Efferent Feedback Research Articles

Relationships between the expectations based on the regularity of preceding sound sequences and the medial olivocochlear reflex.

Rhythms are the most natural cue for temporal anticipation because many sounds in our living environment have rhythmic structures. Humans have cortical mechanisms that can predict the arrival of the next sound based on rhythm and periodicity. Herein, we showed that temporal anticipation, based on the regularity of sound sequences, modulates peripheral auditory responses via efferent innervation. The medial olivocochlear reflex (MOCR), a sound-activated efferent feedback mechanism that controls outer hair cell motility, was inferred noninvasively by measuring the suppression of otoacoustic emissions (OAE). First, OAE suppression was compared between conditions in which sound sequences preceding the MOCR elicitor were presented at regular (predictable condition) or irregular (unpredictable condition) intervals. We found that OAE suppression in the predictable condition was stronger than that in the unpredictable condition. This implies that the MOCR is strengthened by the regularity of preceding sound sequences. In addition, to examine how many regularly presented preceding sounds are required to enhance the MOCR, we compared OAE suppression within stimulus sequences with 0-3 preceding tones. The OAE suppression was strengthened only when there were at least three regular preceding tones. This suggests that the MOCR was not automatically enhanced by a single stimulus presented immediately before the MOCR elicitor, but rather that it was enhanced by the regularity of the preceding sound sequences.

Age-related changes in olivocochlear efferent innervation in gerbils.

Age-related hearing difficulties have a complex etiology that includes degenerative processes in the sensory cochlea. The cochlea comprises the start of the afferent, ascending auditory pathway, but also receives efferent feedback innervation by two separate populations of brainstem neurons: the medial olivocochlear and lateral olivocochlear pathways, innervating the outer hair cells and auditory-nerve fibers synapsing on inner hair cells, respectively. Efferents are believed to improve hearing under difficult conditions, such as high background noise. Here, we compare olivocochlear efferent innervation density along the tonotopic axis in young-adult and aged gerbils (at ~50% of their maximum lifespan potential), a classic animal model for age-related hearing loss. Efferent synaptic terminals and sensory hair cells were labeled immunohistochemically with anti-synaptotagmin and anti-myosin VIIa, respectively. Numbers of hair cells, numbers of efferent terminals, and the efferent innervation area were quantified at seven tonotopic locations along the organ of Corti. The tonotopic distribution of olivocochlear innervation in the gerbil was similar to that previously shown for other species, with a slight apical cochlear bias in presumed lateral olivocochlear innervation (inner-hair-cell region), and a broad mid-cochlear peak for presumed medial olivocochlear innervation (outer-hair-cell region). We found significant, age-related declines in overall efferent innervation to both the inner-hair-cell and the outer-hair-cell region. However, when accounting for the age-related losses in efferent target structures, the innervation density of surviving elements proved unchanged in the inner-hair-cell region. For outer hair cells, a pronounced increase of orphaned outer hair cells, i.e., lacking efferent innervation, was observed. Surviving outer hair cells that were still efferently innervated retained a nearly normal innervation. A comparison across species suggests a basic aging scenario where outer hair cells, type-I afferents, and the efferents associated with them, steadily die away with advancing age, but leave the surviving cochlear circuitry largely intact until an advanced age, beyond 50% of a species' maximum lifespan potential. In the outer-hair-cell region, MOC degeneration may precede outer-hair-cell death, leaving a putatively transient population of orphaned outer hair cells that are no longer under efferent control.

Genetic tools for studying cochlear inhibition.

Efferent feedback to the mammalian cochlea includes cholinergic medial olivocochlear neurons (MOCs) that release ACh to hyperpolarize and shunt the voltage change that drives electromotility of outer hair cells (OHCs). Via brainstem connectivity, MOCs are activated by sound in a frequency- and intensity-dependent manner, thereby reducing the amplification of cochlear vibration provided by OHC electromotility. Among other roles, this efferent feedback protects the cochlea from acoustic trauma. Lesion studies, as well as a variety of genetic mouse models, support the hypothesis of efferent protection from acoustic trauma. Genetic knockout and gain-of-function knockin of the unique α9α10-containing nicotinic acetylcholine receptor (nAChR) in hair cells show that acoustic protection correlates with the efficacy of cholinergic inhibition of OHCs. This protective effect was replicated by viral transduction of the gain-of-function α9L9'T nAChR into α9-knockout mice. Continued progress with "efferent gene therapy" will require a reliable method for visualizing nAChR expression in cochlear hair cells. To that end, mice expressing HA-tagged α9 or α10 nAChRs were generated using CRISPR technology. This progress will facilitate continued study of the hair cell nAChR as a therapeutic target to prevent hearing loss and potentially to ameliorate associated pathologies such as hyperacusis.

