Stimulating the brain can improve the effectiveness of cochlear implants

People with cochlear implants can recognize speech within hours of implantation, while others take months or years. Photo by Michael Matthey/dpa/Alamy

The researchers found that stimulating neurons associated with alertness helped rats implanted with cochlear implants learn to recognize tunes quickly. The findings suggest that activity in the blue spot (LC) region of the brain can improve hearing in deaf rodents. The researchers say the findings are important for understanding how the brain processes sounds, but this approach is still a long way from helping humans. The study was published Dec. 21 in Nature.

Cochlear implants use electrodes in the inner ear area called the cochlea, a device that converts sound into electrical signals that stimulate the auditory nerve, and the brain learns to process these signals to understand the auditory world.

Some people with cochlear implants learn to recognize speech within hours of implanting the device, while others take months or years. “This problem has been around since the advent of cochlear implants and shows no signs of a solution.” Gerald Loeb of the University of Southern California in Los Angeles says he helped develop the first generation of cochlear implants.

The researchers said a person’s age, the duration of hearing loss, and the type of processor and electrodes in the implant did not explain the difference, meaning the brain may be the source of the difference. “It’s kind of like a black box.” Daniel Polley, an otolaryngologist at Harvard Medical School in Boston, Massachusetts, said most previous research has focused on improving cochlear devices and implant procedures.

Trying to improve the brain’s ability to use the device, Polley said, opens a path to improved communication between the ear and the brain.

To explore this relationship, the team trained 16 rats to respond to music. When a particular tune is heard, the animal can stick its nose into the box to get a reward. When they hear other tunes, the food doesn’t appear, and the rats learn not to stick their noses out.

The researchers then surgically deaf the rat and implanted a cochlear implant with 8 electrodes in the rat’s ear, each encoding a tune. The authors repeated the task, only this time, when the researchers stimulated the electrodes, the animals heard tunes that matched the food reward.

The authors report that after 15 days, the rats all learned to distinguish between tunes that offered a reward and those that did not. The researchers noticed that the activity of LC neurons increased when the animals responded correctly to the tune, while the activity of LC neurons decreased when they poked and made mistakes randomly.

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

In another test, the authors stimulated LC in one group of rats and none in another. Unstimulated rats took 9 days to complete the task, while LC stimulated rats took only 3 days to learn. Froemke was amazed at how powerful LC can be in helping animals use cochlear implants.

Researchers warn that stimulating LC in humans could be dangerous. This area sends signals to many brain regions and regulates the fight-or-flight response. Graeme Clark, an otolaryngologist at the Graeme Clark Institute at the University of Melbourne in Australia who developed the first multichannel cochlear implant in the 70s of the 20th century, said that stimulating a person’s LC “raises blood pressure and heart rate, and induces other autonomic responses.”

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

Co-author Robert Froemke, an otolaryngologist at New York University School of Medicine, said that in the future, tools that use technology to improve hearing perception may be attached to cochlear implants, which people can use to train themselves. (Source: Li Muzi, China Science News)

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