Anatomists divide the cerebellum into three functional regions. They are: the vestibulo-cerebellum (thought to control balance and eye movement); the spino-cerebellum (thought to adjust ongoing movements); and the cerebro-cerebellum (thought to coordinate planning of movements of arms and legs and participate in motor learning.)

For hard-core brain junkies, here's more:

• The vestibulocerebellum is the oldest part of the cerebellum, appearing first in invertebrate evolution. It projects without relays -- a straight shot -- into the so-called vestibular "labyrinth." This part of the cerebellum in humans governs eye movements and body equilibrium when one stands and walks. It receives information from the semicircular canals to signal change in head position and from the otolith organs to signal orientation of head with respect to gravity. It also receives information from various eye-processing areas of brain and sends visual information back to the vestibular areas. Thus it's important for hand-eye coordination, maintenance of equilibrium and balance control. (Important for video-game players as well as tennis players. Should be "confused by life in zero-g?)
• The spinocerebellum receives sensory information from the body via numerous pathways, particularly the spinal cord. It plays a major role in the ongoing execution of body movement, adjusting movement as it's happening. This area gets data from the auditory, visual and vestibular systems as well. It is organized somatotopically. That is, it contains a number of body maps, multiple representations of the same body parts. Many brain areas have body maps but they are generally "smooth," that is, with arm next to hand, tongue next to lip. But in the cerebellar maps these body parts are often mixed around, not adjacent to each other, but arrayed in a kind of mosaic, say, of foot and mouth, arm and eye. This unusual arrangement is called "fractured somatotopy."
• It is thought that, as the spinocerebellum receives information about what's going on in one specific body part, it may simultaneously receive information from distant parts, all the better to coordinate the total movement.
• The spinocerebellum sends feedback signals to control muscle tone and the execution of movement. This works to compensate, for example, in the change or shifting weight in a load you are carrying. (rearranging that shopping bag that's slipping off your shoulder.) It controls the small oscillations in starting and stopping a movement. (You begin, say, to swing the baseball bat at the precise millisecond you think you see a strike. Then "instantly" you stop the swing when you perceive that, in fact, the ball is high and outside, you stop your motion without flying off home plate or corkscrewing yourself into the ground.)

Two-way communication pathways between the motor areas of the cerebral cortex and the cerebellum and brain stem.

• The Cerebrocerebellum is the center of a complex feedback circuit that fine-tunes motor commands from the higher brain. It receives information from the cerebral cortex, modulates it and sends it back up from whence it came in the cortex. The cerebrocerebellum is thought to play a specific role in the planning and initiation of movement. (Will I swing at a fastball or bunt?) It, too, is involved in the precision control of rapid movements and skills requiring fine dexterity. It is especially important for multi-joint movements. Part of this region is also involved with timing of performance. People with brain damage in these areas may not be able to tap out a simple rhythm, or judge elapsed time or the speed of a moving object.
• Is the cerebellum involved in Learning, Motor or Otherwise? Is it necessary to have a cerebellum to learn to ride a bike or do jazz dancing? Does the cerebellum participates in learning and memory beyond motor skills? Is it part of the thinking machine? Is it part of the emotional machine? Is the cerebellum a director of the on-going "movie" that is a person's sense of reality? Again, the Bower lab thinks the cerebellum might play a role in all of the above. But the controversy rages.

-- Linda Marsa