In my last post, “Vocal Practice is for the Birds” examined one similarity between human and songbird procedural learning: the necessity for practice before performance. Zebra finches sing a series of introductory notes to prepare before beginning their mating song, much like we warm up before playing an instrument or before an athletic competition. This is but one of the many similarities found between human and songbird behaviors. In fact, scientists have been using songbirds to study many common behaviors, like spatial memory and social interactions in addition to procedural learning. Songbirds are the ideal model system for studying the neurogenetic basis of vocal learning due to the similarity of the neural structures underlying this relatively rare behavior.
Vocal learning is the ability to repeat or make new sounds with one’s voice. This ability is relatively rare. Humans are the only living primates with this ability, and there is no definitive evidence for vocal learning in rodents. Other groups of mammals do have vocal learning, including elephants, seals, walruses, whales, dolphins, and some bat species, typically used in mating or territorial disputes. However, since these vocal learning mammals are not easy to study in the lab, many scientists choose to instead study songbirds.
Songbirds like the zebra finch learn to sing in much the same way we learn to speak. First, they listen to and memorize a tutor’s song. Next, they begin to make rudimentary sounds to try to mimic the song. This is similar to the way human babies babble before they say their first word. As the young zebra finches get older, the babbling sounds more like the tutor’s song, until adulthood when the song matures and stabilizes.
Only the zebra finch males sing, and they are restricted to learning a single song for their entire life. This is not always the case for songbirds. For some other species of songbirds, females also sing. Anyone who has been woken up by a mockingbird will tell you that they can learn more than one song. Some birds can learn a song at any point in their lifetime, but zebra finches have a restricted period in the first 3 months of life to learn. For zebra finch females, the main reason they do not develop song is that the neural circuitry underlying song learning is not completely connected.
The song production system in songbirds is composed of well-defined neural structures called song nuclei. In female zebra finches, the song nuclei are smaller than in males, and not fully connected to one another. These nuclei, though made up of the same cell types as the surrounding areas of the brain, are dedicated to song learning and production. This is a great advantage for neuroscientists, since the activity of the neurons in these song nuclei are known to affect only singing and not other behaviors, such as flying or preening.
Songbirds have been used to the advantage of neuroscientists interested in the genetic, molecular, and cellular mechanisms of learning and memory. By comparing the brains of juvenile zebra finches still in the learning stage to those of adults, we gain insights into what genes are activated or repressed and what kinds of cellular activity corresponds to learning. This has been done extensively for the gen FoxP2, known as a gene related to human speech. FoxP2 is found in high levels in the zebra finch striatum of both males and females at all stages of life. Interestingly, when a male bird sings the levels of FoxP2 go down only in area X, the striatal song nucleus. This suggests that FoxP2 is regulated by singing activity. FoxP2 is a transcription factor that regulates the levels of other genes. Some genes targeted by FoxP2 have also been studied in the zebra finch, including the el autismo susceptibility gene Cntnap2. Of note, one of the core deficits in autism is language regression. Studying these genes can give us insights into the genetic basis of human vocal learning and speech. These studies may lead to novel therapies for speech and language disorders, including those associated with autism spectrum disorder.
Escrito por Michael C. Condro
Doupe A.J. & Kuhl P.K. (1999). Birdsong and Human Speech: Common Themes and Mechanisms, Annual Review of Neuroscience, 22 (1) 567-631. DOI: 10.1146/annurev.neuro.22.1.567
White S.A. (2010). Genes and vocal learning, Brain and Language, 115 (1) 21-28. DOI:10.1016/j.bandl.2009.10.002
Images adapted from WikimediaCommons.