“Grey matter” is synonymous with smarts, but in fact only half of the human brain is grey matter. White matter, the “other brain tissue”, is rarely mentioned. Neurons in the cerebral cortex are packed into in the top layers of the brain, where they are connected together through synapses. Learning takes place in the grey matter by linking neurons together into new circuits by strengthening synapses or forming new ones.
But beneath the topsoil of the brain lies a dense network of fibers packed into a spaghetti-like snarl that is so complicated it is difficult to study or comprehend. These fibers are the wire-like axons projecting out from neurons in grey matter that transmit electrical impulses. Like buried telephone lines, these tightly bundled cables transmit information over long distances to communicate between distant regions of the cerebral cortex that are specialized to carry out different aspects of a complex cognitive function.
To understand the importance of white matter, consider what is happening under the baseball cap of a left fielder leaping over the wall to snatch a baseball in mid air. Visual processing in the back of his brain perceives and tracks the flying object and at the same time it monitors all the other objects on the field as the athlete runs to catch the ball. Then the motor control centers in the parietal region of his brain engage to launch his body on a running trajectory to intercept the projectile. Finally, precisely timed fine motor control extends his arm into space with millimeter precision to clench fingers at the right instant to pluck the speeding ball out of the sky. All the while the player simultaneously perceives the fluid situation on the field as runners advance and strategies unfold so that he can make critical split-second decisions—“Do I hold the ball or hurl it to home plate?” This higher level decision making is calculated in the frontal lobes, just behind the eye brows. All this vital communication sweeps across the entire brain from the back of the skull to the front to activate different regions of cerebral cortex specialized in executing individual aspects of the skill.
That’s the job of white matter—long distant speedy communication. The tissue is white because many axons are coated with tightly wrapped layers of electrical insulation called myelin. This insulation, made by non-neuronal cells (called oligodendrocytes), speeds the transmission of electrical impulses 100 times faster than transmission rates through bare axons. The complex skill of catching a baseball is a far cry from Pavlov and his slobbering dog learning to associate the sound of a bell with food. Skill learning is likely to involve different mechanisms. The kind of complex learning involved in mastering new skills such as catching a fly ball, takes time to learn and repetition over the course of days,weeks or years. This type of learning is what these neuroscientists dared to tackle.
In the first study, Jan Scholz and colleagues at the University of Oxford, England, used MRI brain imaging to obtain a detailed scan of the brain of 48 right-handed adults. Then they taught half of them to juggle. Anyone who has tried to master the three-ball-toss knows how difficult juggling is and how much practice it takes to learn it. But as in learning to ride a bike, once the complicated skill is mastered, suddenly everything “clicks” and the process becomes mysteriously automatic. Learning to read is like that too, which is what the second research group investigated, but first let’s have a look at the fascinating study peering into the brain of jugglers.
