cluttered speech

[gusl]

I would like to see some neuroscience research as to why nervousness/anxiety seems to cause cluttered speech / stuttering. Does attending to others' perceptions of you use up resources, like a musician who can't keep up with the fast tempo? Does an analogous thing happen with musical performances? Do people improve when they don't have to look at the audience? (I know that many stutterers have no problem singing, probably because it involves no improvisation)

This must be related to speech production being one of those things that isn't 100% deliberate.