The influence of the cochlear efferent system on chronic acoustic trauma

Medial Olivocochlear Reflex Strength in Ears With Low-to-Moderate Annual Noise Exposure

PURPOSE

Studies in lower mammals demonstrate enhancement of the medial olivocochlear reflex (MOCR) following noise exposure. A similar effect may occur in humans, and there is some evidence of an individual's acoustic history affecting the MOCR. The current work evaluates the relationship between an individual's annual noise exposure history and their MOCR strength. Given the potential role of the MOCR as a biological hearing protector, it is important to identify factors associated with MOCR strength.

METHOD

Data were collected from 98 normal-hearing young adults. Annual noise exposure history was estimated using the Noise Exposure Questionnaire. MOCR strength was assayed using click-evoked otoacoustic emissions (CEOAEs) measured with and without noise presented to the contralateral ear. MOCR metrics included the MOCR-induced otoacoustic emission (OAE) magnitude shift and phase shift. A CEOAE signal-to-noise ratio (SNR) of at least 12 dB was required for estimation of the MOCR metrics. Linear regression was applied to evaluate the relationship between MOCR metrics and annual noise exposure.

RESULTS

Annual noise exposure was not a statistically significant predictor of the MOCR-induced CEOAE magnitude shift. However, annual noise exposure was a statistically significant predictor of the MOCR-induced CEOAE phase shift-the MOCR-induced phase shift decreased with increasing noise exposure. Additionally, annual noise exposure was a statistically significant predictor of OAE level.

CONCLUSIONS

Findings contrast with recent work that suggests MOCR strength increases with annual noise exposure. Compared with previous work, data for this study were collected using more stringent SNR criteria, which is expected to increase the precision of the MOCR metrics. Additionally, data were collected for a larger subject population with a wider range of noise exposures. Whether findings generalize to other exposure durations and levels is unknown and requires future study.

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

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

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.

Dose-Dependent Pattern of Cochlear Synaptic Degeneration in C57BL/6J Mice Induced by Repeated Noise Exposure

It is widely accepted that even a single acute noise exposure at moderate intensity that induces temporary threshold shift (TTS) can result in permanent loss of ribbon synapses between inner hair cells and afferents. However, effects of repeated or chronic noise exposures on the cochlear synapses especially medial olivocochlear (MOC) efferent synapses remain elusive. Based on a weeklong repeated exposure model of bandwidth noise over 2-20 kHz for 2 hours at seven intensities (88 to 106 dB SPL with 3 dB increment per gradient) on C57BL/6J mice, we attempted to explore the dose-response mechanism of prolonged noise-induced audiological dysfunction and cochlear synaptic degeneration. In our results, mice repeatedly exposed to relatively low-intensity noise (88, 91, and 94 dB SPL) showed few changes on auditory brainstem response (ABR), ribbon synapses, or MOC efferent synapses. Notably, repeated moderate-intensity noise exposures (97 and 100 dB SPL) not only caused hearing threshold shifts and the inner hair cell ribbon synaptopathy but also impaired MOC efferent synapses, which might contribute to complex patterns of damages on cochlear function and morphology. However, repeated high-intensity (103 and 106 dB SPL) noise exposures induced PTSs mainly accompanied by damages on cochlear amplifier function of outer hair cells and the inner hair cell ribbon synaptopathy, rather than the MOC efferent synaptic degeneration. Moreover, we observed a frequency-dependent vulnerability of the repeated acoustic trauma-induced cochlear synaptic degeneration. This study provides a sight into the hypothesis that noise-induced cochlear synaptic degeneration involves both afferent (ribbon synapses) and efferent (MOC terminals) pathology. The pattern of dose-dependent pathological changes induced by repeated noise exposure at various intensities provides a possible explanation for the complicated cochlear synaptic degeneration in humans. The underlying mechanisms remain to be studied in the future.

Auditory brainstem responses in adults with autism spectrum disorder

OBJECTIVE

To investigate possible differences in the auditory peripheral and brainstem functions between adults with autism spectrum disorder (ASD) and neurotypical (NT) adults.

METHODS

Click-evoked auditory brainstem responses (ABRs) were obtained from 17 high-functioning ASD adults (aged 21-38 years) and 20 NT adults (aged 22-36 years). A relatively large number of stimulus presentations (6000) were adopted, and ABRs by horizontal and vertical electrode montages were evaluated, in order to allow precise evaluations of early ABR components.

RESULTS

Waves I, II, III, and V were identified in the vertical electrode montage, and wave I and the summating potential (SP) in electrocochleograms were identified in the horizontal electrode montage. There were no significant group differences in the wave I, II, III, and V latencies or the interpeak latencies (IPLs) in the vertical electrode montage. In the horizontal montage, the ASD adults exhibited significantly shortened SP latencies compared with the NT adults, whereas there was no significant group difference in the wave I latency.

CONCLUSION

The ASD adults may have the abnormalities of processing more in the peripheral auditory system than in the brainstem.

SIGNIFICANCE

The current study suggests that the peripheral abnormality is associated with ASD.

Distortion-product otoacoustic emissions in workers exposed to urban stressors

The study aimed to compare the distortion-product otoacoustic emissions (DPOAEs) with the pure-tone audiometry and to analyze the changes in participants exposed to noise. The results show a prevalence of participants with impaired DPOAEs higher than the prevalence of participants with impaired audiometries in the total sample, in men, and in outdoor workers and a prevalence of impaired DPOAEs and of impaired audiometries in men higher than in women and in outdoor workers higher than in indoor workers. The comparison of mean values between outdoor and indoor workers shows a significant difference only on some frequencies, in the DP-gram. The results suggest a higher effectiveness of DPOAEs compared with the pure-tone audiometry in identifying the presence of any damage in individuals exposed to noise at an early stage.

Musicianship enhances ipsilateral and contralateral efferent gain control to the cochlea

Human hearing sensitivity is easily compromised with overexposure to excessively loud sounds, leading to permanent hearing damage. Consequently, finding activities and/or experiential factors that distinguish "tender" from "tough" ears (i.e., acoustic vulnerability) would be important for identifying people at higher risk for hearing damage. To regulate sound transmission and protect the inner ear against acoustic trauma, the auditory system modulates gain control to the cochlea via biological feedback of the medial olivocochlear (MOC) efferents, a neuronal pathway linking the lower brainstem and cochlear outer hair cells. We hypothesized that a salient form of auditory experience shown to have pervasive neuroplastic benefits, namely musical training, might act to fortify hearing through tonic engagement of these reflexive pathways. By measuring MOC efferent feedback via otoacoustic emissions (cochlear emitted sounds), we show that dynamic ipsilateral and contralateral cochlear gain control is enhanced in musically-trained individuals. Across all participants, MOC strength was correlated with the years of listeners' training suggested that efferent gain control is experience dependent. Our data provide new evidence that intensive listening experience(s) (e.g., musicianship) can strengthen the ipsi/contralateral MOC efferent system and sound regulation to the inner ear. Implications for reducing acoustic vulnerability to damaging sounds are discussed.

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.

