A novel mechanism of cochlear excitation during simultaneous stimulation and pressure relief through the round window

Abstract

The round window (RW) membrane provides pressure relief when the cochlea is excited by sound. Here, we report measurements of cochlear function from guinea pigs when the cochlea was stimulated at acoustic frequencies by movements of a miniature magnet which partially occluded the RW. Maximum cochlear sensitivity, corresponding to subnanometre magnet displacements at neural thresholds, was observed for frequencies around 20 kHz, which is similar to that for acoustic stimulation. Neural response latencies to acoustic and RW stimulation were similar and taken to indicate that both means of stimulation resulted in the generation of conventional travelling waves along the cochlear partition. It was concluded that the relatively high impedance of the ossicles, as seen from the cochlea, enabled the region of the RW not occluded by the magnet, to act as a pressure shunt during RW stimulation. We propose that travelling waves, similar to those owing to acoustic farfield pressure changes, are driven by a jet-like, near-field component of a complex pressure field, which is generated by the magnetically vibrated RW. Outcomes of research described here are theoretical and practical design principles for the development of new types of hearing aids, which use near-field, RW excitation of the cochlea.

Drug distribution along the cochlea is strongly enhanced by low-frequency round window micro vibrations

“…The RW probe diameter (0.5 mm) and its cross-sectional area (0.2 mm 2 ) were much smaller than the dimensions and area of the RW in guinea pigs (Ghiz et al, 2001;Wysocki et al, 2005), and the probe covered only a small part of the RW. Under these conditions, most of the pressure relief during the probe movement was through the RW area not occluded by the probe (Weddell et al, 2014) and the average alternating far-field pressure, M , generated within the cochlea was small. This was confirmed by the absence of stapes responses, which were below the measurement noise floor (~ 0.1 nm)…”

“…during the probe vibrations at RW either at 2 or 4 Hz. The magnitude of M will depend on the stiffness, , of the freely moving area, , of the RW and its volume velocity, , as (Weddell et al, 2014) = 2 .…”

“…Vibrations of the partially occluded RW at acoustic frequencies excite the basilar membrane with conventional travelling waves. A jet-like, near-field component, N , of a complex pressure field near the RW is the proposed mechanism of stimulation (Weddell et al, 2014). N is proportional to the fluid density, , and to the acceleration of the probe, , and an indicative overall magnitude of N can then be defined as = .…”

“…Under these conditions, most of the pressure relief during the probe movement is through the RW area not occluded by the probe. The generated far-field pressure component is small and cochlear excitation is due mainly due to RW near-field pressure, which excites a conventional travelling wave at acoustic frequencies (Weddell et al, 2014). Had a significant far-field pressure been generated, it would cause a stapes movement.…”

Cochlear excitation by the near-field component during stimulation through the partially occluded round window

“…1b in which an inner piston with a radius of has a velocity of and an annular piston with an inner radius of and an outer radius of has a velocity of . The total nearfield pressure is the superposition of those due to the inner and outer pistons (for detailed derivation see [17])…”