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Just a few weeks after they hatch, baby male zebra finches begin to babble, spending much of the day testing their vocal chords. Dad helps out, singing to his hatchlings during feedings, so that the babies can internalize his tune, the same mating refrain shared by all male zebra finches.
Soon, these tiny Australian birds begin to rehearse the song itself, repeating it up to 10,000 times a day, without any clear reward other than their increasing perfection of the melody.
The baby birds’ painstaking devotion to mastering their song led Duke University neuroscientist Richard Mooney and his Duke colleague John Pearson to wonder whether the birds could help us better understand the nature of self-directed learning. In humans, language and musical expression are thought to be self-directed—spontaneous, adaptive and intrinsically reinforced.
Dopamine was always elevated when the birds sang, regardless of song quality.
In a study recently published in Nature, the scientists tracked the brain signals and levels of dopamine, a neurotransmitter involved in reward and movement, in the brains of five male baby Zebra finches while they were singing. They also measured song quality for each rendition the birds sang, in terms of both pitch and vigor, as well as the quality of song performance relative to the bird’s age. What they found is that dopamine levels in the baby birds’ brains closely matched the birds’ performance of the song, suggesting it plays a central role in the learning process.
Scientists have long known that learning that is powered by external rewards, such as grades, praise or sugary treats, is driven by dopamine—which is thought to chart the differences between expected and experienced rewards. But while they have suspected that self-directed learning is likewise guided by dopamine, it had been difficult to test that hypothesis until now.
“It’s hard to study self-directed behavior in animals,” says Pearson, whose research combines neurobiology with biostatistics. “There are few examples of really important learned behaviors that involve the necessary degree of skill,” he says. To run a successful experiment, they also needed to work with an animal that would repeat a behavior consistently many times. Baby zebra finches fit the bill.
In the study, the scientists found that the more mature the bird’s rendition of the song relative to their age, the more dopamine was released in the basal ganglia, a cluster of nuclei in the neocortex involved in control of movement as well as reward and cognition. When the birds performed less well relative to their age, dopamine levels dropped. And when the scientists blocked dopamine release, the birds’ performance relative to their age also declined. Dopamine was always elevated above baseline levels when the birds sang, regardless of song quality, the scientists found, which suggests it may help to reward spontaneous warbling.
To do their experiment, the researchers relied on optogenetics, which allowed them to measure dopamine release precisely enough to track it against behavior. They injected the baby birds with a genetically modified dopamine receptor, which glows when it’s hit with a laser light. “When the bird begins to sing we can see this huge increase in the glow, which means that dopamine has been released,” says study author John Pearson, a neurobiologist at Duke University.
Vikram Gadagkar, a Columbia University neuroscientist who was not involved in the study, says the findings are strong, and echo those of a study he authored, also in Nature, around the same time. His team at Columbia was able to show that dopamine release guides specific song-learning behaviors, such as adjustments to pitch.
The findings of the two studies could give scientists insights into not just self-driven learning but also Parkinson’s disease, obsessive compulsive disorder, and substance use disorder, which all seem to be driven by a faulty intrinsic reward system, says Gadagkar. “This study directly connects dopamine with the learning and performance of a natural behavior which may in the future provide insights for how to treat these kinds of diseases,” he says.
As for the baby Zebra finches, the intrinsic reward is the song itself. In the lab and in the wild, even when no other birds are around to listen, they continue practicing their song until they can get it to sound just right. 
Lead photo: Lucia Kohutova / Shutterstock