Adaptation of distortion product otoacoustic emissions predicts susceptibility to acoustic over-exposure in alert rabbits

A noninvasive test was developed in rabbits based on fast adaptation measures for 2f1-f2 distortion-product otoacoustic emissions (DPOAEs). The goal was to evaluate the effective reflex activation, i.e., "functional strength," of both the descending medial olivocochlear efferent reflex (MOC-R) and the middle-ear muscle reflex (MEM-R) through sound activation. Classically, it is assumed that both reflexes contribute toward protecting the inner ear from cochlear damage caused by noise exposure. The DP-gram method described here evaluated the MOC-R effect on DPOAE levels over a two-octave (oct) frequency range. To estimate the related activation of the middle-ear muscles (MEMs), the MEM-R was measured by monitoring the level of the f1-primary tone throughout its duration. Following baseline measures, rabbits were subjected to noise over-exposure. A main finding was that the measured adaptive activity was highly variable between rabbits but less so between the ears of the same animal. Also, together, the MOC-R and MEM-R tests showed that, on average, DPOAE adaptation consisted of a combined contribution from both systems. Despite this shared involvement, the amount of DPOAE adaptation measured for a particular animal's ear predicted that ear's subsequent susceptibility to the noise over-exposure for alert but not for deeply anesthetized rabbits.

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

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

Alternative splice isoforms of small conductance calcium-activated SK2 channels differ in molecular interactions and surface levels

Small conductance Ca²⁺-sensitive potassium (SK2) channels are voltage-independent, Ca²⁺-activated ion channels that conduct potassium cations and thereby modulate the intrinsic excitability and synaptic transmission of neurons and sensory hair cells. In the cochlea, SK2 channels are functionally coupled to the highly Ca²⁺ permeant α9/10-nicotinic acetylcholine receptors (nAChRs) at olivocochlear postsynaptic sites. SK2 activation leads to outer hair cell hyperpolarization and frequency-selective suppression of afferent sound transmission. These inhibitory responses are essential for normal regulation of sound sensitivity, frequency selectivity, and suppression of background noise. However, little is known about the molecular interactions of these key functional channels. Here we show that SK2 channels co-precipitate with α9/10-nAChRs and with the actin-binding protein α-actinin-1. SK2 alternative splicing, resulting in a 3 amino acid insertion in the intracellular 3′ terminus, modulates these interactions. Further, relative abundance of the SK2 splice variants changes during developmental stages of synapse maturation in both the avian cochlea and the mammalian forebrain. Using heterologous cell expression to separately study the 2 distinct isoforms, we show that the variants differ in protein interactions and surface expression levels, and that Ca²⁺ and Ca²⁺-bound calmodulin differentially regulate their protein interactions. Our findings suggest that the SK2 isoforms may be distinctly modulated by activity-induced Ca²⁺ influx. Alternative splicing of SK2 may serve as a novel mechanism to differentially regulate the maturation and function of olivocochlear and neuronal synapses.

Auditory cortex electrical stimulation suppresses tinnitus in rats

Recent clinical studies have demonstrated that auditory cortex electrical stimulation (ACES) has yielded promising results in the suppression of patients' tinnitus. However, the large variability in the efficacy of ACES-induced suppression across individuals has hindered its development into a reliable therapy. Due to ethical reasons, many issues cannot be comprehensively addressed in patients. In order to search for effective stimulation targets and identify optimal stimulation strategies, we have developed the first rat model to test for the suppression of behavioral evidence of tone-induced tinnitus through ACES. Our behavioral results demonstrated that electrical stimulation of all channels (frequency bands) in the auditory cortex significantly suppressed behavioral evidence of tinnitus and enhanced hearing detection at the central level. Such suppression of tinnitus and enhancement of hearing detection were respectively demonstrated by a reversal of tone exposure compromised gap detection at 10-12, 14-16, and 26-28 kHz and compromised prepulse inhibition at 10-12 and 26-28 kHz. On the contrary, ACES did not induce behavioral changes in animals that did not manifest any behavioral evidence of tinnitus and compromised hearing detection following the same tone exposure. The results point out that tinnitus may be more related to compromised central auditory processing than hearing loss at the peripheral level. The ACES-induced suppression of behavioral evidence of tinnitus may involve restoration of abnormal central auditory processing.

Auditory function and hearing loss in children and adults with Williams syndrome: cochlear impairment in individuals with otherwise normal hearing

Hearing loss is common in school-age individuals with Williams syndrome (WS) and extensive in adults. Prior studies with relatively small sample sizes suggest that hearing loss in WS has an early onset and may be progressive, yet the auditory phenotype and the scope of the hearing loss have not been adequately characterized. We used standard audiometric tools: Otoscopy, tympanometry, air-conduction (bone conduction when available) behavioral testing, and distortion product otoacoustic emissions (DPOAEs) to measure hearing sensitivity and outer hair cell function. We tested 81 individuals with WS aged 5.33-59.50 years. Sixty-three percent of the school-age and 92% of the adult participants had mild to moderately-severe hearing loss. The hearing loss in at least 50% was sensorineural. DPOAE testing corroborated behavioral results. Strikingly, 12 of 14 participants with hearing within normal limits bilaterally had 4,000-Hz DPOAE input/output (DPOAE IO) functions indicative of outer hair cell damage and impaired cochlear compression. Our results indicate that hearing loss is very common in WS. Furthermore, individuals with WS who have "normal" hearing as defined by behavioral thresholds may actually have sub-clinical impairments or undetected cochlear pathology. Our findings suggest outer hair cell dysfunction in otherwise normal hearing individuals. The DPOAE IO in this same group revealed growth functions typically seen in groups with noise-induced damage. Given this pattern of findings, individuals with WS may be at increased risk of noise-induced hearing loss. Recommendations regarding audiological testing for individuals with WS and accommodations for these individuals in both academic and nonacademic settings are provided.

Lack of nAChR activity depresses cochlear maturation and up-regulates GABA system components: temporal profiling of gene expression in alpha9 null mice

Background

It has previously been shown that deletion of chrna9, the gene encoding the α9 nicotinic acetylcholine receptor (nAChR) subunit, results in abnormal synaptic terminal structure. Additionally, all nAChR-mediated cochlear activity is lost, as characterized by a failure of the descending efferent system to suppress cochlear responses to sound. In an effort to characterize the molecular mechanisms underlying the structural and functional consequences following loss of α9 subunit expression, we performed whole-transcriptome gene expression analyses on cochleae of wild type and α9 knockout (α9^−/−) mice during postnatal days spanning critical periods of synapse formation and maturation.

Principal Findings

Data revealed that loss of α9 receptor subunit expression leads to an up-regulation of genes involved in synaptic transmission and ion channel activity. Unexpectedly, loss of α9 receptor subunit expression also resulted in an increased expression of genes encoding GABA receptor subunits and the GABA synthetic enzyme, glutamic acid decarboxylase. These data suggest the existence of a previously unrecognized association between the nicotinic cholinergic and GABAergic systems in the cochlea. Computational analyses have highlighted differential expression of several gene sets upon loss of nicotinic cholinergic activity in the cochlea. Time-series analysis of whole transcriptome patterns, represented as self-organizing maps, revealed a disparate pattern of gene expression between α9^−/− and wild type cochleae at the onset of hearing (P13), with knockout samples resembling immature postnatal ages.

