Solution of the inverse problem for a linear cochlear model: a tonotopic cochlear amplifier

The extraordinary fine-tuning characteristic of normal mammalian hearing is attributed to physiological mechanisms collectively known as the cochlear amplifier (CA), which amplifies and sharpens the basilar membrane (BM) vibration response to incoming acoustic pressure oscillations. Electromechanical properties of outer hair cells (OHCs) are believed to be the critical component of the CA, but its "circuitry" as yet remains unknown. Here, the required frequency-space response characteristics of the CA are computationally determined when typical in vivo tuning data are introduced as input to a linear hydroelastic cochlear model whose cross-sectional dynamics are represented by two coupled vibrational degrees of freedom. It is assumed that the CA senses motion at the tectorial membrane (TM) reticular lamina (RL) and applies proportional, equal, and opposite forces to the BM and the RL. The results show the CA to be tonotopically tuned, meaning it conforms to a space-frequency similarity principle like other cochlear dynamical responses. This requires that the active mechanism use information distributed along the cochlear partition. The physiological mechanism responsible for such behavior remains unknown, but here the computed CA characteristics can be qualitatively reproduced by a circuit spanning the length of the cochlea. This does not preclude other mechanisms, but is intended to motivate closer experimental investigation of extracellular and intercellular ionic flow pathways.