How do musicians compose? Do they hear the music in their head?

I'm a synsthete, a person who perceives and conceives multisensorily. So when I see a painting, I feel and hear movement and music, and vice versa.

When I'm developing a composition I have more of an image in mind; an improvisation follows, and parts of the improvisation become more significant. The patterns which emerge become the seeds for the scored composition. A lot of my compositions were never put into scores, but were performed improvisationally from charts with instructions, like a game plan.