By volume, gray matter makes up roughly half the human brain. The other half is white matter, consisting of filament-like neural projections wrapped in a fatty material called myelin; such a high proportion of white matter appears to be unique to humans. As we “evolved from worms to humans,” says George Bartzokis, a professor of psychiatry at UCLA, the number of non-neural cells in the brain increased 50 times more than the number of neurons. He adds, “My hypothesis has always been that what gives us our cognitive capacity is not actually the number of neurons, which can vary tremendously between human individuals, but rather the quality of our connections.”
Thanks to their layer of insulation, which prevents leakage of electrical impulses, myelinated nerve fibers can send signals about 100 times as fast as unmyelinated ones. The myelin also allows more information to be sent per second by reducing the waiting time between signals. The result is that neurons can process 3,000 times as much information as would otherwise be possible. That capacity, Bartzokis believes, is crucial for speaking and processing language.
The type of MRI typically used for medical scans does not show the finer details of the brain’s white matter. But with a technique called diffusion tensor imaging (DTI), which uses the scanner’s magnet to track the movement of water molecules in the brain, scientists have developed ways to map out neural wiring in detail. While water moves randomly within most brain tissue, it flows along the insulated neural fibers like current through a wire.
Most DTI scans break the MRI image into tiny areas and measure the diffusion of water molecules through each one in six to 12 directions, which is sufficient for detecting thick bundles of neural fibers. But places where wiring overlaps appear as a blur. Newer variations of diffusion imaging measure diffusion in 50 to 500 directions. Computer algorithms synthesize this data into a three-dimensional picture showing the most likely paths of nerve fibers through each area, and then stitch together the information from multiple points to create a wiring map.
The strength of the diffusion signal–the extent to which it reveals a clear direction–is used to gauge how organized the fibers of the white matter are. A stronger diffusion signal may indicate more fibers or thicker myelin; scientists don’t yet know. But the newer diffusion imaging methods have revealed a strong correlation between the strength of this signal–what researchers refer to as the “integrity” of the white matter–and performance on a standard IQ test. “DTI turns out to be one of the most sensitive MRI measures we have for cognitive function,” says Vincent Schmithorst, a neuroscientist at Cincinnati Children’s Hospital.
Thompson refers to his diffusion maps as “pictures of mental speed.” Previous research has repeatedly linked IQ to processing speed, and other studies show that processing speed in turn is tightly linked to the quality of one’s white matter. Does that mean intelligence is determined by how fast the brain works? If so, does finding the key to processing speed in the brain mean researchers have finally found the secret to intelligence?
In reality, speed is probably not the only determinant of IQ. “One of the things that is important for IQ is frontal-lobe function, which is involved in planning, decision making, and weighing evidence,” Thompson says. “I wouldn’t think of those skills as being entirely reliant on mental speed.”
Some of the newest theories of intelligence suggest that the crucial factor may be how efficiently information moves around the brain, rather than just how quickly. In a recent study led by Martijn P. van den Heuvel, a neuroscientist at University Medical Center Utrecht, in the Netherlands, researchers defined efficiency as the number of links it takes to get from one node to another–both in specific brain areas and all over the brain. Just as a direct flight from Paris to Chicago would be considered more efficient than one with a layover in London, a direct link between two parts of the brain would be more efficient than an indirect route.