White matter helps brain learn

The brain's white matter - generally thought to play second fiddle to the neurons that make up the grey matter - expands along with nearby grey matter when a person learns a new task, scientists at Oxford University report.

MRI of a head Image: Wikipedia Commons

Just as new neurons may be formed during learning, corresponding new cells in the white matter may be generated as well, said Jan Scholz, a student in the lab of Heidi Johansen-Berg at Oxford University, at a San Francisco meeting of the Organization for Human Brain Mapping last month.

"It's the first time that experience-related white matter changes have been seen," Scholz told The Scientist.

White matter is often called the wiring of the brain because it connects individual neuron and groups of neurons together. Whether or not it plays a role in learning or cognition, though, is something of a mystery. The Oxford work built on a 2004 Nature paper that showed that cortical grey matter, which consists largely of neuronal cell bodies, expanded in people learning to juggle. In the present study, the Oxford team examined the effects of the same task on the brain's white matter, which comprises connective tissue such as myelin and axons.

Scholz and Johansen-Berg, from Oxford's Centre for Functional Magnetic Imaging of the Brain (FMRIB), taught 24 healthy, right-handed volunteers to juggle. They scanned subjects' brains using diffusion MRI before the six weeks of training, directly after, and then again after four weeks of abstinence from juggling. Diffusion MRI measures how water diffuses within a brain structure, for instance showing how thick myelin in white matter might be.

The researchers observed grey matter "changes in structure" in a part of the parietal lobe associated with spatial coordination -- a change which Scholz told The Scientist probably reflects neurogenesis. Myelin in that region also appeared thicker, which he similarly attributed to myelinogenesis. Those gains then eroded after four weeks of no juggling. This temporal and spatial correlation shows that the two tissue types are working in concert.

"People have long thought of grey matter and white matter as independent structures, but they are clearly actually quite interdependent," said Adeline Vanderver, a neurologist and white matter expert at the National Children's Medical Center in Washington, DC, of the results. "What I think is really interesting is that clearly the cells are working together."

She added, though, that whether or not neurogenesis and myelogenesis were at play was difficult to determine. "You can't say based on the functional imaging and fractional anisotropy what is going on at a cellular level."

Silvia Bunge, a neuroscientist at University of California Berkeley, said that the demonstration of a link between white matter and grey matter is "hugely important" for the study of learning in humans, especially children. Indeed, her PhD student Kirstie Whitaker is looking at similar white matter development in school children.

"White matter is important -- that has been shown over the last 5-10 years. But it is definitely a new area of investigation." Whitaker said. "It's exciting to see a training study where over the course of six weeks they can see changes [in white matter] and reverse those changes."

Pinning down the role of white matter may help scientists understand brain networks. Elucidating the mechanism by which the tissue expands may also provide clues to treatments for diseases such as multiple sclerosis, which results from withering of myelin in the central nervous system.

"It's one of these thing that now that you look back on it you say, 'Oh, that must be happening,' but no one had ever thought of it quite that way," said Vanderver of combined white/grey matter growth. "A better understanding of this interdependency will help us help people's brains."


Related stories:

Brain's neuronal nexus mapped
[1st July 2008]

Decoding the brain
[July 2007]