Protection from acoustic trauma is not a primary function of the medial olivocochlear efferent system

Blast Overpressures as a Military and Occupational Health Concern

PURPOSE

This tutorial reviews effects of environmental stressors like blast overpressures and other well-known acoustic contaminants (continuous, intermittent, and impulsive noise) on hearing, tinnitus, vestibular, and balance-related functions. Based on the overall outcome of these effects, detailed consideration is given to the health and well-being of individuals.

METHOD

Because hearing loss and tinnitus are consequential in affecting quality of life, novel neuromodulation paradigms are reviewed for their positive abatement and treatment-related effects. Examples of clinical data, research strategies, and methodological approaches focus on repetitive transcranial magnetic stimulation (rTMS) and electrical stimulation of the vagus nerve paired with tones (VNSt) for their unique contributions to this area.

RESULTS

Acoustic toxicants transmitted through the atmosphere are noteworthy for their propensity to induce hearing loss and tinnitus. Mounting evidence also indicates that high-level rapid onset changes in atmospheric sound pressure can significantly impact vestibular and balance function. Indeed, the risk of falling secondary to loss of, or damage to, sensory receptor cells in otolith organs (utricle and saccule) is a primary reason for this concern. As part of the complexities involved in VNSt treatment strategies, vocal dysfunction may also manifest. In addition, evaluation of temporospatial gait parameters is worthy of consideration based on their ability to detect and monitor incipient neurological disease, cognitive decline, and mortality.

CONCLUSION

Highlighting these respective areas underscores the need to enhance information exchange among scientists, clinicians, and caregivers on the benefits and complications of these outcomes.

The comprehensive audiological evaluation in young violinists: the medial olivocochlear system, high frequency thresholds, and the auditory figure ground test

PURPOSE

The aim of this study was to examine whether the medial olivocochlear hearing system functions, the high frequency hearing thresholds and speech discrimination in noise performance can guide us in assessing the risk of hearing loss among violinists. It is aimed to investigate possible hearing damage that is not reflected in pure tone hearing thresholds in violinists.

METHODS

The participants (n = 50) who have normal hearing and the ages of 18-30 were included in this study in two groups: violinists and controls who are unrelated to music. High frequency audiometer, auditory figure ground test (AFG) for speech discrimination in noise performance, Distortion Product Otoacoustic Emission (DPOAE) and contralateral suppression on DPOAE for medial olivocochlear system function tests were applied to all participants as well as routine audiological tests.

RESULTS

The high frequency hearing thresholds were obtained higher in violinists compared to the controls. In violinists, the AFG test scores and the suppression amount at 1 kHz were lower than the controls. In addition, DPOAE responses at 4-6 kHz were obtained lower in violinists (p < 0.05).

CONCLUSION

The reason for high frequency hearing loss, decreased DPOAE response amplitudes, and poor medial olivocochlear function in violinists can be explained by the long-term exposure to high-level noise caused by the violin, one of the closest musical instruments. Routine and comprehensive audiological follow-up is crucial for musicians.

Tinnitus development is associated with synaptopathy of inner hair cells in Mongolian gerbils

Human hearing loss (HL) is often accompanied by comorbidities like tinnitus, which is affecting up to 15% of the adult population. Rodent animal studies could show that tinnitus may not only be a result of apparent HL due to cochlear hair cell damage but can also be a consequence of synaptopathy at the inner hair cells (IHCs) already induced by moderate sound traumata. Here, we investigate synaptopathy previously shown in mice in our animal model, the Mongolian gerbil, and relate it to behavioral signs of tinnitus. Tinnitus was induced by a mild monaural acoustic trauma leading to monaural noise induced HL in the animals, quantified by auditory brainstem response (ABR) audiometry. Behavioral signs of tinnitus percepts were detected by measurement of prepulse inhibition of the acoustic startle response in a gap-noise paradigm. Fourteen days after trauma, the cochleae of both ears were isolated, and IHC synapses were counted within several spectral regions of the cochlea. Behavioral signs of tinnitus were only found in animals with IHC synaptopathy, independent of type of HL. On the other hand, animals with apparent HL but without behavioral signs of tinnitus showed a reduction in amplitudes of ABR waves I&II but no significant changes in the number of synapses at the IHC. We conclude-in line with the literature-that HL is caused by damage to the IHC or by other reasons but that the development of tinnitus, at least in our animal model, is closely linked to synaptopathy at the IHC.

Click evoked middle ear muscle reflex: Spectral and temporal aspects

This study describes a time series-based method of middle ear muscle reflex (MEMR) detection using bilateral clicks. Although many methods can detect changes in the otoacoustic emissions evoking stimulus to monitor the MEMR, they do not discriminate between true MEMR-mediated vs artifactual changes in the stimulus. We measured MEMR in 20 young clinically normal hearing individuals using 1-s-long click trains presented at six levels (65 to 95 dB peak-to-peak sound pressure level in 6 dB steps). Changes in the stimulus levels over the 1 s period were well-approximated by two-term exponential functions. The magnitude of ear canal pressure changes due to MEMR increased monotonically as a function of click level but non-monotonically with frequency when separated into 1/3 octave wide bands between 1 and 3.2 kHz. MEMR thresholds estimated using this method were lower than that obtained from a clinical tympanometer in ∼94% of the participants. A time series-based method, along with statistical tests, may provide additional confidence in detecting the MEMR. MEMR effects were smallest at 2 kHz, between 1 and 3.2 kHz, which may provide avenues for minimizing the MEMR influence while measuring other responses (e.g., the medial olivocochlear reflex).

