Portions of a songbird's brain that control how it sings have been
shown to decay within 24 hours of the animal losing its hearing.
The findings, by researchers at Duke University Medical Center, show
that deafness penetrates much more rapidly and deeply into the brain
than previously thought. 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," said senior author Richard Mooney, PhD,
professor of neurobiology at Duke.
"This study provided a laser-like focus on what happens in the living songbird brain, narrowed down to the particular cell type involved."
"This study provided a laser-like focus on what happens in the living songbird brain, narrowed down to the particular cell type involved."
The study was published in Neuron journal online on March 7, 2012.
Like humans, songbirds depend on hearing to learn their mating songs –
males that sing poorly don't attract mates, so hearing a song, learning
it, and singing correctly are all critical for songbird survival.
Songbirds also resemble humans and differ from most other animals in
that their songs fall apart when they lose their hearing, and this
feature makes them an ideal organism to study how hearing loss may
affect the parts of the brain that control vocalization, Mooney said.
"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.
"Our vocal system depends on the auditory system to create
intelligible speech. When people suffer profound hearing loss, their
speech often becomes hoarse, garbled, and harder to understand, so not
only do they have trouble hearing, they often can't speak fluently any
more," Mooney said.
The nerve cells that showed changes after deafening send signals to
the basal ganglia, a part of the brain that plays a role in learning and
initiating motor sequences, including the complex vocal sequences that
make up birdsong and speech.
Although other studies had looked at the effects of deafening on
neurons in auditory brain areas, this is the first time that scientists
have been able to watch how deafening affects connections between nerve
cells in a vocal motor area of the brain in a living animal, said Katie
Tschida, PhD, a postdoctoral research associate in the Mooney laboratory
who led the study.
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.
"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," Tschida said. "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 research was supported by the National Science Foundation and the
National Institute on Deafness and Other Communication Disorders.