The cochlea "transforms" sound-induced vibrations of the middle ear into patterns of neural pulses travelling to the brain along the fibers of the auditory nerve. This "transformation" happens by means of complex and nonlinear coupled mechanical, electrical and chemical mechanisms. To further complicate matters, the mechanical and electro-chemical properties of the cochlea depend upon delicate active processes, rendering the experimental determination of the physical properties of the intact cochlea extremely difficult. For these reasons,although many response properties of the cochlea are well established, their physical interpretation is less so. Despite the difficulties in modelling the detailed functioning of the cochlea, mathematical models of cochlear processing can guide the interpretation of experimental results. Following this idea, this thesis employs mathematical models to shed light on three aspects of cochlear processing that cannot be observed directly. First, this thesis examines the dynamics of the nonlinearity that causes a compressive growth of basilar membrane vibrations in response to increasing sound levels. In particular, studies showing non-instantaneous distortions in basilar membrane recordings suggested that the underlying nonlinearity operates similarly to an automatic gain control characterised by a finite activation time constant. In contrast, the analysis presented here concluded that (i) a finite activation time of the basilar-membrane nonlinearity produces opposite trends than those observed experimentally and (ii) instantaneous nonlinearities are capable of explaining the data well. Second, the differences between the frequency-tuning of the auditory nerve fibers and that of the inner-hair-cell stereocilia are examined using a model of inner-hair-cell mediated mechanical-to-neural transduction. In this way, it is possible to approximately estimate the frequency-tuning of the inner-hair-cell stereocilia in vivo, that is otherwise not directly measurable. Finally, this thesis examines how the nonlinear activation of K+ currents in the inner-hair-cell basolataral membrane affects the responses of the afferent auditory nerve fibers. The present results suggest that the nonlinearities in the inner-hair-cell basolateral membrane, typically neglected in previous models of mechanical-to-neural transduction, play an important role in determining basic response properties of the auditory nerve. In conclusion, the aforementioned findings are employed to improve existing models of the human cochlea, and results are compared against non-invasive measurements in humans.
|Translated title of the contribution||Using models to deduce the functioning of the mammalian cochlea|
|Publication status||Published - 2018|
|MoE publication type||G5 Doctoral dissertation (article)|
- inner hair cells
- auditory nerve