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Introduction: Hearing decline stands as the most prevalent single sensory deficit associated with the aging process. Giving compelling evidence suggesting a protective effect associated with the efferent auditory system, the goal of our study was to characterize the age-related changes in the number of efferent medial olivocochlear (MOC) synapses regulating outer hair cell (OHC) activity compared with the number of afferent inner hair cell ribbon synapses in CBA/J mice over their lifespan. Methods: Organs of Corti of 3-month-old CBA/J mice were compared with mice aged between 10 and 20 months, grouped at 2-month intervals. For each animal, one ear was used to characterize the synapses between the efferent MOC fibers and the outer hair cells (OHCs), while the contralateral ear was used to analyze the ribbon synapses between inner hair cells (IHCs) and type I afferent nerve fibers of spiral ganglion neurons (SGNs). Each cochlea was separated in apical, middle, and basal turns, respectively. Results: The first significant age-related decline in afferent IHC-SGN ribbon synapses was observed in the basal cochlear turn at 14 months, the middle turn at 16 months, and the apical turn at 18 months of age. In contrast, efferent MOCOHC synapses in CBA/J mice exhibited a less pronounced loss due to aging which only became significant in the basal and middle turns of the cochlea by 20 months of age. Discussion: This study illustrates an age-related reduction on efferent MOC innervation of OHCs in CBA/J mice starting at 20 months of age. Our findings indicate that the morphological decline of efferent MOC-OHC synapses due to aging occurs notably later than the decline observed in afferent IHC-SGN ribbon synapses. [ABSTRACT FROM AUTHOR]

IntroductionHearing decline stands as the most prevalent single sensory deficit associated with the aging process. Giving compelling evidence suggesting a protective effect associated with the efferent auditory system, the goal of our study was to characterize the age-related changes in the number of efferent medial olivocochlear (MOC) synapses regulating outer hair cell (OHC) activity compared with the number of afferent inner hair cell ribbon synapses in CBA/J mice over their lifespan.MethodsOrgans of Corti of 3-month-old CBA/J mice were compared with mice aged between 10 and 20 months, grouped at 2-month intervals. For each animal, one ear was used to characterize the synapses between the efferent MOC fibers and the outer hair cells (OHCs), while the contralateral ear was used to analyze the ribbon synapses between inner hair cells (IHCs) and type I afferent nerve fibers of spiral ganglion neurons (SGNs). Each cochlea was separated in apical, middle, and basal turns, respectively.ResultsThe first significant age-related decline in afferent IHC-SGN ribbon synapses was observed in the basal cochlear turn at 14 months, the middle turn at 16 months, and the apical turn at 18 months of age. In contrast, efferent MOC-OHC synapses in CBA/J mice exhibited a less pronounced loss due to aging which only became significant in the basal and middle turns of the cochlea by 20 months of age.DiscussionThis study illustrates an age-related reduction on efferent MOC innervation of OHCs in CBA/J mice starting at 20 months of age. Our findings indicate that the morphological decline of efferent MOC-OHC synapses due to aging occurs notably later than the decline observed in afferent IHC-SGN ribbon synapses.

The lateral superior olive (LSO) is a key structure in the central auditory system of mammals that exerts efferent control on cochlear sensitivity and is involved in the processing of binaural level differences for sound localization. Understanding how the LSO contributes to these processes requires knowledge about the resident cells and their connections with other auditory structures. We used standard histological stains and retrograde tracer injections into the inferior colliculus (IC) and cochlea in order to characterize two basic groups of neurons: (1) Principal and periolivary (PO) neurons have projections to the IC as part of the ascending auditory pathway; and (2) lateral olivocochlear (LOC) intrinsic and shell efferents have descending projections to the cochlea. Principal and intrinsic neurons are intermixed within the LSO, exhibit fusiform somata, and have disk-shaped dendritic arborizations. The principal neurons have bilateral, symmetric, and tonotopic projections to the IC. The intrinsic efferents have strictly ipsilateral projections, known to be tonotopic from previous publications. PO and shell neurons represent much smaller populations (<10% of principal and intrinsic neurons, respectively), have multipolar somata, reside outside the LSO, and have nontopographic, bilateral projections. PO and shell neurons appear to have widespread projections to their targets that imply a more diffuse modulatory function. The somata and dendrites of principal and intrinsic neurons form a laminar matrix within the LSO and share quantifiably similar alignment to the tonotopic axis. Their restricted projections emphasize the importance of frequency in binaural processing and efferent control for auditory perception. This study addressed and expanded on previous findings of cell types, circuit laterality, and projection tonotopy in the LSO of the mouse. [ABSTRACT FROM AUTHOR]