Effects of noise precursors on the detection of amplitude and frequency modulation for tones in noise

Recent studies on amplitude modulation (AM) detection for tones in noise reported that AM-detection thresholds improve when the AM stimulus is preceded by a noise precursor. The physiological mechanisms underlying this AM unmasking are unknown. One possibility is that adaptation to the level of the noise precursor facilitates AM encoding by causing a shift in neural rate-level functions to optimize level encoding around the precursor level. The aims of this study were to investigate whether such a dynamic-range adaptation is a plausible mechanism for the AM unmasking and whether frequency modulation (FM), thought to be encoded via AM, also exhibits the unmasking effect. Detection thresholds for AM and FM of tones in noise were measured with and without a fixed-level precursor. Listeners showing the unmasking effect were then tested with the precursor level roved over a wide range to modulate the effect of adaptation to the precursor level on the detection of the subsequent AM. It was found that FM detection benefits from a precursor and the magnitude of FM unmasking correlates with that of AM unmasking. Moreover, consistent with dynamic-range adaptation, the unmasking magnitude weakens as the level difference between the precursor and simultaneous masker of the tone increases.

Pre-exposure to Lower-Level Noise Mitigates Cochlear Synaptic Loss Induced by High-Level Noise

The auditory sensory organs appear to be less damaged by exposure to high-level noise that is presented after exposure to non-traumatizing low-level noise. This phenomenon is known as the toughening or conditioning effect. Functionally, it is manifested by a reduced threshold shift, and morphologically by a reduced hair cell loss. However, it remains unclear whether prior exposure to toughening noise can mitigate the synaptic loss induced by exposure to damaging noise. Since the cochlear afferent synapse between the inner hair cells and primary auditory neurons has been identified as a novel site involved in noise-induced cochlear damage, we were interested in assessing whether this synapse can be toughened. In the present study, the synaptic loss was induced by a damaging noise exposure (106 dB SPL) and compared across Guinea pigs who had and had not been previously exposed to a toughening noise (85 dB SPL). Results revealed that the toughening noise heavily reduced the synaptic loss observed 1 day after exposure to the damaging noise. Although it was significant, the protective effect of the toughening noise on permanent synaptic loss was much smaller. Compared with cases in the control group without noise exposure, coding deficits were seen in both toughened groups, as reflected in the compound action potential (CAP) by signals with amplitude modulation. In general, the pre-exposure to the toughening noise resulted in a significantly reduced synaptic loss by the high-level noise. However, this morphological protection was not accompanied by a robust functional benefit.

Vestibular suppression of normal bodily sounds

Background: Vestibular-evoked myogenic potentials (VEMPs) are present in almost everyone. The proximity of the cochlear and vestibular end organs strongly indicates an overlap of the stimuli to which they respond.Aims/objectives: To determine the loudness of shouting compared to the external auditory canal (EAC) and explore how hyperstimulation of the cochlea and vestibular structures might be prevented.Materials and methods: We compared the loudness and frequency response of sound during shouting, biting and chewing, as well as the measurement of the loudness and frequency of sound in the external auditory canal (EAC) when a calibrated minishaker is applied to the forehead and to the upper incisor teeth.Results: We showed that vibratory sounds produced by vocalizations and oral activities were attenuated when they reached the ear, so that these sounds would not act as vestibular stimulants.Conclusions and significance: Chewing is known to cause a stapedius reflex which suppresses internal sounds to optimize audition of external sounds, while at the same time suppressing vestibular stimulation, which serves to optimize the sensitivity of the vestibular system, in order that it may respond precisely to a threat.

Short-Term Reliability of Different Methods of Contralateral Suppression of Transient Evoked Otoacoustic Emission in Children and Adults

Purpose This study aimed to investigate the reliability of 3 methods to measure contralateral suppression of transient evoked otoacoustic emissions (TEOAEs) in children and adults. Method Contralateral suppression of TEOAEs was measured in 14 adults and 14 children using 3 methods with and without contralateral acoustic stimulus (CAS). Method-I having "2 s on-off" and Method-II having "10 s on-off" interleaved presentation of white noise. Method-III used "continuous presentation of white noise". Test-retest reliability was checked in adults without removing the probe (same-probe recording) and reinserting the probe (different-probe recording) and in children using a different-probe recording. Results The absolute suppression amplitude of TEOAEs was higher for "continuous noise," followed by "10 s on-off" and "2 s on-off" CAS. There was no significant effect of age across the 2 probe recordings, 3 methods of TEOAEs with and without CAS, and for the absolute suppression amplitude. Also, in adults, there was no significant difference between same-probe and different-probe recordings across the 3 methods. High internal consistency was observed on Cronbach's alpha (α > .9) for the 3 methods and 2 probe recordings. High agreement and correlation between the recordings for all 3 methods were seen using Bland-Altman plots and Pearson product-moment correlation coefficient. Conclusion The study demonstrated that highly reliable contralateral suppression of TEOAE can be measured using the 3 methods in adults and children. However, continuous presentation of CAS resulted in greater TEOAE suppression amplitude compared to interleaved presentation of CAS; hence, the former is recommended.

