The red coloring in these images represents areas where gray matter is abnormally increased in drug users, compared to controls. The blue represents areas where gray matter is abnormally decreased in drug users, compared to controls. The yellow shows white matter tracts, called fractional anisotropy or FA. FA is significantly reduced in both the drug users and in their siblings, which suggests that the white matter tracts work less efficiently. This image relates to a paper that appeared in the Feb. 3, 2012, issue of Science, published by AAAS. The paper, by K.D. Ersche at University of Cambridge in Cambridge, UK, and colleagues was titled, "Abnormal Brain Structure Implicated in Stimulant Drug Addiction."