The lateral superior olive (LSO) is a key structure in the central auditory system of mammals that exerts efferent control on cochlear sensitivity and is involved in the processing of binaural level differences for sound localization. Understanding how the LSO contributes to these processes requires knowledge about the resident cells and their connections with other auditory structures. We used standard histological stains and retrograde tracer injections into the inferior colliculus (IC) and cochlea in order to characterize two basic groups of neurons: (1) Principal and periolivary (PO) neurons have projections to the IC as part of the ascending auditory pathway; and (2) lateral olivocochlear (LOC) intrinsic and shell efferents have descending projections to the cochlea. Principal and intrinsic neurons are intermixed within the LSO, exhibit fusiform somata, and have disk-shaped dendritic arborizations. The principal neurons have bilateral, symmetric, and tonotopic projections to the IC. The intrinsic efferents have strictly ipsilateral projections, known to be tonotopic from previous publications. PO and shell neurons represent much smaller populations (

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.

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. [ABSTRACT FROM AUTHOR]

Lateral olivocochlear (LOC) efferent neurons modulate auditory nerve fiber (ANF) activity using a large repertoire of neurotransmitters, including dopamine (DA) and acetylcholine (ACh). Little is known about how individual neurotransmitter systems are differentially utilized in response to the ever-changing acoustic environment. Here we present quantitative evidence in rodents that the dopaminergic LOC input to ANFs is dynamically regulated according to the animal’s recent acoustic experience. Sound exposure upregulates tyrosine hydroxylase, an enzyme responsible for dopamine synthesis, in cholinergic LOC intrinsic neurons, suggesting that individual LOC neurons might at times co-release ACh and DA. We further demonstrate that dopamine down-regulates ANF firing rates by reducing both the hair cell release rate and the size of synaptic events. Collectively, our results suggest that LOC intrinsic neurons can undergo on-demand neurotransmitter re-specification to re-calibrate ANF activity, adjust the gain at hair cell/ANF synapses, and possibly to protect these synapses from noise damage.

Many users of bilateral cochlear implants (BiCIs) localize sound sources less accurately than do people with normal hearing. This may be partly due to using two independently functioning CIs with fixed compression, which distorts and/or reduces interaural level differences (ILDs). Here, we investigate the potential benefits of using binaurally coupled, dynamic compression inspired by the medial olivocochlear reflex; an approach termed "the MOC strategy" (Lopez-Poveda et al., 2016, Ear Hear 37:e138-e148). Twelve BiCI users were asked to localize wideband (125–6000 Hz) noise tokens in a virtual horizontal plane. Stimuli were processed through a standard (STD) sound processing strategy (i.e., involving two independently functioning sound processors with fixed compression) and three different implementations of the MOC strategy: one with fast (MOC1) and two with slower contralateral control of compression (MOC2 and MOC3). The MOC1 and MOC2 strategies had effectively greater inhibition in the higher than in the lower frequency channels, while the MOC3 strategy had slightly greater inhibition in the lower than in the higher frequency channels. Localization was most accurate with the MOC1 strategy, presumably because it provided the largest and less ambiguous ILDs. The angle error improved slightly from 25.3° with the STD strategy to 22.7° with the MOC1 strategy. The improvement in localization ability over the STD strategy disappeared when the contralateral control of compression was made slower, presumably because stimuli were too short (200 ms) for the slower contralateral inhibition to enhance ILDs. Results suggest that some MOC implementations hold promise for improving not only speech-in-noise intelligibility, as shown elsewhere, but also sound source lateralization. • Sound lateralization for bilateral cochlear-implant users was tested using virtual acoustics. • Binaural sound coding inspired by the medial olivocochlear reflex can enhance inter-aural level differences. • The use of binaurally coupled compression can improve sound lateralization. [ABSTRACT FROM AUTHOR]

