Learning How We Learn—from Songbirds
As humans, we learn new things almost constantly. How to swing a golf club. How to speak a foreign language. How to remember the capital of Djibouti (easy: it’s Djibouti City). Despite our remarkable ability to retain new skills or information, however, researchers don’t yet fully understand how our brains do it.
Mimi Kao, an assistant professor of biology, is untangling that mystery. And in the process, her work might even help researchers better understand neurological disorders like Parkinson’s disease.
In humans, she says, much of our ability to learn comes from a structure called the cortical-basal ganglia (CBG), a two-sided mass deep in the center of the brain. Since the structure also plays a role in emotion, eye movement, cognition, and motor function, though, it’s difficult to tease out which neural circuits are involved just in learning. Doing so would require probing the area in a living brain with electrodes—a feat that is nearly impossible to manage without major surgery.
To get around those limitations, Kao is focusing her efforts not on the brains of humans, but of birds. The common zebra finch, she said, has a part of its brain structure that’s very similar to the CBG. In finches, certain regions are devoted almost exclusively to learning its song from other birds—and since each zebra finch only learns one song for its entire life, studying its brain could provide a clear look at how it eventually masters the song’s performance.
“The learning system in a songbird brain really has two halves,” Kao said. “One is a motor pathway, which is responsible for the bird’s ability to produce a song throughout life. It controls the organ that birds use to make sound. The second system is important for learning and modifying songs. It can vary the signals that the bird sends to that organ, letting it change the song slightly from one attempt to the next.”
These neural circuits work together to let the bird memorize a song and then attempt to match its melody and rhythm to the original through trial and error. Under normal conditions, when juvenile finches repeat the song they hear from their parents, they gradually play with different variations until they hit on the right notes. As they do, they slowly reinforce the muscle memory needed to repeat those notes until they’ve mastered the entire melody.
Kao has verified this in the lab by inserting tiny electrodes into the region of the finch brain that controls learning. From there, she’s able to sense the activity of single neurons, and record exactly when they activate. Sure enough, once a bird learns its song, she says, the neurons in its learning circuit fire predictably at key points while it’s singing, especially where notes or rhythms change.
Surprisingly, though, this pattern only seems to emerge when the bird is “performing”—singing for other birds. When it’s alone, the subtle variations in tone, pitch, and rhythm that it relied on while learning return, and its neurons again fire in seemingly random patterns.
“When it’s alone, it’s almost like it’s singing in the shower,” Kao said. “The pitch tends to vary. The finch is just idly fooling around, having fun with the song.”
This finding initially puzzled Kao. Why would a bird need to vary its song once it had already learned it? To find out, she short circuited the birds’ learning circuits, then tested their brain activity. When she did, she saw that birds still working on mastering their song suddenly seemed stuck in a loop—they were no longer able to make variations on their songs, and whatever progress they had made in learning it up to that point was suddenly frozen in its tracks.
“These experiments have taught us that specific areas of the brain actively help us to learn and explore by using trial and variation. But the brain also is able to turn off that variability in cases when we need to perform something accurately,” Kao said. “What we’d really love to know in the future is how the brain is switching between these different modes of activity, and how it’s adjusting to environmental conditions.”
It’s not just an academic exercise. Understanding these neural systems more deeply could have big implications for human disease, Kao noted. In our brains, the CBG is not just linked to learning—it’s also implicated in several life-threatening neurological disorders, such as Parkinson’s disease and Huntington’s disease.
“The CBG is really the center of motor learning in the body. In the case of Parkinson’s, neurons that project into the CBG die off, so you have problems initiating movement, or your movements are slow,” she said. And not just physical movement like walking, but also talking.
“In the case of Huntington’s disease, there’s too much activity in the CBG, which leads to uncontrollable movements,” Kao said. “If we can understand more about how this circuit works, we might be able to figure out how to prevent the effects of these diseases. With a model system like finches, we can go in and say, ‘How can we fix this? How can we change activity patterns, or stop the cells from dying?’ It’s really exciting.”
David Levin is a Boston-based freelance writer.