Using otoacoustic emissions to evaluate efferent auditory function in humans

The auditory system continually adapts to changes in the acoustic environment over short periods of time. This fine-tuning of its dynamics is mediated in part by the medial olivocochlear (MOC) bundle, a neural feedback loop which aids in the regulation of cochlear micro-mechanics. The ability to measure the response of the MOC system in humans may provide significant insight into unique cochlear functions, such as its sharp frequency selectivity and wide dynamic range. In humans the efferent system can be investigated non-invasively using otoacoustic emissions (OAEs). However, how OAEs can best be used to evaluate efferent function, the pitfalls associated with such measurements, as well as the relationship between OAEs and perception are not fully understood. This dissertation presents three experiments that explore the use of OAEs to assess efferent function in humans. The first study examined the advantages of separating the major components of distortion product otoacoustic emissions (DPOAEs) when evaluating efferent function using contralateral acoustic stimulation (CAS). CAS-induced activation of the medial olivocochlear reflex (MOCR) was found to produce both reductions and enhancements of total DPOAE level. Analysis of the separated components of the DPOAE revealed that these changes could be accounted for by the contribution of an efferent-induced phase change in the reflection component of the DPOAE. In the second, analysis of DPOAE primary level and phase changes over a wide range of CAS levels used to induce MOCR revealed that middle ear muscle reflex (MEMR) activation could be simultaneously monitored. CAS levels commonly used to elicit MOCR could also elicit MEMR responses, which results in contamination of MOCR estimates. Finally, a novel technique to measure simultaneous OAEs and masked behavioral thresholds is presented and used to investigate a perceptual phenomenon thought to be associated with an efferent activation. While a direct association between physiological and behavioral masked thresholds was not observed, a strong relationship was found between the physiological measure of masked thresholds and a measure of CAS-induced efferent suppression, suggesting that although efferent-mediated suppression of basilar membrane mechanics is related to the phenomenon, more central mechanisms may be required to modulate the perceptual response