Jump-starting the brain’s natural ability to adapt to new circumstances, or neuroplasticity, improves how effectively a cochlear implant can restore hearing loss, according to a new study in deaf rats. The survey, the researchers say, may help explain the extreme variation in hearing improvements experienced by implant wearers.
Unlike hearing aids, which amplify, balance, and sharpen incoming sound, cochlear implants send electrical signals that represent sounds directly to the brain. Unfortunately, experts say, it can take time to figure out what the signals mean. Previous studies have shown that while some cochlear implant users understand speech within hours of receiving their device, others take months or years to do so. However, the mechanisms that determine how quickly the brain can adapt to an implant are unclear.
Led by researchers at NYU Langone Health, the new investigation of rats assessed whether stimulation of the locus coeruleus, a major site of neuroplasticity deep in the brainstem of mammals, improved how quickly they learned to use their devices. It showed that within three days of receiving their implants, rodents given an extra boost might effectively perform tasks requiring precise hearing. In contrast, those who were not stimulated needed 16 days to do so.
“Our results suggest that differences in neuroplasticity, particularly in parts of the brain such as the locus coeruleus, may help explain why some cochlear implant users improve faster than others,” says the lead author. of the study and neuroscientist Erin Glennon, PhD, medical student at NYU Grossman School of Medicine.
In an earlier investigation, the research team found that electrical stimulation of the locus coeruleus in rodents increases neuroplasticity and alters how the brain’s auditory system represents sound. However, the new study, published online Dec. 21 in the journal Nature, is the first to demonstrate that stimulation of this region of the brain speeds up hearing in people with cochlear implants, according to Glennon.
For the investigation, the study authors trained normal-hearing rats to press a button following hearing a particular sound and to ignore the button if they heard a different tone. Once deafened, the rats were unable to perform the task. Then they received cochlear implants and were retrained to complete the same challenge relying on the device.
Among the results, the study showed that the activity of the locus coeruleus changed dramatically as the rats learned to use their implants. At first, the brain region was most active when animals received food following hearing the sound and pressing the correct button. As they learned to associate pressing the button with receiving the reward, the activity peaked when they just heard the tones. Notably, the faster this change occurred, the more consistently the rats performed on the task.
“Our results suggest that improving neuroplasticity in the locus coeruleus can accelerate and enhance the effectiveness of cochlear implants,” says study co-lead author and neuroscientist Robert Froemke, PhD, Foundation professor of genetics. Skirball in the Department of Neuroscience and Physiology at NYU Langone.
Froemke says the team next plans to explore ways to stimulate the region of the brain in humans that don’t require invasive surgery. Froemke is also a professor in the Department of Otorhinolaryngology – Head and Neck Surgery at NYU Langone.
“Since our goal is to activate the locus coeruleus, we need to determine what noninvasive mechanisms can be used to trigger the brain region,” says study co-lead author Mario Svirsky, PhD. Svirsky is the Noel L. Cohen Professor of Hearing Sciences in the Department of Otorhinolaryngology – Head and Neck Surgery at NYU Langone.
Svirsky, also a professor in the Department of Neuroscience and Physiology at NYU Langone, warns that rat hearing has been examined using simple sounds in a simple task, while humans must respond to nuanced speech patterns in noisy environments. Further research, he says, is needed in other brain regions that may be involved.
Funding for the study was provided by National Institutes of Health grants F30DC017351, T32GM007308, R01DC003937, R01DC012557, P30CA016087, and P41EB017183. Additional financial support was provided by Cochlear Ltd, an NYU supplier, which also sells equipment and technical assistance to NYU Langone. The terms of these agreements are managed in accordance with health system policies.
