Measurement of Slow Nanometer Movements in the Living Inner Ear

We present a novel technique for measuring sub-micrometre movements in the fluid-filled inner ear, using changes in the resistance of a saline-filled glass micropipette that passes through the middle ear and round window to touch inner ear structures of interest (including the basilar membrane). Our goal was to monitor slow mechanical changes to study cochlear autoregulation. The resistance-displacement transfer function of the microelectrode probe was approximately exponential, with a pseudo-linear range of many hundreds of nanometers. To eliminate mechanical drift, the animal's skull was stabilized using analog PID (proportional integral derivative) feedback, so that the noise floor of in vivo skull displacement was less than 10 nm peak to peak (broadband, raw trace), or much less than 1 nm using filtering or spectral analysis. The skull actuator also allowed landing of the electrode with sub-micrometre accuracy under electronic control, and a sinusoidal vertical skull movement ('the pilot') that served as a mechanical calibration. A novel ventilator was also developed using a subwoofer speaker to minimize breathing and pulse artifact. Examples of acoustic movements in the middle and inner ears are presented, including very slow movements of the basilar membrane in response to low-frequency tones. (C) 2018 The Author(s). Published by S. Hirzel Verlag.EAA.