Auditory Function at the Base of the Human Cochlea

Northwestern University, Evanston IL

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Abstract

DESCRIPTION (provided by applicant): Hearing loss due to aging and other damaging exposures often progresses from high to low frequencies. The ability to detect these progressive and cumulative forms of hearing loss, as well as to evaluate strategies for prevention or intervention, therefore requires diagnostic tools that are capable of probing cochlear status up to the highest range of human hearing. However, our knowledge of the physiology and mechanics underlying auditory function at the basal end of the human cochlea, where high frequencies are encoded, is limited. In addition, the basic properties and potential utility of objective measures f hearing function, such as otoacoustic emissions (OAEs), faint sounds recorded in the ear canal that originate from active, outer hair cell- mediated processes in the cochlea, have been relatively unexplored at frequencies above the conventional audiometric range (> 8 kHz). In particular, there are few reports of OAEs evoked by single tones, termed stimulus-frequency OAEs (SFOAEs), at high frequencies. It is unknown whether SFOAE generation mechanisms extend to the basal end of the cochlea, how SFOAE properties are related to cochlear status above 8 kHz, and whether the amount of SFOAE energy reaching the ear canal is especially limited at high frequencies by characteristics of middle-ear transmission. To address these questions, we propose to (1) determine how SFOAE amplitudes are related to the level and configuration of behavioral pure-tone hearing thresholds at frequencies above 8 kHz, and (2) compare SFOAE amplitudes to the strength of a related audiometric phenomenon, termed threshold fine structure, across a wide range of frequencies. This fine structure is thought to arise when a portion of the SFOAE evoked by the test tone is not transmitted out to the ear canal, but instead is reflected back at the middle-ear boundary, thus re-stimulating the cochlea. Through detailed investigation of these phenomena in clinically normal-hearing adults, we expect that SFOAE amplitudes will be significantly correlated with behavioral hearing thresholds, and will be markedly reduced in the presence of sloping threshold configurations, suggesting that SFOAEs are highly dependent on cochlear status. In addition, we expect that SFOAE amplitudes and the strength of threshold fine structure will co-vary in a similar fashion both above and below 8 kHz, suggesting that transmission via the middle ear is not especially limiting at high frequencies. The results will critically inform theories of OAE generation, cochlear mechanics and middle-ear transmission, and will aid in the development of OAE-based diagnostic tools for assessing cochlear function at frequencies above the conventional audiometric range.