Hearing loss spreads to vocal nerves

How quickly and - and deeply - can deafness penetrate the brain? For a study on this, researchers at Duke University Medical Center turned to male zebra finches.

Songbirds differ from most animals in that males' mating songs fall apart when they lose their hearing. This feature makes them an ideal organism to study how hearing loss may affect the parts of the brain that control vocalization, said Richard Mooney, Ph.D., professor of neurobiology at Duke. He is the senior author of the study's report.

Portions of a songbird's brain that control how it sings began to decay within 24 hours of the animal losing its hearing. As the size and strength of nerve-cell connections visibly changed under a microscope, researchers could even predict which songbirds would have worse songs in coming days.

"When hearing was lost, we saw rapid changes in motor areas in that control song, the bird's equivalent of speech," Mooney said. "This study provided a laser-like focus on what happens in the living songbird brain, narrowed down to the particular cell type involved."

Using a protein isolated from jellyfish that can make songbird nerve cells glow bright green when viewed under a laser-powered microscope, they were able to determine that deafening triggered rapid changes to the tiny connections between nerve cells, called synapses, which are only one thousandth of a millimeter across.

The study was published last week in Neuron journal online.

"I will go out on a limb and say that I think similar changes also occur in human brains after hearing loss, specifically in Broca's area, a part of the human brain that plays an important role in generating speech and that also receives inputs from the auditory system," Mooney said.

About 30 million Americans are hard of hearing or deaf. This study could shed light on why and how some people's speech changes as their hearing starts to decline, Mooney said.