Although many a college student hopes that his brain will continue cramming for an exam while asleep, scientific proof that that can happen has been scarce. But now scientists report new evidence that songbirds may polish their songs during their sleeping hours. The work, described in tomorrow's Science, may also provide new clues to human language learning.
Like novice opera singers, young male songbirds first learn their songs by imitating other males. Each bird later adds flourishes that make his rendition unique. Some researchers thought that the embellishment happens while the bird is awake and singing, based on what they found when they recorded the activity of neurons in HVc and RA, the two brain regions that control bird songs. The HVc neurons send signals to the RA neurons, which in turn control the singing muscles, and researchers had found that the two regions fire not just when the birds sing, but when they listen to recordings of their own songs. This led to suggestions that during singing, the neurons modify their activity to improve the song. But those results came from anesthetized birds. In the new work, Daniel Margoliash and his colleagues at the University of Chicago recorded HVc and RA neuron activities in awake birds.
The researchers found that HVc neurons responded to recordings of the birds' own songs, as they had in previous experiments. But RA neurons did not, firing in a monotonous pattern instead. As the birds fell asleep, however, both the HVc and the RA neurons began firing in response to the recordings. Then, as the birds woke up, the RA neurons returned to the monotonous firing pattern. To Margoliash, the wide-open communication between HVc and RA during sleep suggests that that's when the birds learn to refine their songs, although other neurobiologists caution that there is as yet no direct proof for that idea.
The work might provide insights into human language learning, says Richard Mooney, who studies bird song learning at Duke University Medical Center. Among other things, he notes, the Chicago team linked the shutdown of HVc-RA communication to the neurohormone norepinephrine, and because the sex hormones may lead to increases in local norepinephrine concentrations, the finding may provide an explanation for why songbirds lose the ability to refine their songs at puberty. And from there he says, it would "not be a big leap" to consider that a similar mechanism might lead to the problems humans have in learning languages fluently after puberty.