Cochlear implants enable deaf people to hear. However, how quickly and how well the implant improves hearing varies greatly from person to person. A study with rats now indicates that differences in the adaptability of the brain determine how quickly the brain can integrate the signals from the implant. When the researchers stimulated the activity of a brain region that is important for neuroplasticity in rats with cochlear implants, the animals were able to solve acoustic tasks after just a few days. Further studies should clarify to what extent similar strategies can also improve the response to cochlear implants in humans.
Ordinary hearing aids, which only amplify sound, are of no use to people who were born deaf or are completely deaf. However, if her auditory nerve is intact, a cochlear implant is an option for her. This hearing prosthesis stimulates the auditory nerve directly via implanted electrodes. As part of a hearing training after the implantation, those affected train their brain to interpret the new signals and to perceive them as noises, tones and speech. However, there are large individual differences. While some people understand speech just hours after receiving the device, it takes others years. Although it is already known that people who are recently deaf are usually the fastest to adapt to the implant, the exact mechanisms by which the brain adapts to the implant are still unknown.
Rats with cochlear implants
A team led by Erin Glennon from New York University in the USA has now researched in rats which mechanisms play a role in getting used to the cochlear implant and how effectiveness can be increased. To do this, the researchers first trained normal-hearing rats to perform a task that required precise hearing: if the animals heard a specific sound, they had to press a button to receive food. If they mistakenly pressed the button with a different sound, further attempts were blocked for a while - and with them their reward.
After the rats were able to perform this task reliably, the researchers destroyed their hearing and gave them a cochlear implant. In addition, they placed a fine glass fiber in the animals' locus coeruleus. This is a brain region in the mammalian brainstem that produces and releases the stimulating neurotransmitter norepinephrine, thereby contributing to neuroplasticity. With the help of fluorescent markers, the researchers made sure that this region glows when there is activity. In this way, they were able to track the activity of the locus coeruleus through the fiber optics leading to the outside.
Faster learning thanks to brain stimulation
After this operation, the researchers retrained the rats to perform the previously learned task. Some animals were able to complete the task with the help of the implant after just one day, while others needed more than two weeks. Similar individual differences were found as in human cochlear implant recipients. At the same time, the researchers found that the locus coeruleus did appear to be involved in the learning process: initially, the region was most active when the rats received the food after a correct response. However, after the animals had learned to associate the sound with the reward, the peak activity of the locus coeruleus occurred as soon as the sound was heard. The sooner this change occurred, the faster the rats could reliably distinguish right from wrong sounds.
"We therefore hypothesized that if we artificially activate the locus coeruleus early in the training, as soon as the correct tone is heard, cochlear implant habituation could be accelerated," the researchers explain. And indeed: if Glennon and her team stimulated the locus coeruleus in a new group of rats whenever the right tone sounded, all animals treated in this way were able to solve the task reliably and correctly after three days at the latest.
Long way to applicability in humans
"Our results suggest that improving neuroplasticity in the locus coeruleus can accelerate and enhance the effectiveness of cochlear implants," says Glennon's colleague Robert Froemke. Next, the team plans to find ways to stimulate this brain region in humans as well, potentially allowing the brain to better adapt to a cochlear implant.
On the one hand, the challenge is to find non-invasive possibilities for stimulation. On the other hand, it must first be clarified to what extent the results from the rats can be transferred to humans at all. While the rats in the experiment only had to distinguish simple sounds from one another, the requirements for humans are significantly higher, for example when they recognize differentiated speech patterns or want to focus on a specific conversation partner in a noisy environment. Further studies are needed to better understand the processes involved.
Source: Erin Glennon (New York University, USA) et al., Nature, doi: 10.1038/s41586-022-05554-8