Conclusions

We have taken a systems biology approach to provide insight into molecular programs influenced by the loss of nicotinic receptor-based cholinergic activity in the cochlea and to identify candidate genes that may be involved in nicotinic cholinergic synapse formation, stabilization or function within the inner ear. Additionally, our data indicate a change in the GABAergic system upon loss of α9 nicotinic receptor subunit within the cochlea.

Especially prominent cochlear microphonic activity in the auditory brainstem response

Recent recommendations to record cochlear microphonic (CM) activity in auditory brainstem response (ABR) waveforms are being driven by reports of 'especially prominent' (Starr et al, 2001, p. 92) CM activity in ABR waveforms that were absent or grossly abnormal. This paper adds to these recommendations by providing the first description of especially prominent CM activity in ABR waveforms that were present and not grossly abnormal. The implications of this description are discussed via a review of the possible non-pathophysiological and pathophysiological causes of especially prominent CM activity in auditory evoked potentials.

Expression of the neuron-specific potassium chloride cotransporter KCC2 in adult rat cochlear

Auditory transduction in the cochlear is subject to modulate higher auditory centers in the brain via the efferent systems, which provide protection against damage caused by excessive excitation during auditory over stimulation. GABA is a proven inhibitory neurotransmitter in the efferent systems in mammalian cochlear. KCC2 is a neuron-specific potassium chloride cotransporter whose role in mature central neurons is to maintain the low intracellular Cl(-) concentrations required for the hyperpolarizing responses to the inhibitory amino acids GABA and glycine. However, there is a lack of information concerning KCC2 expression in the mammalian cochlear. In this study, reverse-transcription polymerase chain reaction (RT-PCR) and immunohistochemistry were used to detect the expression and localization of KCC2 in the mammalian cochlear. The results showed that these neuron-specific KCC2 transporters were present in most spiral ganglion neurons (SGNs) corresponding to the distribution of GABA(A)Rs. In addition, less intense reactions were observed on the organ of Corti, stria vascularis, and fibrocytes of the spiral ligament. These data suggest that KCC2 may play an important role in the modulation of a GABA neurotransmitter's function in a mammalian cochlear. Moreover, the presence of KCC2 on the organ of Corti and its surrounding tissues may contribute to maintaining normal K+ cycling. It is also presumed to be related to Cl(-) transportation in hair cells.

Hyperactive auditory efferent system and lack of acoustic reflexes in Williams syndrome

The auditory efferent system and acoustic reflexes have been investigated in patients with Williams syndrome (WS). Twenty-one patients aged 6-26 years with a genetically confirmed diagnosis of WS and with reported hyperacusis were compared with 21 normally developing age-matched subjects. The medial olivocochlear (MOC) efferent system was tested by stimulation of the contralateral ear with increasing levels of white noise, while recording transient evoked otoacoustic emissions (TEOAE) in the ipsilateral ear. The suppression effect on the amplitudes of the TEOAE was computed for each contralateral stimulus level. This measure reflects the strength of the MOC efferent system. In addition, the thresholds of ipsilateral and contralateral acoustic reflexes in response to 1, 2 and 4 kHz tones as well as to broadband stimuli were also recorded. Results showed that patients with WS had a significantly higher suppression effect of the MOC reflex on TEOAE. Ipsilateral and contralateral acoustic reflexes to tonal and broadband stimuli presented at maximum stimulus intensities were absent in 62-86% of the patients with WS. In the remainder, acoustic reflexes were elicited at lower auditory sensation thresholds than in controls. Hyperexcitability of the MOC efferent system coupled with absence of acoustic reflexes may contribute to the hyperacusis in WS and the consequent high-tone hearing loss induced by environmental noise. Both measures can be used for objective detection and thus, intervention of hyperacusis in the early stages of life.

Experimental and clinical aspects of the efferent auditory system

The discovery of active mechanisms in the cochlea and the efferent auditory pathways from the brain to the cochlea demonstrated the existence of a modulation of the auditory input in the central nervous system (CNS). Otoacoustic emissions (OAEs) are weak signals that can be recorded in the ear canal and are considered a byproduct of an active process from the outer hair cells (OHCs) to the basilar membrane. The efferent auditory system plays an inhibitory role on the activity of OHCs; its stimulation reduces auditory nerve response, basilar membrane motility and OAEs amplitude. Indirect stimulation by contralateral sound is also inhibitory; a reduction of OAEs amplitude can be recorded and such an effect disappears after olivocochlear bundle section. The efferent system seems to play a role in detection of signals in noise, protection in noise-induced cochlear damage, development of hearing and processing of complex auditory signals. With respect to clinical application, OAEs suppression after contralateral auditory stimulation seems to be the only objective and non-invasive method for evaluation of the functional integrity of the medial efferent system, and, therefore, for evaluation of the structures lying along its course, at least up to the level of inferior colliculi.

Contralateral suppression of distortion product otoacoustic emissions: effect of the primary frequency in Dpgrams

The amplitude of the 2f1-f2 distortion product otoacoustic emission (DPOAE) can be suppressed by presenting contralateral acoustic stimulation. To test the hypothesis that DPOAE contralateral suppression is influenced by the primary frequency in DPgrams, DPgrams were recorded at resolutions of 1, 8, and 17 pts/octave, in the absence and presence of contralateral broadband noise (BBN). Participants were 20 normal-hearing human adults. In DPgrams with higher frequency resolutions, DPOAE suppression at amplitude peaks in DPgrams (8 pts/octave: Mean = - 0.92 dB, SD = 0.71 for BBN at 60 dB SPL; 17 pts/octave: Mean = - 0.25 to -1.44 dB, SD = 0.51 to 0.86 for BBN at 40 to 70 dB SPL, respectively) was larger than the suppression at the dips in DPgrams (8 pts/octave: Mean = - 0.13 dB, SD = 1.00; 17 pts/octave: Mean = - 0.03 to -0.73 dB, SD = 0.55 to 0.91). A larger intersubject variability in DPOAE contralateral suppression was observed at the dips. The results suggest that measuring DPOAE contralateral suppression at the primary frequencies corresponding to the peaks in DPgrams with higher frequency resolutions may improve the assessment of the efferent system function.

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.

Susceptibility to tinnitus revealed at 2 kHz range by bilateral lower DPOAEs in normal hearing subjects with noise exposure

We investigated potential markers of susceptibility to tinnitus in a group of normal hearing young pilots aged 25-35 years and with 8 +/- 5 years of aircraft noise exposure. 316 pilots were interviewed about their tinnitus status and were tested for hearing thresholds (audiograms) and distortion products otoacoustic emissions (DPOAE-grams). There was no subject with permanent tinnitus. 23% reported having occasionally perceived tinnitus after flight missions and 77% reported never having experienced tinnitus after flight missions. General discomfort in the ears to noise was higher in the occasional tinnitus group (15 vs. 6%). The major finding was that difference of susceptibility to tinnitus in normal hearing subjects exposed to noise on a daily basis seemed to be clearly related to lower DPOAEs, bilaterally, in the 1500- to 2800-kHz range. However, no difference could be observed between groups on audiograms at the 2-kHz frequency range. This study provided evidence of outer hair cell dysfunctions in normal hearing subjects exposed to noise and susceptible to tinnitus. Hypersensitivity to noise and decreased DPOAEs in a non-noise-specific frequency range support the idea of another alteration mechanism than noise itself. This point was discussed in the light of recent publications.

