Brain stimulation increases hearing in rats with ear implants

A girl wears a cochlear implant in a musical performance for hard of hearing and deaf children with cochlear implants.

Some people with cochlear implants can recognize speech within hours of implanting the device, but for others it can take months or years.Credit: Michael Matthey/dpa/Alamy

Stimulating neurons that are linked to alertness helps rats with cochlear implants learn to quickly recognize melodies, researchers have found. The results suggest that activity in a region of the brain called the locus coeruleus (LC) improves auditory perception in deaf rodents. The researchers say the insights are important for understanding how the brain processes sound, but caution that the approach is a long way from helping people.

“It’s like we gave them a cup of coffee,” says Robert Froemke, an otolaryngologist at New York University School of Medicine and co-author of the study, published in Nature December 211.

Cochlear implants use electrodes in the region of the inner ear called the cochlea, which is damaged in people with total or severe hearing loss. The device converts acoustic sounds into electrical signals that stimulate the auditory nerve, and the brain learns to process these signals to make sense of the auditory world.

inner ear implants

Some people with cochlear implants learn to recognize speech within hours of implanting the device, while others may take months or years. “This problem has been around since the dawn of cochlear implants and shows no sign of being resolved,” says Gerald Loeb of the University of Southern California in Los Angeles, who helped develop one of the first cochlear implants.

The researchers say that a person’s age, the duration of their hearing loss, and the type of processor and electrodes in the implant do not explain this variation, but they suggest that the brain could be the source of the differences. “It’s kind of a black box,” says Daniel Polley, an otolaryngologist at Harvard Medical School in Boston, Massachusetts. Most of the previous research has focused on improving the cochlear device and the implantation procedure. Attempts to improve the brain’s ability to use the device open up an avenue to improve communication between the ear and the brain, Polley says.

To explore this relationship, the research team trained 16 rats to respond to melodies.1. Upon hearing a specific tune, the animals could stick their noses into a box to receive a treat. When they heard other tunes, the treat was not presented and the rats learned not to pick their noses.

The researchers then deafened the rats surgically and attached a cochlear implant with eight electrodes, each encoding a melody, into the animals’ ears. The authors repeated the task, but this time, when the researchers stimulated an electrode, the animal would listen to the melody that was paired with a food reward.

tune in training

The authors reported that all the rats learned to distinguish between rewarding and non-rewarding tunes after 15 days. The researchers noted that the activity of neurons in the LC increased when the animals responded correctly to the tunes and decreased when they randomly prodded and made mistakes.

Although the LC is not part of the auditory system, it supplies the system with the neurotransmitter norepinephrine, which increases alertness; LC also has a role in cognition, learning and memory, and attention. When the LC releases norepinephrine throughout the auditory system and the attention and learning circuits of the brain, this accelerates learning and processing of auditory signals.

In another test, the authors stimulated the LC in one group of rats and did not in another group. Animals that were not stimulated took up to nine days to perform the task, while those with stimulated LC learned it in just three days. Froemke was surprised at how powerful LC was in helping animals use cochlear implants. “Each animal that received an implant learned to do it very well.”

way to people

The researchers caution that stimulating the LC in humans could be dangerous. The region sends signals to many regions of the brain and regulates the fight or flight response. Stimulating the LC in people would “increase blood pressure and heart rate, and [induce] other autonomic responses,” says Graeme Clark, an otolaryngologist at the Graeme Clark Institute at the University of Melbourne, Australia, who developed the first multichannel cochlear implant in the 1970s. the sorrow”.

But there are other ways to activate brain circuits to work with the device, such as introducing neurotransmitters at the cochlear level, which is much safer, Clark says.

And in the future, tools that use technologies to improve auditory perception could be connected to a cochlear implant, which people could use to train themselves, Froemke says.

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