Plotting diffusion: Water molecules in the brain diffuse along neural wires, allowing scientists to create, essentially, circuit diagrams. First, the most likely directions of diffusion are calculated for each two-cubic-millimeter patch of the brain, generating 3-D shapes for each point, as shown above. Specialized software calculates the path of neural wires on the basis of these shapes. This method can distinguish overlapping wires, while more conventional imaging cannot.
Van den Heuvel and colleagues found that people with above-normal IQs of 120 and up had the most efficient brain networks. “Our hypothesis is that IQ is about how the human brain can integrate different types of information, how easily it can get information from one brain region to another,” van den Heuvel says. “These activity patterns are highly influenced by white-matter structures in the brain, how the brain is connected.”
Richard Haier and his collaborators are now working on a new method of measuring information flow around the brain using magnetoencephalography, or MEG. MEG measures the magnetic fluctuations around neurons as they fire, allowing scientists to track the millisecond-scale sequence of neural signaling in the brain as people perform different tasks, such as pressing a button in response to a light. Researchers hope to figure out how the flow of these signals differs with intelligence–whether smarter people follow the same sequence but faster, for example, or whether their brains skip a few steps in a circuit. “When you add the timing of the nodes and networks,” says Jung, “then we’re really talking about how the brain works in real time.”
If white matter plays a key role in intelligence, is there a way to enhance it? Does it give us ways to make ourselves smarter, or to help people with neurological and psychiatric disorders that affect cognitive skills?
It’s likely that the quality of white matter is at least partly genetically determined and, therefore, difficult to change. The size of the corpus callosum, the thick tracts of white matter connecting the two hemispheres of the brain, is about 95 percent genetic. And about 85 percent of the white-matter variation in the parietal lobes, which are involved in logic and visual-spatial skills, can be attributed to genetics, according to Thompson. But only about 45 percent of the variation in the temporal lobes, which play a central role in learning and memory, appears to be inherited.
Thompson is now trying to identify specific genes that are linked to the quality of white matter. The top candidate so far is a gene for a protein called BDNF, which promotes cell growth. People with one variation have better-organized fibers, he says.
But environmental factors also play a role. Rodents raised in a stimulating environment have more white matter. And research suggests that the apparent IQ difference between people who were breast-fed and bottle-fed as babies may arise because breast milk contains omega-3s, fatty acids involved in the production of myelin; as a result, some baby formula now includes these compounds.