Hair cells in the cochlea must tune resonant modes to the edge of instability without destabilizing collective modes

TL;DR

This paper models how hair cells in the cochlea tune resonant modes to stay near instability, enabling high sensitivity without destabilizing collective modes, revealing the balance of local and global feedback mechanisms.

Contribution

It introduces a discretized cochlear model with active hair cell feedback, identifying conditions for local amplification and stability of collective modes.

Findings

Localized modes can be amplified independently by hair cells.

Collective extended modes can be destabilized by local feedback.

Limits on molecular feedback mechanisms are imposed by stability constraints.

Abstract

Sound produces surface waves along the cochlea's basilar membrane. To achieve the ear's astonishing frequency resolution and sensitivity to faint sounds, dissipation in the cochlea must be canceled via active processes in hair cells, effectively bringing the cochlea to the edge of instability. But how can the cochlea be globally tuned to the edge of instability with only local feedback? To address this question, we use a discretized version of a standard model of basilar membrane dynamics, but with an explicit contribution from active processes in hair cells. Surprisingly, we find the basilar membrane supports two qualitatively distinct sets of modes: a continuum of localized modes and a small number of collective extended modes. Localized modes sharply peak at their resonant position and are largely uncoupled. As a result, they can be amplified almost independently from each other by local hair cells via feedback reminiscent of self-organized criticality. However, this amplification can destabilize the collective extended modes; avoiding such instabilities places limits on possible molecular mechanisms for active feedback in hair cells. Our work illuminates how and under what conditions individual hair cells can collectively create a critical cochlea.