The cochlea-generated sounds, otoacoustic emissions (OAEs), have been routinely measured as a non- invasive tool for diagnosing hearing loss in humans and for studying cochlear mechanisms in experimental animals. However, underlying mechanical mechanisms of OAE generation and suppression remain unclear. Recent technological breakthroughs in low-coherence interferometry allow us to measure vibrations inside the cochlear partition in living cochleae and genetically engineered mouse models make it possible to study molecular mechanisms of cochlear micromechanics. The objective of this study is to determine the cellular origin and sub-cellular mechanisms responsible for generation of distortion product (DP) OAE (DPOAE), the most commonly used OAE, by conducting a series of novel in vivo experiments using a custom-built scanning heterodyne low-coherence interferometer. Our overarching hypothesis is that, in mammals, nonlinear mechanoelectrical transduction of outer hair cells generates electrical DPs and somatic motility converts them into mechanical DPs and DPOAEs. DPOAE suppression results from suppression of the primary tone-induced traveling waves, and the DPOAE suppression tuning curve (STC) is related to but different from cochlear mechanical tuning. This central hypothesis will be tested by conducting the following experiments. Experiment One will determine the cellular origin of DPs by measuring vibrations from the reticular lamina (RL) at the apical ends of outer hair cells and from the basilar membrane (BM) at DP frequencies in healthy cochleae. Data showing that RL DPs are larger than BM DPs will be the first in vivo demonstration that DPOAEs are generated by outer hair cells. Experiment Two will determine whether or not somatic motility of outer hair cells generates DPs in vivo by measuring RL and BM responses to electrical stimulation with two-frequency currents in alpha-tectorin protein mutant (TectaC1509G/C1509G) mice. Since these mice have functional somatic motility and ineffective hair-bundle motility and mechanoelectrical transduction due to the deformed tectorial membrane, data showing the lack of electrically evoked RL and BM DPs will indicate that somatic motility does not generate DP in vivo. The third experiment will study the mechanical mechanism of DPOAE suppression and determine the relationship between the DPOAE STC and cochlear mechanical tuning. The STC of DPOAE at 2f1-f2 will be measured and compared to the STCs of the BM f2 and RL f2 and iso-response curves of the BM and RL. The proposed experiments will demonstrate the origin of DPOAEs and reveal mechanical mechanisms of DPOAE suppression. Results on the relation of the DPOAE STC to cochlear mechanical tuning can potentially benefit patients and basic science research by gaining precise information on the cochlear active process through objective and noninvasive DPOAE measurement.
Using an innovative technique, this project will measure acoustically and electrically evoked mechanical distortion products inside the cochlear partition in normal and genetically modified cochleae. The proposed experiments will demonstrate the cellular and sub-cellular origins and the suppression mechanism of distortion product otoacoustic emissions. Results from this study can benefit patients and auditory research by precisely assessing cochlear mechanical conditions through objective and noninvasive otoacoustic measurements.
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