The University of Melbourne Departments of Otolaryngology and Electrical Engineering (UMDOLEE) receiving and stimulating component of a multiple-electrode cochlear implant hearing prosthesis produces constant stimulation. It has a stimulating pulse shape that minimizes the production of toxic substances and loss of metal from the electrodes, and this is achieved with a biphasic rectangular waveform where the first phase is negative with respect to ground. The duration of each stimulus phase in 180 msec, which is long enough to allow low levels of current stimulation, and short enough to permit rates of 1000 pulses/second to be achieved. In order to be consistent with our present understanding of the perception of pitch, the device permits the independent stimulation of a number of electrodes. Furthermore, to electrically isolate the stimulus to small areas, there is the capacity to vary the current and set the threshold independently at individual electrodes. The phase and amplitude of the pulses to neighbouring electrodes with also be varied to assist in localizing the current flow. The pattern to stimulation to individual or groups of electrodes can also be altered to enable studies to be carried out to determine ways of conveying frequency and intensity information over a more normal dynamic range.
The ability of spiral ganglion cells to survive long-term electrical stimulation is a precondition for the success of cochlear prostheses. In this study 10 cats were implanted bilaterally with bipolar scala tympani electrodes, and stimulated for periods of up to 2029 hours using charge balanced biphasic current pulses. The status of the auditory nerve was monitored periodically by recording electrically evoked auditory brainstem responses. At the conclusion of the stimulation program, spiral ganglion cell survival was assessed for stimulated and control cochleas; comparison of the two groups showed no statistically significant difference. The results of this study indicate that long-term intracochlear electrical stimulation, using carefully controlled biphasic pulses, does not adversely affect the spiral ganglion cell population. Acta Otolaryngol (Stockh), Suppl. 399 Acta Otolaryngol Downloaded from informahealthcare.com by Flinders University of South Australia on 02/06/15 For personal use only. Acta Otolaryngol (Stockh), Suppl. 399 Acta Otolaryngol Downloaded from informahealthcare.com by Flinders University of South Australia on 02/06/15 For personal use only.
The multichannel cochlear prosthesis requires an electrode stimulus configuration which produces a stimulus field spatially localized to each electrode. In this paper, a three-dimensional discrete resistance model of the cochlea was developed which exhibits electrical response properties similar to those observed during electrical stimulation of the cochlea. The model results suggest that the spatial attenuation of current within the cochlea varies greatly in magnitude, depending on the stimulus configuration. In addition, the model suggests that the spatial attenuation of current in both the auditory nerve fiber endings in the organ of Corti and in the myelinated fibers within the cochlear ground paths is different from the voltage attenuation in the scalar fluids. Therefore the efficacy with which a particular stimulus configuration differentially excites local terminal auditory nerve fiber populations cannot be deduced from scalar voltage measurements which have previously been recorded in the literature. Consequently physiological experiments were performed in the cat to measure the current distributions in the terminal nerve fiber region for monopolar and bipolar stimulation of the scala tympani, and also for stimulation between the scala tympani and the scala vestibuli. The mean length constants measured in the basal turn for these stimuli were found to be 12, 3, and 7.5 mm, respectively.
Animal experimental studies have shown length constants of 2-4 mm for bipolar and 8-16 mm for monopolar stimulations. Studies in models using saline-solution-filled tubes have allowed us to examine the radial and longitudinal current distribution for pseudobipolar stimulation and have demonstrated that current localization is the same for bipolar and pseudobipolar stimulation over a 6-10-dB operating range. With coincident pseudobipolar multiple-channel stimulation there was suppression of the current between the stimulus maxima and enhancement at the edges leading to less stimulus interaction. Experiments performed with a pseudobipolar electrode implanted into the human cochlea showed that there was significant spread of current along the ground electrode because the electrode ground impedance was significantly greater than the cochlear tissue impedances. Because this leads to less current returned at each ground electrode, the pseudobipolar array will result in less interaction for coincident stimulation.
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