Feasibility of direct promontory stimulation by bone conduction: A preliminary study of frequency-response...

As an alternative pathway to air conduction, bone conduction is a multipathway process that transmits sound energy to the inner ear through the skull in general. Based on this mechanism, bone conduction devices (BCDs) have been used widely in the rehabilitation of hearing loss. Although great efforts have been devoted to improving BCDs, drawbacks still exist in most categories of BCDs due to the complicated process of bone conduction. We hypothesized that if a bone conduction transducer was placed on the cochlea to stimulate it directly, the attenuation would be minimized, and the frequency dependency would be different from that of the vibratory response induced by traditional BCDs. This study aimed to explore the feasibility of direct promontory stimulation and to investigate its frequency-response characteristics.

Measurements were conducted in twelve cat ears. To stimulate the promontory directly, the floating mass transducer (FMT) of the Vibrant Soundbridge^© (VSB) implant was glued to the promontory coupled with an oval window (OW) coupler. Auditory brainstem response (ABR) and laser Doppler vibrometry (LDV) measurements were used to evaluate the auditory response induced by the FMT. In both measurements, the FMT was driven by direct voltage stimuli.

ABR waves could be induced under direct promontory stimulation by the FMT. In the frequency range of 1-12 kHz, the variation in the voltage threshold level were limited to 16 dB SPL with a maximum of 0.2 V at 1 kHz and a minimum of 0.04 V at 10 kHz. In the LDV measurements and the relative motion of the round window membrane (RWM) and the promontory were used to evaluate the cochlear response. The LDV results indicated a weak frequency dependency from 1 to 12 kHz.

Different from traditional stimulation via transcranial bone conduction, direct promontory stimulation is a new method in which a small bone conduction transducer stimulates the cochlear shell directly. The current experimental data demonstrate that it is feasible to generate sensations through bone conduction by stimulating the cochlea directly. Furthermore, the cochlear response induced by this type of stimulus in cats was weakly frequency dependent at frequencies ranging from 1 to 12 kHz. This study may provide a basis for the design of new transducers that can perform well over a wide range of frequencies.