Chapter 12 - Auditory Brainstem and Midbrain Implants

Abstract

Persons who lack an auditory nerve, e.g., as a result of vestibular schwannoma surgery, cannot benefit from cochlear implants, but a prosthesis utilizing an electrode array implanted on the surface or in the cochlear nucleus, an auditory brainstem implant (ABI) can restore some hearing. Existing ABI devices use the same processing strategies as cochlear implants. Both surface and penetrating electrodes may be used. Some speech understanding without lip reading is obtained with ABIs, but on average at most 30% of words in sentences, and only in nontumor patients. The auditory midbrain implant (AMI) is a new central auditory prosthesis designed for stimulation of the human inferior colliculus and bypassing the brainstem, which may be damaged by the tumor or the surgery to remove it. From the initial studies, showing pitch perception after long-term use but no speech understanding, it became clear that tonotopic stimulation of the inferior colliculus was insufficient. Electrophysiological studies in animals suggested the need for simultaneously stimulating different parts of the central nucleus of the inferior colliculus (ICC), whereas psychoacoustic studies into the temporal processing of AMI stimulation indicated the different refractory behavior of the ICC compared to auditory nerve fibers. Solving these problems is ongoing.