Pupil size in people with synesthesia changed depending on how bright or dark the perceived colors were.

Have you ever tasted a word, or seen colors while listening to music? If you have, you may be among the 1% to 4% of people who have a fascinating trait known as synesthesia.

Vladimir Nabokov first noticed, at age 7, his special gift of synesthesia when playing with colorful alphabet blocks that "were all the wrong color." Source: Contributor: Neil Overy/Alamy Stock Photo.

Richard Cytowic, a pioneering researcher who returned synesthesia to mainstream science, traces the historical evolution of our understanding of the phenomenon. By Richard E. Cytowic / MIT Press ...