Cochlea implants can help deaf people gaining acoustic sensations, if the deafness is caused by a mechanical defect of the inner ear and if the auditory nerve is intact insofar as the transfer of neural signals is possible. An intracochlear implant necessitates the positioning of several stimulating electrodes inside of the scala tympani.
To achieve a favorable result for the process of understanding speech signals, it is necessary to investigate the position of the electrodes before the prosthesis is implanted. Due to the extracellular stimulation the idea of the activating function is applicable. The calculation of the activating function gives a first approach of the influence of the extracellular potential on each compartment of the nerve fiber with few computational effort in relation to the necessity to numerically solve a system of non-linear differential equations.
The simulated response of a central axon of a human auditory nerve fiber to a stimulating current which is applied by a spherical electrode is examined at different distances from the electrode. Due to the fact that sound is a oscillation of air pressure, the response to sineshaped functions is simulated. The investigation of the activating function values leads to the knowledge about the direct proportionality of the distance between electrode and axon to the width of the region where the influence of the extracellular potential is greatest. There is also a linear proportionality between the strength of the stimulating current and the value of the activating function if the distance between electrode and axon is unvaried. The initiation site of action potentials are examined, and a relation of the activating function and virtually injected current is established.
The results are applied to the investigation of nerve fiber responses to combined pure tones which are sampled by rectangular charge-balanced pulse trains for stimulation. Within this thesis, it is examined that the duration of the stimulating current has great influence on the nerve fiber responses due to the level of the corresponding threshold currents and therefore has to be considered to gain beneficial results. It is also found out that the activating function provides applicable information about the neuronal response to a certain position of the electrode.