PhD defence by Niclas Alexander Janssen

Cochlear implants (CI), a form of neural prostheses, are used to aid patients with extensive hearing loss, representing sounds in a way fundamentally different compared to normal hearing. Our daily life exposes us to a complex sound environment, and the human brain has to identify which sound components belong to the same sound source, such as a voice or melody. We are usually not aware of the processes that make this work and combine what we hear with our two ears (“binaural integration”). But if our brains would not combine the signals from both ears, we would hear everything twice, separately for each ear. Bimodal CI patients have a CI in one ear and hearing aid (HA) on the other ear. This means that one sound is represented very different across ears. To a lesser extend this is also true for Bilateral CI patients, who are implanted in both ears. This thesis investigates whether the potential differences in a sound’s representation across ears for these CI patients influence if and how their brain can perform this binaural integration.

In a first study, bimodal CI patients were interviewed and only a minority indicated to perceive sounds from one source as a simple, uniform sound object. Most participants reported to perceive something more complex, non-uniform, instead, with some describing their percepts as two entirely separate sounds with strong differences in pitch and loudness across ears. The second study dealt with the development of a listening experiment to test the listeners’ capabilities for binaural integration. This new method was then used to investigate if bilateral and bimodal CI patients would integrate sounds like normal-hearing listeners. Results show that at least some bilateral CI listeners can integrate, but none of the bimodal CI patients. This mirrors the patient’s reports from the first study. This suggests that the extended daily use of the bimodal devices has changed how the brain of the bimodal CI patients processes sounds, so that they do not integrate in the same way as normal-hearing listeners or the bilateral CI patients would. These results could help to guide the decisions over who should receive a CI, whether in bimodal or bilateral configuration, how the devices are representing sounds, and how they could be optimally adjusted to the individual patient.