Stimulating the brainstem may make cochlear implants more effective

More than 50 years after cochlear implants were invented, researchers may be closer to understanding why the devices don’t promote hearing in every deaf or hard of hearing recipient. A study in rats suggests that the activity of neurons in part of the brain influences how the animals respond to sounds after an implant is fitted.

Cochlear implants are made up of two parts. One is worn like a hearing aid or clipped on to clothing, while the second is implanted into the cochlea bone in the inner ear. Here it turns sounds into electrical signals that stimulate the auditory nerve, providing the sensation of hearing.

For some people, cochlear implants are effective almost immediately, while in others this can take weeks or even years.

To better understand why these varying responses occur, Erin Glennon at the New York University School of Medicine and her colleagues implanted cochlear implants into 16 rats with induced deafness.

The researchers then monitored the activity of neurons in a structure of the rats’ brainstem called the locus coeruleus (LC).

“This brain area is the main source of noradrenaline,” says Robert Froemke at NYU Langone Health. “LC activity and noradrenaline release seems to act sort of like the brain’s alarm clock, increasing arousal and helping us pay attention to the world around us.”

The rats had varied responses to the cochlear implants initially, with the researchers noticing the animals’ LC activity predicted when they began responding to sounds.

When this brain region was then artificially stimulated in the same rats, the differences to their LC activity disappeared and they all responded to sounds.

According to Froemke, activity in the LC promotes neuroplasticity, the brain’s ability to alter its structure and function, making it more sensitive to sounds.

This study suggests that noradrenaline released by the LC shapes neuroplasticity in the brain’s auditory cortex and helps to promote hearing after a cochlear implant, says Victoria Bajo at the University of Oxford.

“The results open the possibility of using noradrenaline to promote the outcome of cochlear implants,” she says.