Perceptual Adaptation Following Cochlear Implantation

Cochlear implants (CIs) are neural prosthetic devices that successfully restore hearing. However, understanding speech from CIs does not happen instantaneously. Speech understanding and sound quality do improve after initial CI activation, but the process may take weeks, months, or even years. A major element of this adaptation process in postlingually deaf CI users (those who learned oral language before losing their hearing) involves overcoming anatomical frequency mismatch, the discrepancy between the normal cochlear frequency-place function and that imposed by the CI. This can result in perceptual frequency mismatch, defined as the difference between pitch percepts elicited by a sound in a normal ear and a CI- stimulated ear. Importantly, higher levels of perceptual frequency mismatch are associated with poorer speech perception. Fortunately, the human auditory system is plastic and perceptual frequency mismatch decreases over time. The current standard of care is based on the assumption that such adaptation can completely compensate for anatomical frequency mismatch. However, the literature and our pilot data indicate that this assumption is often incorrect. An alternative approach to improve speech perception and sound quality is to use frequency-place maps that match the physiological frequency-place function, with the goal of making adaptation unnecessary. This approach may also be suboptimal. Given typical insertion depth of CI electrodes, matching the physiological frequency-place function can only be done at the cost of failing to deliver low-frequency information. This unexplored tradeoff will be studied in the aims that follow. Our overall goal is to study the extent and possible limitations of the adaptation process that underlies speech perception improvement, and to test personalized medicine approaches that seek to accelerate and improve adaptation outcomes after CI activation. Aim 1 will study adaptation to frequency mismatch in single-sided deaf (SSD) CI users longitudinally, starting at CI activation. SSD-CI users are ideal for this purpose as they can provide a precise comparison between normal acoustic stimulation and CI stimulation. This will be done using two innovative tools: one that is speech- based, and another one that represents a generalization of electroacoustic pitch matching. Aim 2 will examine adaptation over time using a broader set of central and peripheral measures in a more traditional population of CI users who lack usable residual hearing. Lastly, and as a complement to the observational studies in Aim 1, Aim 3 will evaluate different interventional approaches to overcome frequency mismatch in CI users, including SSD CI users (Aim 3a) and CI users without usable residual hearing (Aim 3b). The proposed experiments will provide foundational information about the nature, extent, and potential limitations of perceptual adaptation after CI activation, as well as a path for translation of these results to clinical practice.