Kick-starting the brain’s natural ability to adapt to new circumstances, known as neuroplasticity, improves how effectively a cochlear implant can restore hearing loss, a new study in deaf rats has found. The researchers say the survey may help explain why some implant recipients respond so much better to treatment than others.
Unlike hearing aids, which amplify, balance, and sharpen incoming sound, cochlear implants send electrical signals that represent sounds directly to the brain. Unfortunately, it takes time to understand the meaning of the signals. While some cochlear implant users begin to understand speech hours after receiving their device, others take months or years to do so. The mechanisms that determine how quickly the brain can adapt to an implant are unclear.
Led by NYU Langone Health researchers, the new rat investigation – published online Dec. 21 in the journal Nature— assessed whether stimulation of the locus coeruleus, a major site of neuroplasticity found 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 could 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. »
Erin Glennon, PhD, study lead author and neuroscientist, 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 is the first to show that stimulation of this region of the brain speeds up hearing in people with cochlear implants, according to Dr. Glennon.
For the investigation, the study authors trained rats with normal hearing to press a button after hearing a particular sound and to ignore the button if they heard another. 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 after hearing the sound and pressing the correct button. As they learned to associate pressing the button with receiving the reward, brain 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 C. Froemke, PhD, professor of genetics at the Skirball Foundation in the Department of Neuroscience and Physiology at NYU Langone.
Dr Froemke says the team next plans to explore ways to stimulate the region of the brain in humans that don’t require invasive surgery. Dr. Froemke is also a professor in NYU Langone’s department of otolaryngology – head and neck surgery.
“Since our goal is to activate the locus coeruleus, we need to determine what non-invasive mechanisms can be used to trigger the brain region,” says study co-lead author Mario A. Svirsky, PhD, Professor Noel L. Cohen in auditory science. in the department of otolaryngology-cervicofacial surgery.
Dr Svirsky, also a professor in the Department of Neuroscience and Physiology, warns that rat hearing has been tested 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.
In addition to Dr. Glennon, Dr. Froemke, and Dr. Svirsky, other study researchers include Youssef Z. Wadghiri, PhD, associate professor in the Department of Radiology at NYU Langone; Silvana Valtcheva, PhD, senior researcher at the University of Cologne, Germany; and Angela Y. Zhu, MD, assistant professor of clinical ophthalmology at Bascom Palmer Eye Institute, Miami.