How the act of writing changes the mental vision

Some months (years?) ago, triciasullivan asked me to comment on this quote by Samuel Delany, which I suspect is from About Writing:

When I am writing I am trying to allow/construct an image of what I want to write about in my mind's sensory theater. Then I describe it as accurately as I can. The most interesting point I've noticed is that the *writing down* of words about my imagined vision (or at least the choosing/arranging of words to write down) causes the vision itself to change.

I can't c...