Synesthesia

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Synesthesia (also spelled synæsthesia or synaesthesia, plural synesthesiae or synaesthesiae)—from the Ancient Greek s?? (syn), meaning "with," and a?s??s?? (aisthesis), meaning "sensation"'—is a neurological condition in which two or more bodily senses are coupled. In one common form of synesthesia, known as grapheme ? color synesthesia, letters or numbers are perceived as inherently colored, while in ordinal linguistic personification, numbers, days of the week and months of the year evoke personalities. 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 have a three-dimensional view of a year as a map (clockwise or counterclockwise).

While cross-sensory metaphors (e.g., "loud shirt", "bitter wind" or "prickly laugh") are sometimes described as "synesthetic", true neurological synesthesia is involuntary. It is estimated that synesthesia may be as prevalant as 1 in 23 persons across its range of variants (Simner et al. 2006). It runs strongly in families, possibly inherited as an X-linked dominant trait. Synesthesia is also sometimes reported by individuals under the influence of psychedelic drugs, after a stroke, or as a consequence of blindness or deafness. Synesthesia that arises from such non-genetic events is referred to as adventitious synesthesia to distinguish it from the more common congenital forms of synesthesia. Adventitious synesthesia involving drugs or stroke (but not blindness or deafness) apparently only involves sensory linkings such as sound ? vision or touch ? hearing; there are few if any reported cases involving culture-based, learned sets such as graphemes, lexemes, days of the week, or months of the year.

Although synesthesia was the topic of intensive scientific investigation in the late 1800s and early 1900s, it was largely abandoned in the mid-20th century, and has only recently been rediscovered by modern researchers. Psychological research has demonstrated that synesthetic experiences can have measurable behavioral consequences, while functional neuroimaging studies have identified differences in patterns of brain activation (for a review see Hubbard & Ramachandran 2005).

Many people with synesthesia use their experiences to aid in their creative process, and many non-synesthetes have attempted to create works of art that may capture what it is like to experience synesthesia. Psychologists and neuroscientists study synesthesia not only for its inherent interest, but also for the insights it may give into cognitive and perceptual processes that occur in everyone, synesthete and non-synesthete alike.

Definitional criteria

Although referred to as a "neurological condition", synesthesia is not listed in either the DSM-IV or the ICD classifications, since synesthesia does not, in general, interfere with normal daily functioning. Indeed most synesthetes report that their experiences are neutral, or even pleasant (Day 2005). Rather, like color blindness or perfect pitch, synesthesia is a difference in perceptual experience and is referred to as a neurological condition to reflect 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 synaesthetic experiences were entirely different from synaesthete to synaesthete, but recent research has shown that there are underlying similarities that can be observed when large numbers of synaesthetes are examined together. For example, sound-colour synaesthetes, as a group, tend to see lighter colours for higher sounds (Ward et al, 2006) and grapheme-colour synaesthetes, as a group, share significant preferences for the colour of each letter (e.g., A tends to be red; O tends to be white or black; S tends to be yellow etc., Simner et al., 2005; Rich et al., 2005; Day, 2005). 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 not aware that their experiences have a name. 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 ( Cytowic 2002, pp. 67-69; Cytowic 2003, pp. 76-77):

1. Synesthesia is involuntary and automatic.
2. Synesthetic images are spatially extended, meaning they often have a definite 'location'.
3. Synesthetic percepts are consistent and generic (i.e. simple rather than imagistic).
4. Synesthesia is highly memorable.
5. Synesthesia is laden with affect.
6. Synesthesia is not easily forgotten.

Experiences

Synesthetes often report that they were unaware their experiences were unusual until they realized other people did not have them, while others report feeling as if they had been keeping a secret their entire lives. The automatic and ineffable nature of a synesthetic experience means that the pairing may not seem out of the ordinary. This involuntary and consistent nature helps define synesthesia as a real experience. Most synesthetes report that their experiences are pleasant or neutral although, in rare cases synesthetes report that their experiences can lead to a degree of sensory overload (Day 2005).

