This is a Discovery Research Grant awarded to Dr Bob Carlyon at the University of Cambridge in 2020.
Cochlear implants restore hearing to deaf people by electrically stimulating the auditory nerve. When the implant is first turned on, many people report that everything sounds strange and hard to understand. After several months, however, speech perception becomes much easier. Interestingly, we do not know why this is.
Part of the improvement is likely due to the person learning what speech sounds like through their implant. However, research in animals suggests that when hearing is restored by a cochlear implant, following long periods of deafness, the ways in which the brain processes sounds change. Another significant change that occurs during the first few months after an implant is switched on is that sounds become quieter. An audiologist is often required to retune the implant so that sounds are loud enough to hear again. This reduction in sound loudness over time is very large and corresponds to about one third of the loudness difference between the softest and loudest sounds one can hear.
The research team aims to investigate whether the changes in brain activity seen in animals also occur in humans. If they do, then the team will further investigate whether the changes contribute to the improvements seen in speech perception during the first few months of implant use. To do this, the research team will recruit volunteers who have recently had their cochlear implant switched on to perform some listening tests and non-invasive brain-recording (EEG) experiments. These experiments will investigate whether the brain also gets better at analysing non-speech sounds, which are unlikely to benefit from “speech learning”.
The team will also investigate the changes in loudness that people with new cochlear implants experience. Surprisingly, researchers do not know where in the brain these changes are occurring. The team will find this out by performing different types of EEG recordings that can measure activity in different parts of the brain.
In addition, they will also study another type of loudness change which occurs in some patients, for whom the current level in their implant is turned up so high (in order to get a sufficiently loud sound) that it produces non-auditory sensations, such as facial twitches. These effects can be so uncomfortable that they prevent the patient from using their implant. The team have previously found that turning the implant off for a few weeks can allow someone to hear sounds again, at a lower current level and without these unwanted sensations. The research team will study more people who are affected by these unwanted sensations to find out how many people this solution works for, how long it takes for the improvements to happen, and where in the brain the change occurs.
The results of this research may be useful, for example, in understanding why a particular person’s ability to understand speech is not improving over time, even when using a cochlear implant. The results will also give us important new information on how the brain adapts to hearing again after a period of deafness.
The findings may also allow researchers to develop a good objective measure of loudness changes, which could be used when adjusting implant settings for children or adults who cannot make reliable verbal reports of loudness.