Studies of cochlear mechanics: otoacoustic emissions

DESCRIPTION (provided by applicant): The normal ear not only detects but also generates sounds. These inner ear-generated sounds, i.e., otoacoustic emissions, can be measured using a tiny microphone in the ear canal. Among the various otoacoustic emissions, the cubic distortion product otoacoustic emission (DPOAE) has been widely used for infant hearing screening, the diagnosis and monitoring of hearing disorders, and for studying hearing mechanisms. It is widely believed that the DPOAE results from a reverse-propagating traveling wave along the cochlear partition from apex to base. Traveling waves at primary frequencies f1 and f2 interact in their overlap region and create acoustical energy at the 2f1-f2 frequencies, which propagates in both directions along the cochlear partition. The backward-traveling wave propagates to the stapes and the ear canal and appears as a component of the emissions with a relatively short delay. The forward-traveling wave propagates to the 2f1-f2 CF sites, where it is partially reflected and forms a second backward-traveling wave, which generates an emission component with a long delay. The emission measured in the ear canal is the sum of the long and short delay components. However, the proposed reverse-propagating traveling wave has not yet been demonstrated due to the lack of experimental means to directly measure the propagation direction of the traveling wave in sensitive cochleae. Using a recently developed scanning laser interferometer microscope, the following experiments will be conducted in this study: 1) measurement of the longitudinal pattern and the wave propagation direction and speed of the basilar membrane vibration at the emission frequency of 2f1-f2; 2) observation of the time relationship between the cochlear partition vibration near the f2 CF site and the stapes vibration at the emission frequency; 3) measurement of the wave propagation direction and speed of electrically evoked basilar membrane vibration to test if an introcochlear acoustic source induces a backward traveling wave. Findings of this study will advance our understanding on how the otoacoustic emission is transmitted in the cochlea, improve the interpretation of clinical otoacoustic emission test results, and contribute to general issues of cochlear mechanics.