of these individuals had DP levels that were outside of the 95% confidence range for the control group. The average slopes of the I/O growth functions for each of the patient groups were consistently steeper than those for the control group; however, the slope values were indistinguishable among the patient groups. About 60% of the patients' DPOAE responses (in each group) were categorized as abnormal based on their slope values. Thus, DPOAE measures can be used with at least partial success to distinguish controls from patients with tinnitus, hyperacusis, or both tinnitus and hyperacusis, but not to discriminate among the respective patient groups. These findings suggest that the pathology represented among the patient groups is consistent at the level of the cochlea; however, diagnostic tests targeted at higher centers of processing are needed if the individuals in these groups are to be distinguished among themselves. In all participants, DPOAEs with fixed primary levels as a function of frequency were measured. There was a consistent trend for the three patient groups to have decreased average DP-gram levels at 4000 and 6000 Hz; this notch in the DP-gram was not observed in the response configuration for the control group. In the three patient groups, 51 to 74% of these individuals had DP levels that were outside of the 95% confidence range for the control group. Thus, DPOAE measures can be used with at least partial success to distinguish controls from patients with tinnitus, hyperacusis, or both tinnitus and hyperacusis, but not to discriminate among the respective patient groups. These findings suggest that the pathology represented among the patient groups is consistent at the level of the cochlea; however, diagnostic tests targeted at higher centers of processing are needed if the individuals in these groups are to be distinguished among themselves.

Dynamics of real time DPOAE contralateral suppression in chinchillas and humans Dinámica de la supresión contralateral de las DPOAE en tiempo real en chinchillas y humanos

The dynamics of contralateral acoustic suppression were studied using real time (millisecond resolution) distortion product otoacoustic emissions (DPOAEs) in chinchillas and humans. Latency of DPOAE suppression onset is 26 ms in chinchillas and 45 ms in humans. After onset, suppression builds over time before tending to plateau, reflecting a temporal integration process with a time constant of 100 ms (chinchillas). In chinchillas, suppression persists for 40 ms even when elicited by stimuli as short as 5 ms. With stimuli >40 ms, offset and onset latencies are similar and duration of suppression equals that of the contralateral stimulus. A comparison of DPOAE suppression onset latency with neural latency data from the pathways involved suggests the following timing scheme: stimulus onset to activity in (ventral) cochlear nucleus, 4ms (15% of delay); transfer to olivocochlear efferents, 9 ms (35%); efferent conduction to presynaptic OHC site, 4ms (15%); synaptic and mechanical events at OHCs, 9 ms (35% of delay).

[Efferent suppression test--sensitivity and specificity]

INTRODUCTION

The amplitude of click evoked otoacoustic emissions can be suppressed (reduced) with contralateral acoustic stimulation (CAS). This is thought to be due to the inhibitory control that the medial efferent auditory nerve exerts on outer hair cell function. One commonly used test of medial olivocochlear (MOC) function in humans involves measuring the degree of suppression of ipsilateral otoacoustic emissions (OAEs) by CAS. This effect is called contralateral suppression, efferent effect or olivocochlear reflex. Aim of the study. The objective of this study was to measure cut-off level of contralateral suppression, sensitivity and specificity of the efferent test.

MATERIAL AND METHODS

To evaluate the function of MOC system click evoked otoacoustic emissions (CEOAEs) with and without continuous contralateral broadband noise (BBN) stimulation were recorded in 14 patients with multiple sclerosis, 16 patients with unilateral cerebello-pontine angle tumor and control group matched for age and gender. BBN was presented at 50 dB SL and otoacoustic emissions were recorded in response to nonlinear clicks at 70 +/- 3 dB SPL.

RESULTS

The cut-off level was established at -1.4 dB. For 1.4 dB cut-off level of contralateral suppression the sensitivity of the efferent test was 90% and specificity was 70%.

CONCLUSIONS

To conclude, assessment of efferent suppression may be a useful addition to the battery test employed in the investigation of multiple sclerosis and cerebello-pontine angle tumors. The MOC suppression test may provide a tool for an early diagnosis of the brainstem pathology.

