REPROGRAMS ITSELF AFTER STROKE
Functional imaging of the brain demonstrates that this highly complex organ adapts to injury by redistributing its cognitive workload across established neural networks and recruiting local cortical areas to fill in for lost functions like speech and language comprehension.
"Functional MRI [magnetic resonance imaging] indicates that the dogma that some areas of the brain … are not important for normal function is clearly fallacious," said Dr. Keith Thulborn, director of MR research at the University of Illinois at Chicago at the annual meeting of the American Association for the Advancement of Science in San Francisco. "Loss of any brain tissue is likely to compromise the reserve capacity of many large-scale neurocognitive networks that will ultimately be reflected in the performance of more difficult tasks or recovery from subsequent disease processes."
Watching the brain at work with a very-high-field MRI scanner, Thulborn has mapped a two-stage recovery process in patients who lost their language skills after strokes. In one patient suffering from damage to Wernicke's area (the region in the left cortex that controls the understanding of language), functional MRI showed that the brain initially recouped by allocating speech comprehension to an area on the opposite side of the brain. Over time, while Wernicke's area remained damaged, an adjacent area took on this cognitive task.
The ability of the brain to maintain performance by recruiting undamaged portions of the cortex may suggest why functional recovery can occur even after large strokes, said Thulborn.
One key factor in recovery time, Thulborn suggested, is whether white matter has been damaged. White matter consists of myelinated neuronal axons that serve as cables linking the different areas of the cortex. When these are injured, vital connections needed to allocate functions elsewhere are lost.
"The involvement of white matter tracts portends slower and reduced recovery," said Thulborn. "This may reflect reduced capacity to redistribute workload when the connectivity through white matter is disrupted."
While functional MRI has been largely used in research to map brain functions, it is just beginning to find clinical applications. At the UIC Medical Center, Thulborn is collaborating with other physicians, psychologists and therapists to use the technology in designing and monitoring rehabilitation programs aimed at restoring lost cognitive and motor skills. In this role, Thulborn said, functional MRI can guide and refine therapies to enhance the brain's innate plasticity.
The very-high-field MRI scanner works by picking up faint magnetic signals in the underlying tissue. As neurons become increasingly active in specific regions of the brain, blood flow surges to those regions and blood volume expands.
In the process, deoxygenated blood is replaced with oxygenated blood, the two differing in their magnetic properties. The MRI scanner is able to detect this magnetic change, although minute, because of the scanner's high magnetic field strength: 3.0 Tesla, twice that of MRI scanners more commonly deployed in clinical settings. (A Tesla is equivalent to 10,000 gauss; the magnetic field strength of Earth is less than one gauss.) Cross-sectional images are made through the entire brain to create a three-dimensional view. The images must be run through a series of statistical programs so that they can be correctly interpreted.
To obtain images of the working brain, patients are placed on a table and moved into the center of the magnet. The images are taken while patients are engaged in a set of cognitive tasks devised to correlate functional activities with specific areas of the brain. To map the language comprehension network of the brain, for example, patients are given sentences of varying complexity to read and asked to answer true/false questions by pressing a finger switch. To map the motor and sensory areas, patients simply tap a finger.
Thulborn cautioned that these cognitive tasks must be carefully selected if they are to be of value in answering clinical questions. Moreover, they need to be "robust," or reproducible, and appropriate to the patient's level of education and cognitive and motor abilities. "Careful attention to matching the skills of each patient to the stimulus task is required to avoid variable performance that may alter the functional mapping," said Thulborn.
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