Cerebral mapping was a scanning procedure that could determine whether an individual was telling the truth, or what they believed to be the truth. According to Admiral Katrina Cornwell, it was 100% accurate.
In 2257, Cornwell administered the test to Lieutenant Spock while questioning him about his escape from Starbase 5. She found that he was being truthful when he denied killing any starbase personnel. (DIS: "Project Daedalus")