Heightened OAEs in young adult musicians: Influence of current noise exposure and training recency

Otoacoustic emissions (OAEs) are a non-invasive metric of cochlear function. Studies of OAEs in musicians have yielded mixed results, ranging from evidence of diminished OAEs in musicians—suggesting noise-induced hearing loss—to no difference when compared to non-musicians, or even a trend for stronger OAEs in musicians. The goal of this study was to use a large sample of college students with normal hearing (n = 160) to compare OAE SNRs in musicians and non-musicians and to explore potential effects of training recency and noise exposure on OAEs in these cohorts. The musician cohort included both active musicians (who at the time of enrollment practiced at least weekly) and past musicians (who had at least 6 years of training). All participants completed a questionnaire about recent noise exposure (previous 12 months), and a subset of participants (71 musicians and 15 non-musicians) wore a personal noise dosimeter for one week to obtain a more nuanced and objective measure of exposure to assess how different exposure levels may affect OAEs before the emergence of a clinically significant hearing loss. OAEs were tested using both transient-evoked OAEs (TEOAEs) and distortion-product OAEs (DPOAEs). As predicted from the literature, musicians experienced significantly higher noise levels than non-musicians based on both subjective (self-reported) and objective measures. Yet we found stronger TEOAEs and DPOAEs in musicians compared to non-musicians in the ∼1–5 kHz range. Comparisons between past and active musicians suggest that enhanced cochlear function in young adult musicians does not require active, ongoing musical practice. Although there were no significant relations between OAEs and noise exposure as measured by dosimetry or questionnaire, active musicians had weaker DPOAEs than past musicians when the entire DPOAE frequency range was considered (up to ∼16 kHz), consistent with a subclinical noise-induced hearing loss that only becomes apparent when active musicians are contrasted with a cohort of individuals with comparable training but without the ongoing risks of noise exposure. Our findings suggest, therefore, that separate norms should be developed for musicians for earlier detection of incipient hearing loss. Potential explanations for enhanced cochlear function in musicians include pre-existing (inborn or demographic) differences, training-related enhancements of cochlear function (e.g., upregulation of prestin, stronger efferent feedback mechanisms), or a combination thereof. Further studies are needed to determine if OAE enhancements offer musicians protection against damage caused by noise exposure.

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

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

The effect of medial olivocochlear reflex enhancement associated with temporal expectation on listening performance in noise

The medial olivocochlear reflex (MOCR) is efferent feedback activated by acoustic stimulation and plays a role to improve signal detection in noise. We have reported that the MOCR and phase locking value (PLV) of delta oscillations showed a similar decreasing tendency with increasing jitter added to the preceding sound sequence. This suggests that the processing at the cortical regions is involved in the regularity -based enhancement of MOCR. Further, the dependency disappeared as the load of the interfered task increased, suggesting that the cortical predictive control of MOCR requires top-down attention to sounds. Considering the anti-masking effect of MOCR, MOCR enhancement with anticipation may improve listening performance in noise. We extended our previous experiments by measuring listening performance in noise and compared its dependence on the jitter size with MOCR strength and PLV of delta oscillation. In the low load condition, the three indices increased with decreasing the jitter. In the high load condition, by contrast, the dependency on the jitter size disappeared. The similarities of the indices in the dependency on jitter size and attention implicate that the cortical predictive control can facilitate the antimasking of MOCR, therefore improving the listening performance in noise.

Efferent involvement in neural adaptation to acoustical stimuli: Insights from otoacoustic emissions

Neural adaptation to sound level statistics has been demonstrated at various levels of the auditory pathway, including the auditory periphery. Adaptation is thought to improve the efficiency of encoding acoustic stimuli using limited neural resources without compromising accuracy. However, the precise mechanisms underlying the statistical learning of an acoustic environment are not fully understood. This includes the potential contribution to the stimulus-specific modulation of afferent auditory nerve activity by the medial olivocochlear reflex (MOCR), an efferent feedback loop linking the brainstem to the cochlear amplifier. We used otoacoustic emissions (OAEs) to investigate evidence of neural adaptation in the MOCR in response to broadband elicitors of fluctuating intensity whose mean levels were either predictable (alternating low [48 dB SPL] and high [66 dB SPL]) or unpredictable (randomly occurring low and high mean levels). Magnitudes and time-courses of the MOCR obtained in normal-hearing ears were compared for differences based on the presence or absence of an acoustically predictable context. Our findings will provide valuable insights into the role of the efferent auditory system in neural adaptation to acoustical stimuli at the auditory periphery.

