As a young student (some time ago), learning about why artists began making abstract art was the most exciting period in art history to me.
There were quite a few artists who were attracted to "synesthesia," the phenomenon that one sensation can manifest itself in a completely different form. For example, musical notes corresponding to particular colors in the spectrum.
I never really understood this fully, but a story this morning on NPR was very helpful. The reporter talks with Laura Rosser, a synesthetic pianist who sees her notes in color and her color in notes. In other words, they are inextricably bound to one another–one in the same in her mind.