What is synaesthesia?

What colour is the letter "A"? What does the number "1" taste of? Does listening to music, speaking or eating food produce colours, shape or texture?

For most people, questions such as these tend to yield a look of bewilderment. However, research suggests that as many as 1 in 23 people experience some form of synaesthesia.

Synaesthesia is a truly fascinating condition. In its simplest form it is best described as a “union of the senses” whereby two or more of the five senses that are normally experienced separately are involuntarily and automatically joined together. Some synaesthetes experience colour when they hear sounds or read words. Others experience tastes, smells, shapes or touches in almost any combination.

Famous synaesthetes include David Hockney, who perceives music as colour, shape, and configuration, Russian painter Wassily Kandinsky combined four senses: colour, hearing, touch, and smell. Vladimir Nabokov describes his grapheme-colour synaesthesia at length in his autobiography, Speak Memory and portrays it in some of his characters. Composers include Duke Ellington, Franz Liszt, Nikolai Rimsky-Korsakov, and Olivier Messiaen, whose three types of complex colours are rendered explicitly in musical chord structures that he invented. Physicist Richard Feynman describes his coloured equations in his autobiography, What Do You Care What Other People Think?

 
Do you experience synaesthesia?