"One day," I said to my father, "I realized that to make an 'R' all I had to do was first write a 'P' and then draw a line down from its loop. And I was so surprised that I could turn a yellow letter into an orange letter just by adding a line."

– Writer Patricia Lynne Duffy, recalling an early experience."^[1]

Despite the commonalities which permit definition of the broad phenomenon of synesthesia, individual experiences vary in numerous ways. This variability was first noticed early on in synesthesia research (Flournoy 1893) but has only recently come to be re-appreciated by modern researchers. Some grapheme ? color synesthetes report that the colors seem to be "projected" out into the world, while most report that the colors are experienced in their "mind's eye" (Dixon, Smilek & Merikle 2004). Additionally, some grapheme ? color synesthetes report that they experience their colors strongly, and show perceptual enhancement on the perceptual tasks described below, while others (perhaps the majority) do not (Hubbard et al. 2005a), perhaps due to differences in the stage at which colors are evoked. Some synesthetes report that vowels are more strongly colored, while for others consonants are more strongly colored (Day 2005). The descriptions below give some examples of synesthetes' experiences, but do not exhaust their rich variety.

Various forms

Synesthesia can occur between nearly any two senses or perceptual modes. Given the large number of forms of synesthesia, researchers have adopted a convention of indicating the type of synesthesia by using the following notation x ? y, where x is the "inducer" or trigger experience, and y is the "concurrent" or additional experience. For example, perceiving letters and numbers (collectively called graphemes) as colored would be indicated as grapheme ? color synesthesia. Similarly, when synesthetes see colors and movement as a result of hearing musical tones, it would be indicated as tone ? (color, movement) synesthesia.

While nearly every possible combination of experiences is logically possible, several types are more common than others.

Grapheme ? color synesthesia

Main article: Grapheme-color synesthesia

How someone with synesthesia might perceive certain letters and numbers.

In one of the most common forms of synesthesia, grapheme ? color synesthesia, individual letters of the alphabet and numbers (collectively referred to as graphemes), are "shaded" or "tinged" with a color. While synesthetes do not, in general, report the same colors for all letters and numbers, studies of large numbers of synesthetes find that there are some commonalities across letters (e.g., A is likely to be red) ( Day 2005; Simner et al. 2005).

A grapheme ? color synesthete reports, "I often associate letters and numbers with colors. Every digit and every letter has a color associated with it in my head. Sometimes, when letters are written boldly on a piece of paper, they will briefly appear to be that color if I'm not focusing on it. Some examples: 'S' is red, 'H' is orange, 'C' is yellow, 'J' is yellow-green, 'G' is green, 'E' is blue, 'X' is purple, 'I' is pale yellow, '2' is tan, '1' is white. If I write SHCJGEX it registers as a rainbow when I read over it, as does ABCPDEF."^[2]

Another reports a similar experience. "When people ask me about the sensation, they might ask, 'so when you look at a page of text, it's a rainbow of color?' It isn't exactly like that for me. When I read words, about five words around the exact one I'm reading are in color. It's also the only way I can spell. I remember in elementary school remembering how to spell the word 'priority' because the color scheme, in general, was darker than many other words. I would know that an 'e' was out of place in that word because e's were yellow and didn't fit."

Music ? color synesthesia

In music ? color synesthesia, individuals experience colors in response to tones or other aspects of musical stimuli (e.g., timbre or key). Like grapheme ? color synesthesia, there is rarely agreement amongst synesthetes that a given tone will be a certain color. However, consistent trends can be found, such that higher pitched notes are experienced as being more brightly colored (Ward, Huckstep & Tsakanikos 2006). The presence of similar patterns of pitch-brightness matching in non-synesthetic subjects suggests that this form of synesthesia shares mechanisms with non-synesthetes (Ward, Huckstep & Tsakanikos 2006).

