Most people give little thought to the cerebellum, an old, bulging structure at the back of the brain. But for many of the 150,000 Americans with ataxia, a disease that harms movement, this mysterious region of the brain is a big problem. Damage to the cerebellum makes walking difficult, speech slurred, most movements unsteady.
Genetic ataxia, some strokes and tumors, and alcohol all affect the cerebellum, causing patients to stagger like drunks. Many patients carry a doctor's letter so they won't be arrested for public drunkenness.
While ataxia can have many causes, most cases probably stem from damage to the cerebellum, says Donna Gruetzmacher, executive director of the National Ataxia Foundation.
But what exactly does the cerebellum do? "Coordinating movement" is the accepted, if vague, description of this substantial hunk of brain real estate. In fact, the cerebellum seems to be involved in timing of movements, says Rebecca Spencer, a post-doctoral researcher in the department of psychology at the University of California at Berkeley.
Now, she and her colleagues have found that the cerebellum is required for timing stop-and-start movements, but not for continuous motion.
The difference is subtle: Tap your foot to some music, and that's a continuous movement. But if you pause between taps, that's discontinuous movement. In real life, Spencer says most movements are discontinuous, whether it's buttering toast, typing an impenetrable article about brain research, or planing the edge of a door. And while continuous and discontinuous tapping look pretty much the same, they apparently are triggered by different parts of the brain.
Tap, tap, tap
But the correlation crumbled when applied to continuous and discontinuous movements, she said. One test involved drawing circles on paper, and Spencer and Purdue University researcher Howard Zelaznik found that college students who could draw non-stop circles with good timing were not necessarily good at the discontinuous finger-tap. "We thought continuous circle drawing would correlate with discontinuous tapping -- that if you were good at timing the finger-tapping you would be good at timing the circle-drawing," says Spencer, "but we found that there was no correlation."
Another sign that the cerebellum is involved in timing comes from patients with cerebellum disease, who typically have trouble comparing the duration of two time intervals. Yet while the cerebellum was obviously important for timing, Spencer says, nobody understood exactly what it was doing.
Why could this be? Perhaps, she says, in continuous tasks, "We get our arms cycling, and once you get it going, it can go on its own. I can do the task, and read or talk, and my arm won't stop. It's kind of swinging like a pendulum, going on its own momentum."
Discontinuous movements -- think tap, pause, tap -- are different, she says. Once each cycle, you apparently need to refer to an internal clock to restart the process. And if that clock -- apparently located in the cerebellum -- is defective, the result will be impaired movement. Poor coordination. That drunken appearance.
"That's why the cerebellum patients are impaired," Spencer says. "When they have to turn the movement on and off, there are lots of points where the 'clock' can be interrupted."
It's probably a good thing that cerebellum disease does not affect continuous movement, Spencer notes that such movements are rare outside the lab -- or the dance floor.
As we said, most people take advantage of the cerebellum's clock without even recognizing its importance. But to the 150,000 people with genetic atrophy of the cerebellum -- along with an unknown (but probably smaller) number of people with damage due to stroke or cancer -- this business of timing is anything but academic.
-- David Tenenbaum
Disrupted Timing of Discontinuous But Not Continuous Movements by Cerebellar Lesions, R. M. C. Spencer et al, Science, 30 May 2003.