Sensory systems constantly adapt their responses to the current environment. In hearing, adaptation may facilitate communication in noisy settings, a benefit frequently (but controversially) attributed to the medial olivocochlear reflex (MOCR) enhancing the neural representation of speech. Here, we show that human listeners (N = 14; five male) recognize more words presented monaurally in ipsilateral, contralateral, and bilateral noise when they are given some time to adapt to the noise. This finding challenges models and theories that claim that speech intelligibility in noise is invariant over time. In addition, we show that this adaptation to the noise occurs also for words processed to maintain the slow-amplitude modulations in speech (the envelope) disregarding the faster fluctuations (the temporal fine structure). This demonstrates that noise adaptation reflects an enhancement of amplitude modulation speech cues and is unaffected by temporal fine structure cues. Last, we show that cochlear implant users (N = 7; four male) show normal monaural adaptation to ipsilateral noise. Because the electrical stimulation delivered by cochlear implants is independent from the MOCR, this demonstrates that noise adaptation does not require the MOCR. We argue that noise adaptation probably reflects adaptation of the dynamic range of auditory neurons to the noise level statistics. [ABSTRACT FROM AUTHOR]

We have recently proposed a binaural cochlear implant (CI) sound processing strategy inspired by the contralateral medial olivocochlear reflex (the MOC strategy) and shown that it improves intelligibility in steady-state noise (Lopez-Poveda et al., 2016, Ear Hear 37:e138-e148). The aim here was to evaluate possible speech-reception benefits of the MOC strategy for speech maskers, a more natural type of interferer. Speech reception thresholds (SRTs) were measured in six bilateral and two single-sided deaf CI users with the MOC strategy and with a standard (STD) strategy. SRTs were measured in unilateral and bilateral listening conditions, and for target and masker stimuli located at azimuthal angles of (0°, 0°), (−15°, +15°), and (−90°, +90°). Mean SRTs were 2–5 dB better with the MOC than with the STD strategy for spatially separated target and masker sources. For bilateral CI users, the MOC strategy (1) facilitated the intelligibility of speech in competition with spatially separated speech maskers in both unilateral and bilateral listening conditions; and (2) led to an overall improvement in spatial release from masking in the two listening conditions. Insofar as speech is a more natural type of interferer than steady-state noise, the present results suggest that the MOC strategy holds potential for promising outcomes for CI users. [ABSTRACT FROM AUTHOR]

Inhibition of cochlear amplifier gain by the medial olivocochlear (MOC) efferent system has several putative roles: aiding listening in noise, protection against damage from acoustic overexposure, and slowing age-induced hearing loss. The human MOC reflex has been studied almost exclusively by measuring changes in otoacoustic emissions. However, to help understand how the MOC system influences what we hear, it is important to have measurements of the MOC effect on the total output of the organ of Corti, i.e., on cochlear nerve responses that couple sounds to the brain. In this work we measured the inhibition produced by the MOC reflex on the amplitude of cochlear nerve compound action potentials (CAPs) in response to moderate level (52–60 dB peSPL) clicks from five, young, normal hearing, awake, alert, human adults. MOC activity was elicited by 65 dB SPL, contralateral broadband noise (CAS). Using tympanic membrane electrodes, approximately 10 h of data collection were needed from each subject to yield reliable measurements of the MOC reflex inhibition on CAP amplitudes from one click level. The CAS produced a 16% reduction of CAP amplitude, equivalent to a 1.98 dB effective attenuation (averaged over five subjects). Based on previous reports of efferent effects as functions of level and frequency, it is possible that much larger effective attenuations would be observed at lower sound levels or with clicks of higher frequency content. For a preliminary comparison, we also measured MOC reflex inhibition of DPOAEs evoked from the same ears with f 2 's near 4 kHz. The resulting effective attenuations on DPOAEs were, on average, less than half the effective attenuations on CAPs. [ABSTRACT FROM AUTHOR]

As studies of the olivocochlear (OC) efferent system have matured, issues have been identified that need to be taken into account in the design of new studies and in the interpretation of existing work. The need for high signal-to-noise ratios (SNRs), multiple alternations of conditions, and avoiding middle-ear-muscle activation have been previously highlighted. Less well-known issues include: Contralateral medial OC (MOC) effects may not be good proxies for ipsilateral MOC effects; MOC-induced changes in otoacoustic emissions (OAEs) may not accurately show MOC-induced changes in auditory-nerve responses; Measuring OAE differences from before to after psychophysical trials yields the transient OAE change but not tonic MOC activation; Tonic MOC activation may be measurable by several techniques including by OAE differences in trials in which the subject’s judgment was correct versus trials that were incorrect; SNRs can be preserved by Bootstrap statistical tests; Differences in task difficulty may outweigh differences in subject attention; Lateral efferent effects are little understood and may be tied to MOC effects; To assess whether MOC strength predicts protection from acoustic trauma, prospective tests in humans are needed.