Color changes in response to pitch may involve more than just the hue of the color. Brightness (the amount of white in a color; as brightness is removed from red, for example, it fades into a brown and finally to black), saturation (the intensity of the color; firetruck red and sky blue are highly saturated, while grays, white, and black are unsaturated), and hue may all be affected to varying degrees (Campen & Froger 2003). Additionally, music ? color synesthetes, unlike grapheme ? color synesthetes, often report that the colors move, or stream into and out of their field of view.

Number form synesthesia

Main article: Number form

A number form from one of Francis Galton's (1881b) subjects. Note the convolutions, and how the first 12 digits correspond to a clock face.

A number form is a mental map of numbers, which automatically and involuntarily appears whenever someone who experiences number-forms thinks of numbers. Number forms were first documented and named by Francis Galton in The Visions of Sane Persons (Galton 1881a). Later research has identified them as a type of synesthesia ( Seron, Pesenti & Noël 1992; Sagiv et al. 2006b). In particular, it has been suggested that number-forms are a result of "cross-activation" between regions of the parietal lobe that are involved in numerical cognition and spatial cognition ( Ramachandran & Hubbard 2001; Hubbard et al. 2005b). In addition to its interest as a form of synesthesia, researchers in numerical cognition have begun to explore this form of synesthesia for the insights that it may provide into the neural mechanisms of numerical-spatial associations present unconsciously in everyone.

Personification

Main article: Ordinal linguistic personification

Ordinal-linguistic personification (OLP, or personification for short) is a form of synesthesia in which ordered sequences, such as ordinal numbers, days, months and letters are associated with personalities ( Simner & Holenstein 2007; Simner & Hubbard 2006). Although this form of synesthesia was documented as early as the 1890s ( Flournoy 1893; Calkins 1893) modern research has, until recently, paid little attention to this form.

"T's are generally crabbed, ungenerous creatures. U is a soulless sort of thing. 4 is honest, but… 3 I cannot trust… 9 is dark, a gentleman, tall and graceful, but politic under his suavity"

– Synesthetic subject report in Calkins 1893, p. 454

"I [is] a bit of a worrier at times, although easy-going; J [is] male; appearing jocular, but with strength of character; K [is] female; quiet, responsible..."

– Synesthetic subject MT report in Cytowic 2002, p. 298

For some people in addition to numbers and other ordinal sequences, objects are sometimes imbued with a sense of personality, sometimes referred to as a type of animism. This type of synesthesia is harder to distinguish from non-synesthetic associations. However, recent research has begun to show that this form of synesthesia co-varies with other forms of synesthesia, and is consistent and automatic, as required to be counted as a form of synesthesia (Simner & Holenstein 2007).

Lexical ? gustatory synesthesia

Main article: Lexical-gustatory synesthesia

In a rare form of synesthesia, lexical ? gustatory synesthesia, individual words and phonemes of spoken language evoke the sensations of taste in the mouth.

Whenever I hear, read, or articulate (inner speech) words or word sounds, I experience an immediate and involuntary taste sensation on my tongue. These very specific taste associations never change and have remained the same for as long as I can remember.

– James Wannerton^[3]

Jamie Ward and Julia Simner have extensively studied this form of synesthesia, and have found that the synesthetic associations are constrained by early food experiences ( Ward & Simner 2003; Ward, Simner & Auyeung 2005). For example, James Wannerton has no synesthetic experiences of coffee or curry, even though he eats them regularly as an adult. Conversely, he tastes certain breakfast cereals and candies that are no longer sold.

Additionally, these early food experiences are often paired with tastes based on the phonemes in the name of the word (e.g., /I/, /n/ and /s/ trigger James Wannerton's taste of mince) although others have less obvious roots (e.g., /f/ triggers sherbet). To show that phonemes, rather than graphemes are the critical triggers of tastes, Ward and Simner showed that, for James Wannerton, the taste of egg is associated to the phoneme /k/, whether spelled with a c (e.g., accept), k (e.g., York), ck (e.g., chuck) or x (e.g., fax). Another source of tastes comes from semantic influences, so that food names tend to taste of the food they match, and the word blue tastes "inky".

