Olaf Sporns of Indiana University maps the major pathways that link regions of the human brain. He uses a technique called diffusion spectrum imaging, which traces neural connections by following water molecules as they diffuse along axons. The 5,000 blue lines in this image represent bundles of axons, and the 1,000 red dots are synapses where axons meet. Sporns argues that despite the evident tangle of pathways, the brain could be a lot more complicated. "Only a small percentage of the connections that can exist in the brain do exist," he says.