fMRI, the standard brain scanning technique that gauges brain activity, cannot show this organization. But diffusion imaging can, by revealing the major pathways of axons—the long fibers of nerve cells that transmit impulses to the next cells in line. It works by sensing how water molecules found naturally in brain tissue move along axons. Van Wedeen, a neuroscientist at Massachusetts General Hospital and Harvard University who pioneered the high-resolution version of diffusion imaging, explains that the principle is the same as an ink stain on your shirt: the fluid spreads along the fibers in one main direction. "This technique is like a magic wand you can wave over specimens, organs, or organisms," he told me. "And there is the inner logic—digitally, in 3D, in color."