Neurons of the lateral olivocochlear (LOC) system project from the auditory brainstem to the cochlea, where they synapse on radial dendrites of auditory nerve fibers. Selective LOC disruption depresses sound-evoked auditory nerve activity in the guinea pig, but enhances it in the mouse. Here, LOC disruption depressed spontaneous auditory nerve activity in the guinea pig. Recordings from single auditory nerve fibers revealed a significantly reduced proportion of fibers with the highest spontaneous firing rates (SRs) and an increased proportion of neurons with lower SRs. Ensemble activity, estimated using round window noise, also decreased after LOC disruption. Decreased spontaneous activity after LOC disruption may be a consequence of reduced tonic release of excitatory transmitters from the LOC terminals in guinea pigs. [ABSTRACT FROM AUTHOR]

As studies of the olivocochlear (OC) efferent system have matured, issues have been identified that need to be taken into account in the design of new studies and in the interpretation of existing work. The need for high signal-to-noise ratios (SNRs), multiple alternations of conditions, and avoiding middle-ear-muscle activation have been previously highlighted. Less well-known issues include: Contralateral medial OC (MOC) effects may not be good proxies for ipsilateral (ipsi) MOC effects; MOC-induced changes in otoacoustic emissions (OAEs) may not accurately show MOC-induced changes in auditory-nerve (AN) responses; measuring OAE differences from before to after psychophysical trials yields the transient OAE change but not tonic MOC activation; tonic MOC activation may be measurable by several techniques including by OAE differences in trials in which the subject's judgment was correct vs. trials that were incorrect; SNRs can be preserved by Bootstrap statistical tests; differences in task difficulty may outweigh differences in subject attention; lateral efferent effects are little understood and may be tied to MOC effects; to assess whether MOC strength predicts protection from acoustic trauma, prospective tests in humans are needed. [ABSTRACT FROM AUTHOR]

Conclusion. According to the presented data, speech-in-noise intelligibility (SI) does not correlate with olivocochlear efferent activity - as measured by contralateral suppression (CS) of distortion product otoacoustic emissions (DPOAE) - in humans with normal auditory threshold. Objectives. Literature data indicate a possible role of the medial olivocochlear efferents in speech intelligibility, especially in background noise. The objective of this study was to investigate this relationship. Materials and methods. SI was evaluated in three independent sessions by determining the ratio speech level/noise level, at which 50% of the words are understood (i.e. speech reception threshold, SRT). Efferent activity was inferred measuring CS of DPOAE, using two different paradigms with extensive variation of stimulus parameters and duplicate measurements. Results. For optimum measurement of CS, the study was restricted to subjects (n=49) with valid DPOAE down to primary tone levels L1=47/L2=20dB SPL. Average SRT was -6.66 dB (-4.50 to -7.65dB, SD 0.63dB). CS increased with decreasing primary tone levels, with mean absolute CS values in the range of 0.6-6dB SPL. Test-retest repeatability was good. Statistical evaluation revealed no significant relationship between SI and CS of DPOAE. [ABSTRACT FROM AUTHOR]

Hearing impairment is a well-known consequence of closed head injury (CHI). The aim of this study was to elucidate the pathogenesis of CHI-induced hearing loss, using a rabbit model. Twelve New Zealand white rabbits were divided into two groups of 6. In the first group, CHI was induced mechanically, whereas the rabbits of the second group served as controls. Baseline distortion product otoacoustic emissions (DPOAEs), contralateral suppression (CS) of the DPOAEs and auditory brainstem response (ABR) were obtained. The same measurements were performed in the first group after CHI. Three hours later, the animals were sacrificed and their brain was excised and subjected to histopathologic examination. Mean I–III ABR latencies were increased and DPOAE amplitudes and CS values were reduced in the trauma group after CHI, at a statistically significant level. Histopathologic examination of the temporal lobe and brainstem showed multiple hemorrhagic and necrotic areas, with edema in the surrounding region. The vestibulocochlear nerve was severely damaged at its emerging site at the brainstem. In conclusion, both peripheral and central involvement of the auditory pathway was found after CHI. Otoacoustic emissions in conjunction with ABR may provide significant information on both peripheral and central auditory function. Copyright © 2007 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

