That's the conclusion of researchers who found that people who have had strokes were better able to recall words after they silenced parts of the brain that therapies stimulate.
Stroke is caused when a clot forms or a blood vessel ruptures in the brain, cutting off the blood supply and killing neurons within minutes. Although the dead cells are not replaced, people can still recover lost functions such as the ability to recall words. The prevailing explanation is that healthy regions on one side of the brain learn to take over the jobs that were done by the corresponding cells that died in the other hemisphere. This idea is supported by studies using functional magnetic resonance imaging that have revealed increased electrical activity in regions in the intact hemisphere compared with the same area in healthy brains.
Many therapies are designed to boost this reorganisation by stimulating the intact hemisphere. For instance, speech-impaired patients are often encouraged to sing a song containing the word they are seeking. The hope is that embedding it in a heavily prosodic, or tonal, context will engage the right hemisphere-which processes prosody -and bypass the damaged left hemisphere that normally processes speech. In this way, stroke patients learn to recall a stored word by a different route.
Yet despite therapy, many patients fail to recover some lost functions. This suggests that overstimulating the healthy regions of the brain may be stifling recovery rather than aiding it, says Alvaro Pascual-Leone, director of the Laboratory for Magnetic Brain Stimulation at Harvard Medical School in Boston.
His team tested the theory by using magnetic fields to suppress those areas instead. Using a technique called transcranial magnetic stimulation, the researchers apply a magnetic field that induces currents at selected points in the brain. Depending on the frequency of the field, the current can either silence or amplify neural activity.
Pascual-Leone's team gave the treatment to five patients whose left hemispheres had been damaged by stroke. They applied a field with a slow frequency of 1 hertz, which animal studies show has a dampening effect, to the corresponding right hemisphere area for 10 minutes. The aim was to dampen rather than stimulate neural activity. As a result, the patients' ability to put names to pictures shot up by 35 per cent.
"They are not only naming more, they are naming significantly faster," says Pascual-Leone, who presented the unpublished results at a meeting of the Organization for Human Brain Mapping in Sendai, Japan, this month.
Why this happens is unclear. Cognitive neurologist Marcel Mesulam of Northwestern University in Chicago says that transcranial magnetic stimulation is an imprecise technique, and it is not always clear whether the stimulation excites or inhibits neural activity. He says that details will only become clearer with more experiments.
But Randy Buckner, who studies brain repair at Washington University in St Louis, Missouri, says: "These results, while preliminary, are extremely intriguing and suggest a radically different view of recovery from stroke."
Author: Laura Spinney
New Scientist issue: 22nd June 2002
PLEASE MENTION NEW SCIENTIST AS THE SOURCE OF THIS STORY AND, IF PUBLISHING ONLINE, PLEASE CARRY A HYPERLINK TO: http://www.
"These articles are posted on this site to give advance access to other authorised media who may wish to quote extracts as part of fair dealing with this copyrighted material. Full attribution is required, and if publishing online a link to www.newscientist.com is also required. Advance permission is required before any and every reproduction of each article in full - please contact email@example.com. Please note that all material is copyright of Reed Business Information Limited and we reserve the right to take such action as we consider appropriate to protect such copyright."