Grant title: Is impaired binaural hearing in cochlear implantees caused by lack of experience or inappropriate stimulation?
Grant scheme: General Research Fund, project Nr 9042786
Funding amount: HK$ 976,238
Grant duration: 01.01.2020 - 31.12.2022
Abstract: Cochlear implants (CI) enable many profoundly hearing impaired patients to understand speech, but they fall short of providing normal hearing in important respects. In particular CI users cannot process, so-called interaural time differences (ITDs) which arise when sound from one side arrives at one ear a fraction of a milisecond earlier than at the other. ITDs are an important cue to sound source direction. A lack of early binaural experience is often blamed for the inability of CI users to hear ITDs, but recently our lab has shown that neonatally deafened animals without any early hearing experience localise ITDs perfectly well when given precisely synchronised binaural electrical stimuli. Imagine a child recently implanted binaurally with CIs. She needs to pay close attention to amplitude modulation (AM) in sounds as she learns to process speech. At the same time, she also receives useful interaural level differences (ILDs) – another spatial cue more readily accessible with current CIs – which provide information about where the speech is coming from. This brings advantages of spatial attention, and even spatial unmasking (Hess et al. 2018), making hearing in background noise much easier. However, the ITDs delivered by current CIs are uninformative and incongruent with the ILDs. They are a nuisance factor that the child needs to learn to ignore. An insensitivity to ITDs is therefore probably not due to a lack of early experience, it may instead be actively acquired in response to inappropriate stimulation following implantation. We will test this hypothesis with experimental animals, to test whether these phenomena unfold as we have suggested here. We will compare experimental and control groups of neonatally deaf rats fitted with binaural CIs in young adulthood. Both groups will be trained in AM detection to train them in a key auditory skill which has been shown to be important for speech processing. One group will receive additional congruent ITD and ILD cues, as a normally hearing person would. The second group will receive informative ILDs but uninformative ITDs, replicating current clinical CI processors. We predict that, over a training period lasting several weeks, the animals receiving congruent ITDs will not just hone a high sensitivity to ITDs but also come to perform better at AM detection than the group with uninformative ITDs, which will lose their innate ability to process ITDs and learn more slowly. We will test these predictions with behaviour and electrophysiology experiments.