Neuroscientists at Washington University School of Medicine in St. Louis have linked a common post-stroke disability to impaired communication between brain regions.
In the March 15 issue of Neuron, researchers report a tight correlation between the degree to which communication was blocked and the severity of patients' symptoms. This led them to suggest that testing for such communication breakdowns could greatly improve clinical assessment and treatment of stroke and other brain injuries.
"For more than a century, we have linked neurological deficits and their recovery to the damage done to neurons directly affected by a stroke or other injury," says senior author Maurizio Corbetta, M.D., the Norman J. Stupp Professor of Neurology. "However, we are learning that a lesion in one part of the brain can impair the function of brain regions not directly harmed by the lesion. We need to promote use of this more dynamic view of brain changes after damage."
Researchers studied both healthy volunteers and a group of 11 patients who had a stroke on the right side of the brain between the ear and temple. Every year, strokes in this area leave three to five million patients with a condition called spatial neglect. Patients have trouble paying attention to one side -- they may seem to be unaware of their left arm, for example, fail to shave the left side of their face or leave boxes blank on the left side of a form. The condition is most severe in the first few months following a stroke, but in some patients it becomes a chronic problem.
For the study, scientists had patients and healthy volunteers indicate whether they could see an asterisk on the left or right side of a video screen. While they did this, their brains were scanned using functional magnetic resonance imaging (MRI). Researchers then applied an approach known as functional connectivity to the results.
"Functional connectivity MRI examines correlations between spontaneous activity in different brain regions, providing a peek into the fundamental architecture of the brain," says lead author Biyu He, a graduate student in neurosciences.
Patients' brains were scanned within a month of their stroke and again more than six months later, when the symptoms of spatial neglect often have started to fade. Scientists paid particularly close attention to the connection between two brain networks that help control attention, or what part of the continual stream of sensory inputs the brain most tightly focuses on.
Researchers found a consistent link between the severity of spatial neglect symptoms and the degree of impairment in communication within and between those two networks.
The results further reinforce a theory about the effects of brain injury that Corbetta and his colleagues have been building for years. According to the theory, brain injury can damage give-and-take processes between brain regions that are essential for the regions' proper function. Regardless of whether damage directly strikes a brain region in the network or disables circuitry connecting the regions, the net effect leaves one or more brain areas stranded like the sole occupant of a seesaw, unable to operate effectively without its missing partner.
Follow-up studies are planned to see if other stroke-induced deficits, such as the loss of language ability known as aphasia, are linked to similar disruptions of communication between brain regions.
"This approach may be helpful for other conditions in which functional communication is disrupted, such as traumatic brain injury, multiple sclerosis and Alzheimer's disease," Corbetta says.
He BJ, Snyder AZ, Vincent JL, Epstein A, Shulman GL, Corbetta M. Breakdown of functional connectivity in frontoparietal networks underlies behavioral deficits in spatial neglect. Neuron, March 15, 2007.
Funding from the National Institute of Mental Health, the National Institute of Neurological Disorders and Stroke and the James S. McDonnell Foundation supported this research.
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