Increase in Selectivity in a Hydromechanical Cochlea Model with Basilar Membrane and Tectorial Membrane

An enlarged hydromechanical cochlea model has been built in order to investigate the hydromechanical increase in selectivity when compared with the basilar membrane motion. The model includes a basilar membrane (BM) of variable compliance, an organ of Corti (CO) and as an important addition a tectorial membrane (TM). Preliminary experiments with frequencies corresponding to 1500 Hz up to 2500 Hz at the threshold of pain have shown both relative movements between TM and CO and flow of fluid in the channel between CO and TM. The width of the channel varied periodically with the frequency of the stimulus whereas the average width diminished with growing amplitude of the oval window, presumably as a result of fluid motion. At high stimulus level the TM dropped on the moving CO thus closing almost completely the channel between CO and TM. The position of the closed channel moved towards the oval window with rising frequency. The length of the closed channel was about one third to one fourth of the length of the envelope (−3 dB) of the travelling wave on the BM, with the envelope being about one fifth of the total length of the BM. The results are considered as an increase in selectivity in the hydromechanical system of the cochlea model, resulting from interactions of BM with CO, TM and fluid motion.