Abstract: Activity of the medial olivocochlear efferents can be inferred by measuring the change of the level of distortion product otoacoustic emissions (DPOAE) during ipsilateral or contralateral acoustic stimulation, the so-called medial olivocochlear reflex (MOCR). A limitation of this measurement strategy, however, is the distinct variability of MOCR values depending on DPOAE primary tone levels and frequency, which makes selection of the stimulus parameters difficult. The objective of this study was to evaluate the dependence of MOCR values on DPOAE fine structure in humans. MOCR during contralateral acoustic stimulation was measured at frequencies with distinct non-monotonicity (“dip”) in the DPOAE fine structure, and in frequencies with flat fine structure. One hundred and twenty one different primary tone level combinations were used (L 1 =50–60dB SPL, L 2 =35–45dB SPL, 1dB steps). The measurement was repeated on another day. The major findings were: (1) Largest MOCR effects can be found in frequencies which exhibit a distinct dip in DPOAE fine structure. (2) Primary tone levels have a critical influence on the magnitude of the MOCR effect. MOCR changes of up to 23dB following a L 1 change of only 1dB were observed. Averages of the maximum MOCR change per 1dB step were in the 3–5dB-range. Both findings can be interpreted in the light of the DPOAE two-generator model [Heitmann, J., Waldmann, B., Schnitzler, H.U., Plinkert, P.K., Zenner, H.P. 1998. Suppression of distortion product otoacoustic emissions (DPOAE) near 2f1− f2 removes DP-gram fine structure – evidence for a secondary generator. Journal of the Acoustical Society of America 103, 1527–1531]. According to the present results we propose, that assessing MOCR specifically at frequencies with a distinct dip in the DPOAE fine structure, in combination with fine variation of the stimulus tone levels, allows for a more targeted search for maximum MOCR effects. Future studies must show if this approach can contribute to the further clarification of the physiological roles of the olivocochlear efferents. [Copyright &y& Elsevier]

Abstract: Orphanin FQ/nociceptin (OFQ/N) and its receptor (ORL-1) have been proposed to play a role in the regulation of hearing. In this study, we investigate the localization of OFQ/N-like immunoreactivity in the mammalian cochlea. Sprague–Dawley rat temporal bones were harvested and decalcified. The organ of Corti was microdissected, and indirect immunohistochemistry was performed using a rabbit polyclonal antibody raised against OFQ/N. Immunoreactivity was seen in the tunnel crossing fibers and the large boutons terminating onto outer hair cells, and in the fibers terminating onto the afferents to the inner hair cells. The findings are consistent with OFQ/N expression in lateral and medial olivocochlear efferents. [Copyright &y& Elsevier]

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

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, 4 ms (15% of delay); transfer to olivocochlear efferents, 9 ms (35%); efferent conduction to presynaptic OHC site, 4 ms (15%); synaptic and mechanical events at OHCs, 9 ms (35% of delay). [ABSTRACT FROM AUTHOR]

Otoacoustic emissions (OAEs) are useful for studying medial olivocochlear (MOC) efferents, but several unresolved methodological issues cloud the interpretation of the data they produce. Most efferent assays use a ‘probe stimulus’ to produce an OAE and an ‘elicitor stimulus’ to evoke efferent activity and thereby change the OAE. However, little attention has been given to whether the probe stimulus itself elicits efferent activity. In addition, most studies use only contralateral (re the probe) elicitors and do not include measurements to rule out middle-ear muscle (MEM) contractions. Here we describe methods to deal with these problems and present a new efferent assay based on stimulus frequency OAEs (SFOAEs) that incorporates these methods. By using a postelicitor window, we make measurements in individual subjects of efferent effects from contralateral, ipsi-lateral, and bilateral elicitors. Using our SFOAE assay, we demonstrate that commonly used probe sounds (clicks, tone pips, and tone pairs) elicit efferent activity, by themselves. Thus, results of efferent assays using these probe stimuli can be confounded by unwanted efferent activation. In contrast, the single 40 dB SPL tone used as the probe sound for SFOAE- based measurements evoked little or no efferent activity. Since they evoke efferent activation, clicks, tone pips, and tone pairs can be used in an adaptation efferent assay, but such paradigms are limited in measurement scope compared to paradigms that separate probe and elicitor stimuli. Finally, we describe tests to distinguish middle-ear muscle (MEM) effects from MOC effects for a number of OAE assays and show results from SFOAE-based tests. The SFOAE assay used in this study provides a sensitive, flexible, frequency-specific assay of medial efferent activation that uses a low-level probe sound that elicits little or no efferent activity, and thus provides results that can be interpreted without the confound of unintended efferent activation. [ABSTRACT FROM AUTHOR]