Forebrain Dopamine System Regulates Inner Ear Auditory Sensitivity to Socially Relevant Acoustic Signals

Dopamine is integral to attentional and motivational processes, but studies are largely restricted to the central nervous system. In mammals [1, 2] and fishes [3, 4], central dopaminergic neurons project to the inner ear and could modulate acoustic signals at the earliest stages of processing. Studies in rodents show dopamine inhibits cochlear afferent neurons and protects against noise-induced acoustic injury [5-10]. However, other functions for inner ear dopamine have not been investigated, and the effect of dopamine on peripheral auditory processing in non-mammalians remains unknown [11, 12]. Insights could be gained by studies conducted in the context of intraspecific acoustic communication. We present evidence from a vocal fish linking reproductive-state-dependent changes in auditory sensitivity with seasonal changes in the dopaminergic efferent system in the saccule, their primary organ of hearing. Plainfin midshipman (Porichthys notatus) migrate from deep-water winter habitats to the intertidal zone in the summer to breed. Nesting males produce nocturnal vocalizations to attract females [13]. Both sexes undergo seasonal enhancement of hearing sensitivity at the level of the hair cell [14-16], increasing the likelihood of detecting conspecific signals [17, 18]. Importantly, reproductive females concurrently have reduced dopaminergic input to the saccule [19]. Here, we show that dopamine decreases saccule auditory sensitivity via a D2-like receptor. Saccule D2a receptor expression is reduced in the summer and correlates with sensitivity within and across seasons. We propose that reproductive-state-dependent changes to the dopaminergic efferent system provide a release of inhibition in the saccule, enhancing peripheral encoding of social-acoustic signals.

Neural Encoding of Amplitude Modulations in the Human Efferent System

Most natural sounds, including speech, exhibit temporal amplitude fluctuations. This information is encoded as amplitude modulations (AM)-essential for auditory and speech perception. The neural representation of AM has been studied at various stages of the ascending auditory system from the auditory nerve to the cortex. In contrast, research on neural coding of AM in the efferent pathway has been extremely limited. The objective of this study was to investigate the encoding of AM signals in the medial olivocochlear system by measuring the modulation transfer functions of the efferent response in humans. A secondary goal was to replicate the controversial findings from the literature that efferent stimulation produces larger effects for the AM elicitor with 100 Hz modulation frequency in comparison with the unmodulated elicitor. The efferent response was quantified by measuring changes in stimulus-frequency otoacoustic emission magnitude due to various modulated and unmodulated elicitors. Unmodulated, broadband noise elicitors yielded either slightly larger or similar efferent responses relative to modulated elicitors depending on the modulation frequency. Efferent responses to the unmodulated and modulated elicitors with 100 Hz modulation frequency were not significantly different. The efferent system encoding of AM sounds-modulation transfer functions-can be modeled with a first-order Butterworth low-pass filter with different cutoff frequencies for ipsilateral and contralateral elicitors. The ipsilateral efferent pathway showed a greater sensitivity to AM information comparted to the contralateral pathway. Efferent modulation transfer functions suggest that the ability of the system to follow AM decreases with increasing modulation frequency and that efferents may not be fully operating on the envelope of the speech.

Long-standing problems in speech perception dissolve within an information-theoretic perspective

An information theoretic framework is proposed to have the potential to dissolve (rather than attempt to solve) multiple long-standing problems concerning speech perception. By this view, speech perception can be reframed as a series of processes through which sensitivity to information-that which changes and/or is unpredictable-becomes increasingly sophisticated and shaped by experience. Problems concerning appropriate objects of perception (gestures vs. sounds), rate normalization, variance consequent to articulation, and talker normalization are reframed, or even dissolved, within this information-theoretic framework. Application of discriminative models founded on information theory provides a productive approach to answer questions concerning perception of speech, and perception most broadly.

Noise-Induced Hearing Loss in Mice: Effects of High and Low Levels of Noise Trauma in CBA Mice

Acoustic trauma can induce temporary or permanent noise-induced hearing loss (NIHL). Noise exposed animal models allow us to study the effects of various noise trauma insults on the cochlea and auditory pathways. Here we studied the short-term and long-term functional changes occurring in the auditory system following exposure to two different noise traumas. Several measures of hearing function known to change following noise exposure were examined: Temporary (TTS) and permanent (PTS) threshold shifts were measured using auditory brainstem responses (ABR), outer hair cell function was examined using distortion product otoacoustic emissions (DPOAEs), and auditory temporal processing was assessed using a gap-in-noise (GIN) ABR paradigm. Physiological measures were made before and after the exposure (24 hours, 2 weeks, 4 weeks, and 1 year). The animals were perfused and their brain, and cochlea were collected for future biomarker studies. Young adult mice were exposed to 110 dB and 116 dB octave-band noise levels for 45 minutes, and both groups demonstrated significant threshold shifts 1 day post-noise exposure across all frequencies. However 2 weeks postexposure, PTS within the 110 dB group was significantly reduced compared to 1 day post trauma, this improvement in thresholds was not as great in the 116 dB exposure group. At 2 weeks post-trauma, differences between the measured PTS in the two groups was significant for 4 of the 7 measured frequencies. At this 1 year time point after exposure, mice in the 110 dB group showed very minor PTS, but the 116 dB group showed a large PTS comparable to their 2 and 4 week PTS. At this time point, PTS variation between the two groups was significant across all frequencies. DPOAE amplitudes measured 2 weeks post exposure showed recovery for all frequencies within 10 dB (average) of the baseline in the 110 dB group, however for the 116 dB exposure DP amplitudes were elevated by about 30 dB. The differences in DPOAE amplitudes between the 110 dB and 116 dB groups were significant at 2 weeks, 4 weeks, and 1 year post-trauma in the mid frequency range. At 2 weeks, 4 weeks, and 1 year, DPOAE thresholds returned to within 10 dB of the baseline for the 110 dB group in the low and mid frequency range, whereas the 116 dB group still showed shifts of 30 dB for all frequency ranges. For Gap ABRs, there was a significant decrease in both noise burst 1 (NB1) and noise burst 2 (NB2) amplitudes for peaks 1 and 4 in the 116 dB group relative to the 110 dB group when measured at 1 year post trauma. These results indicate that a 6 dB increase in noise exposure intensity results in a significant increased ototrauma in both the peripheral and central auditory systems.

