The next generation in cochlear implant technology
A cochlear implant is a device that can be surgically implanted to help improve the hearing abilities of many profoundly or totally deaf people. It is suitable for people who have sensorineural hearing loss with a functioning auditory nerve. Although it cannot restore natural hearing it does enable a user to experience sensations of sound. Cochlear implants currently work by replacing the function of the auditory “hair” cells which line the snail-shaped cochlea in the inner ear. Ordinarily, hair cells respond to sound waves by converting them into electrical signals, which can then travel along the auditory nerve to be interpreted by the brain.
In cases where some hair cells remain, an external hearing aid can be used to amplify sound, but this won’t work if hair cells are damaged or missing. Instead, a cochlear implant can be surgically inserted to electrically stimulate the auditory nerve in response to sound.
Professor Robin Davis leads the team of researchers in the Department of Cell Biology and Neuroscience at Rutgers’ School of Arts and Sciences whose findings could be important for engineers and surgeons designing new implants. Davis works with mouse cochlear tissue cultured in the laboratory. Her team have discovered that two types of protein called neurotrophins – already known to promote cell survival – play a key role in sending sound signals to the brain.
At one end of the cochlea, neurotrophin-3 (NT-3) occurs in high concentration while the presence of brain-derived neurotrophic factor (BDNF) is low. At the other end of the cochlea the levels are reversed; a high concentration of BDNF dominates a low level of NT-3. These opposing protein gradients give rise to a complex pattern of signals that affect neuronal firing rate.
Where BDNF levels are high, auditory neurons have a fast firing pattern. This means they can convey information about high sound frequencies – those having a high musical pitch – to the brain. Where NT-3 is abundant, neurons have a slower rate of firing and represent lower sound frequencies.
Davis’ work has highlighted the importance of incorporating gradients of neurotrophins into the next generation of cochlear implants. She says “This approach would not only enhance neuron survival, but would tailor the surviving neurons to the specific coding frequency, and thus improve the efficacy of the cochlear implant”.