Attention-related modulation of auditory brainstem responses during contralateral noise exposure

As determinants facilitating attention-related modulation of the auditory brainstem response (ABR), two experimental factors were examined: (i) auditory discrimination; and (ii) contralateral masking intensity. Tone pips at 80 dB sound pressure level were presented to the left ear via either single-tone exposures or oddball exposures, whereas white noise was delivered continuously to the right ear at variable intensities (none--80 dB sound pressure level). Participants each conducted two tasks during stimulation, either reading a book (ignoring task) or detecting target tones (attentive task). Task-related modulation within the ABR range was found only during oddball exposures at contralateral masking intensities greater than or equal to 60 dB. Attention-related modulation of ABR can thus be detected reliably during auditory discrimination under contralateral masking of sufficient intensity.

The effect of a precursor on growth of forward masking

This study examined the effect of an on-frequency precursor on growth-of-masking (GOM) functions measured using an off-frequency masker. The signal was a 6-ms, 4-kHz tone. A GOM function was measured using a 40-ms, 2.8-kHz tone (the off-frequency masker). GOM functions were then measured with an on-frequency, fixed level precursor presented before the off-frequency masker. The precursor was 50 or 60 dB SPL, and 160 ms in duration. For the 60-dB SPL precursor, a 40-ms duration was also used. Two-line functions were fit to the GOM data to estimate the basilar membrane input-output function. The precursors reduced the gain of the input-output function, and this decrease was graded with precursor level. Both precursor durations had the same effect on gain. Changes in masking following a precursor were larger than would be predicted by additivity of masking. The observed decrease in gain may be consistent with activation of the medial olivocochlear reflex by the precursor.

Amplitude modulation of DPOAEs by acoustic stimulation of the contralateral ear

CONCLUSION

Otoacoustic emissions generated by outer hair cells (OHCs) are influenced by stimulation of the contralateral ear via a neural pathway involving the olivo-cochlear efferent system. This is often referred to as a contralateral 'suppression reflex', but we suggest that such a term is inappropriate since distortion product otoacoustic emissions (DPOAEs) can be both enhanced and suppressed, and there is continuous modulation with no threshold effects.

OBJECTIVE

To characterize the continuous amplitude modulation of DPOAEs by contralateral sound stimulation.

MATERIALS AND METHODS

In an animal model (chinchilla), DPOAEs were recorded in real time from one ear during presentation of acoustic stimuli to the opposite ear.

RESULTS

DPOAE amplitude is suppressed by an increase in contralateral stimulation, and enhanced by a decrease in same, i.e. the emissions are continuously modulated by activity in the opposite ear. The input-output function shows a linear relationship to this system over a 40-50 dB range of contralateral stimulus levels. After a neural delay time of approximately 25 ms, DPOAE amplitude closely follows contralateral amplitude signals up to modulation frequencies of approximately 20 Hz. Thus, stimuli to one ear continually modulate the OHC system (and therefore the biomechanical amplification) of the contralateral cochlea.

The alpha10 nicotinic acetylcholine receptor subunit is required for normal synaptic function and integrity of the olivocochlear system

Although homomeric channels assembled from the alpha9 nicotinic acetylcholine receptor (nAChR) subunit are functional in vitro, electrophysiological, anatomical, and molecular data suggest that native cholinergic olivocochlear function is mediated via heteromeric nAChRs composed of both alpha9 and alpha10 subunits. To gain insight into alpha10 subunit function in vivo, we examined olivo cochlear innervation and function in alpha10 null-mutant mice. Electrophysiological recordings from postnatal (P) days P8-9 inner hair cells revealed ACh-gated currents in alpha10(+/+) and alpha10(+/-) mice, with no detectable responses to ACh in alpha10(-/-) mice. In contrast, a proportion of alpha10(-/-) outer hair cells showed small ACh-evoked currents. In alpha10(-/-) mutant mice, olivocochlear fiber stimulation failed to suppress distortion products, suggesting that the residual alpha9 homomeric nAChRs expressed by outer hair cells are unable to transduce efferent signals in vivo. Finally, alpha10(-/-) mice exhibit both an abnormal olivocochlear morphology and innervation to outer hair cells and a highly disorganized efferent innervation to the inner hair cell region. Our results demonstrate that alpha9(-/-) and alpha10(-/-) mice have overlapping but nonidentical phenotypes. Moreover, alpha10 nAChR subunits are required for normal olivocochlear activity because alpha9 homomeric nAChRs do not support maintenance of normal olivocochlear innervation or function in alpha10(-/-) mutant mice.