Cholinergic top-down influences on the auditory brainstem

Descending connections are present in many sensory systems and support adaptive information processing. This allows the sensory brain to code a wider range of inputs. A well characterized descending system is the olivo-cochlear cholinergic innervation of the inner ear, which mediates a reduction of the sensitivity of the inner ear upon perception of intense sounds. Because this inhibits the response to background noise, the olivo-cochlear system supports detection of transient sound events. Olivo-cochlear neurons also innervate the cochlear nucleus through axon collaterals. Here, acetylcholine increases the excitability of central neurons without reducing their temporal precision. Thus their target neurons in the superior olivary complex can more effectively process binaural temporal cues. We argue that the central effect of the olivo-cochlear system augments the peripheral effect. In addition, olivo-cochlear cholinergic neurons are under top-down control of cortical inputs, providing further adaptability of information processing on the level of the auditory brainstem.

Functional Significance of Medial Olivocochlear System Morphology in the Mouse Cochlea

Objectives

Baso-apical gradients exist in various cochlear structures including medial olivocochlear (MOC) efferent system. This study investigated the cochlear regional differentials in the function and morphology of the MOC system, and addressed the functional implications of regional MOC efferent terminals (ETs) in the mouse cochlea.

Methods

In CBA/J mice, MOC reflex (MOCR) was assessed based on the distortion product otoacoustic emission in the absence and presence of contralateral acoustic stimulation. High, middle, and low frequencies were grouped according to a mouse place-frequency map. Cochlear whole mounts were immunostained for ETs with anti-α-synuclein and examined using confocal laser scanning microscopy. The diameters of ETs and the number of ETs per outer hair cell were measured from the z-stack images of the basal, middle and apical regions, respectively.

Results

The middle cochlear region expressed large, clustered MOC ETs with strong MOCR, the base expressed small, less clustered ETs with strong MOCR, and the apex expressed large, but less clustered ETs with weak MOCR.

Conclusion

The mouse cochlea demonstrated regional differentials in the function and morphology of the MOC system. Strong MOCR along with superior MOC morphology in the middle region may contribute to ‘signal detection in noise,’ the primary efferent function, in the best hearing frequencies. Strong MOCR in spite of inferior MOC morphology in the base may reflect the importance of ‘protection from noise trauma’ in the high frequencies.

Neuroanatomical Evidence for Catecholamines as Modulators of Audition and Acoustic Behavior in a Vocal Teleost

The plainfin midshipman fish (Porichthys notatus) is a well-studied model to understand the neural and endocrine mechanisms underlying vocal-acoustic communication across vertebrates. It is well established that steroid hormones such as estrogen drive seasonal peripheral auditory plasticity in female Porichthys in order to better encode the male's advertisement call. However, little is known of the neural substrates that underlie the motivation and coordinated behavioral response to auditory social signals. Catecholamines, which include dopamine and noradrenaline, are good candidates for this function, as they are thought to modulate the salience of and reinforce appropriate behavior to socially relevant stimuli. This chapter summarizes our recent studies which aimed to characterize catecholamine innervation in the central and peripheral auditory system of Porichthys as well as test the hypotheses that innervation of the auditory system is seasonally plastic and catecholaminergic neurons are activated in response to conspecific vocalizations. Of particular significance is the discovery of direct dopaminergic innervation of the saccule, the main hearing end organ, by neurons in the diencephalon, which also robustly innervate the cholinergic auditory efferent nucleus in the hindbrain. Seasonal changes in dopamine innervation in both these areas appear dependent on reproductive state in females and may ultimately function to modulate the sensitivity of the peripheral auditory system as an adaptation to the seasonally changing soundscape. Diencephalic dopaminergic neurons are indeed active in response to exposure to midshipman vocalizations and are in a perfect position to integrate the detection and appropriate motor response to conspecific acoustic signals for successful reproduction.

The olivocochlear system and protection from acoustic trauma: a mini literature review

Large intersubject variability in the susceptibility to noise-induced hearing loss (NIHL) is known to occur in both humans and animals. It has been suggested that the olivocochlear system (OCS) plays a significant role in protecting the cochlea from exposure to high levels of noise. A mini literature review about the scientific evidence from animal and human studies about the association between the function of the OCS and susceptibility to NIHL was carried out. Animal data consistently show that de-efferented ears exhibit larger temporary threshold shift (TTS) and permanent threshold shift (PTS) than efferented ears. Data from human studies do not consistently show a correlation between the strength of the OCS function and amount of TTS. Further research on human subjects is required to determine how the OCS function could be used to predict susceptibility to NIHL in individual subjects.

