Cochlear Macromechanical Modelling

Contemporary research into the manifestations and origins of nonlinear, active cochlear processes often takes place in a context in which linear, passive cochlear mechanics are poorly understood and poorly communicated. The distinctions among models of one-, two-, and three-dimensional fluid motion in the cochlear scala--models popularized by (among others) Zwislocki, Ranke, and Steele, respectively --are confounded by fuzzy use of terms such as "long-wave model" or "short-wave model." Models are frequently evaluated by comparing their place responses with experimentally observed frequency responses; their global impedance parameters are sometimes chosen solely to secure fit to some local measurement. And Steele's WKB (phase-integral) approach is treated, more often than not, as just another technique for solving cochlear dynamical equations, rather than as a conceptual framework yielding significant insight into cochlear phenomena. In this thesis, I present cochlear dynamical equations for one-, two-, and three-dimensional fluid motion in a box-cochlea model, and I discuss the conditions under which such fluid motion is appropriately described as long wave, short wave, or as something in between. I describe the phase-integral approximate solution to these equations and discuss the utility of this framework for explaining cochlear phenomena. I develop generalized representations for both cochlear-partition impedance and cochlear-gain response that highlight the distinctions and similarities between the place response at a single frequency and the frequency response at a single place. The generalized representations clarify which aspects of partition impedance determine global phenomena, such as cochlear maps, and which aspects determine local features, such as magnitude -response peakiness and phase-response steepness. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253 -1690.).