Cochlear micromechanical mechanisms underlying psychoacoustic phenomena

Project Summary/Abstract Although the pitch of missing fundamentals has been studied for more than a century, and psychophysical tuning curves and gap detection have been used as non-invasive tests for diagnosing auditory disorders, the cochlear mechanical mechanisms underlying these psychoacoustic phenomena remain unclear. This project aims to study the cochlear micromechanical mechanisms contributing to the pitch of missing fundamentals, psychophysical tuning curves, and gap detection by measuring the reticular lamina (RL) vibration from apical ends of the outer hair cells (OHCs) and the auditory nerve potential in living gerbil cochleae. Our overarching hypothesis is that the mechanical missing fundamental is generated by OHCs and can be detected at the cochlear locations tuned to stimulus frequencies. Masker-induced suppression and enhancement and poststimulation oscillation of the RL vibration contribute to the neural tuning curves and gap detection. This hypothesis will be tested by conducting the following experiments. Experiment One will measure the cochlea- generated fundamentals in the RL vibration and auditory nerve potential and observe the effect of a noise masker on the RL and auditory nerve fundamentals. The unmaskable RL and auditory nerve fundamentals will indicate that the cochlea can generate the fundamental, and this cochlea-generated fundamental can be detected at cochlear locations tuned to stimulus frequencies. Experiment Two will measure the RL mechanical tuning curves, the auditory nerve simultaneous and forward masking tuning curves, and the masker-induced changes in the RL probe response. The results from this experiment will determine whether the simultaneous or forward masking tuning curve can accurately reflect the RL mechanical tuning curve and whether the masker-induced changes in the RL probe response contribute to the cochlear compound action potential (CAP) tuning curves. Experiment Three will measure the RL poststimulation vibration and compare the duration of this persistent vibration to the gap detection threshold. The expected result from this experiment will indicate that the RL poststimulation can excite the inner hair cells and auditory nerve fibers during the gap period, which affects the auditory nerve response to the stimulus after the gap. Thus, the results from this study will reveal cochlear micromechanical mechanisms underlying the pitch of missing fundamentals, psychoacoustic tuning curves, and gap detection. The knowledge gained from this project can benefit patients by optimizing procedures and improving the interpretation of psychoacoustic tests in clinics.