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The use of spontaneous and evoked otacoustic emissions is now a standard clinical tool for diagnosis of the function of the inner ear. However, it is not possible to extract this information over the entire, functionally relevant frequency range because of imperfect coupling of: (1) stapedial to ear-drum vibrations through the ossicular chain of the middle ear and (2) ear-drum vibrations to air in the external auditory meatus. The problem could be circumvented if it were possible to measure the vibration of the stapes and ear drum. The ear drum can be visualized non-invasively, whereas the stapes is only accessible intra-operatively. Therefore, we designed a laser-interferometric system to non-invasively measure the vibration of the human ear drum. Vibrations were measured with a laser Doppler velocimeter (Polytec OFV-302) coupled into the side arm of an operating microscope (Zeiss OPMI MDM). The wavelength was 633 nm and emitted power was less than 1 mW. Direct coupling through the optics of the operating microscope, instead of through glass fibers, enabled a larger signal-to- noise ratio (20 - 30 dB) due to collection of more reflected light. This coupling scheme avoids the problems associated with having to place a reflecting material on the ear drum. The developed vibration measurement system allows non-invasive, fast and reproducible characterization of the dynamics of the human ear drum and as such can be used for clinical diagnostics.

Recent advances in optical techniques for vibration measurement have made it feasible to purchase commercially available laser Doppler velocimeters that, in combination with a light microscope, are capable of detecting velocities of poorly reflecting biological materials down to fractions of a micrometers s-1. Thus, in our particular application, the aim is to understand the micromechanics of the inner ear, particularly in the nanometer region where the ear is first able to detect sound. With currently available devices, it is now possible to make the necessary vibration measurements without need of introducing reflecting materials onto the object of interest. This not only avoids inertial loading, but also allows focusing through cellular layers onto otherwise inaccessible structures. Here we present the results of micromechanical experiments for the in vitro inner ear of the guinea pig, the most commonly used model in auditory physiology. Our experiments concentrate on the third and fourth cochlear turns because in this region the different cellular structures of the organ of Corti are optically accessible with present technology. The mechanical responses of all examined structures, including the basilar membrane, were equally frequency selective, but linear below 90 dB SPL, suggesting that active mechanical tuning is not as pronounced in the apex as in the base of the cochlea. This has important consequences for signal processing in the inner ear.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.