Inherent envelope fluctuations in forward masking: Effects of age and hearing loss.

Forward masking is generally greater for Gaussian noise (GN) than for low-fluctuation noise maskers, i.e., GN disruption. Because the minimal hearing loss that is associated with older age may affect GN disruption differently than more significant hearing loss, the current study explored the contribution of minimal hearing loss associated with older age to GN disruption. GN disruption was measured using three masker-signal delays (25, 75, and 150 ms) for three adult groups: younger participants with normal hearing (NH), older participants with minimal hearing loss, and older participants with sensorineural hearing loss. The role of underlying mechanisms was tested using a computational model for midbrain neurons. The primary result suggests that older listeners with mild threshold elevations that typically occur with age may be more susceptible to the deleterious effects of masker envelope fluctuations than younger listeners with NH. Results from the computational model indicate that there may be a larger influence of efferent feedback and saturation of inner hair cells on forward masking and GN disruption than previously considered.

Synaptopathy in Guinea Pigs Induced by Noise Mimicking Human Experience and Associated Changes in Auditory Signal Processing.

Noise induced synaptopathy (NIS) has been researched extensively since a large amount of synaptic loss without permanent threshold shift (PTS) was found in CBA mice after a brief noise exposure. However, efforts to translate these results to humans have met with little success—and might not be possible since noise exposure used in laboratory animals is generally different from what is experienced by human subjects in real life. An additional problem is a lack of morphological data and reliable functional methods to quantify loss of afferent synapses in humans. Based on evidence for disproportionate synaptic loss for auditory nerve fibers (ANFs) with low spontaneous rates (LSR), coding-in-noise deficits (CIND) have been speculated to be the major difficulty associated with NIS without PTS. However, no robust evidence for this is available in humans or animals. This has led to a re-examination of the role of LSR ANFs in signal coding in high-level noise. The fluctuation profile model has been proposed to support a role for high-SR ANFs in the coding of high-level noise in combination with efferent control of cochlear gain. This study aimed to induce NIS by a low-level, intermittent noise exposure mimicking what is experienced in human life and examined the impact of the NIS on temporal processing under masking. It also evaluated the role of temporal fluctuation in evoking efferent feedback and the effects of NIS on this feedback.

Expectations of the timing and intensity of a stimulus propagate to the auditory periphery through the medial olivocochlear reflex.

Expectations concerning the timing of a stimulus enhance attention at the time at which the event occurs, which confers significant sensory and behavioral benefits. Herein, we show that temporal expectations modulate even the sensory transduction in the auditory periphery via the descending pathway. We measured the medial olivocochlear reflex (MOCR), a sound-activated efferent feedback that controls outer hair cell motility and optimizes the dynamic range of the sensory system. MOCR was noninvasively assessed using otoacoustic emissions. We found that the MOCR was enhanced by a visual cue presented at a fixed interval before a sound but was unaffected if the interval was changing between trials. The MOCR was also observed to be stronger when the learned timing expectation matched with the timing of the sound but remained unvaried when these two factors did not match. This implies that the MOCR can be voluntarily controlled in a stimulus- and goal-directed manner. Moreover, we found that the MOCR was enhanced by the expectation of a strong but not a weak, sound intensity. This asymmetrical enhancement could facilitate antimasking and noise protective effects without disrupting the detection of faint signals. Therefore, the descending pathway conveys temporal and intensity expectations to modulate auditory processing.

