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NIDCD-supported Scientists Use Cochlear Implants to Restore Auditory Synapses in Deaf Cats
Scientists supported by the National Institute on Deafness and Other Communication Disorders have demonstrated that cochlear implants can restore the structure of synapses—the connecting space between neurons—along the auditory nerve in deaf cats. Because untreated congenital (at birth) deafness is believed to cause permanent changes in the auditory system, this finding may explain why cochlear implants work best in young children before irreversible abnormalities occur.
In mammals with normal hearing, electrical signals generated in the inner ear travel along the auditory nerve to the auditory cortex, the part of the brain that interprets the signals as sound. When mammals are born deaf, the lack of electrical stimulation causes abnormal synapses to form at the endings of the auditory nerve. The presence of normal synapses is believed to enable the transmission of signals throughout the auditory system.
The investigators designed their experiments to compare the synapses of deaf cats whose auditory nerves are electrically stimulated by a cochlear implant to the synapses of deaf cats with no implants and cats with normal hearing. Previously, cochlear implants have been shown to improve hearing in the auditory cortex. The researchers found that following stimulation, the synapses of deaf cats with cochlear implants closely resembled the synapses of cats with normal hearing, as opposed to the abnormal synapses found in deaf cats with no cochlear implant.
The scientists speculate that deafness in humans is characterized by, among other things, synaptic abnormalities, similar to those found in other mammals. As with the cats, synaptic changes in young deaf children are believed to occur after cochlear implantation and may play a role in the success of these children in acquiring communication skills before abnormal changes become permanent.
This research appeared in the December 2 issue of Science and was conducted by scientists from the Department of Otolaryngology-Head and Neck Surgery and Department of Neuroscience at the Johns Hopkins University Center for Hearing and Balance.