Modelling the physiology of spiral ganglion neurons

The efficiency of electrical stimulation in cochlear implantation with a CI depends critically on the state of the nerve fibers interfacing with the array of electrodes inserted into the inner ear. Neural morphology and degeneration, ionic channels, electrode position and stimulus polarity and shape have all demonstrated to be important factors of the response of SGNs to electrical stimulus. Previous modelling and human studies have shown that some underlying interactions in electrical stimulation of the AN could be at the origin of patient performance variability that could not be explained by CI candidacy factors alone. In this thesis, a biophysical model of the human SGN , that includes voltage-gated hyperpolarization-activated cation (HCN) and low threshold potassium voltaged- gated delayed-rectifier potassium (KLT) channels, was used to model single neuron and population responses to different stimulus conditions and strategies. The results show that healthy SGNs which have preserved peripheral processes prefer cathodic leading stimulation while SGNs with degenerated peripheral processes prefer anodic leading. Also, another degree of degeneration was modelled by partially removing myelin on the peripheral process, resulting in altered spike latencies which could in turn destroy benefits of bilateral CI hearing. Interesting phenomenon of "cathodic blocking" and "spike-rate adaptation blocking" that could explain non-monotonic loudness growth functions were also introduced. Further, population simulations were produced to model Electrically evoked Compound Action Potential (ECAP) responses and pulse train responses. The results show that ECAPs for various polarities and stimulation levels could be used as an assessment tool for neural health and predictions of best stimulation strategies. However, the results also showed that the ECAP responses were often decorrelated from the ability of a neuron population to encode pulse train stimulations. Overall, the thesis presents interesting results with regards to how best to design patient-oriented stimulation strategies. By evaluating the neural health using ECAP and pulse train ECAPs, and performance of individual patients with regards to pulse shapes, preferred polarity and pulse rate, stimulation strategies could be designed beyond the typical thresholds and Most Comfortable Level (MCL). This might create an opportunity to reduce the CI user variability in performance that is currently observed.