Functional MRIs have shown that people with the disorder have greater activity in the orbitofrontal cortex, cingulate cortex, and caudate nucleus, along with other structures that affect how the brain learns, makes decisions, and processes emotion. These areas don’t respond to serotonin — a chemical that affects mood and helps different brain parts communicate — in the same way a brain without OCD does, researchers say. The structures also become less active after patients undergo therapy.