The Rochester Review, University of Rochester, Rochester, New
Finding out how the tiny zebra finch learns to sing can teach us a thing or two about human learning and memory, say two Rochester researchers.
For most of us, the sound of birds singing, piercing winter's cold veil of silence, is a welcome sign of spring. For two Rochester scientists, however, bird song has come to mean much more -- a key to defining the physical essence of learning and memory in humans and animals.
The researchers are Professors Ernest and Kathy Nordeen, a husband-and-wife team of neurobiologists. For the last 10 years they have been studying the zebra finch, a bird so small it would fit in the palm of a child's hand. Commonly seen in pet stores, the males are distinguished by their chestnut cheek patches and the zebra striping on their chests from which they take their name.
Through their work in observing what happens in the zebra finch's brain as it learns to sing, the Nordeens may one day help to explain why we are more receptive to learning during some stages of our development than we are at others. Beyond that, this work may also help us better understand why memory sometimes fails, as it does in victims of Alzheimer's disease.
In their lab, the Nordeens track the microscopic changes that take place in the brains of hatchling finches as they grow, hear their elders sing the song of their species, and then memorize it to reproduce it themselves. By documenting and analyzing those changes, the Nordeens are actually mapping the brain's circuitry for learning and memory.
There are tantalizing parallels between birds and humans, research in the field has suggested.
Those who study birds have known for years that chicks must hear and practice the song of their species during a window of opportunity called the "critical period." If that doesn't happen, the bird simply will never learn it properly, and no amount of extra coaching later on will help. The Nordeens have shown that during these critical learning periods, the brain undergoes a remarkable transformation: The body produces in abundance a certain kind of neuron. When these neurons communicate with other brain cells to form its circuitry, the brain seems to be especially sensitive to experience -- primed to absorb and remember sounds. It is as if the bird is a quick study as a song-learner at this time of life but then loses that aptitude.
Many other species also are able to learn certain things -- or develop certain skills -- only if they are exposed to the right experiences at the right time. For example, if strabismus (crossed eyes) is not corrected in early childhood, a youngster won't ever develop normal binocular vision. If humans aren't exposed to a language by about age 7, their ability to learn to use that language begins to decline.
Bird brains may be small, but the Nordeens believe that the neural machinery our feathered friends use in learning to sing works essentially the same way in other animals that also learn and memorize. Neuroscientists theorize that the brain creates a memory by strengthening the communication between cells that are activated by a particular experience. Looking at a flower, for example, or smelling it, touching it, or even hearing its name, can trigger communication between cells.
Brain cells, the Nordeens explain, communicate with each other by secreting chemical neurotransmitters that plug into receptors on another cell. There are many different kinds of such receptors, and one called the NMDA receptor appears to be especially important in forging the long-term changes in brain-cell communication that are the stuff of memory. The NMDA receptor is widely found in the neural tissue of intelligent creatures, so what the Nordeens discover as they study zebra finches could open the door to a fundamental understanding of how the brain processes experience into learning and memories.
In order for birds to master songs, the Rochester scientists have discovered, NMDA receptors must be activated. Young males learn to sing by hanging out with tutors -- older males from whom they copy the song. However, when the Nordeens put young birds together with an older male, but blocked NMDA receptors with chemicals, they found that the youngsters copied correctly only fragments of the song -- getting only about 20 percent of it right.
Although NMDA receptors appear to play a key role in learning, the brain's ability to learn and create memory is influenced by other processes too, the Nordeens believe. In birds, the role of sex hormones seems especially evident in shaping brain development and function.
In most bird species, males sing more than females do, and this difference is reflected in the brain. At hatching, most areas used for making song are the same size in both sexes. But as the hatchlings mature, females start losing many of the neurons involved in song production, while males hang on to them. By adulthood, male brains have larger song-production areas than do their female counterparts. By trying to understand the role sex hormones play in the life and death of these neurons, the Nordeens hope to illuminate some of the fundamental reasons behind the life or death of brain cells.
In zebra finches, the birds the Nordeens study, the surge in production of "song-related" neurons happens only once -- during the critical period of development. But this isn't true of all species. In canaries, for instance, singing behavior changes with the season, and their neural systems also adapt. In the spring, when males stake out and protect mating territory, longer days trigger the production of more testosterone. Through summer, males sing often, and their songs are stable. Come fall, when birds sing less, their songs are less consistent. These seasonal changes in behavior are accompanied by seasonal fluctuations in the addition and replacement of song neurons.
For now, the discoveries the Nordeens are making have enriched understanding of the marvelously intricate neural machinery through which experience is stored in the brain. Eventually, their research may increase knowledge about how the propensity to learn changes with age.
Jan Fitzpatrick frequently writes about research activities of College faculty.
Copyright 1996, University of Rochester