Characterization of Bone-Conduction Mechanisms in Chinchilla Using in Vivo Measurements and Impedance Models

Abstract

The mechanisms of bone-conduction hearing in chinchilla that result from vibration of the skull, include ear-canal compression, relative motion between the middle-ear bones and the inner ear, compression of the cochlear bone, and the transmission of intracranial sound pressures into the inner ear via fluid-filled connecting pathways. This work aims to characterize these mechanisms in terms of the magnitude and phase of vibration-driven sound pressure and volume-velocity sources within the auditory periphery. A lumped-element circuit model of air-conduction hearing in chinchilla is developed, which serves as the basis for our bone-conduction model. The air-conduction model is adapted from a model of hearing in humans, developed by Zwislocki (1962). The chinchilla model is extended by the addition of an ear canal that both contains multiple external-ear bone-conduction sources, and imposes natural impedances on motions of the TM produced by vibration-driven sources within the external, middle and inner ear. The model is further modified by the addition of realistic cochlear scalae, a helicotrema and vestibular and cochlear aqueducts, all of which are defined by the analysis of micro-CT scans of a chinchilla ear. The multiple vibration-driven bone-conduction sources are characterized by measurements of vibration-induced mechanical, acoustic, and/or neurological responses, in normal- or manipulated-ear conditions. The measurements under the various conditions enable separation of system responses resulting from individual sources. Two external-ear bone-conduction sources, which define the contribution of the bony and cartilaginous walls of the ear canal to vibration-driven sound pressures within the canal, are fully characterized. These sources are shown to dominate the vibration-induced sound pressures within the ear canal. The effect of vibration-driven intracranial sound pressures transmitted to the inner ear via the vestibular and cochlear aqueducts is estimated from measurements and the model. Our analyses suggest this mechanism does not play a significant role in vibration-induced hearing mechanics. A method for differentiating the contributions of cochlear compression and cochlear-fluid inertia to bone-conduction hearing is offered, and an application of this method is demonstrated using a proposed cochlear network that includes such mechanisms.