What does it mean to hear music in colors, to taste voices, to see each letter of the alphabet as a different color? These uncommon sensory experiences are examples of synesthesia, when two or more senses cooperate in perception. Once dismissed as imagination or delusion, metaphor or drug-induced hallucination, the experience of synesthesia has now been documented by scans of synesthetes' brains that show "crosstalk" between areas of the brain that do not normally communicate. In The Hidden Sense, Cretien van Campen explores synesthesia from both artistic and scientific perspectives, looking at accounts of individual experiences, examples of synesthesia in visual art, music, and literature, and recent neurological research.