[edit] Research history

Main article: History of synesthesia research

Although there were previous mentions of synesthesia, the phenomenon was first brought to the attention of the scientific community in the 1880s by Francis Galton ( Galton 1880a; Galton 1880b; Galton 1883). Following these initial observations, research into synesthesia proceeded briskly, with researchers from England, Germany, France and the United States all investigating the phenomenon. However, due to the difficulties in assessing and measuring subjective internal experiences, and the rise of behaviorism in psychology, which banished any mention of internal experiences, the study of synesthesia gradually waned during the 1930s.

In the 1980s, as the cognitive revolution had begun to make discussion of internal states and even the study of consciousness respectable again, scientists began to once again examine this fascinating phenomenon. Led in the United States by Larry Marks and Richard Cytowic, and in England by Simon Baron-Cohen and Jeffrey Gray, research into synesthesia began by exploring the reality, consistency and frequency of synesthetic experiences. In the late 1990s, researchers began to focus on grapheme ? color synesthesia, one of the most common ( Day 2005; Rich, Bradshaw & Mattingley 2005) and easily studied forms of synesthesia. In 2006, the journal Cortex published a special issue on synesthesia, composed of 26 articles. Synesthesia has been the topic of numerous scientific books, as well as novels and short films that include characters who experience some form of synesthesia.

During the 1990s, with the rise of the internet, synesthetes started to contact each other, and create many web pages relating to the condition (see External links below). These early internet and e-mail contacts have now grown into several international organizations for synesthetes, including the American Synesthesia Association, the UK Synaesthesia Association, the Belgian Synaesthesia Association, and the now defunct International Synaesthesia Association.

Prevalence and genetic basis

Estimates of the prevalence of synesthesia have varied widely (from 1 in 20 to 1 in 20,000). However, these studies all suffered from the methodological shortcoming of relying on self-selected samples. That is, the only people included in the studies were those who reported their experiences to the experimenter. Simner et al., (2006) conducted the first random population study, arriving at a prevalence of 1 in 23. Recent data suggests that grapheme ? color, and days of the week ? color variants are most common ( Day 2005; Simner et al. 2006).

Almost every study that has investigated the topic has suggested that synesthesia clusters within families, consistent with a genetic origin for the condition. The earliest references to the familial component of synesthesia date to the 1880s, when Francis Galton first described the condition in Nature. Since then, other studies have supported this conclusion. However, early studies ( Baron-Cohen et al. 1993; Baron-Cohen et al. 1996) which claimed a much higher prevalence in women than in men (up to 6:1) most likely suffered from a sampling bias due to the fact that women are more likely to self-disclose than men. More recent studies, using random samples find a sex ratio of 1.1:1 (Simner et al. 2006).

The observed patterns of inheritance have suggested an X-linked mode of inheritance, although research into the genetics of synesthesia is still preliminary. There are no documented instances of father-to-son transmission, while other forms of transmission (father-to-daughter, mother-to-son and mother-to-daughter) are quite common ( Baron-Cohen et al. 1996; Cytowic 2002; Ward & Simner 2005). Pairs of identical twins have been identified where only one member of the pair experiences synesthesia ( Smilek et al. 2002b; Smilek, Dixon & Merikle 2005) and it has been noted that synesthesia can skip generations within a family (Hubbard & Ramachandran 2003), consistent with models of incomplete penetrance. Additionally, Ward and Simner (2005) note that it is quite common for synesthetes within a family to experience different types of synesthesia, suggesting that the gene or genes involved in synesthesia do not lead to specific types of synesthesia. Rather developmental factors such as gene expression and environment must also play a role in determining which types of synesthesia an individual synesthete will experience.

Objective verification

Proof that someone is a synesthete is easy to come by, and hard to "fake." The simplest test involves test-retest reliability over long periods of time. Synesthetes consistently score higher on such tests than non-synesthetes (either with color names, color chips or even a color picker providing 16.7 million color choices). Synesthetes may score as high as 90% consistent over test-retest intervals of up to one year, while non-synesthetes will score 30-40% consistent over test-retest intervals of only one month, even if warned that they are going to be retested (e.g., Baron-Cohen et al. 1996).

