| Frequency analysis is the first important stage in the process of hearing. The spirally-shaped cochlea located within the inner ear separates the various frequency components of the incoming sound signal, representing them along the length of the organ of Corti (OC). The specific details of this spatial frequency representation are determined by the “frequency-place map”, a scheme by which the OC vibrates selectively and maximally to high frequencies at the base of the cochlea, and to low frequencies at the apex. In this thesis, experimental measurements and modeling studies were combined to understand the various factors that provide the basis for the frequency-place map.; Fundamental mechanical properties of the OC were first characterized experimentally. In particular, the stiffness of the OC was measured along the cochlea. The data indicated that the stiffness changed by a factor of 100 from base to apex. The nature of longitudinal coupling within the basilar membrane (BM) was also studied. These results indicated not only that the BM was longitudinally coupled, but also that the degree of coupling increased from base to apex. The implications of these experimental results were explored using a biophysically based 1-dimensional cochlear model. The resulting model predictions were in good agreement with various types of experimental data.; Finally, a model of the BM was proposed based on its specialized architecture, which resembles that of a reinforced slab of concrete. The radial tension acting on the BM was then calculated from the stiffness of the OC. The changing tension together with the changing geometry [Edge et al, 1998; Schweitzer et al, 1997] of the OC determined a frequency-place map that was in good agreement with physiological data from the adult gerbil Müller, 1996]. Assuming the absence of tension during early development, the map of the young gerbil was predicted using geometry alone, and compared with that of the adult. The difference between the two maps was consistent with the frequency shift observed as the cochlea matures [Echteler et al, 1989; Müller, 1996] and suggests that the tension is established during development. These compelling results support a simple and direct mechanism that potentially explains the origin of frequency-place map. |
