Contralateral Acoustic Stimulation Modulates Low-Frequency Biasing of DPOAE: Efferent Influence on Cochlear Amplifier Operating State?

Assessing Medial Olivocochlear Reflex Strengths via Auditory Brainstem Response: Measurement and Variability in Normal-Hearing Individuals

PURPOSE

Optimal measurement settings to measure the medial olivocochlear reflex (MOCR) in humans have not yet been defined. The purpose of this study was to advance the representation of the MOCR in auditory brainstem response (ABR) as an addition to the current diagnostic portfolio.

PARTICIPANTS AND METHOD

Twelve female and 14 male normal-hearing adults participated in the study. Potential effects of a contralateral acoustic stimulus (CAS) on amplitude changes were investigated by recording ABR waveform profiles on the left side at click intensities of 50/60/70 dB nHL with and without CAS (60 dB SPL). Secondly, to detect potential chronological order influences, measurement settings were rearranged on the right side and measurements were repeated. Additionally, ABR thresholds were recorded with and without a CAS in 10 patients.

RESULTS

When the effect of contralateral suppression was analyzed on the basis of amplitude changes, there was a change under administration of the CAS signal that was statistically significant. Interestingly, the order of recordings affected the degree of amplitude change. In three out of 10 patients, reproducible suppression effects on ABR thresholds were detectable upon CAS presentation.

CONCLUSIONS

To our knowledge, this is the largest study dealing with the recording of the MOCR elicited by a contralateral noise via ABR in normal-hearing individuals. Effects of MOCR are measurable via amplitude changes upon CAS administration. Chronological orders influence the impact of this effect on amplitude changes. Optimal measurement settings have not yet been defined. However, experiments such as this study may help to further improve measurements, and thus advance the representation of the MOC reflex in ABR as an addition to the current diagnostic portfolio.

Brainstem Auditory Evoked Potentials in Raccoon Dogs (Nyctereutes procynoides)

Raccoon dogs (Nyctereutes procynoides) are canids indigenous to Eastern Asia being one of the most invasive non-native animals in Europe and potential vectors for several hazardous parasitic and viral diseases. To present, there is a lack of studies regarding objective techniques used to appreciate the integrity and functionality of the nervous system in this species. Brainstem auditory evoked potentials (BAER) is a cost-effective, quick and noninvasive technique to assess the functionality of nervous system. The aim of the study is to provide reference values and evaluates the reliability of BAER recording with needle electrodes in clinically healthy raccoon dogs. Nine individuals were investigated for BAER under xylazine and ketamine general anesthesia. Four waves (I, II, III and V) were identified and evaluated for latencies, morphologically similarity to those previously reported for other species (dogs, horses, cats, and ferret). Our data can be used in further studies to asses objectively the auditory system function in raccoon dogs.

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.

Slow oscillatory changes of DPOAE magnitude and phase after exposure to intense low-frequency sounds

Sensitive sound detection within the mammalian cochlea is performed by hair cells surrounded by cochlear fluids. Maintenance of cochlear fluid homeostasis and tight regulation of intracellular conditions in hair cells are crucial for the auditory transduction process but can be impaired by intense sound stimulation. After a short, intense low-frequency sound, the cochlea shows the previously described "bounce phenomenon," which manifests itself as slow oscillatory changes of hearing thresholds and otoacoustic emissions. In this study, distortion product otoacoustic emissions (DPOAEs) were recorded after Mongolian gerbils were exposed to intense low-frequency sounds (200 Hz, 100 dB SPL) with different exposure times up to 1 h. After all sound exposure durations, a certain percentage of recordings (up to 80% after 1.5-min-long exposure) showed oscillatory DPOAE changes, similar to the bounce phenomenon in humans. Changes were quite uniform with respect to size and time course, and they were independent from sound exposure duration. Changes showed states of hypo- and hyperactivity with either state preceding the other. The direction of changes was suggested to depend on the static position of the cochlear operating point. As assessed with DPOAEs, no indication for a permanent damage after several or long exposure times was detected. We propose that sensitivity changes occur due to alterations of the mechanoelectrical transduction process of outer hair cells. Those alterations could be induced by different challenged homeostatic processes with slow electromotility of outer hair cells being the most plausible source of the bounce phenomenon. NEW & NOTEWORTHY Low-frequency, high-intensity sound can cause slowly cycling activity changes in the mammalian cochlea. We examined the effect of low-frequency sound duration on the degree of these alterations. We found that cochlear changes showed a stereotypical biphasic pattern independent of sound exposure duration, but the probability that significant changes occurred decreased with increasing sound duration. Despite exposure durations of up to 1 h, no permanent or transient impairments of the cochlea were detected.