Influence of sympathetic fibers on noise-induced hearing loss in the chinchilla

The influence of the sympathetic efferent fibers on cochlear susceptibility to noise-induced hearing loss is still an open question. In the current study, we explore the effects of unilateral and bilateral Superior Cervical Ganglion (SCG) ablation in the chinchilla on hearing loss from noise exposure, as measured with inferior colliculus (IC) evoked potentials, distortion product otoacoustic emissions (DPOAE), and outer hair cell (OHC) loss. The SCG was isolated at the level of the bifurcation of the carotid artery and removed unilaterally in 15 chinchillas. Another eight chinchillas underwent bilateral ablation. Twelve animals were employed as sham controls. Noise exposure was a 4kHz octave band noise for 1h at 110dB SPL. Results showed improved recovery of DPOAE amplitudes after noise exposure in ears that underwent SCGectomy, as well as lower evoked potential threshold shifts relative to sham controls. Effects of SCGectomy on OHC loss were small. Results of the study suggest that sympathetic fibers do exert some influence on susceptibility to noise, but the influence may not be a major one.

Bandwidth determines modulatory effects of centrifugal pathways on cochlear hearing desensitization caused by loud sound

Centrifugal olivocochlear (OC) pathways modulate cochlear hearing losses induced in cats by loud sounds varying in bandwidth from tones to clicks and noise bands, in a variety of conditions. The general effect, always to reduce hearing damage, can be a net effect resulting from complex interactions between OC subcomponents (crossed and uncrossed OC pathways). The interactions between these subcomponents vary with type of loud sound, suggesting that sound bandwidth may be important in determining how OC pathways modulate loud sound-induced hearing loss. This dependency was examined and here it is reported that OC pathways do not alter cochlear hearing losses caused by loud noise with a 2-kHz-wide bandwidth intermediate between the loud sounds of previous studies. Increasing stimulus bandwidth even slightly more, to use a loud 3.5-kHz-wide bandwidth noise as the damaging sound, once again revealed OC modulation of cochlear hearing loss. The fact that OC pathways do not modulate cochlear hearing losses induced by loud 2-kHz-wide noise was demonstrated in three very different test conditions in which OC pathways modulate hearing losses caused by narrower or broader bandwidth sounds. This confirmed that the absence of centrifugal modulation of hearing loss to this particular sound was a robust phenomenon not related to test condition. The absence of overall centrifugal effects was also true at the level of subcomponent pathways; neither crossed nor uncrossed OC pathways individually modulated cochlear hearing losses to the loud 2-kHz-wide noise. This surprising frequency dependency has general implications for centrifugal modulation of cochlear responses.

Functional role of GABAergic innervation of the cochlea: phenotypic analysis of mice lacking GABA(A) receptor subunits alpha 1, alpha 2, alpha 5, alpha 6, beta 2, beta 3, or delta

The olivocochlear efferent system is both cholinergic and GABAergic and innervates sensory cells and sensory neurons of the inner ear. Cholinergic effects on cochlear sensory cells are well characterized, both in vivo and in vitro; however, the robust GABAergic innervation is poorly understood. To explore the functional roles of GABA in the inner ear, we characterized the cochlear phenotype of seven mouse lines with targeted deletion of a GABA(A) receptor subunit (alpha1, alpha2, alpha5, alpha6, beta2, beta3, or delta). Four of the lines (alpha1, alpha2, alpha6, and delta) were normal: there was no cochlear histopathology, and cochlear responses suggested normal function of hair cells, afferent fibers, and efferent feedback. The other three lines (alpha5, beta2, and beta3) showed threshold elevations indicative of outer hair cell dysfunction. Alpha5 and beta2 lines also showed decreased effects of efferent bundle activation, associated with decreased density of efferent terminals on outer hair cells: although the onset of this degeneration was later in alpha5 (>6 weeks) than beta2 (<6 weeks), both lines shows normal efferent development (up to 3 weeks). Two lines (beta2 and beta3) showed signs of neuropathy, either decreased density of afferent innervation (beta3) or decreased neural responses without concomitant attenuation of hair cell responses (beta2). One of the lines (beta2) showed a clear sexual dimorphism in cochlear phenotype. Results suggest that the GABAergic component of the olivocochlear system contributes to the long-term maintenance of hair cells and neurons in the inner ear.

[Correlation between the contralateral supressor effect and the auditory fatigue using transitory acoustic otoemissions]

OBJECTIVE

The purpose of this study is the identification of susceptible individuals to develop noise induced hearing loss. For it we use the capacity of the transitory otoacoustic emissions (TEOAE) for the identification of functional alterations at different levels from the auditory system.

MATERIAL AND METHODS

67 normal hearing subjects have been studied, we have recorded TEOAE in one ear in three different conditions: in normal conditions (SIN register), simultaneously to the stimulation of the contralateral cochlea (EAC register), and after stimulation we can observe (EAI register). We compare the amplitude of the TEOAE with the SIN and EAC registers to quantity the contralateral suppresor effect, and the SIN and EAI, register to study the auditory fatigue.

RESULTS

We can observe a reduction in the amplitude of the TEOAE obtained by acoustic stimulation of the ipsilateral ear (0.954 dB SPL), or after the ipsilateral acoustic stimulation (0.75 dB SPL), being able to see an inverse correlation between both values.

CONCLUSIONS

The study of the TEOAE obtained in different conditions of acoustic stimulation, ipsilateral or contralateral, and the inverse correlation obtained between the magnitude of the auditory fatigue and the contralateral suppresor effect, provides arguments for the identification of individuals susceptible of developing noise induced hearing loss.

A Guinea Pig Strain with Recessive Heredity of Deafness, Producing Normal-Hearing Heterozygotes with Resistance to Noise Trauma

A new strain of waltzing guinea pigs arose spontaneously in a guinea pig breeding facility in Germany in 1996. In addition to obvious vestibular dysfunction, the waltzing animals appear deaf already at birth. Histological analysis revealed that the waltzers lack an open scala media due to the collapse of Reissner's membrane onto the surface of the hearing organ. Subsequent breeding has shown that this strain has a recessive mode of inheritance. The homozygotes are deaf and display a waltzing behaviour throughout their lives while the heterozygotes show no significant signs of inner ear injury despite being carriers of this specific mutated gene of hearing impairment. However, the heterozygous animals offer the opportunity to study how hereditary factors interact with auditory stress. In the present study, the susceptibility of the carriers to noise was investigated. Auditory brainstem responses were obtained prior to and after noise exposure (4 kHz, 110 dB, 6 h). The carriers were significantly less affected by the noise as compared to control animals. This difference was still significant at 4 weeks following noise exposure. It is suggested that the heterozygous animals have an endogenous resistance to auditory stress.

