Intelligence Explained

A series of black-and-white snapshots is splayed across the screen, each capturing a thin slice of my brain. The gray-scale pictures would look familiar to anyone who has seen a brain scan, but these images are different. Andrew Frew, a neuroscientist at the University of California, Los Angeles, uses a cursor to select a small square. Thin strands like spaghetti appear, representing the thousands of neural fibers passing through it. A few clicks of the cursor and Frew refines the tract of fibers pictured on the screen, highlighting first my optic nerve, then the fibers passing through a part of the brain that's crucial for language, then the bundles of motor and sensory nerves that head down to the brain stem.

Frew is giving me a tour of my white matter--the tissue connecting the neurons, or nerve cells, that make up gray matter. Something about the twisting, turning neural wires that ferry information between the neurons--their individual thickness, perhaps, or their abundance, or the specific paths they take from one part of the brain to another--may explain, at least in part, the variations in human intelligence.

Scientists have been searching more than two centuries for the source of intelligence--the general cognitive ability often quantified in the form of IQ. With the advent of technologies such as magnetic resonance imaging (MRI), researchers concentrating mainly on gray matter have been able to map the parts of the brain that appear to play a role. But this has taken them only so far, and the focus on gray matter has not told the whole story. Not until the last few years, as new variations of MRI home in on the brain's white matter, has a deeper understanding begun to emerge. "Scientists are now able to switch the focus from particular regions of the brain to the connections between those regions," says Sherif Karama, a psychiatrist and a neuroscientist at McGill University's Montreal Neurological Institute. Their initial findings have led Karama and others to believe that neural wiring and the way it carries information around the brain may be crucially important to IQ.