Synesthesia: A film by Jonathan Fowler

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Although sometimes spoken of as a “neurological condition,” synesthesia is not listed in either the DSM-IV or the ICD classifications, since it most often does not interfere with normal daily functioning. It has, however, appeared for many years in both Dorland’s and Steadman’s medical dictionaries. Indeed most synesthetes report that their experiences are neutral, or even pleasant.[19] Rather, like color blindness or perfect pitch, synesthesia is a difference in perceptual experience and the term “neurological” simply reflects the brain basis of this perceptual difference. To date, no research has demonstrated a consistent association between synesthetic experience and other neurological or psychiatric conditions, although this is an active area of research (see below for associated cognitive traits). It was once assumed that synesthetic experiences were entirely different from synesthete to synesthete, but recent research has shown that there are underlying similarities that can be observed when large numbers of synesthetes are examined together. For example, sound-color synesthetes, as a group, tend to see lighter colors for higher sounds[20] and grapheme-color synesthetes, as a group, share significant preferences for the color of each letter (e.g., A tends to be red; O tends to be white or black; S tends to be yellow etc.,[19][21][22]). Nonetheless, there are a great number of types of synesthesia, and within each type, individuals can report differing triggers for their sensations, and differing intensities of experiences. This variety means that defining synesthesia in an individual is difficult, and indeed, the majority of synesthetes are completely unaware that their experiences have a name.[19] However, despite the differences between individuals, there are a few common elements that define a true synesthetic experience. Neurologist Richard Cytowic identifies the following diagnostic criteria of synesthesia:[1][2][3] Synesthesia is involuntary and automatic. Synesthetic perceptions are spatially extended, meaning they often have a sense of “location.” For example, synesthetes speak of “looking at” or “going to” a particular place to attend to the experience. Synesthetic percepts are consistent and generic (i.e., simple rather than pictorial). Synesthesia is highly memorable. Synesthesia is laden with affect. Cytowic’s early cases included individuals whose synesthesia was frankly projected outside the body (e.g., on a “screen” in front of one’s face). Later research showed that such stark externalization occurs in a minority of synesthetes. Refining this concept, Cytowic and Eagleman[3] differentiate between “localizers” and “non-localizers” to distinguish those synesthetes whose perceptions have a definite sense of spatial quality.

(from Wikipedia)