Synesthesia (American English) or synaesthesia (British English) is a perceptual phenomenon in which stimulation of one sensory or cognitive pathway leads to involuntary experiences in a second sensory or cognitive pathway.[3][4][5][6] People who report a lifelong history of such experiences are known as synesthetes. Awareness of synesthetic perceptions varies from person to person.[7] In one common form of synesthesia, known as grapheme–color synesthesia or color–graphemic synesthesia, letters or numbers are perceived as inherently colored.[8][9] In spatial-sequence, or number form synesthesia, numbers, months of the year, or days of the week elicit precise locations in space (for example, 1980 may be "farther away" than 1990), or may appear as a three-dimensional map (clockwise or counterclockwise).[10][11] Synesthetic associations can occur in any combination and any number of senses or cognitive pathways.[12]

Other namesSynaesthesia[1][2]
How someone with synesthesia might perceive certain letters and numbers. Most synesthetes see characters just as others do (in whichever color actually displayed) but may simultaneously perceive colors as associated with or evoked by each one.
SpecialtyPsychiatry, neurology

Little is known about how synesthesia develops. It has been suggested that synesthesia develops during childhood when children are intensively engaged with abstract concepts for the first time.[13] This hypothesis  referred to as semantic vacuum hypothesis  explains why the most common forms of synesthesia are grapheme–color, spatial sequence, and number form. These are usually the first abstract concepts that educational systems require children to learn.

Difficulties have been recognized in adequately defining synesthesia.[14][15] Many different phenomena have been included in the term synesthesia (based on the Greek words σύν and αἴσθησις meaning "union of the senses"), and in many cases the terminology seems to be inaccurate. A more accurate but less common term may be ideasthesia.[citation needed]

The earliest recorded case of synesthesia is attributed to the Oxford University academic and philosopher John Locke, who, in 1690, made a report about a blind man who said he experienced the color scarlet when he heard the sound of a trumpet.[16] However, there is disagreement as to whether Locke described an actual instance of synesthesia or was using a metaphor.[17] The first medical account came from German physician Georg Tobias Ludwig Sachs in 1812.[17][18][19] The term is from the Ancient Greek σύν syn, 'together', and αἴσθησις aisthēsis, 'sensation'.[16]