Different contributions of efferent and reafferent feedback to sensorimotor temporal recalibration

Adaptation to delays between actions and sensory feedback is important for efficiently interacting with our environment. Adaptation may rely on predictions of action-feedback pairing (motor-sensory component), or predictions of tactile-proprioceptive sensation from the action and sensory feedback of the action (inter-sensory component). Reliability of temporal information might differ across sensory feedback modalities (e.g. auditory or visual), which in turn influences adaptation. Here, we investigated the role of motor-sensory and inter-sensory components on sensorimotor temporal recalibration for motor-auditory (button press-tone) and motor-visual (button press-Gabor patch) events. In the adaptation phase of the experiment, action-feedback pairs were presented with systematic temporal delays (0 ms or 150 ms). In the subsequent test phase, audio/visual feedback of the action were presented with variable delays. The participants were then asked whether they detected a delay. To disentangle motor-sensory from inter-sensory component, we varied movements (active button press or passive depression of button) at adaptation and test. Our results suggest that motor-auditory recalibration is mainly driven by the motor-sensory component, whereas motor-visual recalibration is mainly driven by the inter-sensory component. Recalibration transferred from vision to audition, but not from audition to vision. These results indicate that motor-sensory and inter-sensory components contribute to recalibration in a modality-dependent manner.

Behavioral Measures of Cochlear Gain Reduction Depend on Precursor Frequency, Bandwidth, and Level.

Sensory systems adjust to the environment to maintain sensitivity to change. In the auditory system, the medial olivocochlear reflex (MOCR) is a known physiological mechanism capable of such adjustment. The MOCR provides efferent feedback between the brainstem and cochlea, reducing cochlear gain in response to sound. The perceptual effects of the MOCR are not well understood, such as how gain reduction depends on elicitor characteristics in human listeners. Physiological and behavioral data suggest that ipsilateral MOCR tuning is only slightly broader than it is for afferent fibers, and that the fibers feed back to the frequency region of the cochlea that stimulated them. However, some otoacoustic emission (OAE) data suggest that noise is a more effective elicitor than would be consistent with sharp tuning, and that a broad region of the cochlea may be involved in elicitation. If the elicitor is processed in a cochlear channel centered at the signal frequency, the growth of gain reduction with elicitor level would be expected to depend on the frequency content of the elicitor. In the current study, the effects of the frequency content and level of a preceding sound (called a precursor) on signal threshold was examined. The results show that signal threshold increased with increasing precursor level at a shallower slope for a tonal precursor at the signal frequency than for a tonal precursor nearly an octave below the signal frequency. A broadband noise was only slightly more effective than a tone at the signal frequency, with a relatively shallow slope similar to that of the tonal precursor at the signal frequency. Overall, these results suggest that the excitation at the signal cochlear place, regardless of elicitor frequency, determines the magnitude of ipsilateral cochlear gain reduction, and that it increases with elicitor level.

Dynamic Heterogeneity Shapes Patterns of Spiral Ganglion Activity.

Neural response properties that typify primary sensory afferents are critical to fully appreciate because they establish and, ultimately represent, the fundamental coding design used for higher-level processing. Studies illuminating the center-surround receptive fields of retinal ganglion cells, for example, were ground-breaking because they determined the foundation of visual form detection. For the auditory system, a basic organizing principle of the spiral ganglion afferents is their extensive electrophysiological heterogeneity establishing diverse intrinsic firing properties in neurons throughout the spiral ganglion. Moreover, these neurons display an impressively large array of neurotransmitter receptor types that are responsive to efferent feedback. Thus, electrophysiological diversity and its neuromodulation are a fundamental encoding mechanism contributed by the primary afferents in the auditory system. To place these features into context, we evaluated the effects of hyperpolarization and cAMP on threshold level as indicators of overall afferent responsiveness in CBA/CaJ mice of either sex. Hyperpolarization modified threshold gradients such that distinct voltage protocols could shift the relationship between sensitivity and stimulus input to reshape resolution. This resulted in an "accordion effect" that appeared to stretch, compress, or maintain responsivity across the gradient of afferent thresholds. cAMP targeted threshold and kinetic shifts to rapidly adapting neurons, thus revealing multiple cochleotopic properties that could potentially be independently regulated. These examples of dynamic heterogeneity in primary auditory afferents not only have the capacity to shift the range, sensitivity, and resolution, but to do so in a coordinated manner that appears to orchestrate changes with a seemingly unlimited repertoire.SIGNIFICANCE STATEMENT How do we discriminate the more nuanced qualities of the sound around us? Beyond the basics of pitch and loudness, aspects, such as pattern, distance, velocity, and location, are all attributes that must be used to encode acoustic sensations effectively. While higher-level processing is required for perception, it would not be unexpected if the primary auditory afferents optimized receptor input to expedite neural encoding. The findings reported herein are consistent with this design. Neuromodulation compressed, expanded, shifted, or realigned intrinsic electrophysiological heterogeneity to alter neuronal responses selectively and dynamically. This suggests that diverse spiral ganglion phenotypes provide a rich substrate to support an almost limitless array of coding strategies within the first neural element of the auditory pathway.

