To learn its signature melody, the male
songbird uses a trial-and-error process to mimic the song of its father,
singing the tune over and over again, hundreds of times a day, making subtle changes
in the pitch of the notes.
For the
male Bengalese finch, this rigorous training process begins around the age of
40 days and is completed about day 90, just as he becomes sexually mature and
ready to use his song to woo females.
To
accomplish this feat, the finch's brain must receive and process large
quantities of information about its performance and use that data to precisely
control the complex vocal actions that allow it to modify the pitch and pattern
of its song.
Now,
scientists at UCSF have shown that a key brain structure acts as a learning
hub, receiving information from other regions of the brain and figuring out how
to use that information to improve its song, even when it's not directly
controlling the action. These insights may help scientists figure out new ways
to treat neurological disorders that impair movement such as Huntington's disease
and Parkinson's
disease.
The
research is reported as an advanced online publication on May 20, 2012 by the
journal Nature, and will appear at a later date in the
journal's print edition.
Years
of research conducted in the lab of Michael Brainard, PhD, an associate
professor of physiology at UCSF, has shown that adult finches can keep track of
slight differences in the individual "syllables," or notes,
they play and hear, and make mental computations that allow them to alter the
pitch.
For
previous experiments, Brainard and his colleagues developed a training process
that induced adult finches to calibrate their song. They created a computer program that
could recognize the pitch of every syllable the bird sang. The computer also
delivered a sound the birds didn't like—a kind of white noise—at the very
moment they uttered a specific note. Within a few hours, the finches learned to
alter the pitch of that syllable to avoid hearing the unpleasant sound.
In the
new research, the UCSF neuroscientists used their technology to investigate how
the learning process is controlled by the brain. A prevailing theory suggests
that new learning is controlled by a "smart" brain structurecalled
the basal ganglia, a cluster of interconnected brain regions involved in motor
control and learning.
"It's
the first place where the brain is putting two and two together," said
Jonathan Charlesworth, a recent graduate of UCSF's neuroscience PhD program and
the first author of the new paper. "If you remove the basal ganglia in a
bird that hasn't yet learned to sing, it will never learn to do so."
Once a
basic, frequently repeated skill such as typing, singing the same song or
shooting a basketball from the free-throw line is learned, the theory suggests,
control of that activity is carried out by the motor pathway, the part of the
nervous system that transmits signals from the brain to muscles. But for the
basic routine to change—for a player to shoot from another spot on the
basketball court or a bird to sing at a different pitch—the basal ganglia must
again get involved, providing feedback that allows learning based on trial and
error, the theory suggests.
What
remained unclear is what makes the basal ganglia so "smart" and
enables them to support such detailed trial-and-error learning. Was it
something to do with their structure? Or were they getting information from
elsewhere?
The
scientists sought to answer this question by blocking the output of a key basal
ganglia circuit while training male finches to alter their song using the
white-noise blasts. As long as the basal ganglia were kept from sending signals
to the motor pathway, the finches didn't change their tune or show signs of
learning. But when Brainard's team stopped blocking the basal ganglia,
something surprising happened: the finches immediately
changed the pitch of their song, with no additional practice.
"It's
as if a golfer went to the driving range and was terrible, hitting the ball
into the trees all day and not getting any better," said Charlesworth.
"Then, at the end of the day, you throw a switch and all of a sudden
you're hitting the fairway like you're Tiger Woods."
Normally,
you'd expect improvement in skill performance like this to take time as the
basal ganglia evaluates information, makes changes and gets new feedback, Brainard
said.
"The
surprise here is that the basal ganglia can pay attention, observe what other
motor structures are doing and get information even when they aren't involved
in motor control," Brainard said. "They covertly learned how to
improve skill performance and this explains how they did it."
These
findings suggest that the basal ganglia's "smartness" is due in large
part to the steady flow of information they receive about the commands of other
motor structures.
It also
portrays the basal ganglia as far more versatile than previously understood,
able to learn how to calibrate fine-motor skills by acting as a specialized hub
that receives information from various parts of the brain and responds to that
information with new directives.
The
findings also support the notion that problems in the basal ganglia circuit's
ability to receive information and learn from it may help trigger the movement
disorders that are symptoms of Huntington's and Parkinson's, Brainard said.
Provided
by University
of California, San Francisco
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