Auditory efferent feedback system deficits precede age-related hearing loss: contralateral suppression of otoacoustic emissions in mice

The C57BL/6J mouse has been a useful model of presbycusis, as it displays an accelerated age-related peripheral hearing loss. The medial olivocochlear efferent feedback (MOC) system plays a role in suppressing cochlear outer hair cell (OHC) responses, particularly for background noise. Neurons of the MOC system are located in the superior olivary complex, particularly in the dorsomedial periolivary nucleus (DMPO) and in the ventral nucleus of the trapezoid body (VNTB). We previously discovered that the function of the MOC system declines with age prior to OHC degeneration, as measured by contralateral suppression (CS) of distortion product otoacoustic emissions (DPOAEs) in humans and CBA mice. The present study aimed to determine the time course of age changes in MOC function in C57s. DPOAE amplitudes and CS of DPOAEs were collected for C57s from 6 to 40 weeks of age. MOC responses were observed at 6 weeks but were gone at middle (15-30 kHz) and high (30-45 kHz) frequencies by 8 weeks. Quantitative stereological analyses of Nissl sections revealed smaller neurons in the DMPO and VNTB of young adult C57s compared with CBAs. These findings suggest that reduced neuron size may underlie part of the noteworthy rapid decline of the C57 efferent system. In conclusion, the C57 mouse has MOC function at 6 weeks, but it declines quickly, preceding the progression of peripheral age-related sensitivity deficits and hearing loss in this mouse strain.

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.

Effects of Aromatic Solvents on Acoustic Reflexes Mediated by Central Auditory Pathways

From previous in vivo investigations, it has been shown that toluene can mimic the effects of cholinergic receptor antagonists and may thereby modify the response of protective acoustic reflexes. The current study aimed to define the relative effects of aromatic solvents on the middle ear and inner ear acoustic reflexes. Toward this end, the cochlear microphonic (CMP) elicited with a band noise centered at 4 kHz, and the compound action potential (CAP) elicited with 4-kHz tone pips was measured in rats. Both potentials were recorded before, during, and after triggering the protective reflexes by a 110-dB SPL contralateral octave band noise centered at 12.5 kHz (12.5 kHz-OBN). In several rats, the middle ear muscles were severed to identify the relative effects of toluene on the two reflexes. While the reflex elicitor was capable of decreasing both the CMP and CAP amplitudes, an injection of 116.2 mM toluene cancelled this suppressor effect induced by the contralateral sound. In the rats with nonfunctional middle ear muscles, a solvent injection did not modify the electrophysiological responses of the cochlea. Different solvents were tested to study the relationship of the chemical structure of the solvents on the acoustic reflexes. The present study showed that aromatic solvents can inhibit the action of the middle ear reflex by their anticholinergic effect on the efferent motoneurons. An aromatic nucleus and the presence of one side chain of no more than 3 C seem to be required in the solvent structure to inhibit the efferent motoneurons.

Bandwidth dependency of cochlear centrifugal pathways in modulating hearing desensitization caused by loud sound

Centrifugal olivocochlear (OC) pathways modulate cochlear hearing desensitization induced by loud sounds, but there is a null point, determined by sound bandwidth, for this effect. In a previous study, using loud sounds from the region of greatest hearing sensitivity in cats, OC pathways did not affect desensitization induced by 2-kHz wide noise, but did to narrower bandwidth (tones) or broader bandwidth (3.5 kHz-wide or 5 kHz-wide noise) trauma from the same cochlear region. The bandwidth null-point effect occurred in three very different conditions in which OC pathways modulated losses to narrower or broader bandwidth traumata, confirming the robustness of this phenomenon, and was also true for sub-component OC pathways: neither crossed nor uncrossed OC pathways individually modulated desensitization to that 2 kHz-wide noise. The medial olivocochlear system (MOCS) that is most likely to have modulated desensitization in that study, varies in its cochlear distribution; in cats, densest innervation is in the region of greatest hearing sensitivity and the decrease away from that region means MOCS effects there may not translate to other regions. This hypothesis was now tested in lower- (around 4 kHz) and higher- (around 18 kHz) frequency cochlear regions. Across this fairly large cochlear swath, no OC modulation of desensitization occurred to 2-kHz-wide bandwidth sounds, but did to broader bandwidth; thus the bandwidth dependency was constant across this swath. However, when OC effects did occur, the pattern of effects of OC sub-components could be idiosyncratic to sound bandwidth and cochlear region even for similar net OC effects.

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.

