Frequency structure in intracochlear voltage supports the concept of tectorial membrane mechanical resonance

Abstract

The mechanical and electrical responses of the mammalian cochlea to acoustic stimulus are nonlinear and tuned. This is reflective of the electromechanical response of the outer hair cells (OHC) which are responsible for mediating the active process necessary for normal hearing. In this paper, we use the experimental data in conjunction with simulations to study nonlinear amplification in the cochlea. The sound-evoked voltage inside the scala tympani was measured for a range of frequencies and pressure levels. The experimental data show a notch in the voltage response at frequencies below the characteristic frequency (CF). This notch is seen to locate the onset of nonlinear amplification (in the frequency domain). In order to interpret the experimental data, a comprehensive three-dimensional model of the cochlea is used. Our model predicts the notch in the extracellular voltage as well as the phase relations observed from the experiments. The notch frequency in the model corresponds to the tectorial membrane (TM) radial resonance obtained when the TM is attached to the limbus but uncoupled from the OHC cilia. At this resonance, the shearing force applied to the HB by the TM is minimum, resulting in smaller HB deflection, current transductions, and the notch in voltage. Our analysis of the model results shows that the phase shift around this notch introduces the correct phasing between mechanical and electrical responses for effective power amplification. Hence, the model results implicate a central role of the TM in amplification.