Two Los Angeles rappers, Michael Eagle and Daniel Rizik-Baer, had their brains scanned while performing a freestyle and a memorized song. Researchers discovered the brains had strong action in the area that motivates action and thought and these changes were primarily in the left hemisphere of the brain, which for most right-handed people, is where language is processed. - Check out NPR for more.
Photo by: NIDCD (orange colors show active parts of the brain during a freestyle rap. The blue part represent activity during a memorized song.