WINSTON-SALEM, N.C. – Scientists at Wake Forest University Baptist Medical Center have developed an explanation for a bizarre but not uncommon medical condition in which patients fail to see -- neglect -- half of their visual world.
The condition follows traumatic brain injury, stroke or other brain damage. Despite the inability of patients to "see" half of what is before their eyes, many visual areas of the brain are still largely intact, said John G. McHaffie, Ph.D., associate professor of neurobiology and anatomy and senior author. "People have tried for a long time to understand why this happens."
McHaffie and his colleagues now have an explanation. They report in the current (June 26) issue of Nature that signals from a brain area called the basal ganglia may be preventing those intact visual areas from functioning properly.
"The potential site of treatment has now been identified," said Barry E. Stein, Ph.D. professor and chairman of the Department of Neurobiology and Anatomy and a co-investigator. "Once you know where the site is, you can speculate about cures and strategies for therapy."
One likely treatment is to disrupt some actions of the basal ganglia so the remaining intact visual areas can function again. The malady itself is among the most bizarre of medical conditions. "These people have very strange perceptions," said McHaffie. "They can go so far as to say, 'that's not my arm.'"
McHaffie said patients who have the malady may eat only half a plate of food, or when they draw pictures, only depict half the scene, such as half a tree or a fork with only two tines.
Still these patients are not blind in the traditional sense, said Stein. "They deny the existence of that side of space."
Estimates of the incidence vary widely. Neglect of the left side is most common following a stroke or brain injury involving the right hemisphere of the brain and occurs in perhaps 10 percent of those patients. Often the malady resolves itself. But for others, no treatment exists.
McHaffie said their research shows that there are two kinds of cells in the basal ganglia. Some work on the same side of the brain, some on the other side. Some of these cells in the basal ganglia "have properties unlike those that neuroscientists have previously seen," he said.
Microelectrodes enabled the team -- which also included Huai Jiang,, Ph.D., assistant professor -- to observe individual cells of the basal ganglia in action. They determined what kind of neurotransmitters the cells used when they fired, and which other cells got the message.
"We used electrophysiology to listen to these cells and we used neuroanatomy to determine who they talked to and what kinds of chemicals are in these particular cells."
They found the basal ganglia are simultaneously enhancing and suppressing visual activity in the superior colliculi -- which are centers for visual reflexes and eye movements -- on both sides of the brain. "We discovered a push-pull system in the basal ganglia that regulates activity on the two sides of the brain," McHaffie said.
The system ordinarily is carefully balanced by other visual elements. But when damage occurs to the visual cortex through a stroke or brain injury, the balance is disrupted and neglect may result. "The brain can't use the remaining parts of the visual system that are in fact there to use," said McHaffie.
So, said Stein, the solution may be to disrupt the activity of the basal ganglia. "There is a population of cells in the basal ganglia that have been identified by this research and appear to be responsible. If you disrupt their function, that would allow the remaining portion of the visual brain on the damaged side to function."
The balance would be restored, and the visual neglect would disappear.
"The position of the basal ganglia in the brain is very central, and that fits into the idea that it is coordinating a lot of activity for any particular behavior," McHaffie said.
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