PhD defence A. (Anna) Vavakou

Dissertation in short:

Our ears are able to cope with an enormous range of sound intensities, spanning 6 orders of pressure magnitude. To achieve that, the cochlea compresses its mechanical responses to sound. This is partly regulated by the function of the Outer hair Cells (OHCs), mechanosensors inside our ears that can change their length in response to mechanical stimulation. However, we still do not know how the motile responses of the OHCs to sound compress the mechanical responses of the organ. In the current thesis, I used Optical Coherence Tomography to measure sound-induced vibrations in the intact cochlea. The results show that the OHCs vibrate differently than the surrounding structures, in ways that have not been anticipated by previous modeling work. In particular, the OHCs are found to show elliptical motion. Additionally, in contrast to common belief, OHC motility does not keep up with frequencies above 3 kHz that are well within the hearing range of mammals. These findings are discussed in relation to contemporary and older findings. The OHCs are often thought to amplify vibrations in a cycle-by-cycle manner, and thus, to improve the sensitivity of the sensory organ. However, it is shown here that instead of amplifying the incoming acoustic energy, carefully controlled damping is actually useful for the local regulation of sensitivity. Finally, a comparative study between insects hearing organs and the mammalian cochlea reveals that despite the miniscule size of the former, its mode of operation strongly resembled that of vertebrate ears.