Olivocochlear Efferents in Animals and Humans: From Anatomy to Clinical Relevance

Olivocochlear efferents allow the central auditory system to adjust the functioning of the inner ear during active and passive listening. While many aspects of efferent anatomy, physiology and function are well established, others remain controversial. This article reviews the current knowledge on olivocochlear efferents, with emphasis on human medial efferents. The review covers (1) the anatomy and physiology of olivocochlear efferents in animals; (2) the methods used for investigating this auditory feedback system in humans, their limitations and best practices; (3) the characteristics of medial-olivocochlear efferents in humans, with a critical analysis of some discrepancies across human studies and between animal and human studies; (4) the possible roles of olivocochlear efferents in hearing, discussing the evidence in favor and against their role in facilitating the detection of signals in noise and in protecting the auditory system from excessive acoustic stimulation; and (5) the emerging association between abnormal olivocochlear efferent function and several health conditions. Finally, we summarize some open issues and introduce promising approaches for investigating the roles of efferents in human hearing using cochlear implants.

Efferent modulation of pre-neural and neural distortion products

Distortion product otoacoustic emissions (DPOAEs) and distortion product frequency following responses (DPFFRs) are respectively pre-neural and neural measurements associated with cochlear nonlinearity. Because cochlear nonlinearity is putatively linked to outer hair cell electromotility, DPOAEs and DPFFRs may provide complementary measurements of the human medial olivocochlear (MOC) reflex, which directly modulates outer hair cell function. In this study, we first quantified MOC reflex-induced DPOAE inhibition at spectral fine structure peaks in 22 young human adults with normal hearing. The f1 and f2 tone pairs producing the largest DPOAE fine structure peak for each subject were then used to evoke DPFFRs with and without MOC reflex activation to provide a related neural measure of efferent inhibition. We observed significant positive relationships between DPOAE fine structure peak inhibition and inhibition of DPFFR components representing neural phase locking to f2 and 2f1-f2, but not f1. These findings may support previous observations that the MOC reflex inhibits DPOAE sources differentially. That these effects are maintained and represented in the auditory brainstem suggests that the MOC reflex may exert a potent influence on subsequent subcortical neural representation of sound.

Efferent Modulation of Stimulus Frequency Otoacoustic Emission Fine Structure

Otoacoustic emissions, sounds generated in the inner ear, have become a convenient non-invasive tool to examine the efferent modulation of cochlear mechanics. Activation of the medial olivocochlear (MOC) efferents has been shown to alter the magnitude of these emissions. When the effects of efferent activation on the detailed spectral structures of these emissions have been examined, a shift of the spectral patterns toward higher frequencies has been reported for distortion product and spontaneous otoacoustic emissions. Stimulus frequency otoacoustic emissions (SFOAEs) have been proposed as the preferred emission type in the study of efferent modulation due to the simplicity of their production leading to the possibility of clearer interpretation of results. The effects of efferent activation on the complex spectral patterns of SFOAEs have not been examined to the best of our knowledge. We have examined the effects of activating the MOC efferents using broadband noise in normal-hearing humans. The detailed spectral structure of SFOAEs, known as fine structure, was recorded with and without contralateral acoustic stimulation. Results indicate that SFOAEs are reduced in magnitude and their fine structure pushed to higher frequencies by contralateral acoustic stimulation. These changes are similar to those observed in distortion product or spontaneous otoacoustic emissions and behavioral hearing thresholds. Taken together with observations made about magnitude and phase changes in otoacoustic emissions and hearing thresholds upon contralateral acoustic stimulation, all changes in otoacoustic emission and hearing threshold fine structure appear to be driven by a common set of mechanisms. Specifically, frequency shifts in fine structure patterns appear to be linked to changes in SFOAE phase due to contralateral acoustic stimulation.

