Listening is an important component of speech. When we speak, we use data collected by our ears to help us modulate our spoken tone, pitch and rhythm. That's why people who experience hearing loss often develop speech impediments, and may decide to stop speaking entirely. A new study, published March 7 in Neuron, reveals that the vocalization problems of the deaf may also have roots in the neural reorganization that occurs after hearing loss.
“Our broad interest is to understand how the brain uses auditory information to learn how to vocalize and maintain a stable vocal pattern,” said Duke University neurobiologist Richard Mooney, an author on the new study. “What we really wanted to know is, could auditory feedback be influencing the part of the brain that controls vocalizations?"
To find out, Mooney and his colleagues studied zebra finches, which are birds that learn their vocalization patterns from their social environment, like humans do. Zebra finch vocalizations, like human vocalizations, become less accurate soon after the onset of deafness. Mooney thought that these changes might be reflected in an area of the bird’s brain called the HVC. ("HVC" is not an abbreviation.) The HVC is similar to the Broca's area in a human brain, Mooney says. The two structures evolved separately in birds and humans, but each controls the motor movements required for vocalization. If a bird's HVC is damaged, it cannot sing. Analogously, if a person's Broca area is damaged, he cannot speak.
To see what happens in the HVC after a finch loses its hearing, Mooney and his team actually built tiny windows into the birds' brains, and covered the one-millimeter-wide holes with glass. Next, they used viruses to inject green fluorescent proteins into the neurons there. Shining a laser light through the window allowed the researchers to observe where the neurons were located and what they looked like during experiments.
At the beginning of the experiment, one particular finch sounded like this:
But then the researchers deafened each bird by removing its cochleae. Two weeks later, this is what the deafened bird’s song sounded like:
Mooney was surprised to find that the neurons controlling the bird's singing began to degrade within 24 hours after deafening, even though birds might not stop singing altogether for days or weeks. As the connections among neurons are lost, their signals weaken. A "use it or lose it" mechanism could be involved, Mooney thinks.
"The motor system must use auditory information to keep the behavior in good shape," he says. "When auditory behavior is removed, the system doesn’t have a way of evaluating its performance. So it falls apart."
The study corroborates a growing tendency among neuroscientists to no longer think of the brain as operating in simple reflexive arcs, where sensory information goes into the brain and motor commands come back out. Instead, the latest research suggests that sensory input and motor output are tangled up in feedback loops. In the case of the birds, the effects of deafening penetrated deeply into the zebra finches' brain circuitry. Mooney says that a similar phenomenon may happen when a human loses hearing. If so, then understanding the distributed effects of hearing loss could eventually help to combat it.
The study also has interesting evolutionary implications. Roosters don’t learn to crow, rather they are born with the ability. And unlike songbirds, who do learn to sing and modify their songs in response to their environments, roosters will keep crowing even after they’ve been deafened. Mooney thinks that hearing oneself must be more important in species with the ability to learn and modify their vocalizations. Understanding the neural circuits involved could help scientists to understand why humans and songbirds are unique in their vocalization flexibility.
"There’s something during songbird evolution that has resulted in the building of a neural circuit for singing and song learning," Mooney says. "In humans, there’s something in us that allows us to learn how to speak. Chimps and gorillas don’t have that flexibility. Why learn to vocalize?"
Up next, Mooney says he wants to understand how the motor areas of the HVC help songbirds to learn and remember the songs that they hear.