Human medial olivocochlear reflex: Contralateral activation effect on low and high frequency cochlear response

Human Olivocochlear Effects: A Statistical Detection Approach Applied to the Cochlear Microphonic Evoked by Swept Tones

The human medial olivocochlear (MOC) reflex was assessed by observing the effects of contralateral acoustic stimulation (CAS) on the cochlear microphonic (CM) across a range of probe frequencies. A frequency-swept probe tone (125-4757 Hz, 90 dB SPL) was presented in two directions (up sweep and down sweep) to normal-hearing young adults. This study assessed MOC effects on the CM in individual participants using a statistical approach that calculated minimum detectable changes in magnitude and phase based on CM signal-to-noise ratio (SNR). Significant increases in CM magnitude, typically 1-2 dB in size, were observed for most participants from 354 to 1414 Hz, where the size and consistency of these effects depended on participant, probe frequency, sweep direction, and SNR. CAS-related phase lags were also observed, consistent with CM-based MOC studies in laboratory animals. Observed effects on CM magnitude and phase were in the opposite directions of reported effects on otoacoustic emissions (OAEs). OAEs are sensitive to changes in the motility of outer hair cells located near the peak region of the traveling wave, while the effects of CAS on the CM likely originate from MOC-related changes in the conductance of outer hair cells located in the basal tail of the traveling wave. Thus, MOC effects on the CM are complementary to those observed for OAEs.

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

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

Middle ear muscle and medial olivocochlear activity inferred from individual human ears via cochlear potentials

The peripheral auditory system is influenced by the medial olivocochlear (MOC) and middle ear muscle (MEM) reflexes. When elicited by contralateral acoustic stimulation (CAS), these reflexes reduce cochlear amplification (MOC reflex) and limit low-frequency transmission through the middle ear (MEM reflex). The independent roles of these reflexes on auditory physiology and perception are difficult to distinguish. The amplitude of the cochlear microphonic (CM) is expected to increase or decrease when the MOC and MEM reflexes are elicited by CAS, respectively, which could lead to a straightforward interpretation of what reflex is dominant for a given CAS level. CM and ear canal sound pressure level (SPL) were measured for a 500 Hz, 90 dB SPL probe in the presence of contralateral broadband noise (CBBN) for levels ranging from 45-75 dB SPL. In most subjects, CM amplitude increased for CBBN levels of 45 and 55 dB SPL, while no change in ear canal SPL was observed, consistent with eliciting the MOC reflex. Conversely, CM amplitude decreased, and ear canal SPL increased in the presence of 65 and 75 dB SPL CBBN, consistent with eliciting the MEM reflex. A CM-based test of the MOC reflex may facilitate detection of MEM effects and the assessment of adults with cochlear hearing loss.

Firing Rate Adaptation of the Human Auditory Nerve Optimizes Neural Signal-to-Noise Ratios

Several physiological mechanisms act on the response of the auditory nerve (AN) during acoustic stimulation, resulting in an adjustment in auditory gain. These mechanisms include-but are not limited to-firing rate adaptation, dynamic range adaptation, the middle ear muscle reflex, and the medial olivocochlear reflex. A potential role of these mechanisms is to improve the neural signal-to-noise ratio (SNR) at the output of the AN in real time. This study tested the hypothesis that neural SNRs, inferred from non-invasive assessment of the human AN, improve over the duration of acoustic stimulation. Cochlear potentials were measured in response to a series of six high-level clicks embedded in a series of six lower-level broadband noise bursts. This paradigm elicited a compound action potential (CAP) in response to each click and to the onset of each noise burst. The ratio of CAP amplitudes elicited by each click and noise burst pair (i.e., neural SNR) was tracked over the six click/noise bursts. The main finding was a rapid (< 24 ms) increase in neural SNR from the first to the second click/noise burst, consistent with a real-time adjustment in the response of the auditory periphery toward improving the SNR of the signal transmitted to the brainstem. Analysis of cochlear microphonic and ear canal sound pressure recordings, as well as the time course for this improvement in neural SNR, supports the conclusion that firing rate adaptation is likely the primary mechanism responsible for improving neural SNR, while dynamic range adaptation, the middle ear muscle reflex, and the medial olivocochlear reflex played a secondary role on the effects observed in this study. Real-time improvements in neural SNR are significant because they may be essential for robust encoding of speech and other relevant stimuli in the presence of background noise.

