Brain scans reveal our moods and thought patterns as clearly as an X-ray reveals our bones; we can watch a person's brain literally light up as it registers a joke, or glow dully when it recalls an unhappy memory. Exploring a fascinating frontier in medical research, Rita Carter shows how these scans can be used to help explain how behavioral eccentricities can be traced to abnormalities in an individual brain. As Carter details here, dyslexia may be caused by a short-circuit in the messages converting sound to visual cues, while addictions might stem from dysfunction in the brain's reward system.