Synesthesia is a neurologically based condition in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway1,2,3,4. One person out of every thousand has synaesthesia & surprisingly, there are people who can smell sounds, see smells or hear colors5.
The most common types of synesthesia are:
Grapheme/Colour Synesthesia: The most common form, where letters & numbers are shaded with different colours
Sound/Colour Synesthesia: Environmental sounds, voices & music are perceived as fireworks & colour shapes
Number Form Synesthesia: A mental map or vision is perceived when thinking of numbers
Personification Synesthesia: Ordinal numbers, days & months have personalities
1. Cytowic, Richard E. (2002). Synesthesia: A Union of the Senses (2nd edition). Cambridge, Massachusetts: MIT Press. ISBN 0-262-03296-1. OCLC 49395033
2. Cytowic, Richard E. (2003). The Man Who Tasted Shapes. Cambridge, Massachusetts: MIT Press. ISBN 0-262-53255-7. OCLC 53186027.
3. Cytowic, Richard E; Eagleman, David M (2009). Wednesday is Indigo Blue: Discovering the Brain of Synesthesia (with an afterword by Dmitri Nabokov). Cambridge: MIT Press. pp. 309. ISBN 0-262-01279-9.
4. Harrison, John E.; Simon Baron-Cohen (1996). Synaesthesia: classic and contemporary readings. Oxford: Blackwell Publishing. ISBN 0-631-19764-8. OCLC 59664610
5. http://www.science20.com/news_releases/synaesthesia_smelling_a_sound_or_...