Switching on a lightbulb is a visual cliché for creativity. But a different kind of switch, made of molecules, affects a number of other critical mental processes. Life’s experiences add chemicals to the genes that control brain activity, dialing up or down the expression of various features. A special two-article section explores how these molecular mechanisms change our brains. “The New Genetics of Mental Illness,” by psychiatrist Edmund S. Higgins, starting on page 40, looks at how the environment influences our susceptibility to depression, anxiety and drug addiction. “Unmasking Memory Genes,” by neuroscientist Amir Levine, explains how such molecules shape memory and learning; see page 48. How does our unified conscious experience emerge from the activity of billions of brain cells and numerous processing “modules” (brain regions associated with certain types of thought)? The mystery has long tantalized researchers. In “Spheres of Influence,” neuroscientist Michael S. Gazzaniga finds some clues from his studies of split-brain patients, whose connective tissue between their two hemispheres has been separated. Are two brains better than one for learning about consciousness? Find out beginning on page 32.