May 19, 2008

How Songbirds Learn Their Songs

Photo of two zebra finches

Like human babies, juvenile zebra finches have their own way of babbling called subsong. A new study has found that subsong is driven by different brain circuits than those that control an adult bird’s song. The study’s insights into how birds explore sounds to learn songs may shed light on how humans learn new things as we develop.

When young animals are learning how to communicate and control their bodies, they use a trial and error process that looks like play. Scientists call it “exploratory behavior.” It’s an important way for the young to learn the relationships between actions and their effects. But researchers haven’t understood what brain areas are involved in this learning process.

A research team led by Dr. Michale Fee at the Massachusetts Institute of Technology set out to discover how the seemingly random noises of subsong transform into the precise patterns of an adult zebra finch’s song. The scientists focused on 2 brain circuits known to be involved in generating song: the high vocal center (HVC)-motor pathway and the anterior forebrain pathway. Their study, funded in part by NIH, was published in the May 2, 2008, issue of Science.

The researchers found that juvenile birds still sang subsong when the HVC was eliminated. Older birds also sang after the HVC was eliminated, but their songs lost structure and became more like subsong. After pharmacological inactivation of the HVC in adult birds, the loss of song structure was fast (within 20 minutes) and reversible, suggesting that the HVC imposes structure on the older birds’ singing.

Inactivating part of the anterior forebrain pathway—the lateral magnocellular nucleus of the nidopallium (LMAN)—entirely abolished subsong production in younger birds. It didn’t block singing, however, in most older birds.

To confirm the role of the LMAN in subsong production, the researchers recorded the activity of single LMAN neurons in young birds as they sang. The exact relationship of neuron activity to song varied, but 65% of the 31 tested neurons fired just before vocal output, indicating their possible involvement in song production. The results were similar in young birds with and without the HVC.

This research shows that separate brain circuits control singing at different stages of zebra finch development. The anterior forebrain pathway generates subsong in young birds, whereas the HVC-motor pathway generates the more ordered song patterns of older birds. There’s a period in between called the intermediate plastic-song stage, in which the pathways interact to generate partially structured songs. How developing brains make this transition will doubtless be the subject of future study.

The finding that a separate brain region controls exploratory behavior in bird song development may apply to other forms of exploratory behavior as well. “In birds, the exploratory phase ends when learning is complete,” Fee says. “But we humans can always call upon our equivalent of LMAN, the prefrontal cortex, to be innovative and learn new things.”

—by Harrison Wein, Ph.D.

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