Middle ear pressure gain and cochlear input impedance in the chinchilla

Abstract

Measurements of middle ear conducted sound pressure in the cochlear vestibule PV have been performed in only a few individuals from a few mammalian species. Simultaneous measurements of sound-induced stapes velocity VS are even more rare. We report simultaneous measurements of VS and PV in chinchillas. The VS measurements were performed using single-beam laser-Doppler vibrometry; PV was measured with fiber optic pressure sensors like those described by Olson [JASA 1998; 103: 3445-63]. Accurate in-vivo measurements of PV are limited by anatomical access to the vestibule, the relative sizes of the sensor and vestibule, and damage to the cochlea when inserting the measurement device. The small size (170 [mu]m diameter) of the fiber-optic pressure sensors helps overcome these three constraints. PV and VS were measured in six animals, and the middle ear pressure gain (ratio of PV to the sound pressure in the ear canal) and the cochlear input impedance (ratio of PV to the product of VS and area of the footplate) computed. Our measurements of middle ear pressure gain are similar to published data in the chinchilla at stimulus frequencies of 500 Hz to 3 kHz, but are different at other frequencies. Our measurements of cochlear input impedance differ somewhat from previous estimates in the chinchilla and show a resistive input impedance up to at least 10 kHz. To our knowledge, these are the first direct measurements of this impedance in the chinchilla. The acoustic power entering the cochlea was computed based on our measurements of input impedance. This quantity was a good predictor for the audiogram at frequencies below 1 kHz.