Influence of 100Hz amplitude modulation on the human medial olivocochlear reflex

Associations between the medial olivocochlear reflex, middle-ear muscle reflex, and sentence-in-noise recognition using steady and pulsed noise elicitors

The middle-ear muscle reflex (MEMR) and medial olivocochlear reflex (MOCR) modify peripheral auditory function, which may reduce masking and improve speech-in-noise (SIN) recognition. Previous work and our pilot data suggest that the two reflexes respond differently to static versus dynamic noise elicitors. However, little is known about how the two reflexes work in tandem to contribute to SIN recognition. We hypothesized that SIN recognition would be significantly correlated with the strength of the MEMR and with the strength of the MOCR. Additionally, we hypothesized that SIN recognition would be best when both reflexes were activated. A total of 43 healthy, normal-hearing adults met the inclusion/exclusion criteria (35 females, age range: 19-29 years). MEMR strength was assessed using wideband absorbance. MOCR strength was assessed using transient-evoked otoacoustic emissions. SIN recognition was assessed using a modified version of the QuickSIN. All measurements were made with and without two types of contralateral noise elicitors (steady and pulsed) at two levels (50 and 65 dB SPL). Steady noise was used to primarily elicit the MOCR and pulsed noise was used to elicit both reflexes. Two baseline conditions without a contralateral elicitor were also obtained. Results revealed differences in how the MEMR and MOCR responded to elicitor type and level. Contrary to hypotheses, SIN recognition was not significantly improved in the presence of any contralateral elicitors relative to the baseline conditions. Additionally, there were no significant correlations between MEMR strength and SIN recognition, or between MOCR strength and SIN recognition. MEMR and MOCR strength were significantly correlated for pulsed noise elicitors but not steady noise elicitors. Results suggest no association between SIN recognition and the MEMR or MOCR, at least as measured and analyzed in this study. SIN recognition may have been influenced by factors not accounted for in this study, such as contextual cues, warranting further study.

Understanding degraded speech leads to perceptual gating of a brainstem reflex in human listeners

The ability to navigate "cocktail party" situations by focusing on sounds of interest over irrelevant, background sounds is often considered in terms of cortical mechanisms. However, subcortical circuits such as the pathway underlying the medial olivocochlear (MOC) reflex modulate the activity of the inner ear itself, supporting the extraction of salient features from auditory scene prior to any cortical processing. To understand the contribution of auditory subcortical nuclei and the cochlea in complex listening tasks, we made physiological recordings along the auditory pathway while listeners engaged in detecting non(sense) words in lists of words. Both naturally spoken and intrinsically noisy, vocoded speech-filtering that mimics processing by a cochlear implant (CI)-significantly activated the MOC reflex, but this was not the case for speech in background noise, which more engaged midbrain and cortical resources. A model of the initial stages of auditory processing reproduced specific effects of each form of speech degradation, providing a rationale for goal-directed gating of the MOC reflex based on enhancing the representation of the energy envelope of the acoustic waveform. Our data reveal the coexistence of 2 strategies in the auditory system that may facilitate speech understanding in situations where the signal is either intrinsically degraded or masked by extrinsic acoustic energy. Whereas intrinsically degraded streams recruit the MOC reflex to improve representation of speech cues peripherally, extrinsically masked streams rely more on higher auditory centres to denoise signals.

Neural Encoding of Amplitude Modulations in the Human Efferent System

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

Human medial efferent activity elicited by dynamic versus static contralateral noises

The medial olivocochlear reflex (MOCR) modifies cochlear amplifier function to improve encoding of signals in static noise, but conflicting results have been reported regarding how the MOCR responds to dynamic, temporally-complex noises. The current study utilized three MOCR elicitors with identical spectral content but different temporal properties: broadband noise, amplitude-modulated noise, and speech envelope-modulated noise. MOCR activity was assessed using contralateral inhibition of transient-evoked otoacoustic emissions in 27 normal-hearing young adults. Elicitors were presented contralaterally at two intensities of 50 and 60 dB SPL. Magnitude and growth of contralateral inhibition with increasing elicitor intensity were compared across the three elicitor types. Results revealed that contralateral inhibition was significantly larger at the elicitor intensity of 60 dB SPL than at 50 dB SPL, but there were no significant differences in the magnitude and growth of inhibition across the three elicitors, contrary to hypothesis. These results suggest that the MOCR responds similarly to both static and dynamic noise.