An analysis of cochlear response harmonics: Contribution of neural excitation

In this report an analysis of cochlear response harmonics is developed to derive a mathematical function to estimate the gross mechanics involved in the in vivo transfer of acoustic sound into neural excitation (f(Tr)). In a simulation it is shown that the harmonic distortion from a nonlinear system can be used to estimate the nonlinearity, supporting the next phase of the experiment: Applying the harmonic analysis to physiologic measurements to derive estimates of the unknown, in vivo f(Tr). From gerbil ears, estimates of f(Tr) were derived from cochlear response measurements made with an electrode at the round window niche from 85 Hz tone bursts. Estimates of f(Tr) before and after inducing auditory neuropathy-loss of auditory nerve responses with preserved hair cell responses from neurotoxic treatment with ouabain-showed that the neural excitation from low-frequency tones contributes to the magnitude of f(Tr) but not the sigmoidal, saturating, nonlinear morphology.

Influence of ketamine-xylazine anaesthesia on cubic and quadratic high-frequency distortion-product otoacoustic emissions

Ketamine is a dissociative anaesthetic, analgesic drug as well as an N-methyl-D-aspartate receptor antagonist and has been reported to influence otoacoustic emission amplitudes. In the present study, we assess the effect of ketamine-xylazine on high-frequency distortion-product otoacoustic emissions (DPOAE) in the bat species Carollia perspicillata, which serves as model for sensitive high-frequency hearing. Cubic DPOAE provide information about the nonlinear gain of the cochlear amplifier, whereas quadratic DPOAE are used to assess the symmetry of cochlear amplification and potential efferent influence on the operating state of the cochlear amplifier. During anaesthesia, maximum cubic DPOAE levels can increase by up to 35 dB within a medium stimulus level range from 35 to 60 dB SPL. Close to the -10 dB SPL threshold, at stimulus levels below about 20-30 dB SPL, anaesthesia reduces cubic DPOAE amplitudes and raises cubic DPOAE thresholds. This makes DPOAE growth functions steeper. Additionally, ketamine increases the optimum stimulus frequency ratio which is indicative of a reduction of cochlear tuning sharpness. The effect of ketamine on cubic DPOAE thresholds becomes stronger at higher stimulus frequencies and is highly significant for f2 frequencies above 40 kHz. Quadratic DPOAE levels are increased by up to 25 dB by ketamine at medium stimulus levels. In contrast to cubic DPOAEs, quadratic DPOAE threshold changes are variable and there is no significant loss of sensitivity during anaesthesia. We discuss that ketamine effects could be caused by modulation of middle ear function or a release from ipsilateral efferent modulation that mainly affects the gain of cochlear amplification.

Attentional modulation of the inner ear: a combined otoacoustic emission and EEG study

Attending to a single stimulus in a complex multisensory environment requires the ability to select relevant information while ignoring distracting input. The underlying mechanism and involved neuronal levels of this attentional gain control are still a matter of debate. Here, we investigated the influence of intermodal attention on different levels of auditory processing in humans. It is known that the activity of the cochlear amplifier can be modulated by efferent neurons of the medial olivocochlear complex. We used distortion product otoacoustic emission (DPOAE) measurements to monitor cochlear activity during an intermodal cueing paradigm. Simultaneously, central auditory processing was assessed by electroencephalography (EEG) with a steady-state paradigm targeting early cortical responses and analysis of alpha oscillations reflecting higher cognitive control of attentional modulation. We found effects of selective attention at all measured levels of the auditory processing: DPOAE levels differed significantly between periods of visual and auditory attention, showing a reduction during visual attention, but no change during auditory attention. Primary auditory cortex activity, as measured by the auditory steady-state response (ASSR), differed between conditions, with higher ASSRs during auditory than visual attention. Furthermore, the analysis of cortical oscillatory activity revealed increased alpha power over occipitoparietal and frontal regions during auditory compared with visual attention, putatively reflecting suppression of visual processing. In conclusion, this study showed both enhanced processing of attended acoustic stimuli in early sensory cortex and reduced processing of distracting input, both at higher cortical levels and at the most peripheral level of the hearing system, the cochlea.

