Using the technique, researchers at Boston's Beth Israel Deaconess Medical Center are the first to mimic the damage caused by stroke by creating "virtual lesions" in normal brains, according to an article in this month's Nature Neuroscience.

The scientists used a technique known as transcranial magnetic stimulation, or TMS, which uses electromagnetic energy to temporarily deactivate parts of the brain.

The technique, which allows researchers to disable and study specific brain areas without causing permanent harm, may someday lead to new treatments for a wide spectrum of brain dysfunctions.

"This is an extremely valuable research technique with the potential to aid in the determination of what different parts of the brain contribute to brain function," said neuroscientist Dr. Bruce Cohen, director of McLean Hospital's Brain Imaging Center.

The Five-Minute Stroke

TMS works by sending a slight current into the cerebral cortex — the outer layer of the brain — that interferes with the brain's normal electric processes. The brain is typically exposed to the current for only one or two seconds.

The Beth Israel researchers used a new variation of the TMS technique, sending a low frequency current into the brains of their subjects for 10 minutes.

"When the brain is stimulated over and over, it lowers its overall electrical activity. In other words, it goes numb," said researcher and lead author of the Nature Neuroscience article Dr. Claus Hilgetag.

"After 10 minutes of stimulation, the brain remains inactive for a period of 5-15 minutes even though the TMS is turned off. During this period, we can test the subjects and then compare the results to their normal results, which wouldn't be possible with actual stroke victims."

But this "virtual lesion" approach has caused some neuroscientists to worry about potential side effects.

"I would worry" about the long term effects on subjects exposed to this type of experiment, such as the development of a seizure focus or a more subtle cortical injury," said Lee H. Schwamm, associate director of Acute Stroke Service at Partners Health Care.

Other neuroscientists pointed to the lack of safety studies for the "virtual lesion" procedure. Earlier trials using high-frequency TMS have caused grand mal seizures in normal patients.

But Hilgetag emphasized that the experiment was conducted under National Institute of Health guidelines and with very low-frequency TMS. He said that his seven test subjects were researchers at Harvard Medical School and were all well informed about the procedure.

A Strange Effect

Hilgetag and his fellow researchers have already begun to make discoveries about the brain with the TMS technique. They used the technique to mimic hemispatial neglect, a syndrome that results when one of the brain's parietal lobes is damaged, usually due to stroke.

Using visual tests, the researchers found that subjects reacted slower than normal to images shown on the side of the body affected by the brain damage. Strangely, they observed that subjects reacted with above-normal speed when the images were shown on the healthy side.

The finding has major implications for the scientific understanding of how the brain works. It suggests that the brain's sensory structures compete with and inhibit each other to process information from both sides of the body. When one region of the brain becomes disabled, the regions it once inhibited can actually perform better than normal.

"Hopefully, our study will allow scientists to better understand the interaction between different regions of the brain," said Hilgetag. "I'm being optimistic, but one day the TMS technique may be used to correct the competitive balance between damaged and undamaged regions in stroke patients."

Even if TMS is never used to treat stroke patients, it may allow researchers to study a wide variety of brain dysfunctions. According to Dr. Larry B. Goldstein, director of the Duke Center for Cerebrovascular Disease, "as we better understand the principles underlying how the brain works, we may be in a better position to design therapeutic interventions in the future."