The electrical stimulation of the auditory nerve can elicit auditory sensations in the subjects with sensorineural deafness. Each stimulating electrode of an electrode array of the multi-channel cochlear implants may stimulate a distinct neural population. However, a great deal of current spreads from each electrode throughout a lymph because of the high electrical conductivity of the lymph liquid. This phenomenon causes the transmitted information to be reduce due to channel interactions. Even if the number of ele-ctrodes is increased, the transmitted information will be limited because of current spread. We have proposed the Tripolar Electrode Stimulation Method (TESM) which may succeed in narrowing the stimulation region and continuously moving the stimulation site for the cochlear implants.
We evaluate whether or not TESM works according to a theory which is based on the numerical analysis using the auditory nerve fiber model consisted of unmyelinated and myelinated segments. In this simulation, the neural site and the sum of the excited fibers are compared with the compound action potentials which we obtained through animal experiments. As a result, based on the numerical analysis using this model, it is also proved that the anodal/cathodal threshold stimulation current ratio increases by decreasing unmyelinated segment between the inner hair cell and the habenula perforata. Also by comparing the result of the numerical analysis with that of the animal experiment, It is suggested that an effect of the unmyelinated segment is not significant. Based on their results mentioned above, we succeed in narrowing a stimulation region by controlling the sum of the currents emitted from the electrodes on both sides. Also we succeed in continuously moving a stimulation site by changing the ratio of the currents emitted from the electrodes on both sides.
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