Once regarded as a rare condition, synesthesia (literally “joined perception,” from the Greek word syn meaning “join” and aisthesis meaning “perception”) likely far more common that you might imagine. Some scientists speculate that one in twenty-three may experience this "cross wiring" of the senses.
In the art world, perhaps the most notable synesthete was Vassily Kandinsky, who wrote several influential treatises that linked color and music. He titled his paintings as a musician might, using words like "improvisation," "symphony" and "fugue." In his writing, it's clear that color and sound were intimately linked: "Colour is the keyboard, the eyes are the harmonies, the soul is the piano with many strings. The artist is the hand that plays, touching one key or another, to cause vibrations in the soul."
So what IS a synesthete? Here's a quick description, and an update of why researchers find this a promising line of study.