Modelling multiple excitation mechanisms in the cochlea using a 2D finite difference box model

Abstract

The cochlea is very versatile. It can be excited by a multitude of mechanisms in order to produce a hearing sensation, even when it is severely malformed. The versatility of the cochlea can also be used to our advantage in the creation of novel methods of cochlear excitation when the conventional pathway is obstructed, for instance through bone conduction hearing aids and excitation via the round window. It is thus important to have an in-depth understanding of the various mechanisms that can cause a hearing sensation and how they are affected by the condition and structures of the cochlea.

This thesis investigates the underlying physics associated with various cochlear excitation mechanisms using a 2D finite difference box model. The excitation mechanism examined include; piston-like motion of the stapes, rocking of the stapes, the two inertial components of bone conduction hearing and local excitation of the round window. The model predicts the pressure distribution within the cochlea and allows a direct comparison of the effects of these various excitation mechanisms within the same framework.

The effect of the ‘third window’ on the cochlear response due to various cochlear excitation mechanisms was investigated by adapting the model to include the cochlear and vestibular aqueducts. It was found that aqueducts do not affect the cochlear response much for volumetric excitation via the oval window, but were demonstrated to have a significant effect for other excitation mechanisms. In particular it was found that the aqueducts had a large effect when an immobile oval window was modelled, as they allowed a volumetric excitation that significantly increased the response at low frequencies.

An air-bone gap has been clinically found in audiograms of patients who have a large vestibular aqueduct, although the reasons for this phenomenon are not fully understood. The effects of a large vestibular aqueduct on the air conduction and bone conduction hearing thresholds have been separately modelled here. It was found that the air-bone gap is predominately caused by the increase of the air conduction hearing threshold due to the internal cochlear pressure forcing fluid through the enlarged aqueduct, reducing the net volumetric excitation.

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