High-Resolution Models of Human Cochlea for a Study of Neural Activation

With more than one million implanted devices, the cochlear implant (CI) is the most successful neuroprosthesis and one of the most successful achievements of medical technology in general. While CI users usually regain the ability to understand and use spoken language with or without visual aids, CI performance is widely variable after implantation. After reconstructing anatomically realistic cochlea models from high-resolution X-ray microtomography scans of eight human temporal bone specimens contrast-enhanced with osmium tetroxide, we conducted an in-depth analysis of the stimulus amplitudes necessary to elicit the excitation of type I spiral ganglion neurons in the cochlea. For comparison, we performed psychophysical measurements on implanted ears from CI users. Our simulations showed that the length of peripheral axons heavily affected the comparison between intact and degenerated neurons. They also suggested that degeneration might have a stronger influence on the excitation of neurons in the basal and apical turns, for their peripheral axons were longer. Our correlation analysis showed that morphological differences could partly account for the variation in the measurements of electrical dynamic range. The comparison with measurements demonstrated that, while the commonly used 1 mm-width neuron activation might be appropriate to represent the detection threshold, 4 mm-width activation was an underestimate for the maximum comfortable level.