Multiple indices of the 'bounce' phenomenon obtained from the same human ears

Loud low-frequency sounds can induce temporary oscillatory changes in cochlear sensitivity, which have been termed the 'bounce' phenomenon. The origin of these sensitivity changes has been attributed to slow fluctuations in cochlear homeostasis, causing changes in the operating points of the outer hair cell mechano-electrical and electro-mechanical transducers. Here, we acquired three objective and subjective measures resulting in a comprehensive dataset of the bounce phenomenon in each of 22 normal-hearing human subjects. We analysed the level and phase of cubic and quadratic distortion product otoacoustic emissions and the auditory thresholds before and after presentation of a low-frequency stimulus (30 Hz sine wave, 120 dB SPL, 90 s) as a function of time. In addition, the perceived loudness of temporary, tinnitus-like sensations occurring in all subjects after cessation of the low-frequency stimulus was tracked over time. The majority of the subjects (70 %) showed a significant, biphasic change of quadratic, but not cubic, distortion product otoacoustic emissions of about 3-4 dB. Eighty-six percent of the tested subjects showed significant alterations of hearing thresholds after low-frequency stimulation. Four different types of threshold changes were observed, namely monophasic desensitisations (the majority of cases), monophasic sensitisations, biphasic alterations with initial sensitisation and biphasic alterations with initial desensitisation. The similar duration of the three bounce phenomenon measures indicates a common origin. The current findings are consistent with the hypothesis that slow oscillations of homeostatic control mechanisms and associated operating point shifts within the cochlea are the source of the bounce phenomenon.

DPOAE Intensity Increase at Individual Dominant Frequency after Short-Term Auditory Exposure

Previous experiments suggested the possibility of a short-term sound stimulus-evoked and transient increase in DPOAE amplitudes. This phenomenon is possibly due to the complexity of the outer hair cells and their efferent control system and the different time scales of regulatory processes. A total of 100 healthy subjects ranging from 18 to 40 years of age with normal hearing and normal DPOAE values in the range of 781–4000 Hz were recruited in the study. Diagnostic DPOAE measurements were performed after short-term sound exposure. We proposed a 10 sec, 50 dB sound impulse as the most effective stimulus for clinical practice between 40 and 60 sec poststimulus time to detect the aforementioned transient DPOAE increase. We developed a procedure for detection of this transient increase in DPOAE by the application of a short-term sound exposure. The phenomenon was consistent and well detectable. Based on our findings, a new aspect of cochlear adaptation can be established that might be introduced as a routine clinical diagnostic tool. A mathematical model was provided that summarizes various factors that determine electromotility of OHCs and serves as a possible clinical application using this phenomenon for the prediction of individual noise susceptibility.

Auditory Distortions: Origins and Functions

To enhance weak sounds while compressing the dynamic intensity range, auditory sensory cells amplify sound-induced vibrations in a nonlinear, intensity-dependent manner. In the course of this process, instantaneous waveform distortion is produced, with two conspicuous kinds of interwoven consequences, the introduction of new sound frequencies absent from the original stimuli, which are audible and detectable in the ear canal as otoacoustic emissions, and the possibility for an interfering sound to suppress the response to a probe tone, thereby enhancing contrast among frequency components. We review how the diverse manifestations of auditory nonlinearity originate in the gating principle of their mechanoelectrical transduction channels; how they depend on the coordinated opening of these ion channels ensured by connecting elements; and their links to the dynamic behavior of auditory sensory cells. This paper also reviews how the complex properties of waves traveling through the cochlea shape the manifestations of auditory nonlinearity. Examination methods based on the detection of distortions open noninvasive windows on the modes of activity of mechanosensitive structures in auditory sensory cells and on the distribution of sites of nonlinearity along the cochlear tonotopic axis, helpful for deciphering cochlear molecular physiology in hearing-impaired animal models. Otoacoustic emissions enable fast tests of peripheral sound processing in patients. The study of auditory distortions also contributes to the understanding of the perception of complex sounds.

Maturation of the human medial efferent reflex revisited

Past work applying otoacoustic emissions to gauge maturational status of the medial olivocochlear (MOC) reflex in human newborns has produced mixed results. The present study revisits the question while considering the dual nature of the 2f(1) - f(2) distortion product otoacoustic emission (DPOAE) and expanding measures of medial efferent function. Subjects included premature and term-born neonates, 6-month-old infants and young adults. The MOC reflex was elicited with contralateral acoustic stimulation (CAS) while shifts in amplitude and phase of the DPOAE, and its distortion and reflection components, were monitored. Overall, CAS-elicited reductions in DPOAE level did not differ among age groups. For all ages, the MOC reflex was strongest at frequencies below 1.5 kHz, and the reflection component of the DPOAE was most affected, showing maximally reduced amplitude and shallower phase slope when contralateral noise was presented. Results suggest that the MOC reflex likely reaches maturation prior to full-term birth. However, prematurely born neonates show markedly more episodes of CAS-induced DPOAE level enhancement. This may be due to more intrusive component mixing in this age group or disruptions in the formation of the MOC pathway or synapse in the most premature neonates.