Contextual modulation of olivocochlear pathway effects on loud sound-induced cochlear hearing desensitization

This study shows that the cochlear hearing losses [temporary threshold shifts (TTSs)] induced by traumatic sound and the effect of olivocochlear (OC) pathways to the cochlea on these hearing losses depend on the context of the sound. Background atraumatic white noise (WN) has been shown to 1) exacerbate loud-pure-tone-induced TTSs, and 2) promote the modulation of TTSs by the uncrossed OC (UOC) pathways additional to the action on TTSs, elicited by binaural loud tones themselves, by the crossed OC (COC) pathway. Here the same atraumatic WN reduced TTSs caused by loud narrow band sound. It also reduced TTS modulation by OC pathways. The UOC no longer exerted any effects on TTSs, and COC effects were significantly reduced in two discrete frequency bands: low frequencies within the narrow band ("within-band" frequencies) and high frequencies outside the band ("high-side" frequencies). COC effects were unchanged at high frequencies within the band. Despite these reductions in OC effects, because the WN itself reduced TTSs, the total effect of OC pathways and background WN now produced larger TTS reductions, especially at higher frequencies. Thus the modulatory effects of the OC pathways on TTSs depend on how background WN modulates cochlear state. It is postulated that the WN background and the OC pathways both modulate TTSs by acting on the outer hair cells, in a way that promotes the reduction of TTSs caused by the narrow band sound trauma. This joint promotion of a protective end-effect on TTSs to narrow band sound trauma contrasts against the effects seen with pure tone trauma where the same background WN exacerbated TTSs at high-side frequencies.

Temporary DPOAE level shifts, ABR threshold shifts and histopathological damage following below-critical-level noise exposures

DPOAE temporary level shift (TLS) at 2f(1)-f(2) and f(2)-f(1), ABR temporary threshold shift (TTS), and detailed histopathological findings were compared in three groups of chinchillas that were exposed for 24 h to an octave band of noise (OBN) centered at 4 kHz with a sound pressure level (SPL) of 80, 86 or 92 dB (n=3,4,6). DPOAE levels at 39 frequencies from f(1)=0.3 to 16 kHz (f(2)/f(1)=1.23; L(2) and L(1)=55, 65 and 75 dB, equal and differing by 10 dB) and ABR thresholds at 13 frequencies from 0.5 to 20 kHz were collected pre- and immediately post-exposure. The functional data were converted to pre- minus post-exposure shift and overlaid upon the cytocochleogram of cochlear damage using the frequency-place map for the chinchilla. The magnitude and frequency place of components in the 2f(1)-f(2) TLS patterns were determined and group averages for each OBN SPL and L(1), L(2) combination were calculated. The f(2)-f(1) TLS was also examined in ears with focal lesions equal to or greater than 0.4 mm. The 2f(1)-f(2) TLS (plotted at f(1)) and TTS aligned with the extent and location of damaged supporting cells. The TLS patterns over frequency had two features which were unexpected: (1) a peak at about a half octave above the center of the OBN with a valley just above and below it and (2) a peak (often showing enhancement) at the apical boundary of the supporting-cell damage. The magnitudes of the TLS and TTS generally increased with increasing SPL of the exposure. The peaks of the TLS and TTS, as well as the peaks and valleys of the TLS pattern moved apically as the SPL of the OBN was increased. However, there was little consistency in the pattern relations with differing L(1), L(2) combinations. In addition, neither the 2f(1)-f(2) nor f(2)-f(1) TLS for any L(1), L(2) combination reliably detected focal lesions (100% OHC loss) from 0.4 to 1.2 mm in size. Often, the TLS went in the opposite direction from what would be expected at focal lesions. Recovery from TLS and TTS was also examined in seven animals. Both TLS and TTS recovered partially or completely, the magnitude depending upon exposure SPL.

Assessment of medial olivocochlear system function in pre-term and full-term newborns using a rapid test of transient otoacoustic emissions

This study was conducted to investigate maturation of the medial olivocochlear efferent system (MOCS) in pre- and full-term neonates using Quickscreen (Otodynamics Ltd) and to confirm previous findings on transient otoacoustic emission (TEOAE) suppression in neonates. MOCS maturation was investigated in 46 neonates born at the Chaim Sheba Medical Center, Tel Hashomer, Israel, using Quickscreen. All neonates were normal with no family history of general or auditory disease and no risk factors for hearing impairment. MOCS function appears gradually in human pre-term neonates and is considered to reach maturity shortly after term birth. The clinical value of MOCS testing in specific populations of newborns at risk for hearing and/or brainstem function can be legitimately raised as activation of the MOCS clearly alters cochlear output. The present results can be interpreted to support the testing of infants at risk of developing abnormal MOCS function using a commercially available rapid TEOAE measurement system.

Time course of efferent fiber and spiral ganglion cell degeneration following complete hair cell loss in the chinchilla

Ethacrynic acid (EA) is known to interact with aminoglycoside antibiotics such as gentamicin (GM). In the chinchilla, co-administration of GM and EA can produce hair cell lesions ranging from a small loss of outer hair cells (OHCs) in the base of the cochlea to complete destruction of all hair cells, depending on dosing parameters. Although hair cell loss has been characterized, little is known about the fate of efferent fibers or spiral ganglion neurons (SGNs) in this model. To study the time course of efferent fiber and SGN loss, chinchillas were injected with GM (125 mg/kg IM) followed immediately by EA (40 mg/kg IV). Estimates of efferent fiber loss and density changes were made after 3 days or 1, 2, 3, or 4 weeks of survival. Estimates of SGN loss and density changes were made after 15 days or 1, 2, 4, or 6 months of survival. Cochlear function was rapidly abolished and all cochlear hair cells were missing within 24 h after treatment. Inner hair cells (IHCs) in the middle turn of the cochlea died earlier than cells in the apex or base, and OHCs in Rows 1 and 2 died earlier than OHCs in Row 3. Degeneration of efferent nerve fibers began 3-7 days post-injection, versus 15-30 days for SGNs, and the loss of efferent fibers was essentially complete within 1 month, versus 2-4 months for SGNs. The rapid time course of efferent fiber and SGN loss in the chinchilla may make it a practical model for studying mechanisms of neural loss and survival in the mammalian inner ear.

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

The medial olivocochlear (MOC) efferent system is an important component of an active mechanical outer hair cell system in mammals. An extensive neurophysiological literature demonstrates that the MOC system attenuates the response of the cochlea to sound by reducing the gain of the outer hair cell mechanical response to stimulation. Despite a growing understanding of MOC physiology, the biological role of the MOC system in mammalian audition remains uncertain. Some evidence suggests that the MOC system functions in a protective role by acting to reduce receptor damage during intense acoustic exposure. For the MOC system to have evolved as a protective mechanism, however, the inner ears of mammals must be exposed to potentially damaging sources of noise that can elicit MOC-mediated protective effects under natural conditions. In this review, we evaluate the possibility that the MOC system evolved to protect the inner ear from naturally occurring environmental noise. Our survey of nonanthropogenic noise levels shows that while sustained sources of broadband noise are found in nearly all natural acoustic environments, frequency-averaged ambient noise levels in these environments rarely exceed 70 dB SPL. Similarly, sources reporting ambient noise spectra in natural acoustic environments suggest that noise levels within narrow frequency bands are typically low in intensity (<40 dB SPL). Only in rare instances (e.g., during frog choruses) are ambient noise levels sustained at moderately high intensities (~70-90 dB SPL). By contrast, all experiments in which an MOC-mediated protective effect was demonstrated used much higher sound intensities to traumatize the cochlea (100-150 dB SPL). This substantial difference between natural ambient noise levels and the experimental conditions necessary to evoke MOC-mediated protection suggests that even the noisiest natural acoustic environments are not sufficiently intense to have selected for the evolution of the MOC system as a protective mechanism. Furthermore, although relatively intense noise environments do exist in nature, they are insufficiently distributed to account for the widespread distribution of the MOC system in mammals. The paucity of high-intensity noise and the near ubiquity of low-level noise in natural environments supports the hypothesis that the MOC system evolved as a mechanism for "unmasking" biologically significant acoustic stimuli by reducing the response of the cochlea to simultaneous low-level noise. This suggested role enjoys widespread experimental support.

