Middle-ear mechanics in chinchilla when stimulated in reverse: Predictions and measurements

Abstract

The properties of the middle ear (ME) when driven from the cochlea (“in reverse”) are important for evaluating otoacoustic emissions (OAEs) and may be quite different from middle-ear function with normal (“forward”) sound transmission. In chinchilla, a species commonly used for auditory research (especially for noise hazard and OAE studies), we measured ear-canal and inner-ear sound pressures and stapes velocity while stimulating the middle ear with sound or in reverse with an actuator on the round window. We compute (1) admittances at the border between the middle and inner ear: the cochlear input admittance YC, the load seen by the ME with normal sound transmission, and the reverse middle-ear input admittance YMER, the load the ME exerts on the cochlea for reverse transmission such as OAEs, and (2) a metric of middle-ear function with reverse stimulation: The reverse ME pressure gain GMEPR between the cochlear vestibule and the ear canal, for different ear canal conditions. The sensitivity of GMEPR and YMER to changes in ear canal termination provides insight into the effect of ear-canal conditions on OAEs and flexibility in the middle ear, while a comparison of the admittances provides an estimate of power absorption and reflection at the boundary between the middle and inner ear. Measurements support predictions from a middle-ear two-port transmission-matrix model based on measurements with forward stimulation.