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DURHAM, N.C. -- By asking subjects to direct their attention
to particular areas in space while their brains were being
scanned by MRI, researchers have mapped brain regions active in
the high-level neural control of attention. Like an initial
satellite reconnaissance of new terrain, this first mapping
represents a key step toward understanding the detailed
topography and function of brain regions involved in high-level
"executive" control of attention.

The researchers reported their findings in an article in the
March Nature
Neuroscience. They are postdoctoral fellow Joseph Hopfinger
and Associate Professor Michael Buonocore of the University of
California at Davis, and George R. Mangun, professor of
cognitive neuroscience and psychology at Duke University. Their
research was sponsored by the National Institute of Mental
Health, the Human Frontier Science Program and the National
Science Foundation.

According to Mangun, basic understanding of attentional
control could provide insights into the pathology of such
problems as Attentional Deficit Hyperactive Disorder (ADHD),
schizophrenia and disorders of attention following brain damage
from stroke. Such understanding could also allow measurements
of the therapeutic activity of drug treatments in improving
attentional functioning.

"Before we can understand how such patients are different in
their attentional control, we have to know how the process
functions normally," said Mangun, who is director of the Duke
Center for Cognitive Neuroscience. "With this finding, we are
laying some important basic groundwork in mapping the areas
involved in attentional control and ultimately understanding
their computational structure."

Mangun added that the study of such "executive control" in
the brain constitutes an important new direction for cognitive
neuroscience, which has often focused mainly on how the brain
processes sensory input during attention.

Mangun and his colleagues used an analytical technique known
as "event-related functional MRI" to distinguish brain regions
active during attentional control. MRI uses harmless magnetic
fields to map the brain, detecting regions of increased blood
flow that reflect increased activity of brain cells called
neurons.

In their experiments, the researchers placed subjects in an
MRI machine and had them watch for an arrow to pop up on a
video screen. Depending on whether the arrow pointed left or
right, after a pause they were to direct their attention,
without moving their eyes, to a checkerboard to the left or
right of the arrow. Occupying their attention was a task of
determining whether the black-and-white checkerboard included
any gray squares.

By integrating the results of large numbers of such trials,
the scientists determined that certain discrete brain areas of
the cortex invariably showed activity during the attentional
tasks. Principal among these areas are the superior frontal,
inferior parietal and superior temporal cortex. The cortex is
the thin layer of brain tissue overlying the brain that is
responsible for integrating sensory and motor information and
for higher brain function.

While previous studies of the brain had implicated regions
of the cortex in attentional control, Mangun said, the studies
had not distinguished between the act of volitional orienting
of attention and the subsequent selective processing of sensory
inputs that are attended or ignored.

"We wanted to distinguish between the neural networks that
activate when you initially tell someone to pay attention to
something, from those involved in processing what happens as a
result," Mangun said. "Thus, in our study, we were able to
distinguish the brain regions involved with the initial command
to pay attention from the orienting of attention to a spatial
location. "It's similar to the distinction in the brain's motor
system between what happens when a person decides to reach out
for an object and the subsequent neural signals to activate
muscle contraction to actually reach out."

According to Mangun, the experiment had to be designed to
separate the two tasks significantly in time, because of a
slight lag between increased neuronal activity and the change
in blood flow that would show up on MRI scans.

"When you measure blood flow changes, as we do in
neuroimaging, the blood flow response to the cue may occur over
seconds. And so, if presentation of the cue and the target are
separated by only milliseconds, it is very difficult to
distinguish the responses."

Such a lag meant that even with a 10-second separation of
the elements of the experiment, careful analysis of the rise
and fall of the "hemodynamic response" was still necessary to
unequivocally reveal the brain regions strictly involved in
attentional control.

Mangun emphasized that the new findings represent only the
beginning of efforts to define the brain regions involved in
attentional control. Further experiments will use more powerful
functional MRI techniques to map the active regions at higher
resolution, like distinguishing finer and finer objects in
satellite images.

The researchers also plan to combine MRI mapping with a
complementary technique of electrical recording of brain waves
during attentional tasks, a method first reported by Mangun and
his colleagues in 1994. While such electrical recording cannot
distinguish active regions of the brain as well as MRI, it can
offer far more precise measurement of the timing of brain
region activity.

"Now, we can distinguish the brain regions that are active,
but we need to understand in detail which ones are active
first, second and third," he said. "Our objective is to
distinguish the different mental operations involved,
ultimately to understand the detailed computational process of
attention."

According to Mangun, new experiments also are underway that
vary the nature of the attentional task; for example, paying
attention to color rather than a spatial location. Such
experiments should yield further insight into the basic brain
mechanisms of attentional control.

The Mangun paper was one of two complementary papers on
attentional control published in the issue of Nature
Neuroscience. The other paper was by Maurizio Corbetta and
colleagues at Washington University School of Medicine in St
Louis. In that report, the authors tested the idea that the
junction between the temporal and parietal areas played a role
in reorienting attention toward stimuli at unexpected
locations; and that another region, called the intraparietal
sulcus, is involved in voluntary orientation and maintenance of
attention at cued locations.

While both papers investigated the two major components of
attention - the top-down attentional control processes, and the
resulting modulations of perceptual processing - the Mangun
paper isolated and demonstrated the two components.

In a News and Views article on the paper, co-authors Roger
Tootell and Nouchine Hadjhikhani of Massachusetts General
Hospital wrote "... these two papers demonstrate the power of
new imaging techniques to resolve complex cognitive operations
into their component steps, and to reveal the structures
involved in each step.