Cochlear implant modelling: stimulation and power consumption

Abstract

Cochlear implants have been shown to successfully restore hearing to the profoundly deaf. Despite this achievement, issues remain concerning the power consumption and the accuracy of stimulation. This thesis is mainly concerned with investigating the spread of stimulation voltage within the cochlea. The power required to generate the stimulus is also investigated, as is the feasibility of powering a fully implanted cochlear implant by harvesting energy from head motion.

Several different models have been used to study the voltage distribution within the cochlea due to electrical stimulation from individual electrodes of a cochlear implant. A resistive cable model is first used to illustrate the fall-off of the voltage with distance at the electrode positions along the cochlea. A three-dimensional finite element model of the cochlea is then developed to obtain the voltage distribution at positions closer to the site of neural stimulation. This model is used to demonstrate the way that the voltage distribution varies with the geometry of the cochlea and the electrode array. It was found that placing the return electrode of the implant within the modiolus, as opposed to outside the cochlea, resulted in higher stimulation for the same current input, which reduces the power requirements. The model has also been used to investigate the consequences of a current-steering, or stimulation focussing, strategy that has previously been proposed. A generalisation of this strategy is suggested, whereby impedance information at the neural level, along the path of the spiral ganglion, was used to optimise the focussed voltage distribution at the target neurons.

The power consumption of various stimulation strategies is then estimated in order to assess their energy efficiency. Strategies are defined by parameters such as stimulation rate and number of active channels. The feasibility has also been investigated of harvesting electrical energy from head motion, to power a fully-implanted cochlear implant. It was demonstrated that more power could be harvested from higher harmonics but that this would be sensitive to walking speed. The practical approach is to have a heavily damped device that is insensitive.

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