Cellular signaling protective against noise-induced hearing loss – A role for novel intrinsic cochlear signaling involving corticotropin-releasing factor?

Hearing loss afflicts approximately 15% of the world's population, and crosses all socioeconomic boundaries. While great strides have been made in understanding the genetic components of syndromic and non-syndromic hearing loss, understanding of the mechanisms underlying noise-induced hearing loss (NIHL) have come much more slowly. NIHL is not simply a mechanism by which older individuals loose their hearing. Significantly, the incidence of NIHL is increasing, and is now involving ever younger populations. This may predict future increased occurrences of hearing loss. Current research has shown that even short-term exposures to loud sounds generating what was previously considered temporary hearing loss, actually produces an almost immediate and permanent loss of specific populations of auditory nerve fibers. Additionally, recurrent exposures to intense sound may hasten age-related hearing loss. While NIHL is a significant medical concern, to date, few compounds have delivered significant protection, arguing that new targets need to be identified. In this commentary, we will explore cellular signaling processes taking place in the cochlea believed to be involved in protection against hearing loss, and highlight new data suggestive of novel signaling not previously recognized as occurring in the cochlea, that is perhaps protective of hearing. This includes a recently described local hypothalamic-pituitary-adrenal axis (HPA)-like signaling system fully contained in the cochlea. This system may represent a local cellular stress-response system based on stress hormone release similar to the systemic HPA axis. Its discovery may hold hope for new drug therapies that can be delivered directly to the cochlea, circumventing systemic side effects.

The biological role of the medial olivocochlear efferents in hearing: separating evolved function from exaptation

Cochlear outer hair cells (OHCs) are remarkable, mechanically-active receptors that determine the exquisite sensitivity and frequency selectivity characteristic of the mammalian auditory system. While there are three to four times as many OHCs compared with inner hair cells, OHCs lack a significant afferent innervation and, instead, receive a rich efferent innervation from medial olivocochlear (MOC) efferent neurons. Activation of the MOC has been shown to exert a considerable suppressive effect over OHC activity. The precise function of these efferent tracts in auditory behavior, however, is the matter of considerable debate. The most frequent functions assigned to the MOC tracts are to protect the cochlea from traumatic damage associated with intense sound and to aid the detection of signals in noise. While considerable evidence shows that interruption of MOC activity exacerbates damage due to high-level sound exposure, the well characterized MOC physiology and evolutionary studies do not support such a role. Instead, a MOC protective effect is well explained as being a byproduct of the suppressive nature of MOC action on OHC mechanical behavior. A role in the enhancement of signals in noise backgrounds, on the other hand, is well supported by (1) an extensive physiological literature (2) examination of naturally occurring environmental acoustic conditions (3) recent data from multiple laboratories showing that the MOC plays a significant role in auditory selective attention by suppressing the response to unattended or ignored stimuli. This presentation will argue that, based on the extant literature combining the suppression of background noise through MOC-mediated rapid adaptation (RA) with the suppression of non-attended signals, in concert with the corticofugal pathways descending from the auditory cortex, the MOC system has one evolved function-to increase the signal-to-noise ratio, aiding in the detection of target signals. By contrast, the MOC system role in reducing noise damage and the effects of aging in the cochlea may well represent an exaptation, or evolutionary "spandrel".

Efferent feedback slows cochlear aging

The inner ear receives two types of efferent feedback from the brainstem: one pathway provides gain control on outer hair cells' contribution to cochlear amplification, and the other modulates the excitability of the cochlear nerve. Although efferent feedback can protect hair cells from acoustic injury and thereby minimize noise-induced permanent threshold shifts, most prior studies focused on high-intensity exposures (>100 dB SPL). Here, we show that efferents are essential for long-term maintenance of cochlear function in mice aged 1 year post-de-efferentation without purposeful acoustic overexposure. Cochlear de-efferentation was achieved by surgical lesion of efferent pathways in the brainstem and was assessed by quantitative analysis of immunostained efferent terminals in outer and inner hair cell areas. The resultant loss of efferent feedback accelerated the age-related amplitude reduction in cochlear neural responses, as seen in auditory brainstem responses, and increased the loss of synapses between hair cells and the terminals of cochlear nerve fibers, as seen in confocal analysis of the organ of Corti immunostained for presynaptic and postsynaptic markers. This type of neuropathy, also seen after moderate noise exposure, has been termed "hidden hearing loss", because it does not affect thresholds, but can be seen in the suprathreshold amplitudes of cochlear neural responses, and likely causes problems with hearing in a noisy environment, a classic symptom of age-related hearing loss in humans. Since efferent reflex strength varies among individuals and can be measured noninvasively, a weak reflex may be an important risk factor, and prognostic indicator, for age-related hearing impairment.