Activation of tyrosine hydroxylase in the lateral efferent terminals by sound conditioning

Preconditioning to sound is a well-documented strategy to provide protections against a subsequent acoustic trauma. In the present study, preconditioning (1.0 kHz tone at 81 dB sound pressure level (SPL) for 24 h) protected ABR thresholds by 17-28 dB from an acoustic trauma (2.7 kHz, 103 dB SPL, 30 min) that resulted in a temporary threshold shift. The protection afforded by sound conditioning was shown to be blocked by the administration of 6-hydroxydopamine which disrupts tyrosine hydroxylase in the nerve terminals of the lateral efferent fibers. Furthermore, tyrosine hydroxylase immunoreactivity was up-regulated both by sound conditioning alone, and by the combined treatment of sound conditioning and acoustic trauma. In contrast, acoustic trauma alone resulted in a reduction in tyrosine hydroxylase immunoreactivity compared to unexposed controls. These findings are the first demonstration that tyrosine hydroxylase in the lateral efferents are up-regulated during sound conditioning and suggests a role for the lateral efferent system in protecting against acoustic trauma by sound conditioning.

Patterns of GABA-like immunoreactivity in efferent fibers of the human cochlea

Olivocochlear efferent neurons originate in the superior olivary complex of the brainstem and terminate within sensory cell regions of the organ of Corti. Components of this complex include the lateral olivocochlear bundle whose unmyelinated axons synapse with radial afferent dendrites below inner hair cells and the medial olivocochlear bundle, from which myelinated axons form a direct synaptic contact with outer hair cells. gamma-Aminobutyric acid (GABA), a major neurotransmitter of the central nervous system believed to be responsible for most fast-inhibitory transmissions, has been demonstrated with interspecies variation between mammal and primate auditory efferents. In the present study, we evaluate the immunocytochemical presence of GABA in 10 human cochleae using light and electron microscopy. GABA-like immunostaining could be observed in inner spiral fibers, tunnel spiral fibers, tunnel-crossing fibers, and at efferent endings synapsing with outer hair cells. To approximate medial efferent fiber quantifications, we counted labeled terminals at the base of each outer hair cell and then compared this sum with the number of tunnel crossing fibers. We found a 'branching ratio' of 1:2 implicating a doubling in quantifiable efferent fibers at the level of the outer hair cell. In human, the distribution of GABA-like immunoreactivity showed a consistent presence throughout all turns of the cochlea. A new method for application of immunoelectron microscopy on human cochleae using a pre-embedding technique is also presented and discussed.

The effect of vestibular nerve section upon tinnitus

This paper reviews the published evidence regarding the effect of vestibular nerve section upon tinnitus. This is of relevance not only for those performing and undergoing this procedure, but also for those considering the hypothesis that auditory efferent system dysfunction may be influential in tinnitus perception. The auditory medial efferent fibres within the internal auditory canal run within the inferior vestibular nerve, only joining the cochlear nerve at the anastomosis of Oort, a bundle of 1300 fibres running from the saccular branch of the inferior vestibular nerve to the cochlear nerve. Vestibular nerve section procedures therefore section this efferent olivocochlear pathway, and ablate efferent influence upon that cochlear. If auditory efferent dysfunction is involved in tinnitus perception, this ablation might influence the tinnitus status of that patient. A literature search identified 18 papers mentioning tinnitus status after vestibular nerve section, describing the experiences of a total of 1318 patients. The proportion of patients in whom tinnitus was said to be exacerbated postoperatively ranged from 0% to 60%, with a mean of 16.4% (standard deviation 14.0). The proportion of patients in whom tinnitus was unchanged was 17% to 72% (mean 38.5%, standard deviation 15.6), and in whom tinnitus was said to be improved was 6% to 61% (mean 37.2%, standard deviation 15.2). In the majority of patients undergoing this procedure, ablation of auditory efferent input (and thus total efferent dysfunction) to the cochlea was not associated with an exacerbation of tinnitus. The finding of this review is that efferent dysfunction after vestibular nerve section does not consistently worsen tinnitus.

Variation in inter-animal susceptibility to noise damage is associated with alpha 9 acetylcholine receptor subunit expression level

Large intersubject variabilities in acoustic injury are known to occur in both humans and animals; however, the mechanisms underlying such differences are poorly understood. The olivocochlear efferent system has been hypothesized to play a significant role in protecting the cochlea from noise overexposure. In this study, we demonstrate that a newly developed test for determining average efferent system strength can predict intersubject variations in acoustic injury. In addition, the intersubject variability in cochlear expression of the alpha9 subunit of the nicotinic acetylcholine receptor was found to be proportional to an animals average efferent strength. Therefore, the inter-animal variability in the alpha9-containing acetylcholine receptor expression may be one mechanism contributing to the inter-animal variability in acoustic injury.

Response adaptation of medial olivocochlear neurons is minimal

Response adaptation is a general characteristic of neurons. A number of studies have investigated the adaptation characteristics of auditory-nerve fibers, which send information to the brain about sound stimuli. However, there have been no previous adaptation studies of olivocochlear neurons, which provide efferent fibers to hair cells and auditory nerve dendrites in the auditory periphery. To study adaptation in efferent fibers, responses of single olivocochlear neurons were recorded to characteristic-frequency tones and noise, using anesthetized guinea pigs. To measure short-term adaptation, stimuli of 500 ms duration were presented, and the responses were displayed as peristimulus time histograms. These histograms showed regular peaks, indicating a "chopping" pattern of response. The rate during each chopping period as well as the general trend of the histogram could be well fit by an equation that expresses the firing rate as a sum of 1) a short-term adaptive rate that decays exponentially with time and 2) a constant steady-state rate. For the adaptation in medial olivocochlear (MOC) neurons, the average exponential time constant was 47 ms, which is roughly similar to that for short-term adaptation in auditory-nerve fibers. The amount of adaptation (expressed as a percentage decrease of onset firing rate), however, was substantially less in MOC neurons (average 31%) than in auditory-nerve fibers (average 63%). To test for adaptation over longer periods, we used noise and tones of 10 s duration. After the short-term adaptation, the responses of MOC neurons were almost completely sustained (average long-term adaptation 3%). However, in the same preparations, significant long-term adaptation was present in auditory-nerve fibers. These results indicate that the MOC response adaptation is minimal compared with that of auditory-nerve fibers. Such sustained responses may enable the MOC system to produce sustained effects in the periphery, supporting a role for this efferent system during ongoing stimuli of long duration.

