Duke University Medical Center scientists crowded around a laser-powered microscope in a darkened room to peer into the brain of an anesthetized juvenile songbird right after he heard an adult tutors' song for the first time.
Specifically, they wanted to see what happened to the connections between nerve cells, or synapses, in a part of the brain where the motor commands for song are thought to originate.
In the first experiment of its kind, they employed high resolution imaging to track changes to individual dendritic spines, important points of contact between nerve cells.
"We expected to see the building of new spines and loss of old spines accelerate when the juvenile heard a tutor's song for the first time," said senior author Richard Mooney, Ph.D., a Duke professor of neurobiology. "Instead, we saw exactly the opposite: hearing a tutor song rapidly stabilized previously dynamic synapses."
Juveniles with initially higher levels of spine turnover before hearing the tutor song subsequently learned more from their tutors. Because the scientists studied birds during their late adolescence, some may have been past their optimal learning period. "Juveniles in which spines were already highly stable weren't able to learn from their tutors," said Todd Roberts, Ph.D., a postdoctoral fellow in the Department of Neurobiology who is lead author on the study, which was published online in the journal Nature on Feb. 17.
In the "learners," hearing a tutor song rapidly stabilized spines.
Roberts said they were expecting to find higher "plasticity," the brain's ability to remodel connections in response to learning or injury. "We thought we would see an initial stage of higher plasticity, because it can take weeks or even months for a juvenile to copy the tutor song." .
The findings provide fundamental insight into how the brain changes during the juvenile's critical periods for behavioral learning. They also can guide future research aimed at restoring plasticity to synapses after the critical period closes, an important therapeutic goal in helping people regain function after an injury like hearing loss or stroke, Mooney said.
The researchers studied juvenile male songbirds that were kept only with females, which do not sing. They had been exposed to other calls and noises, but not the critically important song of a male tutor. "The adult male's song is a signal that the juvenile's brain seems to crave," Mooney said.