Efferent feedback minimizes cochlear neuropathy from moderate noise exposure

Although protective effects of the cochlea's efferent feedback pathways have been well documented, prior work has focused on hair cell damage and cochlear threshold elevation and, correspondingly, on the high sound pressure levels (>100 dB SPL) necessary to produce them. Here we explore the noise-induced loss of cochlear neurons that occurs with lower-intensity exposures and in the absence of permanent threshold shifts. Using confocal microscopy to count synapses between hair cells and cochlear nerve fibers, and using measurement of auditory brainstem responses and otoacoustic emissions to assess cochlear presynaptic and postsynaptic function, we compare the damage from a weeklong exposure to moderate-level noise (84 dB SPL) in mice with varying degrees of cochlear de-efferentation induced by surgical lesion to the olivocochlear pathway. Such exposure causes minimal acute threshold shifts and no chronic shifts in mice with normal efferent feedback. In de-efferented animals, there was up to 40% loss of cochlear nerve synapses and a corresponding decline in the amplitude of the auditory brainstem response. Quantitative analysis of the de-efferentation in inner versus outer hair cell areas suggested that outer hair cell efferents are the most important in minimizing this neuropathy, presumably by virtue of their sound-evoked feedback reduction of cochlear amplification. The moderate nature of this acoustic overexposure suggests that cochlear neurons are at risk even in everyday acoustic environments, so the need for cochlear protection is plausible as a driving force in the design of this feedback pathway.

Critical role for cochlear hair cell BK channels for coding the temporal structure and dynamic range of auditory information for central auditory processing

Large conductance, voltage- and Ca²⁺-activated K⁺ (BK) channels in inner hair cells (IHCs) of the cochlea are essential for hearing. However, germline deletion of BKα, the pore-forming subunit KCNMA1 of the BK channel, surprisingly did not affect hearing thresholds in the first postnatal weeks, even though altered IHC membrane time constants, decreased IHC receptor potential alternating current/direct current ratio, and impaired spike timing of auditory fibers were reported in these mice. To investigate the role of IHC BK channels for central auditory processing, we generated a conditional mouse model with hair cell-specific deletion of BKα from postnatal day 10 onward. This had an unexpected effect on temporal coding in the central auditory system: neuronal single and multiunit responses in the inferior colliculus showed higher excitability and greater precision of temporal coding that may be linked to the improved discrimination of temporally modulated sounds observed in behavioral training. The higher precision of temporal coding, however, was restricted to slower modulations of sound and reduced stimulus-driven activity. This suggests a diminished dynamic range of stimulus coding that is expected to impair signal detection in noise. Thus, BK channels in IHCs are crucial for central coding of the temporal fine structure of sound and for detection of signals in a noisy environment.—Kurt, S., Sausbier, M., Rüttiger, L., Brandt, N., Moeller, C. K., Kindler, J., Sausbier, U., Zimmermann, U., van Straaten, H., Neuhuber, W., Engel, J., Knipper, M., Ruth, P., Schulze, H. Critical role for cochlear hair cell BK channels for coding the temporal structure and dynamic range of auditory information for central auditory processing.

The cochlear CRF signaling systems and their mechanisms of action in modulating cochlear sensitivity and protection against trauma

A key requirement for encoding the auditory environment is the ability to dynamically alter cochlear sensitivity. However, merely attaining a steady state of maximal sensitivity is not a viable solution since the sensory cells and ganglion cells of the cochlea are prone to damage following exposure to loud sound. Most often, such damage is via initial metabolic insult that can lead to cellular death. Thus, establishing the highest sensitivity must be balanced with protection against cellular metabolic damage that can lead to loss of hair cells and ganglion cells, resulting in loss of frequency representation. While feedback mechanisms are known to exist in the cochlea that alter sensitivity, they respond only after stimulus encoding, allowing potentially damaging sounds to impact the inner ear at times coincident with increased sensitivity. Thus, questions remain concerning the endogenous signaling systems involved in dynamic modulation of cochlear sensitivity and protection against metabolic stress. Understanding endogenous signaling systems involved in cochlear protection may lead to new strategies and therapies for prevention of cochlear damage and consequent hearing loss. We have recently discovered a novel cochlear signaling system that is molecularly equivalent to the classic hypothalamic-pituitary-adrenal (HPA) axis. This cochlear HPA-equivalent system functions to balance auditory sensitivity and susceptibility to noise-induced hearing loss, and also protects against cellular metabolic insults resulting from exposures to ototoxic drugs. We review the anatomy, physiology, and cellular signaling of this system, and compare it to similar signaling in other organs/tissues of the body.

The medial olivocochlear system attenuates the developmental impact of early noise exposure

The early onset of peripheral deafness profoundly alters the functional maturation of the central auditory system. A prolonged exposure to an artificial acoustic environment has a similar disruptive influence. These observations establish the importance of normal patterns of sound-driven activity during the initial stages of auditory development. The present study was designed to address the role of cochlear gain control during these activity-dependent developmental processes. It was hypothesized that the regulation of auditory nerve activity by the medial olivocochlear system (MOCS) would preserve normal development when the immature auditory system was challenged by continuous background noise. To test this hypothesis, knock-out mice lacking MOCS feedback were reared in noisy or quiet environments and then evaluated with behavioral paradigms for auditory processing deficits. Relative to wild-type controls, noise-reared knock-out mice showed a decreased ability to process rapid acoustic events. Additional anatomical and physiological assessments linked these perceptual deficits to synaptic defects in the auditory brainstem that shared important features with human auditory neuropathy. Our findings offer a new perspective on the potentially damaging effects of environmental noise and how these risks are ameliorated by the protective role of MOCS feedback.

