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February 10, 2005
Bird Brains Show How Trial and Error May Contribute to Learning
The adult male zebra finch knows only one scratchy tune learned in
its youth, which it performs repeatedly and intensely when females are
listening. But occasionally, the finch might improvise, experimenting
with a slower, more sultry variation or emphasizing different
notes.
Neurobiologists studying the finch now say the improvisation arises
from a component of a crucial learning circuit in a section of the
forebrain that seems to generate the trial and error necessary to
master sophisticated motor skills, such as singing in birds or speech
and sports in humans.
“The basal ganglia play a special role in generating motor variability.”
Mimi Kao
"It means this part of the brain is important for instructing or
allowing changes in the song," said Mimi Kao, first author of a paper
in the February 10, 2005, issue of the journal Nature that
demonstrates how the region modulates bird song in real time. Kao, a
Howard Hughes Medical Institute (HHMI) predoctoral fellow, is in the
final months of her doctoral training in the laboratory of co-author
Allison Doupe at the University of California, San Francisco's Keck
Center for Integrative Neuroscience.
A similar brain pathway in humans may explain how children learn to
talk by listening to themselves and others, and how adults learn and
hone new motor skills, such as tennis. The process relies on feedback
about what works and what doesn't, also called experience-dependent or
performance-based learning.
"That all requires paying attention to how we're doing,
experimenting with different things, and gradually getting better,"
said senior author Michael Brainard, assistant professor of physiology
at UCSF, whose lab is funded in part by a grant from HHMI. "It makes
sense that one part of the brain has as part of its job introducing
that kind of variability."
Kao began with an experiment to stimulate the region of the
forebrain called LMAN (lateral magnocellular nucleus of the anterior
nidopallium). In the avian brain, LMAN receives input about complex
movements from the basal ganglia and forwards the information to motor
neurons that participate in song production. Without LMAN, scientists
have long known, a young bird cannot learn its song, but an adult bird
can sing its song without that region of the brain. A postdoctoral
fellow on another project had noticed greater and more variable brain
activity when the finches were singing to themselves compared to when
they were serenading females. Kao wondered whether she could cause
changes in bird song by manipulating this region.
Learning takes some time, so Kao expected to wait for her results.
But stimulating the LMAN had an immediate impact. The tune and rhythm
of the basic song did not change, but a tiny burst of electricity would
show up a few notes later as a change in volume or pitch at a
particular time in the song.
The variations are usually too subtle for human ears, Kao said, but
sensitive recording equipment can detect them. The systematic trials
were possible with the aid of a computer program that could track the
bird twitters and trigger stimulations to LMAN at precise moments to
elicit measurable effects on a predetermined syllable, song after
song.
The researchers found that different areas of LMAN tuned the same
note in different directions, one area raising the pitch of a certain
note and another area lowering its frequency. The moment-by-moment
influence of this brain region on song is a new observation, Brainard
said.
Next, the researchers analyzed the relationship between song and the
natural neural activity of LMAN during the two types of male finch
song, the performance-quality song directed at females, accompanied by
some posturing and feather plumping, and the experimental solo
variations, called "undirected," akin to singing in the shower. The
brain region showed greater activity and more variable signaling during
the undirected song, suggesting that this area is generating the
variations of solo song.
Finally, they showed that birds with damage to that region of the
brain lost their improvisational ability. In birds without a
functioning LMAN, Kao and her colleagues still found differences
between the two types of songs, but the solo song lost its subtle
variations.
"In a nutshell, our paper suggests that the basal ganglia play a
special role in generating motor variability," Brainard said. "It's
been known for a long time that this circuit is important in learning.
Our data supports the hypothesis that one of the things it could be
doing is introducing variability."
Other researchers in the Brainard and Doupe labs are following up to
see if the male mating song can be permanently altered by LMAN
stimulation and whether females prefer the stable or variable song
version.
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