Mathematical model of the auditory nerve response to stimulation by a micro‐machined cochlea

Abstract

We report a novel mathematical model of an artificial auditory system consisting of a micro‐machined cochlea and the auditory nerve response it evokes. The modeled micro‐machined cochlea is one previously realized experimentally by mimicking functions of the cochlea [Shintaku et al, Sens. Actuat. 158 (2010) 183–192; Inaoka et al, Proc. Natl. Acad. Sci. USA 108 (2011) 18390–18395]. First, from the viewpoint of mechanical engineering, the frequency characteristics of a model device were experimentally investigated to develop an artificial basilar membrane based on a spring–mass–damper system. In addition, a nonlinear feedback controller mimicking the function of the outer hair cells was incorporated in this experimental system. That is, the developed device reproduces the proportional relationship between the oscillation amplitude of the basilar membrane and the cube root of the sound pressure observed in the mammalian auditory system, which is what enables it to have a wide dynamic range, and the characteristics of the control performance were evaluated numerically and experimentally. Furthermore, the stimulation of the auditory nerve by the micro‐machined cochlea was investigated using the present mathematical model, and the simulation results were compared with our previous experimental results from animal testing [Shintaku et al, J. Biomech. Sci. Eng. 8 (2013) 198–208]. The simulation results were found to be in reasonably good agreement with those from the previous animal test; namely, there exists a threshold at which the excitation of the nerve starts and a saturation value for the firing rate under a large input. The proposed numerical model was able to qualitatively reproduce the results of the animal test with the micro‐machined cochlea and is thus expected to guide the evaluation of micro‐machined cochleae for future animal experiments.

Oscillation Characteristics of an Artificial Cochlear Sensory Epithelium Optimized for a Micrometer-Scale Curved Structure

“…It has been shown that PVDF-TrFE is more suitable for transducers than PVDF because the electromechanical coupling factor of PVDF-TrFE ( = 0.3 [ 34 ]) is higher than that of PVDF ( = 0.14 [ 35 ]), and the mechanical and dielectric loss tangent of PVDF-TrFE ( = 0.0335 and = 0.089 [ 36 ]) are lower than those of PVDF ( = 0.0789 and = 0.184 [ 36 ]). Three pairs of upper electrodes are used to obtain electrical signals based on biological tonotopy and control the oscillation through feedback control [ 10 , 17 , 18 , 29 ]. Because the electrodes are metallic and the piezoelectric film and elastic layer are organic materials, the adhesion of these materials is difficult.…”

“…However, two commonly known bottlenecks in the development of fully insertable cochlear implants are (1) the lack of power available to stimulate the nerve using biocompatible materials and (2) the difficulty of achieving a reproducible frequency range within the dimensions of the cochlea. We [ 17 , 18 , 29 ] and other groups [ 30 , 31 , 32 , 33 ] addressed the former issue by developing a nonlinear feedback control system that mimics the amplification of outer hair cells. However, there have been few studies of microdevices being implanted in the cochlea, and their performance, particularly in terms of mechanical dynamics, has yet to be thoroughly established.…”