HOW THE BRAIN FINDS WHAT IT'S LOOKING FOR
Despite the barrage of
visual information the brain receives, it retains a remarkable ability to focus
on important and relevant items. This fall, for example, NFL quarterbacks will
be rewarded handsomely for how well they can focus their attention on color and
motion -- being able to quickly judge the jersey colors of teammates and
opponents and where they're headed is a valuable skill. How the brain
accomplishes this feat, however, has been poorly understood
Now, University of
Chicago scientists have identified a brain region that appears central to
perceiving the combination of color and motion. They discovered a unique
population of neurons that shift in sensitivity toward different colors and
directions depending on what is being attended -- the red jersey of a receiver
headed toward an end zone, for example. The study, published Sept. 4 in the
journal Neuron, sheds light on a fundamental neurological process
that is a key step in the biology of attention.
"Most of the
objects in any given visual scene are not that important, so how does the brain
select or attend to important ones?" said study senior author David
Freedman, PhD, associate professor of neurobiology at the University of
Chicago. "We've zeroed in on an area of the brain that appears central to
this process. It does this in a very flexible way, changing moment by moment
depending on what is being looked for."
The visual cortex of
the brain possesses multiple, interconnected regions that are responsible for
processing different aspects of the raw visual signal gathered by the eyes.
Basic information on motion and color are known to route through two such
regions, but how the brain combines these streams into something usable for
decision-making or other higher-order processes remained unclear.
To investigate this
process, Freedman and postdoctoral fellow Guilhem Ibos, PhD, studied the
response of individual neurons during a simple task. Monkeys were shown a rapid
series of visual images. An initial image showed either a group of red dots
moving upwards or yellow dots moving downwards, which served as an instruction
for which specific colors and directions were relevant during that trial. The
subjects were rewarded when they released a lever when this image later
reappeared. Subsequent images were composed of different colors of dots moving
in different directions, among which was the initial image.
Dynamic neurons
Freedman and Ibos
looked at neurons in the lateral intraparietal area (LIP), a region highly
interconnected with brain areas involved in vision, motor control and cognitive
functions. As subjects performed the task and looked for a specific combination
of color and motion, LIP neurons became highly active. They did not respond,
however, when the subjects passively viewed the same images without an
accompanying task.
When the team further
investigated the responses of LIP neurons, they discovered that the neurons
possessed a unique characteristic. Individual neurons shifted their sensitivity
to color and direction toward the relevant color and motion features for that
trial. When the subject looked for red dots moving upwards, for example, a
neuron would respond strongly to directions close to upward motion and to
colors close to red. If the task was switched to another color and direction
seconds later, that same neuron would be more responsive to the new
combination.
"Shifts in
feature tuning had been postulated a long time ago by theoretical
studies," Ibos said. "This is the first time that neurons in the
brain have been shown to shift their selectivity depending on which features
are relevant to solve a task."
Freedman and Ibos
developed a model for how the LIP brings together both basic color and motion
information. Attention likely affects that process through signals from
higher-order areas of the brain that affect LIP neuron selectivity. The team
believes that this region plays an important role in making sense of basic
sensory information, and they are trying to better understand the brain-wide
neuronal circuitry involved in this process.
"Our study
suggests that this area of the brain brings together information from multiple
areas throughout the brain," Freedman said. "It integrates inputs --
visual, motor, cognitive inputs related to memory and decision making -- and
represents them in a way that helps solve the task at hand."
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