Does the human cochlea work like a laser?

Abstract

The sharply tuned sense of hearing in humans is believed to be due to active amplification in the cochlea. One seemingly natural consequence of this ‘cochlear amplifier’ is the existence of spontaneous otoacoustic emissions (SOAEs), narrow-band tones that are detected in the ear canals of 33% to 70% of all normal-hearing individuals (1). The mechanisms underlying the features and generation of SOAEs have long been a subject of debate. Zweig and Shera (2) argue that SOAEs are created by multiple travelling-wave reflections between the middle ear boundary and a dense array of inhomogeneities scattered throughout the cochlea; Shera (3) likens this process to activity in a laser cavity. This theory is contrary to previous ideas which assume independently unstable oscillators in the cochlea. This work uses a state space formulation of the cochlea to test the predictions made by Zweig and Shera (2). The Elliott et al. model of the cat cochlea (4) has been revised to describe characteristics of the human cochlea. Linear instabilities arise across a wide bandwidth of frequencies when the smooth spatial variation of basilar membrane impedance is disturbed. The salient features of Zweig and Shera’s theory (2) are observed in this active model given perturbations in the distribution of feedback gain along the cochlea. A step change in gain is used to demonstrate system instability.

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ORCID for B. Lineton: orcid.org/0000-0003-4784-7762

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