The mammalian auditory system contains descending pathways that originate in the cortex and relay at various intermediate levels before reaching the peripheral sensory organ of Corti. The last link in this chain consists of the olivocochlear bundle. The activity of this bundle can be measured through otoacoustic emissions, which are acoustic signatures of the cochlear biomechanical activity. In the present study, it was hypothesized that frequency-specific activation of the olivocochlear bundle in the contralateral ear would show up as frequency-specific variations in otoacoustic emission amplitude in the ipsilateral ear. Two groups of young adult subjects participated in this experiment. Evoked otoacoustic emissions were recorded in the ipsilateral ear at two test frequencies (1 and 2 kHz). Subjects had to detect probe tones at a given frequency in background noise in the contralateral ear. Larger efferent activation was measured at test frequencies on which attention is focused. This result provides evidence for an influence of attention on the auditory periphery via descending projections. [ABSTRACT FROM AUTHOR]

• We evaluate a binaural cochlear-implant sound-coding strategy inspired by the medial olivocochlear reflex. • The strategy combines dynamic, contralaterally controlled acoustic-to-electric maps with linked automatic gain control and fine structure processing. • Compared to using fixed maps, the strategy can improve speech recognition in noise without affecting two proxies for listening effort. Cochlear implant (CI) users find it hard and effortful to understand speech in noise with current devices. Binaural CI sound processing inspired by the contralateral medial olivocochlear (MOC) reflex (an approach termed the 'MOC strategy') can improve speech-in-noise recognition for CI users. All reported evaluations of this strategy, however, disregarded automatic gain control (AGC) and fine‐structure (FS) processing, two standard features in some current CI devices. To better assess the potential of implementing the MOC strategy in contemporary CIs, here, we compare intelligibility with and without MOC processing in combination with linked AGC and FS processing. Speech reception thresholds (SRTs) were compared for an FS and a MOC-FS strategy for sentences in steady and fluctuating noises, for various speech levels, in bilateral and unilateral listening modes, and for multiple spatial configurations of the speech and noise sources. Word recall scores and verbal response times in a word recognition test (two proxies for listening effort) were also compared for the two strategies in quiet and in steady noise at 5 dB signal-to-noise ratio (SNR) and the individual SRT. In steady noise, mean SRTs were always equal or better with the MOC-FS than with the standard FS strategy, both in bilateral (the mean and largest improvement across spatial configurations and speech levels were 0.8 and 2.2 dB, respectively) and unilateral listening (mean and largest improvement of 1.7 and 2.1 dB, respectively). In fluctuating noise and in bilateral listening, SRTs were equal for the two strategies. Word recall scores and verbal response times were not significantly affected by the test SNR or the processing strategy. Results show that MOC processing can be combined with linked AGC and FS processing. Compared to using FS processing alone, combined MOC-FS processing can improve speech intelligibility in noise without affecting word recall scores or verbal response times. [ABSTRACT FROM AUTHOR]

Lateral olivocochlear (LOC) efferent neurons modulate auditory nerve fiber (ANF) activity using a large repertoire of neurotransmitters, including dopamine (DA) and acetylcholine (ACh). Little is known about how individual neurotransmitter systems are differentially utilized in response to the ever-changing acoustic environment. Here we present quantitative evidence in rodents that the dopaminergic LOC input to ANFs is dynamically regulated according to the animal's recent acoustic experience. Sound exposure upregulates tyrosine hydroxylase, an enzyme responsible for dopamine synthesis, in cholinergic LOC intrinsic neurons, suggesting that individual LOC neurons might at times co-release ACh and DA. We further demonstrate that dopamine down-regulates ANF firing rates by reducing both the hair cell release rate and the size of synaptic events. Collectively, our results suggest that LOC intrinsic neurons can undergo on-demand neurotransmitter re-specification to re-calibrate ANF activity, adjust the gain at hair cell/ANF synapses, and possibly to protect these synapses from noise damage., Competing Interests: JW, EY, MM, HJ, AL, EG No competing interests declared, (© 2020, Wu et al.)