Human middle-ear muscles rarely contract in anticipation of acoustic impulses: Implications for hearing risk assessments.

The current study addressed the existence of an anticipatory middle-ear muscle contraction (MEMC) as a protective mechanism found in recent damage-risk criteria for impulse noise exposure. Specifically, the experiments reported here tested instances when an exposed individual was aware of and could anticipate the arrival of an acoustic impulse. In order to detect MEMCs in human subjects, a laser-Doppler vibrometer (LDV) was used to measure tympanic membrane (TM) motion in response to a probe tone. Here we directly measured the time course and relative magnitude changes of TM velocity in response to an acoustic reflex-eliciting (i.e. MEMC eliciting) impulse in 59 subjects with clinically assessable MEMCs. After verifying the presence of the MEMC, we used a classical conditioning paradigm pairing reflex-eliciting acoustic impulses (unconditioned stimulus, UCS) with various preceding stimuli (conditioned stimulus, CS). Changes in the time-course of the MEMC following conditioning were considered evidence of MEMC conditioning, and any indication of an MEMC prior to the onset of the acoustic elicitor was considered an anticipatory response. Nine subjects did not produce a MEMC measurable via LDV. For those subjects with an observable MEMC (n = 50), 48 subjects (96%) did not show evidence of an anticipatory response after conditioning, whereas only 2 subjects (4%) did. These findings reveal that MEMCs are not readily conditioned in most individuals, suggesting that anticipatory MEMCs are not prevalent within the general population. The prevalence of anticipatory MEMCs does not appear to be sufficient to justify inclusion as a protective mechanism in auditory injury risk assessments.

Adversarial relationship between combined medial olivocochlear (MOC) and middle-ear-muscle (MEM) reflexes and alarm-in-noise detection thresholds under negative signal-to-noise ratios (SNRs).

The role of auditory efferent feedback from the medial olivocochlear system (MOCS) and the middle-ear-muscle (MEM) reflex in tonal detection tasks for humans in the presence of noise is not clearly understood. Past studies have yielded inconsistent results on the relationship between efferent feedback and tonal detection thresholds. This study attempts to address this inconsistency. Fifteen human subjects with normal hearing participated in an experiment where they were asked to identify an alarm signal in the presence of 80 dBA background (pink) noise. Masked detection thresholds were estimated using the method of two-interval forced choice (2IFC). Contralateral suppression of transient-evoked otoacoustic emissions (TEOAEs) was measured to estimate the strength of auditory efferent feedback. Subsequent correlation analysis revealed that the contralateral suppression of TEOAEs was significantly negatively correlated (r = -0.526, n = 15, p = 0.0438) with alarm-in-noise (AIN) detection thresholds under negative signal-to-noise conditions. The result implies that the stronger the auditory efferent feedback, the worse the detection thresholds and thus the poorer the tonal detection performance in the presence of loud noise.

Effects of cochlear synaptopathy on middle-ear muscle reflexes in unanesthetized mice.

Cochlear synaptopathy, i.e. the loss of auditory-nerve connections with cochlear hair cells, is seen in aging, noise damage, and other types of acquired sensorineural hearing loss. Because the subset of auditory-nerve fibers with high thresholds and low spontaneous rates (SRs) is disproportionately affected, audiometric thresholds are relatively insensitive to this primary neural degeneration. Although suprathreshold amplitudes of wave I of the auditory brainstem response (ABR) are attenuated in synaptopathic mice, there is not yet a robust diagnostic in humans. The middle-ear muscle reflex (MEMR) might be a sensitive metric (Valero et al., 2016), because low-SR fibers may be important drivers of the MEMR (Liberman and Kiang, 1984; Kobler et al., 1992). Here, to test the hypothesis that narrowband reflex elicitors can identify synaptopathic cochlear regions, we measured reflex growth functions in unanesthetized mice with varying degrees of noise-induced synaptopathy and in unexposed controls. To separate effects of the MEMR from those of the medial olivocochlear reflex, the other sound-evoked cochlear feedback loop, we used a mutant mouse strain with deletion of the acetylcholine receptor required for olivocochlear function. We demonstrate that the MEMR is normal when activated from non-synaptopathic cochlear regions, is greatly weakened in synaptopathic regions, and is a more sensitive indicator of moderate synaptopathy than the suprathreshold amplitude of ABR wave I.

