PROJECT SUMMARY Auditory brainstem implants (ABI) are the only therapeutic option for deaf patients with abnormal cochlea or auditory nerve. ABIs bypass the auditory nerve and directly stimulate the central auditory pathways using electrodes surgically placed on the cochlear nucleus. Current ABI devices use the same coding strategies as cochlear implants (CI) to encode sound into electrical stimulation. However, despite identical external hardware, signal processing, and stimulation patterns being delivered by ABI and CI electrodes, the two devices have very different outcomes. Only a small percentage of ABI patients can understand speech using their device, whereas CI patients can generally communicate verbally using their device alone. The mechanisms for these disparate outcomes are elusive, but may be related to the differences in electrode- neural interface characteristics between the two devices. The cochlear nucleus, the site of ABI stimulation, is much more complex than the auditory nerve and consists of diverse neuron types. The majority of cochlear nucleus neural types have different biophysical properties than the auditory nerve. Aim 1 is to study electrode- neural interface characteristics in ABI patients and their relation to perceptual outcomes. We will study electrode-neural interface by obtaining electrically-evoked compound action potentials (ECAPs) using the voltage recording capability of the implant device. We will use ECAP to assess the morphology of neural responses as well as neural recovery and adaptation of neural responses. We will also assess neural interactions between different electrodes. We will evaluate the association between electrode-neural interface and behavioral performance, and assess how this association differs between ABI and CI patients. Aim 2 is to evaluate whether the electrode-neural interface measures can guide elimination of poorly functioning electrodes to improve ABI patient outcomes. We will compare speech perception outcomes obtained before and after eliminating the poorly functioning electrodes from the coding strategy. Speech perception outcomes with each new manipulation will be measured when the subjects have had the opportunity to adapt to it for one month. This study will provide critical understanding regarding auditory processing differences between ABI and CI users, which may be related to differences in electrode-neural interface between the cochlea and the cochlear nucleus.