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March 8, 2000
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Brain Cell 'Chorus' Appears as Attention
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
player tracking a fly ball through a cloud-cluttered sky, a
into a pocket to feel for keys, and a high-school student seeking
cafeteria dish that smells edible could all have something in
of the nerve cells in the cortex, the sophisticated outer layer
brain, may be sending messages in unison to allow them to pay
a single stream of sensory input.
"Every second, we get millions or hundreds of millions of bits
information coming in from our senses," says Ernst Niebur,
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
stand out from the crowd of other information," says
author on the paper and a former postdoctoral scholar at the
"Firing synchronously – like singers in a chorus -- is one way to
from the crowd."
Scientists produced the new finding by re-analyzing data gathered
several years. Institute scientists
Ken Johnson and
Steve Hsiao had been
monitoring brain cell activity in monkeys who were performing
that required them to focus their attention on visual or tactile
Tasks included identifying which of three white squares of light
on a video
monitor was beginning to dim, and comparing the shape of raised
figures pressed against a finger.
Applying a technique perfected by Hopkins neuroscientist Vernon
Mountcastle, researchers used seven electrodes to simultaneously
individual brain cell activity in the monkeys as they worked.
originally analyzed the data they gathered for changes in the
of brain cells as the animals switched attention between
When Niebur arrived at Hopkins a few years ago, researchers
about taking another look at the data.
Niebur and other theoretical neuroscientists were speculating
brain might encode information both in the firing of individual
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,"
says. "But it seems to run the risk of confusing signals if you
code for two different things -- the stimulus itself and the
one should pay attention to it -- with one type of signal, the
which neurons are firing."
Niebur says the two different signals have to be connected. What
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
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
independent, but related, second signal.
Hsiao and Johnson had data from three earlier experiments
testing the theory. Steinmetz, now a post-doctoral scholar at
combined currently available computer power with a cutting-edge
technique to determine if nerve cells were firing synchronously
and if the
strength of that synchrony changed when the monkeys needed to pay
"Detecting synchronous firing reliably has been difficult in the
because of the large amounts of data that need to be analyzed,
outcome of the computer revolution has been the ability to
type of testing in reasonable timeframes," Steinmetz says.
The results of the analysis, according to Niebur, suggested that
monkeys were paying close attention to the stimuli, "the amount
synchronous firing appeared to increase in a sizable fraction of
neurons involved in these tasks."
Such a mechanism could have intriguing connections to basic nerve
structure and function, Hsiao notes. Nerve cells frequently
incoming signals from not just one but several different
structures known as dendrites. Unless the signal is very strong,
a signal on any one dendrite doesn't necessarily guarantee that
cell will pass on the message.
"If all the neurons upstream are firing synchronously, though,
strongly increases the possibility that the nerve cell will pass
message on downstream," says Hsiao, an associate professor of
"We were lucky that these three groups could come together for
effort," Hsiao comments. "The Mind-Brain Institute is one of a
places in the U.S. where you could see such a unique and close
collaboration between experimental, theoretical, and
All 3 research groups plan to follow up on the finding in the
"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
synchronized nerve cell firings," Johnson, a professor of
Niebur and Hsiao expressed interest in finding out what happens
synchrony rates if the test subjects fail to successfully
complete the task
they're concentrating on. Steinmetz's current research includes
investigation how strongly the neurons need to synchronize their
Funding for this study came from the National Institutes of
Alfred P. Sloan Foundation and the National Science Foundation.
authors were Arup Roy and Paul Fitzgerald, graduate students in
neuroscience at Krieger Mind-Brain.
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