Modulation of hair cell efferents

Outer hair cells (OHCs) amplify the sound-evoked motion of the basilar membrane to enhance acoustic sensitivity and frequency selectivity. Medial olivocochlear (MOC) efferents inhibit OHCs to reduce the sound-evoked response of cochlear afferent neurons. OHC inhibition occurs through the activation of postsynaptic α9α10 nicotinic receptors tightly coupled to calcium-dependent SK2 channels that hyperpolarize the hair cell. MOC neurons are cholinergic but a number of other neurotransmitters and neuromodulators have been proposed to participate in efferent transmission, with emerging evidence for both pre- and postsynaptic effects. Cochlear inhibition in vivo is maximized by repetitive activation of the efferents, reflecting facilitation and summation of transmitter release onto outer hair cells. This review summarizes recent studies on cellular and molecular mechanisms of cholinergic inhibition and the regulation of those molecular components, in particular the involvement of intracellular calcium. Facilitation at the efferent synapse is compared in a variety of animals, as well as other possible mechanisms of modulation of ACh release. These results suggest that short-term plasticity contributes to effective cholinergic inhibition of hair cells.

Temporal properties of perceptual calibration to local and broad spectral characteristics of a listening context

The auditory system calibrates to reliable properties of a listening environment in ways that enhance sensitivity to less predictable (more informative) aspects of sounds. These reliable properties may be spectrally local (e.g., peaks) or global (e.g., gross tilt), but the time course over which the auditory system registers and calibrates to these properties is unknown. Understanding temporal properties of this perceptual calibration is essential for revealing underlying mechanisms that serve to increase sensitivity to changing and informative properties of sounds. Relative influence of the second formant (F(2)) and spectral tilt was measured for identification of /u/ and /i/ following precursor contexts that were harmonic complexes with frequency-modulated resonances. Precursors filtered to match F(2) or tilt of following vowels induced perceptual calibration (diminished influence) to F(2) and tilt, respectively. Calibration to F(2) was greatest for shorter duration precursors (250 ms), which implicates physiologic and/or perceptual mechanisms that are sensitive to onsets. In contrast, calibration to tilt was greatest for precursors with longer durations and higher repetition rates because greater opportunities to sample the spectrum result in more stable estimates of long-term global spectral properties. Possible mechanisms that promote sensitivity to change are discussed.

The efferent system or olivocochlear function bundle - fine regulator and protector of hearing perception

The efferent system of the ear possesses several distinct functions, in particular noise protection, mediation of selective attention and improvement of signal to noise ratio. It also supports adaptation and frequency selectivity by modification of the micromechanical properties of outer hair cells. There are many differences in anatomy and physiology between the medial and lateral olivocochlear system suggesting that they are functionally separate systems. The efferent system is affected by inner ear stressors, e.g. noise, ototoxic drugs, and might play a key role in tinnitus generation and maintenance. The anatomy, physiology and its realtionships to inner ear pathologies are discussed in this review article.

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.

The role of the terminal nerve and GnRH in olfactory system neuromodulation

Animals must regulate their sensory responsiveness appropriately with respect to their internal and external environments, which is accomplished in part via centrifugal modulatory pathways. In the olfactory sensory system, responsiveness is regulated by neuromodulators released from centrifugal fibers into the olfactory epithelium and bulb. Among the modulators known to modulate neural activity of the olfactory system, one of the best understood is gonadotropin-releasing hormone (GnRH). This is because GnRH derives mainly from the terminal nerve (TN), and the TN-GnRH system has been suggested to function as a neuromodulator in wide areas of the brain, including the olfactory bulb. In the present article we examine the modulatory roles of the TN and GnRH in the olfactory epithelium and bulb as a model for understanding the ways in which olfactory responses can be tuned to the internal and external environments.

[Is the middle ear the first filter of frequency selectivity?]

INTRODUCTION AND OBJECTIVES

The cochlea has traditionally been considered as the first frequency selection filter in the auditory pathway due to the contraction of its external ciliated cells. Yet, much evidence has emerged from work carried out during experiments with animals, some of which is anatomical (connections between the auditory pathway and motor nuclei of the middle ear muscles) and other physiological, which indicates that the middle ear might be the first filter through which specific sounds from noisy environments may initially be isolated.

METHODS

In cooperation with the Department of Mechanical Engineering of the Technical School of Industrial Engineering at the University of Valladolid (UVa) we have developed and refined a new admittance meter capable of evaluating changes in impedance that occur in the human middle ear depending on frequency. Using this device we have measured variation in impedance in 7 otologically healthy volunteers submitted to a varied range of sound environments.

RESULTS

We have found that hearing impedance is not constant but rather that the attention offered by the examined subjects when following a conversation in a noisy environment leads to variations in hearing impedance at high frequencies.

CONCLUSIONS

In the light of these findings we feel that the middle ear does not play a merely passive role in hearing but rather that the contraction of the endotympanic muscles makes possible variations in impedance such that the resonance frequency of the ear shifts towards higher frequencies, thus enhancing sound discrimination in noisy environments.

Auditory color constancy: calibration to reliable spectral properties across nonspeech context and targets

Brief experience with reliable spectral characteristics of a listening context can markedly alter perception of subsequent speech sounds, and parallels have been drawn between auditory compensation for listening context and visual color constancy. In order to better evaluate such an analogy, the generality of acoustic context effects for sounds with spectral-temporal compositions distinct from speech was investigated. Listeners identified nonspeech sounds-extensively edited samples produced by a French horn and a tenor saxophone-following either resynthesized speech or a short passage of music. Preceding contexts were "colored" by spectral envelope difference filters, which were created to emphasize differences between French horn and saxophone spectra. Listeners were more likely to report hearing a saxophone when the stimulus followed a context filtered to emphasize spectral characteristics of the French horn, and vice versa. Despite clear changes in apparent acoustic source, the auditory system calibrated to relatively predictable spectral characteristics of filtered context, differentially affecting perception of subsequent target nonspeech sounds. This calibration to listening context and relative indifference to acoustic sources operates much like visual color constancy, for which reliable properties of the spectrum of illumination are factored out of perception of color.

