Problems in Coclear Micromechanics

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As part of a long-standing collaboration with NIDCD's Dr. Richard Chadwick and his group, we have been instrumental in a number of projects during the past year. Using primarily the atomic force microscope (AFM), we completed the measurements of the elastic modulus of the tectorial membrane (TM) whose role in hearing is critical but exact mechanism of action remains controversial. It was found that the TM is radially inhomogeneous and that such a property is favorable to the shearing efficiency of the hair cells -- the stimulatory action on the sensory cells. Another issue addressed has to do with the characteristic spiral shape of the mammalian cochlea that has mystified hearing researchers for many years. Various studies have been inconclusive and it has been argued that the only purpose of the shape is packaging of a long channel in a small volume. Some recent work, however, showed strong correlation between low frequency sensitivity and number of turns in the spiral. Our NIDCD collaborators were interested in studying a simplified mathematical model of a spiral cochlea to investigate the effect of the shape on various parameters of energy flow along the spiral. In this instance, computations showed that although energy flow along the spiral channel remains constant, the continuously changing curvature concentrates energy toward the outer channel wall. The energy focusing effect becomes progressively stronger toward the spiral apex where low frequencies are analyzed. Such a phenomenon could have profound effect on low frequency thresholds leading to a potential explanation of the evolutionary utility of the spiral shape in mammals. In collaboration with marine mammal experts at Woods Hole Oceanographic Institute we have analyzed histological sections, CT-scans and other anatomical images of a large number of mammalian cochleae to establish the hypothesized correlations between low frequency limits and total curvature change along the spiral. It was found that a simple measure of the spiral cochlea geometry correlates surprisingly well with the low frequency limit of both marine and land mammals. Such a correlation has implications for the study of the evolution of hearing but it may also help guide future cochlear implants toward more effective implementation.