source: NPR, Michigan Radio:

“When pianist Laura Rosser performs, she hears more than sounds. She hears colors — each note has its own associated hue. Rosser has a rare neurological condition called synesthesia. Stimulation of one sense produces the sensation of another.

Synesthesia is rare. Perhaps one person in several thousand has it. Most of these people don’t have the form that allows them to perceive sounds as colors.

Yet a number of famous composers appear to have been synesthetes. They include Franz Liszt, Alexander Scriabin, and Nicolai Rimsky-Korsakov. Some research suggests musical synesthesia is more likely to come with perfect pitch.

Each synesthete has a unique system of associations. That means the periwinkle that Rosser associates with D-flat might be purple for another synesthete. Other synesthetes associate colors with letters and numbers. Rosser has this type of synesthesia as well.

Randolph Blake, a researcher at Vanderbilt University, says the brains of synesthetes appear to be wired in a way that allows signals from one sense to trigger brain circuits usually associated with another sense. But it’s unclear how this alternate wiring takes place.”