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Name that Tune: how Birds learn to Recognize Song


Researchers in a University of Chicago lab are peering inside the
minds of European starlings to find out how they recognize songs.
In the process are providing insights into how the brain
learns, recognizes and remembers complex sounds at the cellular
level. In a study published in the Aug. 7, 2003, issue of Nature,
the researchers show how songs that birds have learned to
recognize trigger responses both in individual neurons and in
populations of neurons in the bird's brain.


"We found that cells in a part of the brain are altered
dramatically by the learning process," said Daniel Margoliash,
Ph.D., professor of organismal biology and anatomy and of
psychology, and co-author of the paper. "As birds learn to
recognize certain songs, the cells in this area become sensitive
to particular sound patterns or auditory objects that occur in
the learned songs, while cells never show such sensitivity to
patterns in unfamiliar songs. Specific cells in the brain become
'tuned' to what the bird is learning."

How the brain perceives and interprets stimuli from the external
world are fundamental questions in neuroscience. There are many
types of memory systems in the brain. Memories of words, sounds
of voices or patterns of music are important components of human
daily experience and are essential for normal communication, yet
"we know little about how such memories are formed in the brain
and how they are retrieved," Margoliash said.




Bird songs have captured the interest of humans for ages.
"Birders can often recognize many species of birds by only their
songs," he said.

For the birds themselves, however, song recognition is no casual
business. The ability to match a singer to a song, often down to
the level of an individual bird, can mean the difference between
"a day spent wrestling through the thicket and one spent enjoying
a sun-soaked perch, or the missed chance at mating with the
healthiest partner around."

Lead investigator of the study Timothy Gentner, Ph.D., a research
associate in the department of organismal biology and anatomy,
has tapped into the recognition abilities of songbirds by
training birds in the lab to recognize songs. The birds were
taught to press different buttons on a small metal panel
depending on the song they heard. The researchers rewarded
correct responses with food and turned the lights off to convey
an incorrect response.

Gentner's earlier research has shown that European starlings
learn to recognize different songs by the individual pieces that
comprise each song.

"If you listen closely to a singing starling, you'll hear that
the song is really composed of much shorter sounds," Gentner
said. "We call these sounds 'motifs,' and to produce a song, the
bird will sing the same motif a few times, then switch to a new
repeated motif, and then another, as long as he can keep it
going. When male starlings sing, they might use only half of the
motifs they know and then mix up the motifs when they sing
another song."

Given this highly variable motif structure, when other starlings
are learning which songs belong to which individuals, they do it
by concentrating on the motifs, he said. "Even one or two
familiar motifs in an otherwise unfamiliar song is enough to
trigger recognition."

To examine the neural mechanisms associated with auditory memory,
Gentner and Margoliash measured the electrical impulses from
single nerve cells in the auditory area of the bird's brain known
as cmHV an area analogous to the higher-order, secondary
auditory cortex in humans in starlings trained to recognize
several songs.

The researchers recorded the response of each neuron to songs the
birds had learned to recognize, to unfamiliar songs the birds had
never heard before and to synthetic sounds such as white noise.
As a population, the cells responded much more strongly to the
songs the birds had learned to recognize than to any of the other
sounds. Individually, a majority of the cells responded to only
one song, and almost all (93 percent) of these cells responded to
one of the songs the bird had learned to recognize. After
examining the data even more closely, they found that many of
these cells only responded to specific motifs in a familiar song.

"The song motifs that drive these cells so strongly are the same
components of sounds that control recognition behavior in the
birds," Gentner said. "It appears that we are seeing the memory
traces for recognition of these complex acoustic patterns. Rather
than representing all motifs equally well at any time, we find
that experience modifies the brain to highlight those motifs that
are the most important to the bird at that time."

"[These motifs] are the books that make up the starlings' library
of memories, and we're learning how the starling represents those
books in his brain," Margoliash said.

"Memories are not permanent," he said. "Do we loose memories
because of disuse or because they are crowded out by other
memories? Our research shows that the context in which you learn
a sound affects how it is memorized. What are the brain
mechanisms that control this process of how a memory is laid
down?"

Margoliash and Gentner believe these are questions that can be
answered in future research with starlings. Neuroscientists, and
children learning language, are interested in the answers.


This story has been adapted from a news release issued by
University Of Chicago Medical Center.


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