Deaf birds warble out of tune

Caren B. Les, caren.les@photonics.com

Music may be an important part of human art and culture, but for birds it’s a matter of species survival and even evolution: If a male bird sings the wrong notes in his mating song, he won’t attract a mate.

Song is deeply implanted in birds’ brains, too – when a songbird loses its hearing, nerve cells in the area of the brain responsible for song will begin to decay within 24 hours.

The short decay time was a surprise to the researchers at Duke University Medical Center in Durham, N.C., who discovered it with the help of longitudinal in vivo imaging. Their goal was to investigate how deafness affects brain function, particularly for vocalizing.

They used a Zeiss laser scanning two-photon microscope 510 to examine nerve cells from deafened zebra finches. The cells were labeled with green fluorescent protein. Under the microscope, they observed that deafening decreased the size of synapses on one cell type in particular, and songbirds that showed more pronounced changes in their synapses also showed more severe changes to their songs in the coming days.

“When hearing was lost, we saw rapid changes in motor areas that control song, the bird’s equivalent of speech,” scientist Richard Mooney said.

The nerve cells that showed changes after deafening send signals to the basal ganglia, the part of the brain involved in learning and initiating motor sequences, including vocal sequences.

“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, who led the study, which was published online in Neuron on March 8.

A zebra finch’s nerve cells are labeled with green fluorescent protein, and other nerve cells in its brain are labeled with red or blue tracers. The small bulbs – dendritic spines – show synapses, places where nerve cells connect and communicate. Researchers found that when these spines shrink over time, the changes precede and predict the severity of vocal changes in the birds following deafness. Images courtesy of Katie Tschida, Duke University Department of Neurobiology.

The findings could apply to human hearing and brain function as well.

Songbirds resemble humans and differ from most other animals in that their vocalizations fall apart when they lose their hearing, and this feature makes them an ideal organism for studying 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. When people suffer profound hearing loss, their speech often becomes harder to understand, he added.