The timbre of the voice as perceived by the singer him-/herself.

Fifteen professional singers sang simple vocal exercises at different pitches. The sung excerpts were recorded, and seven modified versions were created from each recording. The modifications were then played to the participant and (s)he had to assess the similarity of these stimuli to the perception of his/her own voice during the act of singing. Participants rated as most similar those stimuli which were modified by the filter whose frequency response most closely resembled a trapezoid, which was created by taking into account 1) the diffracting air conduction component from the mouth of the singer to his ear channel, 2) the bone conduction component, and also 3) the influence of the stapedius reflex on the sensitivity of his/her auditory system.

Auditory brainstem circuits that mediate the middle ear muscle reflex.

The middle ear muscle (MEM) reflex is one of two major descending systems to the auditory periphery. There are two middle ear muscles (MEMs): the stapedius and the tensor tympani. In man, the stapedius contracts in response to intense low frequency acoustic stimuli, exerting forces perpendicular to the stapes superstructure, increasing middle ear impedance and attenuating the intensity of sound energy reaching the inner ear (cochlea). The tensor tympani is believed to contract in response to self-generated noise (chewing, swallowing) and non-auditory stimuli. The MEM reflex pathways begin with sound presented to the ear. Transduction of sound occurs in the cochlea, resulting in an action potential that is transmitted along the auditory nerve to the cochlear nucleus in the brainstem (the first relay station for all ascending sound information originating in the ear). Unknown interneurons in the ventral cochlear nucleus project either directly or indirectly to MEM motoneurons located elsewhere in the brainstem. Motoneurons provide efferent innervation to the MEMs. Although the ascending and descending limbs of these reflex pathways have been well characterized, the identity of the reflex interneurons is not known, as are the source of modulatory inputs to these pathways. The aim of this article is to (a) provide an overview of MEM reflex anatomy and physiology, (b) present new data on MEM reflex anatomy and physiology from our laboratory and others, and (c) describe the clinical implications of our research.

Tympanometric and acoustic stapedius reflex measures in older adults: the Blue Mountains Hearing Study.

Tympanometric peak pressure, peak compensated static acoustic admittance (peak Ytm) and acoustic stapedius reflex (ASR) thresholds were obtained for a representative sample of 1565 older Australians who were participants in the Blue Mountains Hearing Study (BMHS). No significant age or gender effects were found for tympanometric peak pressure. Peak Ytm measures, however, decreased with age in the left ear only across all age groups and were consistently higher for men than for women. After allowing for hearing loss, the effect of age on ASR thresholds was inconsistent. An increase in ASR thresholds with age was observed at selected frequencies but only when measured contralaterally, and these changes were not clinically significant. Overall, our findings suggest that current normative data for peak Ytm is too restricted for application in the older population, but there is insufficient evidence to warrant alternative normative data for the ASR threshold range in this same population.

Middle ear influence on otoacoustic emissions. I: noninvasive investigation of the human transmission apparatus and comparison with model results.

Evoked otoacoustic emissions (EOAEs) are generated within the cochlea in response to external sounds, and they can be acoustically detected in the external auditory meatus after backward propagation through the middle ear. In addition to being used to probe the cochlear mechanisms, they are expected to be sensitive to minute changes in middle ear impedance. Systematic measurements of the changes in the vectorial components of EOAEs were carried out after various manipulations of the human middle ear in order to characterize the influence of stiffness and inertia of the stapes and tympanic-membrane systems. For this purpose, stapedius muscle contractions were elicited by high-level contralateral sound, controlled changes in middle ear pressure (range +/-100 daPa) were produced and the tympanic membrane was loaded with water droplets. A computer model of the middle ear network was implemented using a standard lumped-element electric analog of the middle ear (Zwislocki's model). Forward and backward transmission changes were simulated and model predictions were compared to experimental data. Apart from the case of positive middle ear pressures, a close qualitative correspondence was found between model and real-ear results. Each of the effects was characterized by a unique pattern of phase and magnitude changes as a function of frequency, in relation to the mechanical characteristics of the involved subsystem (i.e. stapes stiffness, tympanic-membrane stiffness or mass) and its resonance properties. Owing to their high sensitivity, EOAEs could be helpful for an accurate individual multifrequency analysis of middle ear impedance by comparisons under rest and test conditions.