Changes in cochlear function during acute endolymphatic hydrops development in guinea pigs

Previous studies have injected artificial endolymph into scala media in anaesthetized guinea pigs as an acute model of endolymphatic hydrops. Here, we have injected artificial endolymph into scala media in guinea pigs at rates of 40-80 nl/min, whilst monitoring Compound Action Potential (CAP) thresholds, the Summating Potential (SP)/CAP ratio, Cochlear Microphonic (CM) distortion, low-frequency modulated Distortion Product Otoacoustic Emissions (DPOAEs), and the Endocochlear Potential (EP). We found that abrupt recovery of CAP thresholds, SP/CAP ratio, and CM and DPOAE asymmetric distortion could occur several times during a single injection of less than 3 μl, suggesting that endolymph pressure could periodically decrease while the injection was ongoing. Larger volumes are thought to produce a rupture of the membranous labyrinth, however, our results suggest that multiple injections, each larger than 3 μl and within 40 min of each other, cause multiple pressure-related changes, which are difficult to be explained on the basis of a simple labyrinth rupture. We have also examined the morphological changes of the temporal bones ex vivo using X-ray micro-tomography. Both the functional changes and the micro-CT images suggest ruptures of the membranous labyrinth may not always be responsible for abrupt changes in inner ear function. Our results provide a new insight into the changes in cochlear function occurring during acute hydrops development, which compares well to the clinical findings observed in Ménière's Disease. We suggest that hydrops development may be a continual process, yet cause discontinuous functional changes due to mechanisms other than a simple rupture of the membranous labyrinth.

Effect of contralateral pure tone stimulation on distortion emissions suggests a frequency-specific functioning of the efferent cochlear control

Contralateral acoustic stimulation (CAS) with white noise and pure tone stimuli was used to assess frequency specificity of efferent olivocochlear control of cochlear mechanics in the gerbil. Changes of the cochlear amplifier can be monitored by distortion product otoacoustic emissions (DPOAEs), which are a byproduct of the nonlinear amplification by the outer hair cells. We used the quadratic DPOAE f2-f1 as ipsilateral probe, as it is known to be sensitive to efferent olivocochlear activity. White noise CAS, used to evoke efferent activity, had maximal effects on the DPOAE level for f2-stimulus frequencies of 5–7 kHz. The dominant effect during CAS was a DPOAE level increase of up to 13.5 dB. The frequency specificity of the olivocochlear system was evaluated by presenting pure tones (0.5–38 kHz) as contralateral stimuli to evoke efferent activity. Maximal DPOAE level changes were triggered by CAS frequencies close to the frequency of the DPOAE elicitor tones (tested f2 range: 2.5–15 kHz). The effective CAS frequency range covered 1.4–2.4 octaves and was centered 0.42 octaves below the DPOAE elicitor tone f2. The frequency-specific effect of CAS with pure tones suggests a dedicated central control of mechanical adjustments for peripheral frequency processing.

Cochlear efferent innervation and function

PURPOSE OF REVIEW

This review covers topics relevant to olivocochlear-efferent anatomy and function for which there are new findings in papers from 2009 to early 2010.

RECENT FINDINGS

Work within the review period has increased our understanding of medial olivocochlear (MOC) mechanisms in outer hair cells, MOC-reflex tuning, MOC effects on distortion product otoacoustic emissions, the time course of MOC effects, MOC effects in psychophysical tests and on understanding speech, MOC effects in attention and learning, and lateral efferent function in binaural hearing. In addition, there are new insights into efferent molecular mechanisms and their effect on cochlear development.

SUMMARY

Techniques for measuring efferent effects using otoacoustic emissions are now well developed and have promise in clinical applications ranging from predicting which patients are susceptible to acoustic trauma to characterizing relationships between efferent activation and learning disabilities. To realize this promise, studies are needed in which these techniques are applied with high standards.