Development of hyperactivity after hearing loss in a computational model of the dorsal cochlear nucleus depends on neuron response type

Cochlear damage can change the spontaneous firing rates of neurons in the dorsal cochlear nucleus (DCN). Increased spontaneous firing rates (hyperactivity) after acoustic trauma have been observed in the DCN of rodents such as hamsters, chinchillas and rats. This hyperactivity has been interpreted as a neural correlate of tinnitus. In cats, however, the spontaneous firing rates of DCN neurons were not significantly elevated after acoustic trauma. Species-specific spontaneous firing rates after cochlear damage might be attributable to differences in the response types of DCN neurons: In gerbils, type III response characteristics are predominant, whereas in cats type IV responses are more frequent. To address the question of how the development of hyperactivity after cochlear damage depends on the response type of DCN neurons, we use a computational model of the basic circuit of the DCN. By changing the strength of two types of inhibition, we can reproduce salient features of the responses of DCN neurons. Simulated cochlear damage, which decreases the activity of auditory nerve fibers, is assumed to activate homeostatic plasticity in projection neurons (PNs) of the DCN. We find that the resulting spontaneous firing rates depend on the response type of DCN PNs: PNs with type III and type IV-T response characteristics may become hyperactive, whereas type IV PNs do not develop increased spontaneous firing rates after acoustic trauma. This theoretical framework for the mechanisms and circumstances of the development of hyperactivity in central auditory neurons might also provide new insights into the development of tinnitus.

Exploring efferent-mediated DPOAE adaptation in three different guinea pig strains

The aims of this study were to explore the correlation between DPOAE adaptation magnitude in three different guinea pig strains to examine if the genetic component affects the DPOAE adaptation magnitude. It was also to investigate the correlation between strains with certain characteristics i.e. reduced susceptibility to noise, and early onset of age-dependent hearing loss and the DPOAE adaptation magnitude. The animals were anaesthetized and the 2f1-f2 DPOAE (f1=8k Hz, and f2/f1=1.2) adaptation was established with a minimum of 144 combinations of f1; f2 where f1 was held fixed and f2 was varied in 1 dB or 0.4 dB steps. The DPOAE adaptation magnitude was defined as the difference between maximum positive level and the maximum negative level. ABRs were conducted at different age-groups (at 4, 6.3, and 12.5k Hz) to evaluate the progress of hearing thresholds by age. There was a significant difference between strains regarding the hearing loss at one year of age. There was no significant difference in DPOAE adaptation magnitude between strains included in this study and from this we conclude that the DPOAE adaptation magnitude is not a predictor for the susceptibility to noise trauma, or early onset of age-dependent hearing loss, using the methods described in this paper.

Selective removal of lateral olivocochlear efferents increases vulnerability to acute acoustic injury

Cochlear sensory cells and neurons receive efferent feedback from the olivocochlear (OC) system. The myelinated medial component of the OC system and its effects on outer hair cells (OHCs) have been implicated in protection from acoustic injury. The unmyelinated lateral (L)OC fibers target ipsilateral cochlear nerve dendrites and pharmacological studies suggest the LOC's dopaminergic component may protect these dendrites from excitotoxic effects of acoustic overexposure. Here, we explore LOC function in vivo by selective stereotaxic destruction of LOC cell bodies in mouse. Lesion success in removing the LOC, and sparing the medial (M)OC, was assessed by histological analysis of brain stem sections and cochlear whole mounts. Auditory brain stem responses (ABRs), a neural-based metric, and distortion product otoacoustic emissions (DPOAEs), an OHC-based metric, were measured in control and surgical mice. In cases where the LOC was at least partially destroyed, there were increases in suprathreshold neural responses that were frequency- and level-independent and not attributable to OHC-based effects. These interaural response asymmetries were not found in controls or in cases where the lesion missed the LOC. In LOC-lesion cases, after exposure to a traumatic stimulus, temporary threshold shifts were greater in the ipsilateral ear, but only when measured in the neural response; OHC-based measurements were always bilaterally symmetric, suggesting OHC vulnerability was unaffected. Interaural asymmetries in threshold shift were not found in either unlesioned controls or in cases that missed the LOC. These findings suggest that the LOC modulates cochlear nerve excitability and protects the cochlea from neural damage in acute acoustic injury.

Neural population coding of sound level adapts to stimulus statistics

Mammals can hear sounds extending over a vast range of sound levels with remarkable accuracy. How auditory neurons code sound level over such a range is unclear; firing rates of individual neurons increase with sound level over only a very limited portion of the full range of hearing. We show that neurons in the auditory midbrain of the guinea pig adjust their responses to the mean, variance and more complex statistics of sound level distributions. We demonstrate that these adjustments improve the accuracy of the neural population code close to the region of most commonly occurring sound levels. This extends the range of sound levels that can be accurately encoded, fine-tuning hearing to the local acoustic environment.