Middle-ear influence on otoacoustic emissions. II: contributions of posture and intracranial pressure.

Although it seems likely that body tilt or surgically provoked variations in intracranial pressure (ICP) can result in variations of intralabyrinthine pressure, the channels for pressure transmission remain controversial and the reasons why evoked otoacoustic emissions (EOAEs) exhibit attendant modifications are unclear. The theoretical framework implemented in the companion paper [Avan et al. part I, 2000] provides sensitive and non-invasive means to identify the middle-ear mechanism(s) entailed in EOAE changes. It was thus applied to analyze the influence of posture on EOAE phases and magnitudes as a function of frequency, in a series of experiments involving body tilt from sitting to supine (0 degrees or -30 degrees ). Controlled ICP variations were surgically carried out in a series of hydrocephalic patients and the resulting EOAE changes were compared to posture data and model predictions. In all cases, the EOAE changes closely resembled those due to an increase in the stiffness of the stapes' annular ligament, in keeping with the assumption that ICP gets transmitted to intralabyrinthine spaces and modifies the hydrostatic load on the stapes, thereby influencing EOAE features. A small additional contribution of middle-ear pressure to EOAE changes was identified in addition to the main stapes component. Dynamical EOAE measurements showed that sudden ICP changes were transmitted to the inner ear within 8-30 s. The high sensitivity of EOAE phases below 2 kHz to ICP changes, together with the absence of any significant confounding middle-ear effect, favors EOAEs for a reliable non-invasive monitoring of ICP and intralabyrinthine pressures.

[Electrostimulation-induced stapedius reflex. Presentation of a standardized procedure and results of clinical studies]. / Der elektrotaktil ausgelöste Stapediusreflex. Darstellung eines standardisierten Verfahrens und Ergebnisse klinischer Untersuchungen.

The electro-tactile elicitation of the stapedius muscle reflex is well known but not used in clinical practice. Despite a complex reflex pathway in the brainstem, correlations between reflex parameters and neurological impairments are still possible. We have developed a method for eliciting the electro-tactile stapedius muscle reflex and automatic analysis of different reflex parameters by using an analog-digital converter and a personal computer-integrated signal analysis system. Clinical investigations were carried out with a group of 55 healthy persons (control group), 13 patients with central facial palsies, 51 patients with multiple sclerosis (MS group) and 24 patients with absent acoustic-stapedius muscle reflexes. Findings demonstrated that the latency between stimulus and beginning of the reflex (L1) was significantly longer in the MS group compared with the control group. The best parameter for dividing the MS group into groups with different disability scores was S, which was the parameter for the increase in the L1 growth curve at different stimulus levels. Reproduction of the parameter S in repeated tests with different locations of the stimulus-electrode was excellent.

Intra-aural reflexes elicited by a cochlear prosthesis in monkeys.

Objective audiological tests are needed for pre- and postsurgical evaluation of cochlear prosthesis patients who are unable to give reliable subjective responses. In this study we demonstrated that contralateral intra-aural reflexes were elicited by a cochlear prosthesis in the monkey. Reflex variables measured include threshold, latency and amplitude. These findings indicate that the electrically elicited intra-aural reflex response may be useful to evaluate the peripheral auditory system in subjects with sensory deafness.

[Study of the wave-form of the stapedial reflex in response to sonorous stimuli of different intensity]. / Studio della forma d'onda del riflesso stapediale a diverse intensita' sonore.

The threshold of the acoustic reflex and its pattern in response to different intersity stimuli, was investigated by means of the signal-averaging technique in 10 normal ears. Trains of tone bursts between 100 and 0 dB HL were used. The frequencies tested were 500, 1000, 2000 and 4000 Hz. In all subjects the pattern of the acoustic reflex for stimuli between 110 and 100 dB HL was biphasic, with an initial positive plateau followed by a longer negative one. For stimuli < 80 dB HL the pattern of the reflex was monophasic, characterized by a single positive peak (latency between 120 and 170 msec).