Making an effort to listen: mechanical amplification in the ear

Area of Science:

• Auditory Neuroscience
• Bioacoustics
• Mechanobiology

Background:

• The inner ear's active process enhances hearing via amplification, frequency selectivity, compressive nonlinearity, and spontaneous otoacoustic emission.
• These features are explained by mechanoelectrical transduction operating near a Hopf bifurcation.

Purpose of the Study:

• To elucidate the mechanisms underlying the four key features of the inner ear's active process.
• To differentiate the active processes in nonmammalian tetrapods and mammalian cochleae.

Main Methods:

• Mathematical modeling of mechanoelectrical transduction near a Hopf bifurcation.
• Analysis of active hair-bundle motility in nonmammalian tetrapods.
• Investigation of outer hair cell electromotility in mammalian cochleae.

Main Results:

• The Hopf bifurcation naturally explains the four characteristics of the ear's active process.
• Nonmammalian tetrapod active processes rely on hair-bundle motility driven by negative stiffness and adaptation motors.
• Mammalian high-frequency active processes are dominated by outer hair cell electromotility involving prestin.

Conclusions:

• The study provides a unified framework for understanding the active process in the inner ear across different species.
• The findings highlight the distinct yet functionally convergent mechanisms of auditory enhancement in vertebrates.