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March 8, 2000
FOR IMMEDIATE RELEASE
CONTACT: Michael Purdy
mcp@jhu.edu, 410-516-7906
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Brain Cell 'Chorus' Appears as Attention
Increases
The sudden emergence of a brain cell "chorus" from the cacophony
of normal brain cell activity may enable the brain to pay close
attention to one item in a flood of incoming sensory information,
according to a report in this week's Nature.
The report, based on data acquired from monkeys, suggests that a
baseball
player tracking a fly ball through a cloud-cluttered sky, a
driver reaching
into a pocket to feel for keys, and a high-school student seeking
a
cafeteria dish that smells edible could all have something in
common: Some
of the nerve cells in the cortex, the sophisticated outer layer
of the
brain, may be sending messages in unison to allow them to pay
attention to
a single stream of sensory input.
"Every second, we get millions or hundreds of millions of bits
of
information coming in from our senses," says Ernst Niebur,
assistant
professor of
neuroscience at the
Krieger Mind-Brain Institute
at The Johns
Hopkins University. "And we have to decide, every second, which
part of it
is important and which part is not important."
"The nerve cells which represent the important information need a
way to
stand out from the crowd of other information," says
Peter
Steinmetz, lead
author on the paper and a former postdoctoral scholar at the
institute.
"Firing synchronously – like singers in a chorus -- is one way to
stand out
from the crowd."
Scientists produced the new finding by re-analyzing data gathered
over
several years. Institute scientists
Ken Johnson and
Steve Hsiao had been
monitoring brain cell activity in monkeys who were performing
simple tasks
that required them to focus their attention on visual or tactile
stimuli.
Tasks included identifying which of three white squares of light
on a video
monitor was beginning to dim, and comparing the shape of raised
letters or
figures pressed against a finger.
Applying a technique perfected by Hopkins neuroscientist Vernon
Mountcastle, researchers used seven electrodes to simultaneously
monitor
individual brain cell activity in the monkeys as they worked.
They
originally analyzed the data they gathered for changes in the
firing rate
of brain cells as the animals switched attention between
tasks.
When Niebur arrived at Hopkins a few years ago, researchers
started talking
about taking another look at the data.
Niebur and other theoretical neuroscientists were speculating
that the
brain might encode information both in the firing of individual
brain cells
and in the timing of those firings.
"It's been shown in animals that the firing rate of neurons can
go up by a
factor of 2 or 3 when they start to pay attention to a stimulus,"
Niebur
says. "But it seems to run the risk of confusing signals if you
try to
code for two different things -- the stimulus itself and the
degree that
one should pay attention to it -- with one type of signal, the
rate at
which neurons are firing."
Niebur says the two different signals have to be connected. What
your
senses perceive will influence how much attention you pay to
them, but, he
said, "it seems like a good idea if you can have two different
but related
signals that you can use to represent these two things." An
increase in the
number of nerve cells firing in unison could represent just such
an
independent, but related, second signal.
Hsiao and Johnson had data from three earlier experiments
appropriate for
testing the theory. Steinmetz, now a post-doctoral scholar at
Caltech,
combined currently available computer power with a cutting-edge
statistical
technique to determine if nerve cells were firing synchronously
and if the
strength of that synchrony changed when the monkeys needed to pay
attention.
"Detecting synchronous firing reliably has been difficult in the
past
because of the large amounts of data that need to be analyzed,
but one
outcome of the computer revolution has been the ability to
perform this
type of testing in reasonable timeframes," Steinmetz says.
The results of the analysis, according to Niebur, suggested that
when the
monkeys were paying close attention to the stimuli, "the amount
of
synchronous firing appeared to increase in a sizable fraction of
the
neurons involved in these tasks."
Such a mechanism could have intriguing connections to basic nerve
cell
structure and function, Hsiao notes. Nerve cells frequently
receive
incoming signals from not just one but several different
branch-like
structures known as dendrites. Unless the signal is very strong,
receiving
a signal on any one dendrite doesn't necessarily guarantee that
the nerve
cell will pass on the message.
"If all the neurons upstream are firing synchronously, though,
that
strongly increases the possibility that the nerve cell will pass
the
message on downstream," says Hsiao, an associate professor of
neuroscience.
"We were lucky that these three groups could come together for
this team
effort," Hsiao comments. "The Mind-Brain Institute is one of a
very few
places in the U.S. where you could see such a unique and close
collaboration between experimental, theoretical, and
computational
neuroscientists."
All 3 research groups plan to follow up on the finding in the
future.
"I'd like to go back to an earlier stage in this process, and
look for some
type of oscillatory signal that we're thinking could proceed
these
synchronized nerve cell firings," Johnson, a professor of
neuroscience, says.
Niebur and Hsiao expressed interest in finding out what happens
to
synchrony rates if the test subjects fail to successfully
complete the task
they're concentrating on. Steinmetz's current research includes
an
investigation how strongly the neurons need to synchronize their
firing to
be "heard."
Funding for this study came from the National Institutes of
Health, the
Alfred P. Sloan Foundation and the National Science Foundation.
Other
authors were Arup Roy and Paul Fitzgerald, graduate students in
neuroscience at Krieger Mind-Brain.
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