Cochlear Implantation and Older Age: Exploring the Effect of Different Stimulation Rates on Cognitive Performance

The most documented cognitive changes in the aging brain are slower processing speed and decline in executive functions such as working memory (WM), attention and cognitive flexibility. If the stimulation rate (SR) in cochlear implant (CI) settings is not optimised for the aging population, elderly CI listeners need to devote extra cognitive resources for successful speech understanding, with the increase of listening effort. The overall aim of this PhD is to explore how SR in older CI users affects listening effort and cognitive processing, by evaluating the feasibility of using a bimodal task to extract a visual-related neural marker of effort. An exploratory study on 19 normal hearing (NH) controls was carried out to optimise the task design and sensitivity, along with event-related potentials (ERPs) measures. For the main study the bimodal task was performed by 18 elderly CI users at three SRs: manufacturer standard SR, low-SR and high-SR. The bimodal task sequentially engaged audition and vision to investigate the transfer of listening effort across modalities. Over a 3-months period, CI participants were asked to try two new MAPs. For each SR setting, CI users completed a series of tests in various cognitive domains (attention, executive function, spatial working memory and social cognition) and ERP recordings while performing the bimodal task. The NH study highlighted that the P300 amplitude indexed the level of task-related attention. Results on CI users showed that speech scores in noise were significantly poorer at high-SR, and the visual P300 amplitude was larger when accomplishing the bimodal task at high-SR, which might confirm the need for extra attentional resources at this setting. However, there was little or no effect of SR on cognitive tests. This thesis presents a viable approach to identify a visual-related marker of effort in CI users – artefact free – by developing a bimodal task, which could be an appropriate step to effectively tailor CI parameters for each individual.