Noise priming and the effects of different cochlear centrifugal pathways on loud-sound-induced hearing loss

Priming/conditioning the cochlea with moderately loud sound can reduce damage caused by subsequent loud sound. This study examined immediate effects of short-term priming with monaural broadband noise on temporary threshold shifts (TTSs) in hearing caused by a subsequent loud high-frequency tone and the role of centrifugal olivocochlear pathways. Priming caused delay-dependent changes in tone-induced TTSs, particularly or only at frequencies higher than the peak tone-affected frequency, through two general effects: a short-lasting increase in cochlear susceptibility to loud sound and longer-lasting complex end effects of centrifugal pathways. The results indicated the following points. Priming noise had "pure" cochlear effects, outlasting its presentation and declining with delay, that exacerbated tone-induced TTSs at frequencies higher than the peak tone-affected frequency. The centrifugal uncrossed medial olivocochlear system (UMOCS) could prevent this noise exacerbation and as this noise effect declined, could even reduce tone-induced TTSs below those to the unprimed tone. For longer delays, when priming noise no longer had any exacerbative "pure" cochlear effects on TTSs, UMOCS exacerbated TTSs above those to the unprimed tone. The crossed medial olivocochlear system (CMOCS) appeared to show a gradual "build-up" of effects postpriming. A parallel study showed it exercised no end effect on TTSs when noise and tone were concurrent. With priming, CMOCS effects were observed. For the shortest priming delay, the CMOCS blocked a UMOCS effect preventing noise exacerbation of tone-induced TTSs. For longer delays, CMOCS end effects, when present, reduced tone-induced TTSs below those to the unprimed tone. The CMOCS may oscillate between producing these effects and exerting no end-effect. With increasing delay CMOCS protection occurred in a greater proportion of animals. Finally, with a delay of 600 s between primer and loud tone, all these systems appeared to have reset to normal so that TTSs were similar to those in the unprimed condition. Thus the effects of short-term priming are not simple and do not suggest that centrifugal pathways act automatically as a protective system during such priming.

Contralateral Suppression of Transient Evoked Otoacoustic Emissions: Normative Data for a Clinical Test Set-Up

HYPOTHESIS

Contralateral suppression of transient evoked otoacoustic emissions (TEOAEs) can be used in a clinical set-up using a procedure based on a unique and robust parameter to quantify the magnitude of suppression for a subject.

BACKGROUND

TEOAEs can be suppressed by delivering contralateral white noise (WN). This suppression is thought to be mediated via the efferent nerve fibers that innervate the outer hair cells. The ipsilateral TEOAE-eliciting click stimulus level and the contralateral WN level have a strong impact on the recorded level of suppression.

METHODS

TEOAEs were recorded using the nonlinear stimulation mode in two conditions (with and without contralateral WN). An optimal TEOAE-eliciting click stimulus level and contralateral WN level were defined to obtain a unique and robust parameter to quantify the magnitude of suppression.

RESULTS

Suppression of TEOAEs with contralateral WN can be measured in a clinical set-up using nonlinear stimulation, and the level of suppression is of the same order of magnitude as measures using the linear stimulation recording mode. The level of suppression appears to be "locked" to the interaural difference between ipsilateral TEOAE-eliciting broadband click stimulus level and the contralateral WN level.

CONCLUSIONS

A procedure is proposed to record contralateral suppression in a clinical set-up, and normative data are given for a normal-hearing population (n = 60).

Unilateral hearing losses alter loud sound-induced temporary threshold shifts and efferent effects in the normal-hearing ear

In animals with bilaterally normal hearing, olivocochlear pathways can protect the cochlea from the temporary shifts in hearing sensitivity (temporary threshold shifts; TTSs) caused by short-duration intense loud sounds. The crossed olivocochlear pathway provides protection during binaural loud sound, and uncrossed pathways protect when monaural or binaural loud sounds occur in noise backgrounds. Here I demonstrate that when there is a chronic unilateral hearing loss, effects of loud sounds, and efferent effects on loud sound, in the normal-hearing ear differ markedly from normal. Three categories of test animals with unilateral hearing loss were tested for effects at the normal-hearing ear. In all categories a monaural loud tone to the normal-hearing ear produced lower-than-normal TTSs, apparently because of a tonic re-setting of that ear's susceptibility to loud sound. Second, in the two test categories in which the hearing-loss ear was only partly damaged, binaural loud sound exacerbated TTSs in the normal-hearing ear because it caused threshold shifts that were a combination of "pure" TTSs and uncrossed efferent suppression of cochlear sensitivity. (In normal cats, this binaural tone results in crossed olivocochlear protection that reduces TTS.) Binaural loud sound did not produce such uncrossed efferent effects in the test category in which the nontest ear had suffered total hearing loss, suggesting that this uncrossed efferent effect required binaural input to the CNS. It is noteworthy that, in the absence of this uncrossed efferent suppression, the pure loud sound-alone induced TTSs after binaural exposure were low. Thus in the absence of any efferent effect, the normal-hearing cochlea had a reduced susceptibility to loud tone-induced damage. Finally, the results suggest that, with respect to cochlear actions at high sound levels, uncrossed and crossed efferent pathways may exert different effects at the one type of receptor cell.

Plasticity of the superior olivary complex

The superior olivary complex (SOC) is part of the auditory brainstem of the vertebrate brain. Residing ventrally in the rhombencephalon, it receives sensory signals from both cochleae through multisynaptic pathways. Neurons of the SOC are also a target of bilateral descending projections. Ascending and descending efferents of the SOC affect the processing of auditory signals on both sides of the brainstem and in both organs of Corti. The pattern of connectivity indicates that the SOC fulfills functions of binaural signal integration serving sound localization. But whereas many of these connectional features are shared with the inferior colliculus (with the important exception of a projection to the inner ear), cellular and molecular investigations have shown that cells residing in SOC are unique in several respects. Unlike those of other auditory brainstem nuclei, they specifically express molecules known to be involved in development, plasticity, and learning (e.g., GAP-43 mRNA, specific subunits of integrin). Moreover, neurons of the SOC in adult mammals respond to various kinds of hearing impairment with the expression of plasticity-related substances (e.g., GAP-43, c-Jun, c-Fos, cytoskeletal elements), indicative of a restructuring of auditory connectivity. These observations suggest that the SOC is pivotal in the developmental and adaptive tuning of binaural processing in young and adult vertebrates.

Effects of industrial noise exposure on distortion product otoacoustic emissions (DPOAEs) and hair cell loss of the cochlea--long term experiments in awake guinea pigs

Distortion product otoacoustic emissions (DPOAEs), a sensitive detector of outer hair cell (OHC) function, cochlear microphonics (CM), and hair cell loss have been monitored in 12 awake guinea pigs before and after 2 h exposure to specific, played-back industrial noise (105 dB SPL maximal intensity). All animals had stable DPOAE levels before noise exposure. In the first hours after noise exposure DPOAE levels were reduced significantly. In about 70% a partial recovery of the DPOAEs was found within 4 months after noise exposure. In 16% of the investigated ears no recovery of DPOAEs was observed. However, in a few ears increased DPOAEs were observed after noise exposure. Exposure to industrial noise caused both morphological changes in the middle turns of the cochlea and electrophysiological changes in the middle frequency range. A close correlation existed between reduced DPOAE levels, loss in CM potentials, and area of damaged or lost OHCs, but not with the numbers of damaged or lost OHCs in the cochlea. It can be concluded that continuous industrial noise causes a damage to OHCs which differs form the damage caused by impulse noise.

