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.