A small bird chirps a song somewhere in the trees above. The song replays over and over, and other birds join the chorus, each with its own unique melody. While these songs may conjure pastoral, peaceful feelings for many, for assistant professor of biology Todd Troyer they stir up thoughts of complex sequences of brain cell activity.
The first to admit he knows little about ornithology, Troyer maintains a nest of nearly 50 birds whose songs he listens to not for pleasure, but in hopes of shedding light on the mysteries of the human brain.
After receiving his Ph.D. in mathematics from the University of California, Berkeley, Troyer accepted a postdoctoral position in the W.M. Keck Center for Integrative Neuroscience at the University of California, San Francisco. His interests led him to work under behavioral neuroscientist Allison Doupe, an expert in using birds as models for understanding complex learned behavior. In 2007, he joined UTSA’s Department of Biology and began using computational methods based in his background in mathematics to conduct research in the Neuroscience Institute.
Along with bats and aquatic mammals, birds are the only known animals to learn to “speak” the way humans do—by imitating adults. Troyer’s research focuses on zebra finches, a small bird that is native to Australia. While both male and female birds sing, only the males produce a complex song used to attract a mate. Young birds learn their father’s song, although no two birds sound exactly alike.
In the wild, zebra finches live in the grassy plains, feeding on seeds. Unlike most songbirds that breed seasonally, zebra finches breed whenever there is sufficient rain and food, reaching sexual maturity within 65 days. Their songs are fully developed by three months. This short time span makes the zebra finch ideal for research, and they often are called the mouse of the bird world.
Birdsongs are used as a model for understanding how the neurons that control learned behaviors work in human brains. Troyer says there are similarities between bird and human brains both in speech development and in neurological diseases. There are two circuits in particular, he says: those that control learning and our ability to change behavior, and those that produce a certain behavioral task. In normal behavior, the two circuits are balanced; however, with some disorders such as Obsessive Compulsive Disorder (OCD), the ability to switch off the circuit for producing a particular task is impaired. Understanding how the brain balances the firing of the two circuits in birds’ brains may help in understanding how to control OCD and even Parkinson’s disease.
“Previous studies have indicated that the basal ganglia, a set of brain areas compromised in Parkinson’s, play a critical role as birds learn,” says Troyer. “Recent studies suggest that these brain areas may be important for regulating how the bird explores the production of different sounds during the learning process.”
For his research, Troyer compares his own observations of the birds’ natural behaviors with recordings of the birds’ neuronal activity.
He is concerned with the timing structure of the song and how that varies from song to song. Because zebra finches sing the same song hundreds of times each day, Troyer is able to gather a lot of data, which he observes on two levels.
At a broad level, Troyer is looking at what modulates the system for singing: How does the song vary across its entirety? How does it vary depending on the time of day? A key question he asks is what are the motivating factors and the mechanisms for the variations that occur. A number of chemicals that affect the level of hormones, such as norepinephrine and dopamine, send strong signals to the areas of the bird’s brain that control singing, and Troyer wants to know what effects each has on the ability to produce songs.
On a more detailed level, Troyer is looking at the basic units of each song. He created software that allows him to visually display the different sounds and then analyze them quantitatively. Each chunk of the song—commonly called a song “syllable”—is represented as a pattern of orange color on a graph, with silence represented by the black background. Features such as pitch and tempo are represented by small waves or spots within the orange bars. The syllables last anywhere from one-tenth to one-fifth of a second and are separated by silence. One study has shown that although the syllables contain smaller segments, the birds will not stop singing mid-syllable, even when startled.
Troyer is interested in determining the relationship between the small pieces and the whole syllable. If the syllable is sung more slowly, one would think the parts within it are slow by the same percentage. But Troyer has found that the parts are independent of one another—they are not slowed at the same rate as the syllable.
“Imagine a row of dominoes,” Troyer says. “Each domino is the same in size and shape; however, imagine they are placed at varying distances from one another. If you push the first domino over, it will knock down the next and each of the following will do the same in sequence. But the relationship between each domino is different because the space between them is different. This is how the parts of the birds’ songs are. The entire row will fall in sequence, but the space between some is greater, so it affects the following domino differently than those spaced closer together.”
Troyer continues to collect data for his research and has planned two possible experiments this summer. The first would be to block the birds from hearing themselves sing; Troyer would like to know whether it is important for the bird to actually hear itself while singing. The other would be to alter the chemistry levels in the birds’ brains and see how their singing is affected.
Using the zebra finch as a model for understanding human brain circuits, Troyer’s research may lead to a greater understanding of neurons in our own brain, and the neurological disorders we face.