'''Synesthesia''' ([[American English]]) or '''synaesthesia''' ([[British English]]) is a [[Perception|perceptual]] [[phenomenon]] in which [[stimulation]] of one sensory or cognitive pathway leads to involuntary [[Experience|experiences]] in a second sensory or cognitive pathway.<ref name="isbn0-262-03296-1">{{cite book | vauthors = Cytowic RE |title=Synesthesia: A Union of the Senses |publisher=[[MIT Press]] |location=[[Cambridge, Massachusetts]] |year=2002 |isbn=978-0-262-03296-4 |oclc=49395033|edition=2nd }} {{page needed|date=January 2014}}</ref><ref name="isbn0-262-53255-7">{{cite book | vauthors = Cytowic RE |title=The Man Who Tasted Shapes |publisher=[[MIT Press]] |location=[[Cambridge, Massachusetts]] |year=2003 |isbn=978-0-262-53255-6 |oclc=53186027}} {{page needed|date=January 2014}}</ref><ref name="JO MAMACytowic_Eagleman2009">{{cite book | vauthors = Cytowic RE, Eagleman DM |title=Wednesday is Indigo Blue: Discovering the Brain of Synesthesia (with an afterword by Dmitri Nabokov)|publisher=[[MIT Press]] |location=Cambridge |year=2009 |isbn=978-0-262-01279-9}} {{page needed|date=January 2014}}</ref><ref name="isbn0-631-19764-8">{{cite book | vauthors = Harrison JE, Baron-Cohen S |title=Synaesthesia: classic and contemporary readings |publisher=[[Blackwell Publishing]] |location=[[Oxford]] |year=1996 |isbn=978-0-631-19764-5 |oclc=59664610 }} {{page needed|date=January 2014}}</ref> For instance, people with synesthesia may experience colors when listening to music, see shapes when smelling certain scents, or perceive tastes when looking at words. People who report a lifelong history of such experiences are known as '''synesthetes'''. Awareness of synesthetic perceptions varies from person to person with the perception of synesthesia differing based on an individual's unique life experiences and the specific type of synesthesia that they have.<ref>{{Cite web |last=Henry |first=Paige |date=19 September 2003 |title=Synesthesia: Definition, Explanation, And More |url=https://www.betterhelp.com/advice/synesthesia/understanding-the-synesthesia-definition/}}</ref><ref name="campen2009">{{cite journal | vauthors = van Campen C | date = 2009 | title = The Hidden Sense: On Becoming Aware of Synesthesia | journal = Teccogs | volume = 1 | pages = 1–13 | url = http://www.pucsp.br/pos/tidd/teccogs/artigos/pdf/teccogs_edicao1_2009_artigo_CAMPEN.pdf |url-status=dead |archive-url=https://web.archive.org/web/20090708013028/http://www.pucsp.br/pos/tidd/teccogs/artigos/pdf/teccogs_edicao1_2009_artigo_CAMPEN.pdf |archive-date=8 July 2009 }}</ref> In one common form of synesthesia, known as [[grapheme–color synesthesia]] or color–graphemic synesthesia, letters or numbers are perceived as inherently colored.<ref name="pmid11823804">{{cite journal | vauthors = Rich AN, Mattingley JB | title = Anomalous perception in synaesthesia: a cognitive neuroscience perspective | journal = Nature Reviews. Neuroscience | volume = 3 | issue = 1 | pages = 43–52 | date = January 2002 | pmid = 11823804 | doi = 10.1038/nrn702 | type = Review | s2cid = 11477960 }}</ref><ref name="Hubbard_2005">{{cite journal | vauthors = Hubbard EM, Ramachandran VS | title = Neurocognitive mechanisms of synesthesia | journal = Neuron | volume = 48 | issue = 3 | pages = 509–520 | date = November 2005 | pmid = 16269367 | doi = 10.1016/j.neuron.2005.10.012 | type = Review | s2cid = 18730779 | doi-access = free }}</ref> In spatial-sequence, or [[number form]] synesthesia, numbers, months of the year, or days of the week elicit precise locations in space (''e.g.,'' 1980 may be "farther away" than 1990), or may appear as a three-dimensional map (clockwise or counterclockwise).<ref name="galton1880b">{{cite journal| vauthors = Galton F |year=1880|title=Visualized Numerals |url=https://zenodo.org/record/1429243 |journal=Nature|volume=21|issue=543|pages=494–5|doi=10.1038/021494e0|bibcode=1880Natur..21..494G|s2cid=4074444|doi-access=free}}</ref><ref name="pmid1511585">{{cite journal | vauthors = Seron X, Pesenti M, Noël MP, Deloche G, Cornet JA | title = Images of numbers, or "When 98 is upper left and 6 sky blue" | journal = Cognition | volume = 44 | issue = 1–2 | pages = 159–196 | date = August 1992 | pmid = 1511585 | doi = 10.1016/0010-0277(92)90053-K | s2cid = 26687757 }}</ref> Synesthetic associations can occur in any combination and any number of senses or cognitive pathways.<ref>{{Cite web|url=http://science.howstuffworks.com/life/inside-the-mind/emotions/synesthesia1.htm|title=How Synesthesia Works|date=1970-01-01|website=HowStuffWorks|access-date=2016-05-02}}</ref>
