To the unaided eye, the most striking feature of the human brain is its squiggly pattern of bumps and grooves. But within those curves is a latticework of nerve fibers that cross each other at roughly right angles (above), according to a study published in tomorrow’s issue of Science. The researchers used a recently-developed method called diffusion spectrum imaging to infer the position of nerve fibers in the living human brain from the way water flows through and around them. These scans revealed an orderly weave of fibers—a much simpler organization than many scientists would have suspected. Scans in four monkey species found a similar pattern. The researchers suggest that this grid-like organization may be advantageous during brain development, providing the equivalent of highway lane markers to help growing nerve fibers find their way to the appropriate destination.