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BRISTOL: Understanding the mechanism for how zebra finches learn their songs may shed light on how mammals - including humans - develop motor skills.
Songbirds have been studied extensively for over 30 years as they provide a powerful model for how animals learn to make vocalisations, including how humans learn to speak. Most theories have suggested that plasticity in a particular region of the bird brain, known as the arcopallium, is involved with learning this complex skill but, until now, there has been no experimental evidence to support the theory.
Now researchers have shown that stimulation of the brain in a particular pattern of activity weakens the connections between nerve cells in this region of the brain. This provides evidence for the cellular mechanism involved in song learning.
"Bird brains are much more similar to mammal brains than one might think," said David Perkel, from the Departments of Biology and Otolaryngology at the University of Washington and senior author on the study published in the current issue of Proceedings of the National Academy of Sciences.
"The learning pathway for songbirds maps on to mammalian brain circuits; there are corresponding parts, or homologues, in mammals. So understanding how the pathway works for learning in birds could have direct implications for how humans learn skills."
Songbirds as a model
Young male zebra finches learn their song from other adults, usually their father. The young males start to listen to the adult song when they are about 10 days old and begin to memorise it.
When they are a month old, the young males begin to vocalise themselves. They compare their own vocalisations to the memory of their father's song and start to fine-tune it. At first, the song is highly variable, but then it improves to become a good copy of their father's song.
By the time the young bird is three months old, he will have learned the one song he will sing for the rest of his life and it will become crystallised, meaning he will no longer be able to modify it. "It's similar to humans learning a new language, it's hard to sound like a native speaker if you learn the language after puberty," said Perkel.
Adjusting the volume
There are two pathways in the brain of the zebra finch that are involved with the learning and vocalisation of song. The direct motor pathway is necessary for the song production itself, while the anterior forebrain pathway is necessary for song learning.
The arcopallium is a region of interest as it receives input from both the motor pathway and the learning pathway. The researchers induced a change in the strength of collateral nerve cell connections in this region in response to stimulation of brain slices from juvenile birds.
Changes in synapse strength occur in many areas of the central nervous system and are usually a prelude to more permanent changes in the neural connections. Selective weakening, in particular, has been implicated in spatial learning and motor learning in mammals. "Changes in the neuronal connections are like adjusting the volume to the right level. Tweaking the knob is part of the learning process," said Perkel.
