Measuring sound-induced motions of the alligator lizard cochlea

Abstract

The sensitivity and frequency selectivity of the hearing sense are determined primarily by mechanical properties of the cochlea. These mechanical properties are poorly understood in any species. This thesis contributes to our understanding of cochlear mechanics by presenting measurements of sound-induced motion of the alligator lizard cochlea. Novel methods were developed to maintain the cochlea in vitro for the time required to measure three-dimensional motions. Three-dimensional images of cochlear motion were taken by illuminating the cochlea with a light-emitting diode stroboscopically at predetermined phases of the acoustic stimulus. The resulting images were analyzed using computer vision algorithms to extract three-dimensional motions of all visible structures with nanometer precision. The sound-induced motion of the entire basilar papilla and of individual hair bundles of hair cells were simultaneously measured. The basilar papilla, in which the hair cells reside, moved as a rigid body, exhibiting simultaneous translational and rotational modes of motion. Both modes apply shearing forces to hair bundles. A simple mechanical model of the basilar papilla, based on these measurements, provides a physical basis for a mechanical low-pass filter hypothesized in previous models. In the tectorial region of the cochlea, motion of the tips of hair bundles and of the tectorial membrane (TM) were in phase with motion of the basilar papilla. None of the motions had significant frequency dependence, suggesting that this region does not exhibit appreciable mechanical frequency selectivity. In the free-standing region, which has no TM, hair bundle deflection depended on stimulus frequency and hair bundle height.

(cont.) At high frequencies, hair bundle deflection was proportional to basilar papilla displacement. At low frequencies, hair bundle deflection was proportional to a linear combination of basilar papilla velocity and acceleration. Measured hair bundle deflections were well fit by a simple hydrodynamic model (Freeman and Weiss, 1990) of this region of the cochlea. The measurements in this study provide the first characterization of the three-dimensional motion of all structures in a vertebrate cochlea.