June 2, 2003
Study: Stroke victims may retain continuous motion ability
WEST LAFAYETTE, Ind. Stroke victims may retain more motor coordination than previously thought, according to research led by Purdue University. The findings challenge current understanding of brain function and open new possibilities for aiding the physically challenged.
The research team, which included faculty from Purdue and University of California-Berkeley, was led by Purdue professor Howard Zelaznik.
The team found that the cerebellum may not be as fully responsible for the timing of "continuous" motions, such as drawing circles repeatedly on paper, as it is for "discontinuous" motions that have a more start-stop nature, such as tapping your finger rhythmically on a table. Patients with cerebellum damage who participated in the study had difficulty tapping a steady beat, but no such trouble with drawing circles in rhythm. The study indicates that stroke victims may retain some motor skills thought to have been lost to cerebellar damage.
"The basic idea is, if the same part of your brain governs timing of all your muscular activity, you should have equally good rhythm when tapping your finger as when you draw circles," said Zelaznik, professor of kinesiology in Purdue's School of Liberal Arts. "But we found that one skill doesn't predict the other. So if part of your brain gets traumatized through injury, you may not necessarily lose all your motor skills."
The research, which appears in the May 30 edition of the journal Science, was the Purdue dissertation project of Rebecca Spencer, who is now at UC Berkeley. It was co-authored by Zelaznik and her colleagues at UC Berkeley, J. Diedrichsen and R. Ivry.
Under the right and left hemispheres of the cerebrum, which handles higher thinking processes, lies a smaller and more ancient section of the brain the cerebellum. It is here that motor skills which include movements from walking to blinking to catching a ball are governed. Strokes can leave damaging lesions on the cerebellum that make it difficult for victims to control their muscles, especially when it comes to activities that require a sense of timing.
"Trouble with rhythmic movement among stroke victims has been noted widely over the years," Zelaznik said. "This has encouraged brain specialists to conclude that the cerebellum must be the seat of an internal 'clock' that governs all types of bodily coordination. But we noticed in another experiment that patients with cerebellum damage retained some ability to 'keep rhythm,' but only with certain kinds of body movement."
The team set up an experiment in which about a half-dozen patients attempted to perform three different tasks, each of which required rhythmic sense: tapping a beat with a finger; drawing circles at a constant rate, stopping at the same point on the page and continuing; and drawing uninterrupted circles, also at a constant rate but merely passing the same point on the page without stopping. Stroke victims were unable to keep time in the first two tasks, but could draw uninterrupted circles in rhythm without difficulty.
"The key seems to be whether or not your body has to stop and produce a sudden change in direction," Zelaznik said. "A damaged cerebellum appears to have difficulty keeping time only when it has to stop and start, such as when you lift your finger from the table and then push it down again. We refer to these as discontinuous motions."
However, continuous rhythmic motions, such those used in the uninterrupted-circle test, were still possible with cerebellar damage.
"The cerebellum is definitely involved in timing, but only in certain kinds of timing," Zelaznik said. "Somehow, the body knows how to carry out continuous motions even when its motor control center is not performing as it should."
The question, then, is how your muscles "know" how to carry out circular motion if the cerebellum is not telling them. At this point, Zelaznik can only theorize.
"We believe that it's kind of like pushing a kid on a swing," he said. "You have to get the motion started, but once it's going, you don't have to exert a force every time the swing comes back you can leave it alone for awhile and it will continue."
One of the next challenges, Zelaznik said, will be answering that question accurately. He cautions, however, that even his group's current findings need to be confirmed with wider testing.
"A potential problem with our findings is that we have based them on a rather small number of test subjects," Zelaznik said. "This invites the criticism that our study does not conclusively illustrate how the cerebellum is organized. We certainly would like the opportunity to investigate these effects further, and to see other researchers working on them as well."
Zelaznik said the payoff could go beyond attaining knowledge of the brain for its own sake.
"Suppose you are trying to design a device to help a brain-damaged patient interface with the world," he said. "Many such devices make use of 'point-and-click' technology, such as many of us use at our computers. But pointing and clicking are just the sort of interrupted, discontinuous motions that a damaged cerebellum appears to have trouble with."
Zelaznik said his group's research suggests more viable alternatives would be continuous-motion based controllers.
"For individuals with certain kinds of cerebellum damage, we might want to create electric wheelchairs they can steer by varying the speed at which they turn a small crank, for example," he said. "Some of the best societal value of research like this is that it can allow us to consider more innovative and appropriate ways to help people."
Writer: Chad Boutin, (765) 494-2081, firstname.lastname@example.org
Source: Howard Zelaznik, (765) 494-5601, email@example.com
Purdue News Service: (765) 494-2096; firstname.lastname@example.org
Disrupted Timing of Discontinuous but not Continuous Movements by Cerebellar Lesions
Rebecca M. C. Spencer, Howard N. Zelaznik,
Patients with cerebellar damage are known to exhibit deficits in the temporal control of movements. We report that these deficits are restricted to discontinuous movements. Cerebellar patients exhibited no deficit in temporal variability when producing continuous, rhythmic movements. We hypothesize that the temporal properties of continuous movements are emergent and reflect the operation of other control parameters not associated with the cerebellum. In contrast, discontinuous movements require an explicit representation of the temporal goal, a function of the cerebellum. The requirement for explicit temporal representation provides a parsimonious account of cerebellar involvement in a range of tasks.