Synesthesia is the blending of sensory perceptions. Like how you may perceive some sounds to be “sweet” and some pictures to be “heavy”. This is an area I am really interested in and have been extensively reading about.
Here is an anecdote from Mozart on his creative process, somewhat related to synesthesia:
“According to Mozart, he saw his music in his mind’s eye such that it stood, “almost complete and finished in my mind, so that I can survey it, like a fine picture or a beautiful statue, at a glance,,, For this reason the committing to paper is done quickly enough, for everything is, as I have said before, already finished; and it rarely differs on paper from what it was in my imagination.” – Strategies of Genius Vol 1
Finally, this is a website where you can literally “paint” a song