Ever since I was little I've had a very good imagination. Whenever I got a "good movie" going on in my head, so to speak, I would pace back and forth and begin quickly moving my hand and fingers in tune with any action I was imagining. Often I'd become completely engrossed in the mental movie I was creating. It was completely voluntary, it allowed me to be, on some level, physically part of the imagining.
I do artwork now-a-days, and once in a while, I'll be brainstorming some good ideas and I'll find myself doing it, like I did when I was a child. Its odd behavior and I can't seem to find any information anywhere online. What exactly would this be under?