EXPOSING MECHANICAL IMPEDANCE OF THE COCHLEAR PARTITION

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The cochlea is selective to individual frequencies of sound pressure, as sensitive as physical limits allow, and instantaneously adaptive to a wide range of stimulation levels. The basis for cochlear operation must lie in the mechanical and electromechanical properties of the cochlear partition, but is poorly understood. The proposed work seeks to characterize mechanical properties of the partition, especially as they relate to the frequency tuning and sensitivity of cochlear motion. The experimental animal will of the Mongolian gerbil, whose anatomy allows a 0.5mm longitudinal extent of the cochlear partition to be accessed through the round window opening. Two sets of measurements will be performed, both directed at determining the partition's mechanical impedance, which is defined as the ratio of pressure or force across the partition to the motion of the partition. In the first set of measurements, the impedance will be directly measured at a point as the force required to locally vibrate the partition. Point impedance will be measured at nine radial positions spanning the partition, and at two to three longitudinal positions over 0.5 mm. Vibration frequencies will vary from 100 to 1000 Hz. The radial variation in impedance will be directly related to the partition anatomy, indicating which partition elements contribute substantially to the impedance. The longitudinal variation in impedance will be analyzed with respect to the predicted variation, based on longitudinal variations in partition width and characteristic frequency. The second set of measurements map intracochlear pressure in scala tympani in three dimensions. Scala tympani pressure will be measured through the round window opening, close to the cochlear partition, at frequencies through the characteristic frequency (30 - 40 kHz in this region). Scala tympani pressure will be referenced to pressure at one point in scala vestibuli, relatively remote from the partition. The spatial, level, and frequency dependent variations in pressure will inform cochlear models by constraining the form of the partition impedance, and addressing the issue of active-vs-passive cochlear operation. The concerted analysis of the two sets of measurements will suggest modes of partition motion, and the micromechanical basis for that motion. The cochlea is a unique organ, but its nonlinear, dynamic operation reflects general principles and strategies for signal retrieval, sorting and transduction. The goal of these measurements is to understand the complex workings of the cochlea, which, beyond its intrinsic value, is of fundamental value to the understanding of physiological systems, with extension to the design of prosthetics and sensing systems in general.