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Brain Cells' Multitask
A region of the brain known to play a key role in visual and spatial processing also sorts visual information into categories, according to a new study by researchers at the University of Chicago
Primates are known to have a remarkable ability to place visual stimuli into familiar and meaningful categories, such as fruit or vegetables. They can also direct their spatial attention to different locations in a scene and make spatially-targeted movements, such as reaching.
The study, published in the March issue of Neuron, shows that these very different types of information can be simultaneously encoded within the posterior parietal cortex. The research brings scientists a step closer to understanding how the brain interprets visual stimuli and solves complex tasks.
"We found that multiple functions can be mapped onto a particular region of the brain and even onto individual brain cells in that region," said study author David Freedman, PhD, assistant professor of neurobiology at the University of Chicago. "These functions overlap. This particular brain area, even its individual neurons, can independently encode both spatial and cognitive signals."
Freedman studies the effects of learning on the brain and how information is stored in short-term memory, with a focus on the areas that process visual stimuli. To examine this phenomenon, he has taught monkeys to play a simple video game in which they learn to assign moving visual patterns into categories.
"The task is a bit like a baseball umpire calling balls and strikes," he said, "since the monkeys have to sort the various motion patterns into two groups, or categories."
The monkeys master the tasks over a few weeks of training. Once they do, the researchers record electrical signals from parietal lobe neurons while the subjects perform the categorization task. By measuring electrical activity patterns of these neurons, the researchers can decode the information conveyed by the neurons' activity.
"The activity patterns in these parietal neurons carry strong information about the category that each motion pattern gets assigned to during the task," Freedman said.
Over the years, his team's work on categorization has zeroed
in on the lateral intraparietal (LIP) area. Studies have shown
that this area is vital to directing spatial attention and eye
movements. But it was unclear how it could also play a role
in non-spatial functions such as visual categorization.
To compare spatial and category functions, Freedman and his
team added a twist to the monkeys' task. During the category
task, researchers required the monkeys make eye-movements
to visual cues at various positions on the computer screen,
while categorizing the visual patterns at the same time.
The parietal brain cells reflected simultaneous, independent
encoding of both eye-movement along with an ability to
categorize information—multiplexing of information
at the level of single brain cells.
"These signals rode right on top of the eye-movement signals," said the study's first author, Chris Rishel, PhD, a recent graduate from Freedman's laboratory. "We could decode both the eye-movement and the category signals with high accuracy. This tells us that different kinds of information that are usually considered quite unrelated were simultaneously and independently represented by neurons in this particular brain area."
Their results, the study authors note, "support the possibility that LIP plays a key role in transforming visual signals in earlier sensory areas into abstract category signals during category-based decision-making tasks."
What does the brain gain from this territorial arrangement?
"There has long been a tendency to look at the many distinct anatomical areas of the cerebral cortex of the brain and to assume that each area is like a specialized module that plays a very specific function." Freedman said. "Our results support the growing sense that most, if not all, of these brain areas have multiple overlapping roles."
A brain that includes such overlapping functional centers may be more efficient, Freedman suggests. "It makes mapping these regions more complicated for scientists like us, but it may boost the brain's capacity. If each area can do a number of different things, you can squeeze a lot more function into the same space."
A next step is to understand how neuronal category representations develop in LIP neurons during the learning process, the authors said.
The paper, "Independent category and spatial encoding in parietal cortex," will be published online March 6 by the journal Neuron. The National Institutes of Health funded this study with additional support from the National Science Foundation, the McKnight Endowment Fund for Neuroscience, the Alfred P. Sloan Foundation and the Brain Research Foundation. Gang Huang, formerly a research technician in the lab, also contributed to the research.
Original article: http://www.eurekalert.org/pub_releases/2013-03/uocm-ort030513.php
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