Binaural Background Noise Enhances Neuromagnetic Responses from Auditory Cortex

The presence of binaural low-level background noise has been shown to enhance the transient evoked N1 response at about 100 ms after sound onset. This increase in N1 amplitude is thought to reflect noise-mediated efferent feedback facilitation from the auditory cortex to lower auditory centers. To test this hypothesis, we recorded auditory-evoked fields using magnetoencephalography while participants were presented with binaural harmonic complex tones embedded in binaural or monaural background noise at signal-to-noise ratios of 25 dB (low noise) or 5 dB (higher noise). Half of the stimuli contained a gap in the middle of the sound. The source activities were measured in bilateral auditory cortices. The onset and gap N1 response increased with low binaural noise, but high binaural and low monaural noise did not affect the N1 amplitudes. P1 and P2 onset and gap responses were consistently attenuated by background noise, and noise level and binaural/monaural presentation showed distinct effects. Moreover, the evoked gamma synchronization was also reduced by background noise, and it showed a lateralized reduction for monaural noise. The effects of noise on the N1 amplitude follow a bell-shaped characteristic that could reflect an optimal representation of acoustic information for transient events embedded in noise.

The role of cGMP signalling in auditory processing in health and disease.

cGMP is generated by the cGMP-forming guanylyl cyclases (GCs), the intracellular nitric oxide (NO)-sensitive (soluble) guanylyl cyclase (sGC) and transmembrane GC (e.g. GC-A and GC-B). In summarizing the particular role of cGMP signalling for hearing, we show that GC generally do not interfere significantly with basic hearing function but rather sustain a healthy state for proper temporal coding, fast discrimination and adjustments during injury. sGC is critical for the integrity of the first synapse in the ascending auditory pathway, the inner hair cell synapse. GC-A promotes hair cell stability under stressful conditions such as acoustic trauma or ageing. GC-B plays a role in the development of efferent feed-back and gain control. Regarding the crucial role hearing has for language development, speech discrimination and cognitive brain functions, differential pharmaceutical targeting of GCs offers therapeutic promise for the restoration of hearing. LINKED ARTICLES: This article is part of a themed issue on cGMP Signalling in Cell Growth and Survival. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v179.11/issuetoc.

Cochlear protection against noise exposure requires serotonin type 3A receptor via the medial olivocochlear system.

The cochlear efferent feedback system plays important roles in auditory processing, including regulation of the dynamic range of hearing, and provides protection against acoustic trauma. These functions are performed through medial olivocochlear (MOC) neurons. However, the underlying cellular and molecular mechanisms are not fully understood. The serotonin type 3A (5-HT3A) receptor is widely expressed throughout the nervous system, which suggests important roles in various neural functions. However, involvement of the 5-HT3A receptor in the MOC system remains unclear. We used mice in this study and found that the 5-HT3A receptor was expressed in MOC neurons that innervated outer hair cells in the cochlea and was involved in the activation of MOC neurons by noise exposure. 5-HT3A receptor knockout impaired MOC functions, potentiated noise-induced hearing loss, and increased loss of ribbon synapses following noise exposure. Furthermore, 5-HT3 receptor agonist treatment alleviated the noise-induced hearing loss and loss of ribbon synapses, which enhanced cochlear protection provided by the MOC system. Our findings demonstrate that the 5-HT3A receptor plays fundamental roles in the MOC system and critically contributes to protection from noise-induced hearing impairment.

Effect of rhythm-based prediction on medial olivocochlear reflex and delta oscillations

The medial olivocochlear reflex (MOCR) is an efferent feedback activated by acoustic stimulation. Our previous studies showed that a preceding sound expedites MOCR, which suggests that temporal expectation about the timing of stimulus occurrence can modulate MOCR. In contrast, the temporal structure, e.g., regularity and rhythm, of preceding sequences also contains cues for temporal expectation about the timing of upcoming sounds. Here we investigate whether the regularities of preceding sound sequences can modulate MOCR. Simultaneously, we measured electroencephalography (EEG) delta-band oscillations whose phase-locking was reported to play a role in mediating the effects of anticipation on target detection. The predictability of target occurrence was modulated by adding jitter to the inter-stimulus interval (ISI) of the preceding sound sequence. Both the strength of MOCR and the phase synchronization of EEG delta oscillations decreased with increasing the jitter up to 20% of the ISI and are saturated above that percentage. The similar dependence on the amount of the jitter suggest the involvement of delta oscillations in the predictive control of MOCR based on the regularity of preceding sound sequences.

