A popular explanation for recovery from stroke is that the healthy side of the brain assumes control of the functions lost from the damaged side. Most existing therapies are designed to encourage this transfer of function. Now, however, researchers in the United States are suggesting that the brain changes that drive the rearrangement of function are maladaptive, and that suppressing them is more likely to aid recovery.
Patients whose stroke occurs in the brain's left hemisphere often suffer a linguistic impairment called aphasia that affects their ability to use and understand words.
The left hemisphere, and particularly an area in the left frontal lobe known as Broca's area, plays a key role in the comprehension and production of speech. And brain-scan studies using functional magnetic resonance imaging have shown that the right hemisphere equivalent of Broca's area is more active in aphasic patients than in healthy brains.
That finding led scientists to believe that the right hemisphere is somehow compensating for the loss of speech, and they designed therapies to boost what seemed to be the brain's healing plasticity.
Singing therapy, for instance, works on the premise that embedding lost words in a heavily prosodic or tonal context will engage the right hemisphere -- which processes prosody -- and provide an alternative route to the word store, bypassing the damaged left hemisphere.
Yet despite such therapies, many aphasic patients recover only partially -- if at all.
To investigate whether the increased activity of the healthy hemisphere might actually be hindering recovery rather than enhancing it, Alvaro Pascual-Leone, director of the Laboratory of Magnetic Brain Stimulation at the Beth Israel Deaconess Medical Center of Harvard Medical School, Boston, and colleagues used a technique called transcranial magnetic stimulation to suppress the healthy right hemisphere counterpart of Broca's area in five aphasic patients who had suffered severe left hemisphere strokes.
TMS involves holding a magnetic coil close to a person's head and inducing a magnetic field at right angles to it that penetrates the brain and triggers currents at specific points within it. Depending on the frequency of the stimulation, the current can either dampen or enhance the activity of brain cells, and Pascual-Leone chose a slow frequency, 1 hertz, that had been shown in previous animal studies to suppress brain activity.
The researchers applied the current for 10 minutes and tested the patients before and after using a picture-naming task. They reported up to a 35 percent improvement in the patients' performance immediately following TMS -- up on average from six words a minute to nine or ten. But there was no significant effect when the TMS was applied to any other brain site, either in the damaged or in the healthy hemisphere.
"They are not only naming more, they're also naming significantly faster," said Pascual-Leone, who believes there are good grounds for suspecting that singing therapy, among others, might actually be compounding the patients' deficit rather than reducing it. "The [neural] system was not designed to recover from stroke, it was designed to learn new skills," he said, pointing to the plastic changes in the neural architecture that accompany the development of language in childhood.
However, the mechanisms of brain repair are still unclear, and Pascual-Leone's preliminary findings received a cautious welcome last week at the annual meeting of the Organization for Human Brain Mapping in Sendai.
"If a child has a lesion on one side then the other side is definitely important for recovery, but in the adult it's less clear," commented neurologist Jean-Claude Baron of Britain's Cambridge Centre for Brain Repair. "The word 'maladaptive' [used by Pascual-Leone] is hypothetical, but if it is a maladaptation then what he's doing makes sense."
Marcel Mesulam, a cognitive neurologist at Northwestern University, Chicago, added that TMS is a young technique, and it's not always possible to know whether it is enhancing or suppressing activity. But, he said, "The importance of this is that it brings in a new concept. The details will become clearer in time."