Columbia-Presbyterian researchers report that rather than being completely 'hard wired,' structures on one side of the brain can take over certain language functions normally achieved by a damaged region on the opposite side. The study, published in the September 2000 issue of the journal Neuropsychologia, may provide insights into the brain's compensatory responses to injury.
The brain is not a homogeneous structure--various brain functions are each performed by distinct areas, so damage to a brain region can produce specific and lasting difficulties in perception, thinking or movement. Columbia-Presbyterian researchers report, however, that in the brains of three patients with chronic arteriovenous malformations (AVMs) in the left frontal lobe, brain functions involved in speech production were successfully 'relocated.'
AVMs are chronic conditions thought to develop in utero and may begin to cause brain problems, as evidenced by learning disabilities in childhood. As adults, many AVM patients are asymptomatic and show no signs of language and speech deficits, suggesting that their brains may have successfully adapted to any damage caused by the AVM. "An AVM is a tangle of blood vessels in the brain. There's an abnormal connection between artery and vein; rather than passing smoothly through fine capillaries, blood rushes through the AVM, often at the expense of blood flow to the surrounding tissue," said Ronald M. Lazar, associate professor of neuropsychology at Columbia's College of Physicians & Surgeons and director of Columbia's Cerebral Localization Laboratory.
Neuroradiologists and neurosurgeons often map language functions to precise brain locations before undertaking an invasive procedure. In particular, surgeons planning to remove the blood supply to an AVM need to know whether permanently blocking a particular vessel will affect language comprehension or speech. By anesthesizing a small brain region while asking the patients questions (Wada testing), doctors can identify regions important for speech.
The position of a functional brain region and even the side of the brain that dominates during a particular task tend to be similar between individuals. In normal, right-handed individuals, a region of the left temporoparietal cortex, referred to as Wernicke's area, functions in understanding language. Likewise, a region of the left inferior frontal lobe known as Broca's area normally is necessary for producing speech.
In the three patients with AVMs within the left frontal lobe, Lazar and colleagues from the departments of neurology, neurological surgery, radiology and anesthesiology found that anesthesizing the left frontal lobe did not alter speech production as in normal subjects. This suggested that the function had relocated to another part of the brain. Anesthesizing Wernicke's area on the left side had the usual effect of disrupting comprehension.
To test this idea, Lazar and the other researchers used a method called functional magnetic resonance imaging (fMRI) to visualize the brain regions activated as patients came up with lists of words. They found that, in contrast with normal, right-handed subjects, the areas activated were primarily in the right rather than left hemisphere. They also found activation in a small additional area of the left hemisphere not normally activated during language production. "In essence understanding was controlled by the left side of the brain, and language expression was on the right," said Lazar.
Reorganization may follow damage to other regions, as well. "In cases of hemorrhage near large AVMs, a deficit is often not as extreme as would be expected from the size of the hemorrhage," Lazar explained. "We think that displacements [of brain function] are fairly common in people with AVMs." If brain reorganization in AVM patients is as common as the researchers suspect, Lazar suggested, "embolization & stemming the abnormal blood flow with a special glue & to treat AVM may be somewhat less risky than previously thought."
On the basis of their findings, according to Lazar, neuroradiologists and neurosurgeons have considered extending brain function studies, using Wada testing, near the AVM with alternative techniques such as fMRI applied to the side of the brain opposite the AVM. It is not known whether brain reorganization after a stroke follows the same patterns as that seen in AVM. Nevertheless, studies of AVM patients may provide insights into long-term compensatory mechanisms possibly active in other neurological diseases. The research was supported in part by the National Institute of Neurologic Disease and Stroke and by the Doris and Stanley Tananbaum Foundation.
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