Sex difference in the efferent inner hair cell synapses of the aging murine cochlea

Efferent innervation of the inner hair cells changes over time. At an early age in mice, inner hair cells receive efferent feedback, which helps fine-tune tonotopic maps in the brainstem. In adulthood, inner hair cell efferent innervation wanes but increases again in older animals. It is not clear, however, whether age-related inner hair cell efferents increase along the entire range of the cochlear frequencies, or if this increase is restricted to a particular frequency-region, and whether this phenomenon occurs in both sexes. Age-related hearing loss, presbycusis, affects men and women differently. In mice, this difference is also strain specific. In aging black six mice, the auditory brainstem response thresholds increase in females earlier than in males. Here, we study age-related increase of the inner hair cell efferent innervation throughout the cochlea before hearing onset, in one month old and in ten months old and older male and female black six mice. We collected confocal images of immunostained inner hair cell efferents and quantified the labeled terminals in the entire cochlea using a machine learning algorithm. The overall number of the inner hair cell efferents in both sexes did not change significantly between age-groups. The distribution of the inner hair cell efferent innervation did not differ across frequencies in the cochlea. However, in females, inner hair cells received on average up to four times more efferent innervation than in males per each of the frequency regions tested. Sex differences were also found in the oldest age-group tested (≥ 10 months) where on average inner hair cells received six times more efferents in females than in males of matching age. Our findings emphasize the importance of including both sexes in sensorineural hearing loss research.

Conductive hearing loss during development does not appreciably alter the sharpness of cochlear tuning

An increasing number of studies show that listeners often have difficulty hearing in situations with background noise, despite normal tuning curves in quiet. One potential source of this difficulty could be sensorineural changes in the auditory periphery (the ear). Signal in noise detection deficits also arise in animals raised with developmental conductive hearing loss (CHL), a manipulation that induces acoustic attenuation to model how sound deprivation changes the central auditory system. This model attributes perceptual deficits to central changes by assuming that CHL does not affect sensorineural elements in the periphery that could raise masked thresholds. However, because of efferent feedback, altering the auditory system could affect cochlear elements. Indeed, recent studies show that adult-onset CHL can cause cochlear synapse loss, potentially calling into question the assumption of an intact periphery in early-onset CHL. To resolve this issue, we tested the long-term peripheral effects of CHL via developmental bilateral malleus displacement. Using forward masking tuning curves, we compared peripheral tuning in animals raised with CHL vs age-matched controls. Using compound action potential measurements from the round window, we assessed inner hair cell synapse integrity. Results indicate that developmental CHL can cause minor synaptopathy. However, developmental CHL does not appreciably alter peripheral frequency tuning.

Efferent unmasking of speech-in-noise encoding?

Objective The medial olivocochlear (MOC) reflex provides efferent feedback from the brainstem to cochlear outer hair cells. Physiologic studies have demonstrated that the MOC reflex is involved in “unmasking” of signals-in-noise at the level of the auditory nerve; however, its functional importance in human hearing remains unclear. Design This study examined relationships between pre-neural measurements of MOC reflex strength (click-evoked otoacoustic emission inhibition; CEOAE) and neural measurements of speech-in-noise encoding (speech frequency following response; sFFR) in four conditions (Quiet, Contralateral Noise, Ipsilateral Noise, and Ipsilateral + Contralateral Noise). Three measures of CEOAE inhibition (amplitude reduction, effective attenuation, and input-output slope inhibition) were used to quantify pre-neural MOC reflex strength. Correlations between pre-neural MOC reflex strength and sFFR “unmasking” (i.e. response recovery from masking effects with activation of the MOC reflex in time and frequency domains) were assessed. Study sample 18 young adults with normal hearing. Results sFFR unmasking effects were insignificant, and there were no correlations between pre-neural MOC reflex strength and sFFR unmasking in the time or frequency domain. Conclusion Our results do not support the hypothesis that the MOC reflex is involved in speech-in-noise neural encoding, at least for features that are represented in the sFFR at the SNR tested.