This story is a supplement to the feature "Why Migraines Strike" which was printed in the August 2008 issue of Scientific American.

Auras have been found to stem from cortical spreading depression: a wave of excessive signaling across large areas of the brain, followed by abnormal silence in the previously overactive areas. The spreading moves across the cortex at the rate of two to three millimeters a minute and has recently been captured by brain-imaging technology.