In normal hearing, sound reaches the cochlea via the tympanic membrane and middle ear, i.e. air conduction (AC). A secondary mode of hearing, known as bone conduction (BC), bypasses the conductive pathway to cause pressure fluctuations within the cochlea primarily via vibration of surrounding bone. While the contributions of BC to audition under normal listening conditions are minimal, BC signals are readily passed to the cochlea using mechanical transducers, and hearing tests employing such transducers have been used for over a century to diagnose conductive hearing pathologies. For individuals with conductive hearing loss, BC stimulation via modern BC hearing aids can improve audibility in the affected ear(s). BC hearing aids may also be worn in cases of single-sided deafness, wherein a BC hearing aid on the deaf side transmits the signal through the bones and tissues of the head to the opposite (hearing) cochlea. Indeed, transcranial cross-talk is a key feature of BC stimulation, and traditionally even in cases of bilateral conductive loss, only a single BC device is worn. Bilateral BC stimulation results in superposition of two signals at each ear, which is expected to limit binaural disparities and associated perceptual benefits (sound localization, speech-in-noise perception). However, non-zero binaural disparities via bilateral BC have been reported in several biophysical studies, and improved sound localization and speech-in-noise perception with bilateral versus unilateral BC have been reported in several psychophysical studies. While efforts to understand and exploit binaural and spatial hearing via BC have thus increased in recent years, basic constraints on performance remain poorly understood. The current proposal aims to fill this knowledge gap. A first set of experiments (Aim 1) will quantify acoustic information conveyed via bilateral BC devices using measurements of intracochlear pressure, the input drive to the auditory system, in cadaveric specimens. A complementary set of measurements will be made during simultaneous AC and contralateral BC stimulation, simulating the signal presented to single-sided deaf BC hearing aid users, who show variable but generally poor outcomes in spatial tasks. A second set of experiments (Aim 2) will determine the limits of normal-hearing psychophysical sensitivity to binaural information conveyed via bilateral BC across key stimulus parameters, including frequency, bandwidth, and spatial cue type. Additional measurements of BC performance in everyday spatial tasks, including sound localization and speech-in-noise perception, will be completed and related to intracochlear and basic psychophysical data. Collectively, the proposed experiments will evaluate the overarching hypotheses that (1) BC signal superposition within the cochlea(e) generates systematic spatial cues, and that (2) such cues can be used to systematically modulate perception. Data will provide critical new insight on factors that constrain spatial hearing via BC, and point to strategies for improvement of BC hearing aid technology and outcomes.
Hearing loss is a serious health problem that can make communication and everyday tasks more difficult. While many types of hearing loss can be treated with conventional hearing aids, some types of hearing loss are best treated with a bone conduction hearing aid that vibrates the skull behind the ear, typically worn on only one ear even if loss is present in both ears. The goal of this research proposal is to understand what happens when bone conduction signals are delivered to one or both ears, with the goal to improve treatment outcomes for bone conduction hearing aid users in the future.!
