Right side of brain learns language skills after stroke
When a stroke affects the language areas in the left side of the brain, the right side takes over and learns how to perform language tasks, according to research at Washington University School of Medicine in St. Louis. The study found that patients right side of the brain is more active than normal during a verbal language task, and that the right sides activity decreases with practice, similar to what happens on the left side of the brain in healthy individuals.
“This is the first demonstration that learning and, by extension, speech therapy change the way compensatory pathways in the brain work,” says Maurizio Corbetta, M.D., head of stroke and brain injury rehabilitation. “This study supports the hypothesis that brain pathways in the right hemisphere are directly involved in the recovery of language after stroke.”
The study appears in the Sept. 26 issue of the journal Neuron. Corbetta, the studys senior author, also is associate professor of neurology, of radiology and of anatomy and neurobiology. The first author is Valeria Blasi, M.D., a former post-doctoral fellow in neurology at the School of Medicine, who now is at the San Raffaele Scientific Institute in Milan, Italy.
Each year, about 750,000 Americans suffer a loss of blood flow to the brain, a condition known as an ischemic stroke. Since the left side of the brain houses most of the areas responsible for speech and language, a left-sided stroke often causes language problems, a condition known as aphasia. About 1 million people in the United States have aphasia, resulting in an estimated $1.5 billion of lost productivity and other costs each year.
Remarkably, many of those who initially lose language abilities after a stroke significantly recover these abilities within six to 12 months. Several studies suggest that language recovery occurs because the right hemisphere of the brain compensates for the loss of the left hemisphere. For example, language abilities recovered after a stroke to the left hemisphere are lost again if the individual later has a stroke to the right hemisphere.
To test whether there is a direct link between recovered language abilities and activity in the right hemisphere, Corbettas team compared performance scores and brain images from 14 healthy individuals with those of eight stroke patients. The patients all had experienced a stroke at least six months before participating in the study and still had damage near a brain region called Brocas area, located towards the front of the brain and thought to be involved in speech.
The team first measured the two groups performance while they learned a word-stem completion task. Participants saw three letters at a time and were asked to say a word that began with those three letters. For example, if they saw the letters “COU” they might say “cougar.” Each word stem appeared several times during the experiment.
The patient group was slower and less accurate, but both groups learned and improved at the same rate. For example, each group was about 400 milliseconds faster on the seventh block of trials than on the first. Participants in each group also repeated the same answer for a given word stem about 20 percent of the time, a sign that they remembered the word and learned the answer.
The participants then performed the same task while the researchers took brain images using functional magnetic resonance imaging (fMRI). These images identify which brain regions are active during a given task.
The brain normally becomes less active as subjects practice. Because the task becomes familiar and easier, the brain does not have to work as hard. Brain activity decreases in areas that are important for performing the task. In this study, as healthy participants performance improved, language areas on the left side of the brain, including Brocas area, became less active with practice. In addition, areas towards the back of the brain that are involved in vision became less active in both the left and right hemispheres.
Brain images from stroke patients revealed several differences. Language areas damaged by the stroke were not active during the language task. However, areas on the right side of the brain opposite the damaged areas on the left did become active during the task, and that activity decreased with practice. Visual areas on the right side also decreased with practice.
The team also noticed that patients with smaller lesions had slightly different patterns of brain activity. In addition to learning-related changes on the right side of the brain, the areas near the lesion on the left side also improved with practice. These patients also were better at learning the task and ultimately recovered more than the other stroke patients.
According to Corbetta, these results indicate that a stroke produces complex changes in the way both sides of the brain perform during language tasks. “The good news is that these new patterns of brain activity can change through practice, and that this correlates with improved performance,” he says. “This information has direct implications for optimizing rehabilitation and pharmacological treatment after stroke.”
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