June 23, 2024

The plasticity of the brain increases the effectiveness of cochlear implants

Cochlear implants enable deaf people to hear. However, how quickly and how well an implant improves hearing varies greatly from person to person. A study in mice now suggests that differences in the brain’s ability to adapt determine how quickly the brain can integrate signals from the implant. When the researchers stimulated activity in a brain region important for neuronal plasticity in mice with cochlear implants, the animals were able to solve vocal tasks after only a few days. Additional studies should clarify the extent to which similar strategies can also improve response to cochlear implants in humans.

Ordinary hearing aids, which only amplify sound, are of no benefit to people who were born deaf or who are completely deaf. However, if the auditory nerve is healthy, a cochlear implant is an option. This auditory device directly stimulates the auditory nerve via implanted electrodes. As part of post-implant hearing training, sufferers train their brains to interpret and perceive new signals as noises, tones, and speech. However, there are significant individual differences. While some people understand speech only hours after receiving the device, for others it takes years. Although it is already known that people who have recently become deaf are usually the fastest to adapt to the implant, the exact mechanisms by which the brain adapts to the implant are still unknown.

Mice with cochlear implants

A team led by Erin Glennon of New York University, USA, has now researched in mice the mechanisms that play a role in cochlear implant habituation and how effectiveness can be increased. To do this, the researchers first trained mice with normal hearing 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 accidentally press the button with a different sound, further attempts will be blocked for a while – and with them their reward.

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After the mice could reliably perform this task, the researchers destroyed their hearing and implanted cochleas. In addition, they placed fine fiberglass in place of the animals. This is a brain region in the mammalian brainstem that produces and releases the excitatory neurotransmitter norepinephrine, which contributes to neuroplasticity. With the help of fluorescent markers, the researchers made sure that this area lights up when there is activity. In this way, they were able to track the activity of the blue locus through an optical fiber leading outwards.

Learn faster thanks to brain stimulation

After this process, the researchers retrained the rats to perform the previously learned task. Some animals were able to complete the task with the help of the transplant after just one day, while others needed more than two weeks. Similar individual differences were found in human cochlear implant recipients. At the same time, the researchers found that the blue locus appears to be involved in the learning process: initially, the area was more active when the rats received the food after a correct response. However, after the animals had learned to associate the sound with the reward, a peak in locus blue activity occurred as soon as the sound was heard. The earlier this change occurred, the faster the rats could distinguish between true and false sounds.

“We therefore hypothesized that if we artificially activate the locus blue early in training, once the correct tone is heard, habituation to cochlear implants can be accelerated,” the researchers explain. And indeed: If Glennon and her team stimulated the locus aureus in a new group of mice whenever the correct tone appeared, all of the animals treated in this way were able to reliably and correctly solve the task after three days at the latest.

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Long way to apply to humans

“Our results indicate that improving neuroplasticity in the locus cochlear can accelerate and enhance the effectiveness of cochlear implants,” says Glennon’s colleague Robert Fromke. Next, the team plans to find ways to stimulate this brain region in humans as well, which could allow the brain to better adapt to a cochlear implant.

On the other hand, the challenge is to find non-invasive possibilities for stimulation. On the other hand, the extent to which rat findings can be transferred to humans at all must first be demonstrated. While the rats in the experiment only had to distinguish simple sounds from one another, the demands on humans are much higher, for example when they recognize disparate speech patterns or want to focus on a specific conversation partner in a noisy environment. More studies are needed to better understand the processes involved.

Source: Irene Glennon (NYU, USA) et al., Nature, Available Here. doi: 10.1038/s41586-022-05554-8