Centrifugal pathways protect hearing sensitivity at the cochlea in noisy environments that exacerbate the damage induced by loud sound

Loud sounds damage the cochlea, the auditory receptor organ, reducing hearing sensitivity. Previous studies demonstrate that the centrifugal olivocochlear pathways can moderately reduce these temporary threshold shifts (TTSs), protecting the cochlea. This effect involves only the olivocochlear pathway component known as the crossed medial olivocochlear system pathway, originating from the contralateral brainstem and terminating on outer hair cells in the cochlea. Here I demonstrate that even moderate noise backgrounds can significantly exacerbate the cochlear TTSs induced by loud tones, but this is prevented because in such conditions there is additional activation of uncrossed olivocochlear pathways, enhancing protection of cochlear hearing sensitivity. Activation of the uncrossed pathways differs from that of the crossed pathway in that it is achieved only in noise backgrounds but can then be obtained under monaural conditions of loud tone and background noise. In contrast, activation of the crossed pathway is achieved only by binaural loud tones and is not further enhanced by background noise. Thus, conjoint activation of both crossed and uncrossed efferent pathways can occur in noise backgrounds to powerfully protect the cochlea under conditions similar to those encountered naturally by humans.

Predicting vulnerability to acoustic injury with a noninvasive assay of olivocochlear reflex strength

Permanent noise-induced damage to the inner ear is a major cause of hearing impairment, arising from exposures occurring during both work- and pleasure-related activities. Vulnerability to noise-induced hearing loss is highly variable: some have tough, whereas others have tender ears. This report documents, in an animal model, the efficacy of a simple nontraumatic assay of normal ear function in predicting vulnerability to acoustic injury. The assay measures the strength of a sound-evoked neuronal feedback pathway to the inner ear, the olivocochlear efferents, by examining otoacoustic emissions created by the normal ear, which can be measured with a microphone in the external ear. Reflex strength was inversely correlated with the degree of hearing loss after subsequent noise exposure. These data suggest that one function of the olivocochlear efferent system is to protect the ear from acoustic injury. This assay, or a simple modification of it, could be applied to human populations to screen for individuals most at risk in noisy environments.

Cochlear de-efferentation and impulse noise-induced acoustic trauma in the chinchilla

The olivocochlear bundle (OCB) has been shown to protect the ear from acoustic trauma induced by continuous noise or tones. The present study examines the OCB's role in the ear's response to impulse noise (150 dB pSPL, 100 impulses, 50 s total exposure duration). Successful section of the OCB was achieved through a posterior parafloccular fossa approach for the right ears of six out of 15 adult chinchillas. The left ears from the same animals served as efferent-innervated controls. Measurements of inferior colliculus evoked potentials (ICPs) showed that the de-efferented ears incurred similar temporary and permanent threshold shifts as the control ears. Twenty days after noise exposure, depressed ICP amplitudes had virtually recovered to pre-values in the control ears whereas those in the de-efferented ears remained significantly depressed. Greater loss of inner hair cells was seen in the de-efferented ears than in the control ears. Both control and de-efferented ears incurred large loss of outer hair cells, with no statistically significant differences between groups. The current data are intriguing, yielding tentative evidence to suggest that inner hair cells of de-efferented ears are more susceptible to impulse noise than those in efferented control ears. In contrast, outer hair cell vulnerability to impulse noise appears to be unaffected by de-efferentation.

Cochlear microphonics and otoacoustic emissions in chronically de-efferented chinchilla

The effects of eliminating the olivocochlear bundle (OCB) on cochlear electromechanical properties were examined by measuring cochlear microphonics (CM) and distortion product otoacoustic emissions (DPOAEs) in chronically de-efferented chinchillas. The OCB fibers to the right ears were successfully sectioned in six out of 15 adult chinchillas via a posterior paraflocular fossa approach. At the end of the experiment, these ears were histologically verified as being deprived of both lateral and medial OCB fibers. The opposite (left) ears from the animals served as controls. Following de-efferentation, changes of the inter-modulation distortion components (2f(1)-f(2), f(2)-f(1), 3f(1)-2f(2), 3f(2)-2f(1)) varied, depending on the frequencies and levels of the stimuli. DPOAE amplitudes to low-level stimuli were within the 95% confidence intervals around mean DPOAE amplitudes of the control ears at all the frequencies (1-8 kHz). At high stimulus levels, DPOAE amplitudes increased by 5-20 dB at 1 and 2 kHz while remaining in the normal range at 4 and 8 kHz. In contrast, the CM input/output functions to stimuli from 1 to 8 kHz were significantly reduced by approximately 40-50% at all input levels. The results suggest that the OCB may play a role in modulating electrical properties of the outer hair cells and in reducing the magnitude of cochlear distortion to high-level stimuli.

Long-term sound conditioning enhances cochlear sensitivity

Sound conditioning, by chronic exposure to moderate-level sound, can protect the inner ear (reduce threshold shifts and hair cell damage) from subsequent high-level sound exposure. To investigate the mechanisms underlying this protective effect, the present study focuses on the physiological changes brought on by the conditioning exposure itself. In our guinea-pig model, 6-h daily conditioning exposure to an octave-band noise at 85 dB SPL reduces the permanent threshold shifts (PTSs) from a subsequent 4-h traumatic exposure to the same noise band at 109 dB SPL, as assessed by both compound action potentials (CAPs) and distortion product otoacoustic emissions (DPOAEs). The frequency region of maximum threshold protection is approximately one-half octave above the upper frequency cutoff of the exposure band. Protection is also evident in the magnitude of suprathreshold CAPs and DPOAEs, where effects are more robust and extend to higher frequencies than those evident at or near threshold. The conditioning exposure also enhanced cochlear sensitivity, when evaluated at the same postconditioning time at which the traumatic exposure would be delivered in a protection study. Response enhancements were seen in both threshold and suprathreshold CAPs and DPOAEs. The frequency dependence of the enhancement effects differed, however, by these two metrics. For CAPs, effects were maximum in the same frequency region as those most protected by the conditioning. For DPOAEs, enhancements were shifted to lower frequencies. The conditioning exposure also enhanced both ipsilaterally and contralaterally evoked olivocochlear (OC) reflex strength, as assessed using DPOAEs. The frequency and level dependence of the reflex enhancements were consistent with changes seen in sound-evoked discharge rates in OC fibers after conditioning. However, comparison with the frequency range and magnitude of conditioning-related protection suggests that the protection cannot be completely explained by amplification of the OC reflex and the known protective effects of OC feedback. Rather, the present results suggest that sound conditioning leads to changes in the physiology of the outer hair cells themselves, the peripheral targets of the OC reflex.