Singing Finches Tell Us How Dopamine Levels Guide Adolescent Learning

If you’ve ever seen a child learning to walk, you’re likely familiar with the process: stand up, wobble, fall down. Stand up, take a step, fall down. This repeats over and over until the child can reliably waddle around on two legs.

This trial-and-error learning is how we acquire natural behaviors such as walking and talking. Previous research suggested that dopamine — the neurotransmitter involved in feeling pleasure— may motivate this process. Still, up until recently it hasn’t been clear exactly how the neurochemical reinforces trial-and-error learning during development.

A recent study published in Nature reveals that changes in dopamine levels guide the learning process of a type of songbird. Researchers found that dopamine levels increase in juvenile zebra finches during successful attempts at singing and decrease during unsuccessful ones. The findings provide insights into learning in other animals, including humans.

“People have been speculating about how dopamine might be involved in learning natural behaviors for years, but we haven’t had a way to directly test it,” says study lead author Vikram Gadagkar, a Simons Collaboration on the Global Brain (SCGB) investigator at Columbia University. “This is the first demonstration that learning natural behaviors during development is an internally motivated process mediated by dopamine.”

Gadagkar worked on the study alongside SCGB executive committee member Adrienne Fairhall of the University of Washington; Alison Duffy of the University of Washington; and Jonathan Kasdin, Nathan Nadler, Arnav Raha and Kimberly Stachenfeld of Columbia University.

Dopamine Guides Birdsong

Dopamine is known to play a role in the learning process for both externally and internally motivated behaviors. For example, if a child receives candy as a reward for eating their vegetables, their dopamine will spike and help reinforce the behavior as an external motivation. Alternatively, if the child correctly solves a puzzle, their dopamine may spike via an internal motivation because they are proud of their achievement.

However, eating vegetables and solving puzzles aren’t what scientists would call “natural behaviors.” Natural behaviors are evolutionarily essential skills that an animal will instinctively exhibit on its own, such as walking and talking.

In their previous work, Gadagkar and colleagues showed that natural behaviors in zebra finches are also likely mediated by dopamine. In a 2016 study, they examined how dopamine levels changed in the brains of adult zebra finches as they sang, a natural behavior for songbirds.

In that experiment, researchers played a trick on the finches. While the birds sang, the team either allowed the birds to hear their song as it was sung correctly or provided instantaneous feedback in which the pitch of their song was warped — similar to how your voice sounds when you speak into a fan that’s turned on. When birds heard accurate feedback, their dopamine levels increased in the region of their brain dedicated to learning and producing songs, known as ‘Area X.’ When the finches heard the distorted feedback, their dopamine levels decreased. This suggested that dopamine levels were likely mediating natural behaviors in the same way it does for optional behaviors.

But this result came from adult songbirds that had largely mastered their songs. The question remained whether dopamine influences the learning process during development when learning is most dynamic.

“What’s cool about the new study is that it goes back to a period in which the bird is dramatically changing its song every day rather than making small tweaks as an adult,” Fairhall says. “So there’s a lot more variability, and the analysis becomes a lot more powerful.”

Inside a Bird Brain

To see whether dopamine levels also guided song learning in young finches, the researchers employed a technique called dopamine photometry. This method involves introducing a dopamine receptor into their brains that connects to a fiber-optic cable. As dopamine levels increase in the brain, more dopamine molecules bind to the receptor, causing the cable to emit brighter light.

The finches were raised as they would be in the wild, learning to sing by imitating their fathers. As each finch practiced, the team measured how much dopamine was produced in the bird’s Area X.

“When the birds sang renditions that were more similar to the eventual adult version, dopamine increased,” Gadagkar says. “And when they sang renditions that were less similar to the adult version, dopamine decreased. This suggested that dopamine was helping evaluate the quality of the song during learning.”

Listen to the song of a young male zebra finch (top) and an adult male zebra finch (bottom). Credit: Gadagkar lab.

The team then wondered whether dopamine could be predictive of future song changes: Does a dopamine spike during a more accurate rendition mean that birds will shift their song to match the successful attempt?

“Our analysis showed that the song actually moves in the direction to which dopamine points, so it has predictive value,” Fairhall says. “Dopamine seems to be directing the song to move along in the right direction.”

Finally, the scientists wondered if this dopamine-mediated learning was taking into account all of a bird’s past attempts or just its most recent one.

“Dopamine is signaling an expectation that’s based not just on the last song the bird sang, but a whole sequence of past efforts,” Fairhall says. “It integrates the past and compares with the present to help the bird refine its song over time. This lines up with reinforcement learning models and was a very satisfying finding.”

More To Learn About Learning

The results suggest that dopamine may influence humans’ learning process like it does for the finches.

“Zebra finches are a great model for studying natural behaviors, because the way they learn to sing has similarities to the way humans learn to talk, and they have a brain circuit dedicated just to learning and producing song,” Gadagkar says. “So the results may be informative for humans, too.”

The study also raises a few more questions the researchers would like to explore related to learning and social interactions. Do learning and performance vary over the course of a day? How about several days? And could the brain circuits behind evaluating song be similar to those behind learning it?

“We study male songbirds because they are the ones singing to attract a mate, but there’s another side to this, which is how the females evaluate the males’ song,” says Gadagkar. “What are the neural mechanisms behind that? Could it be similar to the neural mechanisms humans use to evaluate each other?”