Simultaneous measurement of noise-activated middle-ear muscle reflex and stimulus frequency otoacoustic emissions

Otoacoustic emissions serve as a noninvasive probe of the medial olivocochlear (MOC) reflex. Stimulus frequency otoacoustic emissions (SFOAEs) elicited by a low-level probe tone may be the optimal type of emission for studying MOC effects because at low levels, the probe itself does not elicit the MOC reflex [Guinan et al. (2003) J. Assoc. Res. Otolaryngol. 4:521]. Based on anatomical considerations, the MOC reflex activated by ipsilateral acoustic stimulation (mediated by the crossed olivocochlear bundle) is predicted to be stronger than the reflex to contralateral stimulation. Broadband noise is an effective activator of the MOC reflex; however, it is also an effective activator of the middle-ear muscle (MEM) reflex, which can make results difficult to interpret. The MEM reflex may be activated at lower levels than measured clinically, and most previous human studies have not explicitly included measurements to rule out MEM reflex contamination. The current study addressed these issues using a higher-frequency SFOAE probe tone to test for cochlear changes mediated by the MOC reflex, while simultaneously monitoring the MEM reflex using a low-frequency probe tone. Broadband notched noise was presented ipsilaterally at various levels to elicit probe-tone shifts. Measurements are reported for 15 normal-hearing subjects. With the higher-frequency probe near 1.5 kHz, only 20% of subjects showed shifts consistent with an MOC reflex in the absence of an MEM-induced shift. With the higher-frequency probe near 3.5 kHz, up to 40% of subjects showed shifts in the absence of an MEM-induced shift. However, these responses had longer time courses than expected for MOC-induced shifts, and may have been dominated by other cochlear processes, rather than MOC reflex. These results suggest caution in the interpretation of effects observed using ipsilaterally presented acoustic activators intended to excite the MOC reflex.

Age-related declines in distortion product otoacoustic emissions utilizing pure tone contralateral stimulation in CBA/CaJ mice

One role of the medial olivocochlear (MOC) auditory efferent system is to suppress cochlear outer hair cell (OHC) responses when presented with a contralateral sound. Using distortion product otoacoustic emissions (DPOAEs), the effects of active changes in OHC responses due to the MOC as a function of age can be observed when contralateral stimulation with a pure tone is applied. Previous studies have shown that there are age-related declines of the MOC when broad band noise is presented to the contralateral ear. In this study, we measured age-related changes in CBA/CaJ mice by comparing DPOAE generation with and without a contralateral pure tone at three different frequencies (12, 22, and 37 kHz). Young (n = 16), middle (n = 10) and old-aged (n = 10) CBA mice were tested. DPOAE-grams were obtained using L1 = 65 and L2 = 50 dB SPL, F1/F2 = 1.25, using eight points per octave covering a frequency range from 5.6-44.8 kHz. The pure tone was presented contralaterally at 55 dB SPL. DPOAE-grams and ABR levels indicated age-related hearing loss in the old mice. In addition, there was an overall change in DPOAEs in the middle-aged and old groups relative to the young. Pure tone stimulation was not as effective as a suppressor compared to broadband noise. An increase in pure tone frequency from 12 to 22 kHz induced greater suppression of DPOAEs, but the 37 kHz was least effective. These results indicate that as the mouse ages, there are significant changes in the efficiency of the suppression mechanism as elicited by contralateral narrowband stimuli. These findings reinforce the idea that age-related changes in the MOC or the operating points of OHCs play a role in the progression of presbycusis - age-related hearing loss in mammals.

Isoflurane increases amplitude and incidence of evoked and spontaneous otoacoustic emissions

The volatile anesthetic isoflurane was tested for its effect on cochlear function by means of measuring distortion product otoacoustic emissions (DPOAE) and spontaneous otoacoustic emissions (SOAE) in the mustached bat (Pteronotus parnellii parnellii). Averaged growth functions of DPOAE and spontaneous otoacoustic emissions were assessed and compared between the control group (no isoflurane application) and the isoflurane group (application of isoflurane at vaporizer settings sof about 1.5-2%). Isoflurane significantly increases the DPOAE amplitude, e.g. at a primary tone level l2 of 40 dB SPL by 10.7 dB. Additionally, the incidence of SOAEs was highly increased during application of isoflurane. The sound-evoked efferent effect on the generation of otoacoustic emissions was significantly reduced in the isoflurane group. We suggest that isoflurane might affect the postsynaptic action of acetylcholine (ACh) released by the efferent terminals of outer hair cells (OHCs). This could lead to the observed decrease of efferent suppression and to a disinhibition of cochlear amplification.