Phase of shear vibrations within cochlear partition leads to activation of the cochlear amplifier.

Jessica S Lamb,Richard S Chadwick

doi:10.1371/journal.pone.0085969

Jessica S Lamb, Richard S Chadwick

Open Access

https://doi.org/10.1371/journal.pone.0085969

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Abstract

Since Georg von Bekesy laid out the place theory of the hearing, researchers have been working to understand the remarkable properties of mammalian hearing. Because access to the cochlea is restricted in live animals, and important aspects of hearing are destroyed in dead ones, models play a key role in interpreting local measurements. Wentzel-Kramers-Brillouin (WKB) models are attractive because they are analytically tractable, appropriate to the oblong geometry of the cochlea, and can predict wave behavior over a large span of the cochlea. Interest in the role the tectorial membrane (TM) plays in cochlear tuning led us to develop models that directly interface the TM with the cochlear fluid. In this work we add an angled shear between the TM and reticular lamina (RL), which serves as an input to a nonlinear active force. This feature plus a novel combination of previous work gives us a model with TM-fluid interaction, TM-RL shear, a nonlinear active force and a second wave mode. The behavior we get leads to the conclusion the phase between the shear and basilar membrane (BM) vibration is critical for amplification. We show there is a transition in this phase that occurs at a frequency below the cutoff, which is strongly influenced by TM stiffness. We describe this mechanism of sharpened BM velocity profile, which demonstrates the importance of the TM in overall cochlear tuning and offers an explanation for the response characteristics of the Tectb mutant mouse.

Highlights

Mammals transduce sound waves to neuronal signals within the fluid-filled cochlea via traveling waves in the organ of Corti (OC)
From this we see that relative phase of vibrating structures strongly affects active force output, enabling us to offer a novel explanation for observations of the Tectb mutant mouse [12]
Previous works have dealt with evanescent wave modes, showing they make a perceptible contribution near the frequency cutoff [17,18], or than they emerge with added components such as longitudinal coupling [19] and fluid chambers [20]

Summary

Introduction

Mammals transduce sound waves to neuronal signals within the fluid-filled cochlea via traveling waves in the organ of Corti (OC). We increased the complexity of the cochlea WKB model by developing it with a modified ‘‘sandwich’’ cross-section [9], allowing the TM, which has a fluid-facing surface, to vibrate as a second degree of freedom and found that such a system can produce a second propagating wave [10]. This model builds on results in [10], the aforementioned model where differential motion of the TM generates a second mode, and [13], which added a nonlinear active force onto a WKB-based model as a perturbation.

Results

Conclusion