Use of new measurements to develop a model of bone conduction in animal and human

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Abstract Bone conduction (BC) is the transmission of sound to the inner ear by way of skull vibration. Both bone conduction and air conduction (AC), the usual pathway by which sound reaches the inner ear, stimulate the organ of hearing within the cochlea in the same manner. Bone conduction is important clinically, as it is used to diagnose and treat chronic and congenital middle-ear disease. BC hearing aids overcome conductive hearing loss by bypassing the middle ear, allowing the inner-ear mechanisms of BC to stimulate the sensory hair cells of the cochlea. Bone-conduction headphones are being used in noisy environments to aid in communication while simultaneously allowing for the use of hearing protection to AC stimulus. Additionally, BC headphones may be used when it is necessary to maintain an open external ear, so as to not compromise AC hearing. In order improve such devices and to develop more-controlled tests for hearing loss, we need a complete model of BC stimulation of the normal and pathological ear. BC hearing comprises three major components that act primarily on the 1) external, 2) middle, and 3) inner ear. Their relative contributions to hearing are not fully understood. We aim to quantify the contributions of the external- and inner-ear components in chinchilla, as they are comprised of several mechanisms, and to develop and test a network model for BC hearing in chinchilla and human. The contribution of the inner-ear mechanisms (fluid inertia, compression by bone, and transmission of sound pressure via cerebrospinal fluid) will be determined from measurements of intracochlear pressures (stapes velocity and cochlear sound pressure) during BC stimulation. Contribution of the external-ear mechanisms (ear canal compression, motion of the tympanic membrane with respect to the skull bone, and vibration of the jaw bone) will be determined from measurements of ear canal sound pressure and tympanic membrane velocity during AC and BC stimulation. For further understanding of BC mechanisms, we optimize a simple model of the middle ear to fit modern AC sound data from chinchilla, an animal with human-like middle-ear structures and frequency range of hearing that has been used to study noise-related hearing loss. We add suitable BC sources to our model, with their frequency dependence defined by our measurements. We will optimize our model parameters using novel techniques we have developed. The model will be generalized to a human BC model, and tested and optimized using existing human BC data. Our model will allow us to investigate the effects of ear disease on hearing and to develop better BC-based devices for both treatment of ear disease and alternative modes of communication.