Researchers using high-resolution imaging to track changes to individual dendritic spines--points of contact between nerve cells--found that behavioral learning results when instructive experience is able to rapidly stabilize and strengthen synapses on sensorimotor neurons, according to a study conducted by Richard Mooney, PhD, professor of neurobiology at Duke University Medical Center in Durham, N.C. and colleagues.
In the study, which was published online in the journal Nature on Feb. 17, the researchers observed connections between nerve cells, or synapses, in the brain of an anesthetized juvenile songbird right after the animal heard an adult tutors' song for the first time.
“Behavioral learning depends on the brain’s capacity to respond to instructive experience and is often enhanced during a juvenile sensitive period," wrote the authors. “How instructive experience acts on the juvenile brain to trigger behavioral learning remains unknown.”
Mooney and colleagues hypothesized that when the juvenile songbird heard a tutor's song for the first time, the building of new spines and loss of old spines would accelerate.
"Instead,” said Mooney, “We saw exactly the opposite: hearing a tutor song rapidly stabilized previously dynamic synapses."
By imaging the brain of an anesthetized juvenile songbird and tracking and photographing the same neurons and spines for approximately one month, the researchers found that juveniles with initially higher levels of spine turnover before hearing the tutor song appeared to learn more from their tutors, and that hearing a tutor song rapidly stabilized the spines.
However, they also wrote that the juveniles in whom spines were already highly stable weren't able to learn from their tutors, noting that these birds may have been past their optimal learning period.
The findings for the brain changes, including the rapid stabilization of dendritic spines during the juvenile songbird’s critical periods for behavioral learning, can translate to the human brain, said the researchers.
"Many skills, including communication skills, require great precision if you want to stay in the gene pool," Mooney said. "A male songbird has to learn to sing precisely or he won't attract a mate."
The research also suggests, wrote the authors, that the stable network of synapses that rapidly form when a young songbird hears the tutor song can act in a young brain to build stable connections between neurons. This process may aid humans in learning new behaviors.
According to the authors, the study’s findings may guide further research aimed at restoring plasticity to synapses after the critical learning period closes, which is a therapeutic goal in helping people regain function after a brain injury.