People who report such experiences are known as synesthetes.
Difficulties have been recognized in adequately defining synesthesia: Many different phenomena have been included in the term synesthesia ("union of the senses"), and in many cases the terminology seems to be inaccurate. A more accurate term may be ideasthesia.
In one common form of synesthesia, known as grapheme → color synesthesia or color-graphemic synesthesia, letters or numbers are perceived as inherently colored. In spatial-sequence, or number form synesthesia, numbers, months of the year, and/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).
Only a fraction of types of synesthesia have been evaluated by scientific research. Awareness of synesthetic perceptions varies from person to person.
Although synesthesia was the topic of intensive scientific investigation in the late 19th and early 20th centuries, it was largely abandoned by scientific research in the mid-20th century. Psychological research has demonstrated that synesthetic experiences can have measurable behavioral consequences, and functional neuroimaging studies have identified differences in patterns of brain activation. Many find that synesthesia aids the creative process. Psychologists and neuroscientists study synesthesia not only for its inherent appeal, but also for the insights it may give into cognitive and perceptual processes that occur in synesthetes and non-synesthetes alike.
|How someone with synesthesia might perceive (not "see") certain letters and numbers. Synesthetes see characters just as others do (in whichever color actually displayed), yet simultaneously perceive colors as associated to each one.|