Just read in today's USA Today that scientists recently scanned musicians' brains and have located the region that lights up when they improvise. It's the medial prefrontal cortex. Turns out this is the same area we all use when we're talking about ourselves (i.e. describing who we are and what interests us). Charles Limb, one of the researchers who is also a jazz saxophonist, thinks this makes perfect sense. He said, "Because the person is spontaneously composing, they really are revealing themselves musically. It's like your own musical autobiography."
And... at the same time the medial prefrontal cortex lights up in the improvising musician, the portion of the brain linked to careful planning and self-censoring (the ol' dorsolateral prefrontal cortex) turns off. Interesting stuff.
I'd like to go on record as saying Offspring #2 (who plays jazz trumpet) has a darn fine medial prefrontal cortex.