This is interesting:

http://www.sciencedaily.com/releases/2012/11/121115133154.htm

They studied the brains of experienced freestyle rappers in an MRI scanner. It turns out that this kind of performance involves simultaneously activating parts of the brain involved in language and emotion while reducing activity in areas that normally monitor and constrain how you think. Rehearsed performances showed a different pattern of activity.

Rapping is an unusually complex kind of improvisation since it involves language. It would be interesting to find out how different purely musical improvisation looks like under MRI.