June 2021 The Hearing Journal
For the first time, researchers have successfully used cochlear implants (CIs) in human adults to record their electroencephalographic (EEG) signals and get a better assessment of their hearing. Brain signals recorded by ear implants provide an objective understanding of how good or bad a person's hearing is, without the need for expensive and cumbersome equipment, according to this new study published in Scientific Reports. The technology could also pave the way for clinicians to remotely conduct CI fittings, and further, for CIs themselves to autonomously perform fittings based on EEG diagnostics.
ELECTRODES AS NEURAL SENSORS
CIs in the market each have an array of electrodes that decode external sounds and translate them into electrical signals that the brain can interpret. This study used an experimental CI with a dedicated electrode for recording EEG. “With this implant, the internal part, like the electrode array that is in the cochlea, was the same as a clinical implant. The only difference was that there was a percutaneous (through-the-skin) connector that connects the implant to the outside,” explained study first author Ben Somers, PhD, an associate professor at the KU Leuven. “Normally, this connection to the behind-the-ear piece is wireless. This connector was very convenient because it allowed us to directly access the electrodes in the cochlea to which we could connect an external EEG recording device.”
Typically, experiments that involve electroencephalography use a set of external equipment, including an electrode cap that is placed on the person's head. Somers's team minimised the requirement for “expensive equipment that is cumbersome to use,” as their paper described.
“The possibility to measure EEG with the implant electrodes would be an elegant solution to replace EEG head caps with gel electrodes that are placed on the scalp,” Somers said. He also noted that CI electrodes have been used as sensors in various measuring and monitoring studies, but using them as EEG-recording sensors is a new approach. “The idea to use CI electrodes as sensors is not new in itself; for example, implants from all major manufacturers are able to measure impedances in the cochlea, or monitor responses of the auditory nerve to electrical stimulation (so-called ‘CAPs’ or compound action potentials). These measures are helpful, but they characterise very local, very peripheral effects in the auditory pathway. EEG is a promising technique to measure higher-level auditory diagnostics; for example, to measure how speech is processed and understood in the brain.”
OBJECTIVE AUDIOMETRY & FUTURE APPLICATIONS
Getting EEG signals from a CI user could be a more objective way to assess their hearing, as opposed to just asking them for feedback during their CI fitting. A user's opinion can be subjective, making it challenging for a trained audiologist to adjust their CI settings. Some users may also have extra difficulty in evaluating their own sense of hearing, as may be the case for persons born deaf or those with dementia. In addition, CI fittings are done during in-clinic sessions, which may not take into account variable factors like the user's different listening environments outside the clinic.
The researchers emphasise that a CI user's EEG signals provide “objective audiometry, in which brain responses evoked by auditory stimulation are collected and analysed.” An audiologist could then adjust the CI settings based directly on analysed EEG information. More than that, the researchers are hopeful their findings will be the foundation for more leaps in CI technologies.
“This will make it possible to monitor the hearing status of CI users outside of the clinic, and possibly allow an audiologist to perform remote fitting adjustments,” shared Somers. “This technology also fits within a broader vision for ‘closed-loop’ cochlear implants, which are CIs that will be able to collect EEG-based hearing diagnostics and use them to autonomously adjust their settings to improve the hearing quality for the user.”
However, Somers noted that there are important hurdles to clear along the way. “The main challenge would be to integrate the EEG functionality into cochlear implants. This is a very technical challenge and would mainly be carried out by the manufacturers of CIs,” he said. “However, the setup that we created and the results of this research provide valuable information that will help with this challenge. For instance, we were able to determine advantageous locations to place implanted recording electrodes, and have gained valuable knowledge about the signal characteristics of intracochlear EEG.
“A next step that we are looking into is to record more advanced EEG responses from our set-up: in this first study, we used relatively simple evoked potentials in response to click stimuli. In the future, we will also attempt to record steady-state responses and even responses to speech stimuli, as those could provide even more useful diagnostic information about the hearing system.