Click-Evoked Auditory Efferent Activity: Rate and Level Effects

There currently are no standardized protocols to evaluate auditory efferent function in humans. Typical tests use broadband noise to activate the efferents, but only test the contralateral efferent pathway, risk activating the middle ear muscle reflex (MEMR), and are laborious for clinical use. In an attempt to develop a clinical test of bilateral auditory efferent function, we have designed a method that uses clicks to evoke efferent activity, obtain click-evoked otoacoustic emissions (CEOAEs), and monitor MEMR. This allows for near-simultaneous estimation of cochlear and efferent function. In the present study, we manipulated click level (60, 70, and 80 dB peak-equivalent sound pressure level [peSPL]) and rate (40, 50, and 62.5 Hz) to identify an optimal rate-level combination that evokes measurable efferent modulation of CEOAEs. Our findings (n = 58) demonstrate that almost all click levels and rates used caused significant inhibition of CEOAEs, with a significant interaction between level and rate effects. Predictably, bilateral activation produced greater inhibition compared to stimulating the efferents only in the ipsilateral or contralateral ear. In examining the click rate-level effects during bilateral activation in greater detail, we observed a 1-dB inhibition of CEOAE level for each 10-dB increase in click level, with rate held constant at 62.5 Hz. Similarly, a 10-Hz increase in rate produced a 0.74-dB reduction in CEOAE level, with click level held constant at 80 dB peSPL. The effect size (Cohen's d) was small for either monaural condition and medium for bilateral, faster-rate, and higher-level conditions. We were also able to reliably extract CEOAEs from efferent eliciting clicks. We conclude that clicks can indeed be profitably employed to simultaneously evaluate cochlear health using CEOAEs as well as their efferent modulation. Furthermore, using bilateral clicks allows the evaluation of both the crossed and uncrossed elements of the auditory efferent nervous system, while yielding larger, more discernible, inhibition of the CEOAEs relative to either ipsilateral or contralateral condition.

Cochlear Delay and Medial Olivocochlear Functioning in Children with Suspected Auditory Processing Disorder

Behavioral manifestations of processing deficits associated with auditory processing disorder (APD) have been well documented. However, little is known about their anatomical underpinnings, especially cochlear processing. Cochlear delays, a proxy for cochlear tuning, measured using stimulus frequency otoacoustic emission (SFOAE) group delay, and the influence of the medial olivocochlear (MOC) system activation at the auditory periphery was studied in 23 children suspected with APD (sAPD) and 22 typically developing (TD) children. Results suggest that children suspected with APD have longer SFOAE group delays (possibly due to sharper cochlear tuning) and reduced MOC function compared to TD children. Other differences between the groups include correlation between MOC function and SFOAE delay in quiet in the TD group, and lack thereof in the sAPD group. MOC-mediated changes in SFOAE delay were in opposite directions between groups: increase in delay in TD vs. reduction in delay in the sAPD group. Longer SFOAE group delays in the sAPD group may lead to longer cochlear filter ringing, and potential increase in forward masking. These results indicate differences in cochlear and MOC function between sAPD and TD groups. Further studies are warranted to explore the possibility of cochlea as a potential site for processing deficits in APD.

Influence of the stimulus presentation rate on medial olivocochlear system assays

Click evoked otoacoustic emissions (CEOAEs) are commonly used both in research and clinics to assay the medial olivocochlear system (MOC). Clicks presented at rates >50 Hz in the contralateral ear have previously been reported to evoke contralateral MOC activity. However, in typical MOC assays, clicks are presented in the ipsilateral ear in conjunction with MOC elicitor (noise) in the contralateral ear. The effect of click rates in such an arrangement is currently unknown. A forward masking paradigm was used to emulate typical MOC assays to elucidate the influence of ipsilateral click presentation rates on MOC inhibition of CEOAEs in 28 normal hearing adults. Influence of five click rates (20.83, 25, 31.25, 41.67, and 62.5 Hz) presented at 55 dB peSPL was tested. Results indicate that click rates as low as 31.25 Hz significantly enhance contralateral MOC inhibition, possibly through the activation of ipsilateral and binaural MOC neurons with potential contributions from the middle ear muscle reflex. Therefore, click rates ≤25 Hz are recommended for use in MOC assays, at least for 55 dB peSPL click level.