A cochlear-bone wave can yield a hearing sensation as well as otoacoustic emission.

Tatjana Tchumatchenko,Tobias Reichenbach

doi:10.1038/ncomms5160

Tatjana Tchumatchenko, Tobias Reichenbach

Open Access

https://doi.org/10.1038/ncomms5160

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Abstract

A hearing sensation arises when the elastic basilar membrane inside the cochlea vibrates. The basilar membrane is typically set into motion through airborne sound that displaces the middle ear and induces a pressure difference across the membrane. A second, alternative pathway exists, however: stimulation of the cochlear bone vibrates the basilar membrane as well. This pathway, referred to as bone conduction, is increasingly used in headphones that bypass the ear canal and the middle ear. Furthermore, otoacoustic emissions, sounds generated inside the cochlea and emitted therefrom, may not involve the usual wave on the basilar membrane, suggesting that additional cochlear structures are involved in their propagation. Here we describe a novel propagation mode within the cochlea that emerges through deformation of the cochlear bone. Through a mathematical and computational approach we demonstrate that this propagation mode can explain bone conduction as well as numerous properties of otoacoustic emissions.

Highlights

A hearing sensation arises when the elastic basilar membrane inside the cochlea vibrates
Through mathematical and numerical methods we find that the deformation of the cochlear bone can evoke a traveling wave on the basilar membrane as well, and elicit a hearing sensation
A force that acts on the basilar membrane must besides the basilar-membrane wave, stimulate a second degree of freedom: the wave on the cochlear bone

Summary

Introduction

A hearing sensation arises when the elastic basilar membrane inside the cochlea vibrates. Otoacoustic emissions, sounds generated inside the cochlea and emitted therefrom, may not involve the usual wave on the basilar membrane, suggesting that additional cochlear structures are involved in their propagation. The pressure oscillation propagates as a surface wave on the basilar membrane, an elastic structure that separates two fluid-filled compartments in the cochlea. The nonlinearity arises from mechanical activity of hair cells that reside on the basilar membrane These cells can produce mechanical forces that greatly amplify weak stimuli; large vibrations are amplified less. A cubic nonlinearity yields a response at frequencies such as 2f1 À f2 or 2f2 À f1 when stimulated at two frequencies f1 and f2 Such distortion products arise prominently in the cochlea. The cubic distortion frequencies 2f1 À f2 or 2f2 À f1, for instance, are only created at a significant amplitude

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