Neurological maps hold key
to how brain learns and forgets
BY KRISTIN WEIDENBACH
When the mind forgets, it may not mean the brain has
forgotten -- at least that's the case with barn owls. For
the wise birds, learning new skills then reverting back
to old methods requires nothing more than pulling up the
correct neurological map, according to Stanford
researchers, who have studied the owls in their quest to
find out how the brain learns and forgets.
By outfitting the owls with a pair of prism eyeglasses
that give them a skewed view of the world, the scientists
have been able to analyze how neurons in the owls' brains
adjust to the confusing new environment.
Sight and hearing are intimately connected in barn
owls, so in order to adapt completely to the new
off-kilter visual world, the animal has to learn new
responses to localize the sounds that it hears. It must
also forget the old responses that are no longer
appropriate, said Eric Knudsen, PhD, Sewall Professor of
Neurobiology.
In a series of incremental advances over the last
three years, Knudsen and his team have determined how
cells in the owl's brain learn to respond correctly to
sound. In the May 7 issue of Science, Knudsen and
postdoctoral fellow, Weimin Zheng, PhD, now report that
they have worked out the opposite side of the equation
how cells in the owl's brain forget the incorrect
responses.
Barn owls, which have a much more sensitive auditory
system than humans or other animals, naturally know how
to localize their prey. They quickly locate a chirping
cricket or a squeaking mouse by integrating two
neurological maps in their brain one based on sound,
the other on vision. If the owl first hears a sound in
its left ear, it knows it must turn its head to the left
to see the noisy critter and scoop it up for lunch.
Researchers in Knudsen's lab call the inherent
neurological map that the owls use to do this, the normal
map.
Owls that are fitted with the prism glasses see a
world that is shifted. An owl wearing a pair of
right-shifting glasses sees objects to its left when it
is looking straight ahead. A young bespectacled owl
learns to compensate for this altered perception, turning
its head to the right to see what lies straight ahead.
These owls have developed a new neurological map that the
researchers call the learned map. "For an owl to be
able to localize the sound correctly after wearing the
prisms it has to be able to forget the older map which is
not behaviorally appropriate and follow the new
map," said Zheng.
Previous research in the Knudsen lab has shed light on
how neurons in the owl's brain learn the new map, but
according to Zheng, people knew nothing about the
mechanisms underlying the process of forgetting the old
map. Zheng and Knudsen focused on the ICX region of the
brain, which is a site of auditory learning in the barn
owl. This region of the brain is also rich in a certain
class of inhibitory nerve cells called GABAergic neurons.
The researchers wondered if the normal map, when it was
not appropriate to the owl's situation, was being
specifically suppressed by these neurons. If their theory
was correct, by blocking these neurons the normal map
would move to the forefront, replacing the learned map.
Zheng and Knudsen conducted a series of experiments,
which confirmed their hypothesis. When the GABAergic
neurons were blocked, cells in the owl's brain that had
been showing the learned responses suddenly began to show
the normal responses. The results were important because
they revealed that the normal map had not been lost. The
map and all its information had remained intact, but had
been temporarily suppressed, or forgotten.
When the researchers tried the same approach on adult
owls whose glasses had been removed, they got a very
different result. One month after they removed the
glasses, they blocked the same neurons in owls that had
readjusted to an unadulterated view of the world and were
responding normally to sounds using their normal
neurological maps. Unlike the unveiling of the normal map
in the previous experiment, however, the learned map did
not emerge, indicating that it was not being suppressed
by the inhibitory action of the GABAergic neurons.
This revealed that the "genetically controlled
normal map is different from the learned map controlled
by experience," said Zheng.
However, the results presented the researchers with a
new conundrum that they are now working to solve. Knudsen
and his team knew from their previous work that owls
reared with glasses that had subsequently been removed
could adapt to the glasses successfully once again if
they were reintroduced to the adult owl. These adults
quickly remembered how to accommodate the zany view of
the world that the glasses induced, whereas adult owls
introduced to the glasses for the first time never
learned to adapt.
So although the learned map could not be
experimentally brought forth by suppression of GABAergic
neurons, there must be a trace left in the brain that
allows the owls to reacquire it, said Knudsen. Some
portion of that circuitry is still there, but like a
radio that can be unplugged or have its volume turned all
the way down, the signal is not coming through, he
explained. Now, the task before Knudsen and his
colleagues is to identify the unseen hand that is
controlling this phenomenon.
Stanford University School of Medicine, the McKnight
Foundation and the National Institutes of Health provided
funding for the study. SR
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