Medial Olivocochlear Reflex Effect on Cochlear Response in Humans: Elicitor Side and Level

BACKGROUND

Understanding the functional differences between crossed and uncrossed medial olivocochlear (MOC) neurons has been of interest to researchers for decades. Previous reports revealed conflicting results about which MOC pathway, crossed or uncrossed, is stronger in humans. Both crossed and uncrossed MOC neurons synapse at the base of the outer hair cells (OHCs) in each ear. OHCs generate the cochlear microphonic, which is a major contributor to the cochlear response (CR) PURPOSE:  The current study investigated the effects of eliciting the crossed and uncrossed MOC reflex (MOCR) on CR in humans with three levels of noise.

RESEARCH DESIGN

Normal-hearing, young adults (n = 16) participated in this study. The CR was recorded using 500 Hz tone-burst stimuli presented at 80 dB nHL. To examine the crossed and uncrossed MOCR, CR was recorded without and with continuous ipsilateral or contralateral broadband noise (BBN) at three levels (40, 50, and 60 dB SPL).

DATA ANALYSIS

Analysis of the CR was completed using the amplitude of the response extracted using fast Fourier transform. Statistical analysis was completed using repeated measures analysis of variance and post-hoc analysis.

RESULTS

Compared with baseline, the presentation of BBN, specifically contralaterally, resulted in CR enhancement with no significant difference as a function of the three BBN levels. Greater enhancement of the CR amplitude was observed with contralateral than ipsilateral BBN elicitor.

CONCLUSIONS

The current findings suggest that a contralateral elicitor of the uncrossed MOC pathway results in a larger CR amplitude enhancement compared with an ipsilateral elicitor of the crossed MOC pathway, regardless of the elicitor level. Eliciting the MOCR appears to modulate the OHCs function. Furthermore, assessing the MOCR with the 500 Hz CR with BBN elicitors at moderate levels should separate its effects (i.e., increase response amplitude) from those associated with the middle ear muscle reflex (i.e., reduce response amplitude).

The role of the medial olivocochlear reflex in psychophysical masking and intensity resolution in humans: a review

This review addresses the putative role of the medial olivocochlear (MOC) reflex in psychophysical masking and intensity resolution in humans. A framework for interpreting psychophysical results in terms of the expected influence of the MOC reflex is introduced. This framework is used to review the effects of a precursor or contralateral acoustic stimulation on 1) simultaneous masking of brief tones, 2) behavioral estimates of cochlear gain and frequency resolution in forward masking, 3) the buildup and decay of forward masking, and 4) measures of intensity resolution. Support, or lack thereof, for a role of the MOC reflex in psychophysical perception is discussed in terms of studies on estimates of MOC strength from otoacoustic emissions and the effects of resection of the olivocochlear bundle in patients with vestibular neurectomy. Novel, innovative approaches are needed to resolve the dissatisfying conclusion that current results are unable to definitively confirm or refute the role of the MOC reflex in masking and intensity resolution.

The Role of the Medial Olivocochlear Reflex in Acceptable Noise Level in Adults

BACKGROUND

 The acceptable noise level (ANL) is a measurement used to quantify how much noise a person is willing to accept while listening to speech. ANL has been used to predict success with hearing aid use. However, physiological correlates of the ANL are poorly understood. One potential physiological correlate is the medial olivocochlear reflex (MOCR), which decreases the output of the cochlea and is thereby expected to increase noise tolerance.

PURPOSE

 This study investigates the relationship between contralateral activation of the MOCR and tolerance of background noise.

RESEARCH DESIGN

 This study recruited 22 young adult participants with normal hearing. ANL was measured using the Arizona Travelogue recording under headphones presented at the most comfortable level (MCL) with and without multitalker babble noise. The MOCR strength was evaluated in all participants by measuring the cochlear microphonic (CM) with and without 40 dB sound pressure level contralateral broadband noise (CBBN).

DATA ANALYSIS

 The CM observed in response to a 500-Hz tone was measured with and without CBBN, and changes in response to fast Fourier transform amplitude at 500 Hz were used as an indicator of the MOCR effect. The ANL was calculated by subtracting the maximum acceptable background noise level from the MCL. Participants were divided into two groups based on their ANL: low-ANL (ANL < 7 dB) and moderate-ANL (ANL ≥ 7 dB). An independent samples t-test was used to compare CM enhancement between low-ANL and moderate-ANL groups. Additionally, Pearson's correlation was used to investigate the relationship between the ANL and the MOCR effect on the CM.

RESULTS

 The results indicated that presentation of CBBN increased the CM amplitude, consistent with eliciting the MOCR. Participants in the low-ANL group had significantly larger CM enhancement than moderate-ANL participants. The results further revealed a significant correlation between the ANL and the MOCR effect on the CM.

CONCLUSION

 This study suggests that a stronger MOCR, as assessed using CM enhancement, is associated with greater noise tolerance. This research provides a possible objective measure to predict background tolerance in patients and adds to our understanding about the MOCR function in humans.