Intracochlear distortion products are broadly generated by outer hair cells but their contributions to otoacoustic emissions are spatially restricted

Thomas Bowling,Sebastiaan W F Meenderink,Julien Meaud,Haiqi Wen,Wei Dong

doi:10.1038/s41598-021-93099-7

Abstract

Detection of low-level sounds by the mammalian cochlea requires electromechanical feedback from outer hair cells (OHCs). This feedback arises due to the electromotile response of OHCs, which is driven by the modulation of their receptor potential caused by the stimulation of mechano-sensitive ion channels. Nonlinearity in these channels distorts impinging sounds, creating distortion-products that are detectable in the ear canal as distortion-product otoacoustic emissions (DPOAEs). Ongoing efforts aim to develop DPOAEs, which reflects the ear’s health, into diagnostic tools for sensory hearing loss. These efforts are hampered by limited knowledge on the cochlear extent contributing to DPOAEs. Here, we report on intracochlear distortion products (IDPs) in OHC electrical responses and intracochlear fluid pressures. Experiments and simulations with a physiologically motivated cochlear model show that widely generated electrical IDPs lead to mechanical vibrations in a frequency-dependent manner. The local cochlear impedance restricts the region from which IDPs contribute to DPOAEs at low to moderate intensity, which suggests that DPOAEs may be used clinically to provide location-specific information about cochlear damage.

Highlights

Detection of low-level sounds by the mammalian cochlea requires electromechanical feedback from outer hair cells (OHCs)
For the first time, simultaneous measurements of intracochlear distortion products (IDPs) in the electrical potential due to local OHC activity and in the fluid pressure, both measured in the scala tympani (ST) near the basilar membrane (BM) using a dual sensor that consists of a microelectrode attached to a micro-pressure s ensor[4] (Fig. 1A)
As previously r eported[4], at low sound pressure levels (SPLs), Pst (Fig. 2A) and LCM (Fig. 2E) are tuned to the best frequency (BF) of the measurement location, 23.5 kHz at 20 dB SPL. Both increase with SPL in a compressive nonlinear manner such that their iso-intensity sensitivity curves fan out at frequencies around the BF, similar to what has been observed in the sound-evoked vibrations of the BM22

Summary

Introduction

Detection of low-level sounds by the mammalian cochlea requires electromechanical feedback from outer hair cells (OHCs) This feedback arises due to the electromotile response of OHCs, which is driven by the modulation of their receptor potential caused by the stimulation of mechano-sensitive ion channels. The mammalian cochlea can detect sounds that cause vibrations of less than 1 nm, distinguish frequencies less than 0.4% apart, and operate over a wide dynamic range than spans a trillion-fold range of acoustic energy[1] These amazing characteristics are commonly attributed to an active feedback mechanism linked to the nonlinear electromechanics of outer hair cells (OHCs)[2]. The recent measurements of the IDP response of the RL imply a potentially broad DPOAE generation region This deviates from the current prevailing theories and would have important implications for the development of DPOAEs into an objective, noninvasive tool in the clinical diagnosis of location-specific hearing loss

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