About 750,000 Americans suffer a stroke each year. In many cases, their ability to understand and use language is severely impaired.

As this ScienCentral News video reports, now scientists can see why this ability is not lost for good.

The plastic brain

When a stroke hits the left side of a person’s brain, it can cause large-scale damage to his or her language center. The result, a condition called aphasia, can be a devastating mix of suddenly not being able to speak, or understand what others are saying. According to Dr. Maurizio Corbetta of the Attention and Brain Recovery Lab at Washington University in St. Louis, the initial effects of aphasia can be so disturbing to its victims that they often are mistakenly put in the psych ward.

Once a lesion has occurred in the left-brain, it cannot be repaired. So how is it that aphasics are able to regain the ability to use and understand language? Scientists have long assumed that the right side of the brain took over the task, but until recently there was no concrete evidence.

“Many people have discovered before that there was activity in the right side of the brain when people with aphasia tried to speak after six months,” says Corbetta. “What was not clear was whether this activity on the right side is actually functional or dysfunctional, whether it was actually supporting language recovery or not.”

In other words, scientists knew that the right side of the brain was firing, but there was the possibility that it had nothing to do with language recovery. The activity could easily have represented effort, difficulty, increased alertness or frustration. “Others had suggested that right hemisphere activity was maladaptive—that is, correlates with poor speech or recovery—and might represent 'blocking' of speech or dis-inhibition from the left hemisphere,” says Corbetta.

Using the brain scan technique called magnetic resonance imaging (MRI), Corbetta set out to test whether the right brain activity was functional. While in the MRI, aphasics and control patients performed a word-learning task where they were presented with word stems that they were asked to turn into complete words (for example, “SMO” might become “smoke” and “COU” might become “couple”). The first question was whether stroke patients could learn the task as well as control patients—for instance, when a word stem came around for a second or third time, were they able to remember it and turn it into a complete word faster? The answer was yes.

“And the second question then was, what was happening in their brain, given that they did not have an area that was necessary for learning?” says Corbetta. “And what we saw was that all the changes associated with learning [in the left brain of control subjects] are going on in the right side of the brain [in aphasia patients] in an area that is analogous to the one that has been destroyed by the stroke.”

These changes included a decrease in brain activity in the specified area as the task went on. In other words, the task became easier as the subject learned how to perform it over time, so the decrease in brain activity showed that it was learning and performing the task. “So this very good evidence that this area [in the right brain of stroke patients] is actually compensating for function and is also learning and changing with learning,” he says.

Corbetta thinks this could have implications on rehabilitation for stroke victims.

“Up to now, I think rehabilitation [like speech therapy] is not really based on scientific principle, but has been based more on good common sense,” he says. “And this kind of data, this kind of experiment, demonstrates directly how rehabilitation or learning can actually influence the brain or change the brain.”

Corbetta’s work was funded by the National Institute of Neurological Disorders and Stroke, the J.S. McDonnell Foundation and the Mallinkrodt Institute of Radiology. He now hopes to use MRI to directly test the efficacy of speech therapy over time, and see how it might be changing the brain.