Wednesday Is Indigo Blue

Wednesday Is Indigo Blue: Discovering the Brain of Synesthesia by Richard Cytowic and David Eagleman, MIT Press, 2009. 

Qualia

Qualia are the subjective experience of the senses, such as redness, sweetness or pain. Synesthesia occurs when one sense induces the qualia of a different sense. For example, hearing a sound causing a person to see a colored image. Synesthesia is almost always unidirectional. If hearing a sound causes the person to see an image, then seeing an image does not cause the person to hear a sound. Color being stimulated by seeing words is the most common form of synesthesia, and is called grapheme-color synesthesia. The word itself is not painted that color. Instead, there is a patch of color located somewhere else in space.