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The auditory brainstem implant (ABI) can restore useful hearing to patients in whom both auditory nerves have been destroyed by tumors of the V111th cranial nerve (vestibular schwannomas), and who therefore cannot benefit from cochlear implants The present version of the ABI consists of 8 platinum-iridium electrodes implanted over the ventral cochlear nucleus. However, the overall auditory performance of patients with this surface-electrode ABI is considerably poorer than the average performance of patients with a multi-channel cochlear implants. One possible explanation for this difference is the relatively poor access to the tonotopic axis of the cochlear nucleus, as compared to the cochlear implant. The objective of the present project has been to develop an auditory prosthesis that is based on microstimulation within the human ventral cochlear nucleus (VCN). The functionality of such a device is dependent on its ability to access the tonotopic axis of the human ventral cochlear nucleus in an orderly fashion, and its ability to excite the efferent projections from the VCN for a prolonged period without causing them injury or inducing excessive depression of their electrical excitability. We have addressed all of these issues in studies in which iridium microelectrodes were implanted chronically into the posteroventral cochlear nucleus of adult cats.