Auditory Nerve, Brain Center Can Return to Normal After Inner-Ear Nerve Damage

ST. PETERSBURG, FLA. -- The brain center responsible for hearing retains the ability to reorganize itself and respond normally during periods of reduced activity resulting from damage to the auditory nerve endings in the inner ear, a study by University at Buffalo researchers has shown.

They also found that the damaged nerve endings that transmit impulses from hair cells to the brain can recover from injury, but at a significantly slower rate than the brain.

The findings have important implications for restoring lost hearing in humans.

Results of the study were presented here today (Monday, Feb. 16) at the annual meeting of the Association for Research in Otolaryngology.

"It is not news that the brain can reorganize itself after damage to the peripheral sensory organ," said Sandra McFadden, Ph.D., research scientist in UB's Center for Hearing and Deafness and an author of the study. "That has been shown in many previous studies in which permanent damage has been created by surgery, drugs or aging.

"What is new here," she said, "is our finding that the brain can reorganize itself again after the peripheral sensory organ recovers from damage and sensory input is restored. This may be important with regard to restoring hearing in humans, through the use of hearing aids or cochlear implants, for example, because it demonstrates that the brain remains plastic after a period of sensory deprivation."

The finding of central-auditory-system plasticity also may explain why many hearing-aid users go through an adjustment period before they perceive an improvement, McFadden said.

Researchers in UB's Center for Hearing and Deafness induced reversible damage to the auditory-nerve endings in the cochlea, the primary sensory organ of the inner ear, in eight chinchillas, and monitored auditory-signal transmission between the damaged nerve and the location in the brain that receives its signals.

Measurements of activity at the brain site and at the auditory-nerve fibers were taken at days 1, 5, 10 and 30 following the induced injury.

"Remarkably, we found that the brain recovers sooner than the ear itself," McFadden said. "Specifically, responses recorded from the inferior colliculus recovered to normal in five days, long before the responses recorded from the auditory nerve, which took up to 30 days.

"These results tell us that auditory-nerve fibers carrying impulses from the ear to the brain can regrow, which is essential to the recovery of hearing, and that the central auditory system in the brain reorganizes itself to maintain its function while the nerve fibers are damaged. It then reorganizes itself again as nerve function is restored."

What researchers don't know yet, McFadden said, is how long the brain retains this plasticity -- important clinically to determine how quickly treatment, via hearing aids or cochlear implants, must begin -- or whether a return to normal brain activity means a return to normal hearing.

"We have demonstrated that the brain's ability to respond to sound can be restored," she said, "but we don't know yet how this affects an individual's perception of sound, if at all. We hope to address these questions in the future."

Other researchers involved in this study were Xiang Yang Zheng, research scientist, and Donald Henderson, Ph.D., co-director of the Center for Hearing and Deafness.

This study was supported by a grant from the National Institutes of Health.