Medial olivocochlear efferent reflex inhibition of human cochlear nerve responses

Abstract

Inhibition of cochlear amplifier gain by the medial olivocochlear (MOC) efferent system has several putative roles: aiding listening in noise, protection against damage from acoustic overexposure, and slowing age-induced hearing loss. The human MOC reflex has been studied almost exclusively by measuring changes in otoacoustic emissions. However, to help understand how the MOC system influences what we hear, it is important to have measurements of the MOC effect on the total output of the organ of Corti, i.e., on cochlear nerve responses that couple sounds to the brain. In this work we measured the inhibition produced by the MOC reflex on the amplitude of cochlear nerve compound action potentials (CAPs) in response to moderate level (52–60 dB peSPL) clicks from five, young, normal hearing, awake, alert, human adults. MOC activity was elicited by 65 dB SPL, contralateral broadband noise (CAS). Using tympanic membrane electrodes, approximately 10 hours of data collection were needed from each subject to yield reliable measurements of the MOC reflex inhibition on CAP amplitudes from one click level. The CAS produced a 16% reduction of CAP amplitude, equivalent to a 1.98 dB effective attenuation (averaged over five subjects). Based on previous reports of efferent effects as functions of level and frequency, it is possible that much larger effective attenuations would be observed at lower sound levels or with clicks of higher frequency content. For a preliminary comparison, we also measured MOC reflex inhibition of DPOAEs evoked from the same ears with f2’s near 4 kHz. The resulting effective attenuations on DPOAEs were, on average, less than half the effective attenuations on CAPs.

Efferent modulation of pre-neural and neural distortion products

“…To directly compare the functional effects of each type of inhibition, it would be necessary to derive a level series function and quantify MOC reflex inhibition in terms of “effective attenuation” (i.e., the amount of stimulus gain in dB needed to overcome the effects of inhibition; see Puria et al, 1996; Lichtenhan et al, 2016, and Smith et al, 2017). Because one presentation level was used in this study, we cannot calculate effective attenuation from our data.…”

“…Experiments in laboratory animals are also planned to identify the spatial origin(s) of OAE and FFR distortion product components, and elucidate the hypothesized relationship between pre-neural and neural distortion. For example, slow injection of cell-specific ototoxic pharmaceuticals into the cochlear apex can be used to sequentially manipulate finely spaced cochlear regions, and the time course of distortion product component ablation can identify the region of origination (Lichtenhan et al, 2016). This technique overcomes limitations associated with passive diffusion of ototoxic agents (e.g., limited treatment of the high-frequency, basal half of the cochlear spiral), which is imperative for studying distortion products in OAEs and FFRs from low-frequency stimuli.…”

“…However, a major limitation of this approach is that OAEs are insensitive to MOC reflex effects on the neural ensembles that mediate human hearing, and the functional consequences of this mechanism remain unclear. To directly assess the degree to which efferent-induced changes in inner ear mechanics are maintained beyond the cochlea, it is necessary to understand relationships between pre-neural and neural measurements of the MOC reflex (Lichtenhan et al, 2016; Smith et al, 2017). …”

Behavioral Pure-Tone Threshold Shifts Caused by Tympanic Membrane Electrodes

“…It is possible that the pressure against the TM, as well as the mass of the gel-soaked electrode tip, caused the threshold shifts. TM electrodes likely do not cause a shift in hearing sensitivity when using free field stimuli because expandable foam insert earphones are not used and thus there is less medial pressure imparted on the electrode (e.g., Lichtenhan et al 2015). Pure tone threshold data was solely collected by the first author.…”

“…TM electrodes provide superior signal-to-noise ratios compared to other extra-tympanic electrodes and are a non-invasive alternative to using trans-tympanic electrodes, which require a tympanotomy. Electrocochleography with a TM electrode has been used for a wide variety of clinical (e.g., Ferraro 2010) and basic investigation purposes (e.g., Chertoff et al 2010; Lichtenhan & Chertoff 2008; Lichtenhan et al 2015); however, no experiment has investigated the effects of TM electrode placement and contact location on behavioral hearing thresholds.…”