A male and female zebra finch, with lines suggesting the paths that males use to produce songs for female finches. Photo by Daniel E. Baleckaitis
When a bird sings, what goes on inside its brain to produce those clear, silvery strings of sound? Studies have suggested that something like a metronome worked independently inside bird brains, ticking away to trigger individual notes. But new research, published today in Nature by lead author Ana Amador, a post-doctoral researcher from the University of Chicago, shows that the part of the brain responsible for bird song is far more intricately organized around the exact physical motions that produce a bird’s tunes—possibly hinting at the mechanisms that help to steer our own speech.
By showing that the movements involved in switching notes during bird song are actually made up of a subset of more refined actions, Amador found that each of these motions are encoded in neurons in an area of the brain called the high vocal center—or HVC—that has long been associated with birdsong. But the neurons could have a different role in executing song than the one that has always been recognized, says Daniel Margoliash, the lead neuroscientist in whose lab the research was conducted. This is something he and the team of researchers found by studying the exact timing of the throat motions that produce notes, and the firing of neurons in the brain.
The researchers used zebra finches—birds that are typically reliable candidates, because their song is “very regular, very structured,” says Margoliash, and the male courtship tunes incredibly precise. All of this makes their song a neat, predictable model to work with.
By attaching fine recording wires to the birds’ brains, and using a mathematical model devised by physics professor Gabriel Mindlin at the University of Buenos Aires, to record the small pressures and tensions in the syrinx—a bird’s equivalent of a voice box—the researchers were able to figure out when the finch’s neurons were firing in relation to when the sounds were made. “The bird structures his song based on these little gestures—vocal movements—that include pressure as a function of time and tension as a function of time,” Margoliash said.
The results showed that contrary to the ideas presented in some other studies, neurons don’t directly control the noises that come out of a bird’s beak. Certainly, the HVC region is known to ultimately shape sounds, says Margoliash—but the team was able to show when they matched up the bird’s syrinx motions with the firing neurons, that the neurons fired just as transitions were occurring between one note and another, a little too slowly to actually be driving the change.
The findings suggest that HVC neurons are then essentially “logging the information about what’s expected to be happening,” Margoliash explains. “The idea then is that as the bird is singing, it is getting feedback information.”
But how is this valuable to the finch? Having audio feedback allows a bird to recognize its own tunes; to essentially know where it is in its song repertoire, says Margoliash. “By having a marker of the time when the feedback info occurred…it may be useful in the future, when the action is repeated”—or when the bird has cause to adjust its tune.
The research tells us about the neural coding and structure that exists behind sound and speech, because it essentially shows how our brains are wired to drive complex movements. So, says Margoliash, “problems in speech show problems with structuring the sequence of events. One everyone knows about [is] stuttering.”
Why turn to a finch to tell us more about ourselves? Because humans share some interesting base features with birds. Both song and speech “are learned vocalizations,” Margoliash says. Birds require exposure to song during their early development much like humans do. “Birds have dialects very similar to human dialects,” he adds. And finally, “song birds require auditory feedback to learn their songs”—which is similar to how a baby babbles away to practice the sounds of speech.
Of course, much of what the researchers suggest is hypothesis, based on what they have observed in the lab. Bird song is still a mystery that has much to reveal, and researchers will just keeping adding pieces to the puzzle. Margoliash is one: he’ll keep listening to his zebra finches, which ironically don’t sing the sweetest of tunes, he says. “Its song is a cross between Bugs Bunny and a squeaky door.”