TMS is a non-invasive technique that consists of a magnetic field emanating from a wire coil held outside the head. The magnetic field induces an electrical current in nearby regions of the brain. While TMS is often used to diagnose brain abnormalities, this report shows that it also can influence brain plasticity, or reorganization. This plasticity is thought to be responsible for much of the recovery seen in people who have suffered brain damage due to trauma, stroke, or other problems. The study, conducted by researchers at the National Institute of Neurological Disorders and Stroke (NINDS), is published in the February 1 issue of The Journal of Neuroscience.
Many studies in the last two decades have shown that the brain continually responds to changes in stimuli by reorganizing itself. These changes are often beneficial. For example, people who have been blind from an early age often use part of the brain region normally employed for vision to process sensations from their fingertips, which helps them read Braille. However, in other cases, brain reorganization may lead to problems such as phantom pain, which often develops after amputation. This study suggests that researchers might be able to use TMS or other strategies to enhance plasticity when it is beneficial (as in the blind) and to decrease it when it is harmful (as with phantom pain).
In the new study, researchers Leonardo G. Cohen, M.D., Ulf Ziemann, M.D., and Brian Corwell used a simple tourniquet around the elbow to shut off the blood supply to the forearm, blocking nerve signals to the brain and temporarily mimicking what occurs after amputation. The loss of normal signals caused the brain to reorganize. When the researchers applied TMS to the plastic cortex, or the part of the brain that was undergoing reorganization because of the tourniquet, the brain's responsiveness to stimuli increased, meaning that its plasticity was enhanced. However, when researchers applied TMS to the non-plastic cortex on the other side of the brain, the brain's responsiveness in the plastic cortex decreased. This shows that stimulation of the non-plastic cortex somehow inhibits plasticity on the opposite side of the brain.
"This paper addressed the possibility of modulating reorganization in a non-invasive way," says Dr. Cohen. "The technique used is not as important as the overall concept that we can increase and decrease plasticity." The ability to control brain reorganization after brain damage or injury could allow doctors to accelerate recovery and bring about more successful rehabilitation. Similar strategies could be developed to promote learning, a kind of brain reorganization that occurs every day.
Since the brain reorganization in this experiment occurred rapidly, it probably resulted from short-term strengthening of specific connections between neurons, rather than from sprouting of new connections, Dr. Cohen says. "New techniques or a reformulation of existing techniques might be able to induce more lasting effects," he adds. For example, long-term TMS or drug therapy, combined with practice or physical therapy, may be able to change brain circuitry as well as strengthen connections.
While these results are promising, much more research is needed before these strategies will be ready for clinical use. In most cases, researchers still need to learn whether the changes in brain function that they see after injury play a beneficial role by helping the brain to compensate, or if they are harmful or simply irrelevant, says Dr. Cohen. They also need to understand more about how plasticity occurs so they can design and test new strategies for controlling it.
The NINDS, one of the National Institutes of Health located in Bethesda, Maryland, is the nation's leading supporter of research on the brain and nervous system and a lead agency for the Congressionally designated Decade of the Brain.
This release will be posted on EurekAlert! at http://www.eurekalert.org and on the NINDS home page at http://www.ninds.nih.gov/whtnwhp.htm
1 Ziemann, U.; Corwell, B.; and Cohen, L.G. "Modulation of Plasticity in Human Motor Cortex after Forearm Ischemic Nerve Block." The Journal of Neuroscience, 1998 18(3): 1115-1123.