What is synaesthesia?

Synaesthesia, from the Greek syn (union) and aesthesis (sensation), is a neurological disorder where a normal sensory stimulus triggers a response in another sense or in a different aspect of the same sense. Some synaesthetes say that music, speech, and other sounds evoke colours (sound -> colour synaesthesia). “I choose a Mozart piece … the colours immediately and gently flow over me.”1

Other synaesthetes perceive black text on a page as inherently coloured (eg, seeing the letter q may trigger the perception of a bright green colour) (visual grapheme -> colour). These are the two most common forms of synaesthesia but other forms have also been reported; one ongoing survey describes 40 different sensory combinations including word -> taste and music -> shape.2

Over the past 20 years, researchers have been drawn back into synaesthesia research from a variety of academic disciplines, including neurology, psychiatry, and psychology. Studies of synaesthesia have provided fresh insight into the neural and cognitive aspects of perception, multisensory integration, language development, and memory.