Deafness can have unexpected consequences on speech patterns

Going deaf isn't just about sound - those who lose their hearing also have trouble producing sounds themselves.
Written by Rose Eveleth, Contributing Editor

Conversations have two parts – listening and talking. But those who are deaf don’t simply lose one of those parts; they often have trouble with both. Hearing people speak is how we learn to speak ourselves, picking up on the rhythm and tone of other people’s voices. For those who cannot hear, imitating normal speech is nearly impossible.

To look closer at the connection between hearing and speech, researchers at Duke University examined the songs, and brains, of birds with and without their cochlea – the organ in the ear that picks up sound.

Here’s what the bird – in this case a male zebra finch – sounds like before it has gone deaf.

Birdsong - Before Deafness by RoseEveleth

And here is what that same bird sounds like just two weeks after his cochlea were removed, and he became deaf.

Birdsong - After Deafness by RoseEveleth

You might have to listen a couple times to hear it, but the deaf bird stumbles over his song, it sounds messier.

This has big implications if you’re a bird. Males learn songs, and sing them to attract mates. The prettier the song, the more attractive that male is to the ladies. While humans who can’t sing aren’t out of the mating game, the fact that deafness can effect vocal coordination does have implications for the 30 million Americans who are hearing impaired.

“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," said Richard Mooney, a researcher from Duke, in the press release.

But the most interesting part of this study was how quickly the losses occurred in the finch’s brains. After just 24 hours the researchers saw the size and strength of nerve connections to the basal ganglia, the part of the brain involved in both learning and reproducing motor sequences like the complex bird call.

"I was very surprised that the weakening of connections between nerve cells was visible and emerged so rapidly -- over the course of days these changes allowed us to predict which birds' songs would fall apart most dramatically," Katie Tschida, one of the researchers said in the press release. "Considering that we were only tracking a handful of neurons in each bird, I never thought we'd get information specific enough to predict such a thing."

The experiment had two parts. First, the team built tiny portholes into the brain covered with glass. Then, they stained the songbird’s nerve cells with a glowing protein. When the researchers shined a laser through the holes, the protein would glow, revealing the connections between neurons. After going deaf, these spidery connections shrank.

“The motor system must use auditory information to keep the behavior in good shape,” Mooney told Scientific American. “When auditory behavior is removed, the system doesn’t have a way of evaluating its performance. So it falls apart.”

Top photo: Wikimedia Commons, Maurice van Bruggen
Neuron photo: Duke University, Katie Tschida

This post was originally published on Smartplanet.com

Editorial standards