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| Photo
by Daniel Dubois |
| Jeff Schall |
Neurons
that play truth or consequences
By
David F. Salisbury
October 3, 2003
The
“CEO” in your brain appears to be concerned more about the consequences
of your actions than how hard they are to produce.
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| Illustration
by Barbara Martin |
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| The
anterior cingulate cortex crowns the brainstem and is tucked
between the hemispheres of the brain. It is connected to a number
of different parts of the brain, including the areas in the
cortex that control both thought and action, as well as sub-cortical
structures involved in emotion. Patients with abnormally high
activity levels in the cingulate often display problems such
as obsessive-compulsive disorder or aberrant social behaviors
while reduced levels of activity can lead to depression, diminished
self-awareness and reduced response to pain. |
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That
is the implication of a detailed study of the neuronal activity
in a critical area of the brain, called the anterior cingulate cortex
(ACC),published
in the Oct. 3 issue of the journal Science.It
is the latest in a series of experiments that are beginning to lift
the veil on the brain’s “executive function” – how it monitors its
own performance so that it can regulate behavior. Many cognitive
scientists feel that the ACC may be at the heart of this higher
order system.
Researchers
found the ACC responds to discrepancies between a person’s intentions
and what actually occurs when actions are performed, providing new
support for one popular theory on its function. But they did not
find evidence of neural activity in the ACC when the brain is forced
to change course in mid-action, as predicted by another popular
theory.
| |
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| Courtesy
of Joshua Brown |
| Joshua
Brown |
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“The broad question is, ‘How
does the brain monitor and control intentional actions.’ Our research
indicates that it does so by monitoring the consequences of such actions,
not the actions themselves,” says Jeffrey Schall, Ingram Professor
of Neuroscience, director of Vanderbilt’s Center for Integrative and
Cognitive Neuroscience and Kennedy Center investigator. He directed
the study with doctoral student Shigehiko Ito, post-doctoral fellow
Veit Stuphorn, and Joshua Brown, a research associate at Washington
University.
Brown adds that
“the ACC isn’t the only part of the brain that processes these kinds
of signals. But this finding is important because the ACC plays
a key role in disorders such as schizophrenia and obsessive-compulsive
disorder in humans. Other research here at Washington University
suggests how the ACC uses these signals to help people control their
actions.”
The researchers
investigated the ACC through detailed studies measuring the response
of hundreds of individual neurons in the ACC of macaque monkeys
as the animals performed a task that required self-control. Macaques
serve as the primary animal model for higher cognitive function.
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| Courtesy
Jeff Schall |
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| Macaque
brain with the area of the anterior cingulate cortex where neuronal
activity was measured in blue. |
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The monkeys
were trained to look at a visual target displayed in different positions
on a computer screen, unless they received a stop signal. They were
taught not to look at the target after receiving such a signal.
The monkeys’ eye movements were tracked with enough precision so
that they could be correlated with neuronal activity. The monkeys
were trained by rewarding them with squirts of juice when they correctly
followed instructions.
By requiring
the monkeys to inhibit a movement after their brain had begun preparing
to execute it, Schall and his colleagues created situations that
isolated different types of neural signals. By recording in the
ACC while the monkeys responded to these situations, the researchers
successfully identified neurons that signaled discrepancies between
intentions and actions, what the researchers refer to as errors.
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| Illustration
by Barbara Martil |
|
In the experiments, the macaque monkeys were placed in a specially
designed “high chair” in front of a computer-controlled display
screen. They are trained to respond to visual signals that instruct
them to look or not look at targets that appear on the screen.
When they follow instructions properly they are rewarded with
squirts of juice. During the sessions the macaques’ eye movements
are tracked and the activity in different parts of their brains
are monitored. |
“The elegance
of this paper is that they were able to sort out different cognitive
components or behavioral components that might be driving neural
activity in the cingulate,” says Tomas Paus of McGill University,
a neuroscientist who was not involved in the study but also investigates
this part of the brain.
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| Photo
by Daniel Dubois |
| Record
of neuronal activity |
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This methodology
allowed the researchers to determine whether activity in the anterior
cingulate signaled that the action deviated from what the monkey
had intended or signaled that the consequences of the action differed
from what he anticipated. “We had a few trials where he did the
right movement but we didn’t give him juice. We found that many
of these neurons also fired following the absence of reinforcement,”
says Schall. In these trials the monkey’s action was correct but
the consequence was unexpected. If the ACC were monitoring the actions
alone, the neurons would not have responded.
The
researchers also found that there was an appreciable time lag in the
responses of the neurons in the anterior cingulate compared to that
of neurons in another part of the frontal lobe, the supplementary
eye field – an area that Schall and coworkers had previously shown
to contain neurons signaling success, errors and degree of difficulty
in eye-tracking tasks.
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| Photo
by Daniel Dubois |
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| Viet
Stuphorn |
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“This delay
makes it unlikely that the purpose of the error detection that we
discovered in the ACC is to correct actions as they take place,”
said Schall.
An influential
theory about ACC function has suggested that the brain is sensitive
to the conflict that arises when tasks are too complex and subjects
are being asked to do more than they can without making errors. Schall’s
earlier work on the supplementary eye field found neurons signaling
this conflict, but they failed to find neurons signaling conflict
alone in the ACC. This observation is at odds with certain functional
brain imaging studies in humans.
These results
do not mean that the ACC does not also monitor conflicts, Schall
cautions. Virtually all of the human measurements have been made
using tasks that involve button pushing or other manual tasks, rather
than eye movements. The ACC has no direct connections to the areas
of the brain that control the eyes but it does have direct connections
to those that control the muscles in the hands and arms. So hand
movements may be controlled differently than eye movements. There
is also the possibility that the ACC in humans may function differently
than it does in monkeys.
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| Courtesy
of McGill University |
|
Tomas Paus |
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“These results
are telling us that things are not as simple as some people have thought,”
says Paus, who adds that we won’t really be able to tie these signals
back to cognitive functions until researchers can go beyond the current
stage of simply recording brain activities to that of actually inducing
changes in brain activity – either through the administration of drugs
or electrical stimulation to small groups of neurons – and observing
the changes in behavior that result.
The research
was funded by grants from the National Institute for Mental Health,
the National Eye Institute, McKnight Endowment Fund for Neuroscience
and the Deutscheforschungs Gemeinschaft.
-VU-
Jeff
Schall’s laboratory home page
Joshua
Brown’s home page
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