More specialized tests include using modified versions of the Stroop effect. In the standard Stroop paradigm, it is harder to name the ink color of the word "red" when it is printed in blue ink than if it is presented in red ink. This demonstrates that reading is "automatic." Similarly, if a grapheme ? color synesthete is presented with the digit 4 that he or she experiences as red, but presented in blue ink, he or she is slower to identify the ink color. Not because the synesthete cannot see the blue ink, but rather that the same sort of "response conflict" that is responsible for the standard Stroop effect is also occurring between the color of the ink and the automatically induced color of the grapheme. Similar variants of the Stroop effect can be devised where, for example, a music ? color synesthete is asked to name a red color patch while listening to a tone that produces a blue sensation (Ward, Tsakanikos & Bray 2006), or where a musical key ? taste synesthete is asked to identify a bitter taste while hearing a musical interval that induces a sweet taste (Beeli, Esslen & Jäncke 2005).

An example of a test used to demonstrate the reality of synesthetic experiences (from Ramachandran & Hubbard 2001).

Finally, studies of grapheme ? color synesthesia have demonstrated that synesthetic colors can improve performance on certain visual tasks, at least for some synesthetes. Inspired by tests for color blindness, Ramachandran and Hubbard (2001) presented synesthetes and non-synesthetes with displays composed of a number of 5s, with some 2s embedded among the 5s. These 2s could make up one of four shapes; square, diamond, rectangle or triangle. For a synesthete who sees 2s as red and 5s as green, their synesthetic colors help them to find the "embedded figure". Subsequent studies have explored these effects more carefully, and have found that 1) there is substantial variability among synesthetes ( Dixon, Smilek & Merikle 2004; Hubbard et al. 2005a) and 2) while synesthesia is evoked early in perceptual processing, it does not occur prior to attention (e.g., Edquist et al. 2006; Sagiv, Heer & Robertson 2006a).

Possible neural basis

Main article: Neural basis of synesthesia

Regions thought to be cross-activated in grapheme-color synesthesia (from Ramachandran & Hubbard 2001).

Theories of the neural basis of synesthesia start from the observation that there are dedicated regions of the brain that are specialized for certain functions. Based on this notion of specialized regions, some researchers have suggested that increased cross-talk between different regions specialized for different functions may account for different types of synesthesia. For example, since regions involved in the identification of letters and numbers lie adjacent to a region involved in color processing (V4), the additional experience of seeing colors when looking at graphemes might be due to "cross-activation" of V4 (Ramachandran & Hubbard 2001). This cross-activation may arise due to a failure of the normal developmental process of pruning.

Alternatively, synesthesia may arise though "disinhibited feedback" or a reduction in the amount of inhibition along feedback pathways (Grossenbacher & Lovelace 2001). Normally, the balance of excitation and inhibition are maintained. However, if normal feedback were not adequately inhibited, then signals coming from later multi-sensory stages of processing might influence earlier stages of processing, such that tones would activate visual cortical areas in synesthetes more than in non-synesthetes. In this case, it might explain why some users of psychedelic drugs such as LSD or mescaline report synesthetic experiences while under the influence of the drug.

Functional neuroimaging studies using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) have demonstrated significant differences between the brains of synesthetes and non-synesthetes. Recent studies using fMRI have demonstrated that V4 is more active in both word ? color and grapheme ? color synesthetes ( Nunn et al. 2002; Hubbard et al. 2005a; Sperling et al. 2006). However, these neuroimaging studies do not have the spatial and temporal resolution necessary to distinguish between the pruning and disinhibited feedback theories.

Associated cognitive traits

Very little is known about the overall cognitive traits associated with synesthesia (or, indeed if there are any cognitive traits that are consistently associated with synesthesia). Some studies have suggested that synesthetes are unusually sensitive to external stimuli (see, e.g., Cytowic 2002). Other possible associated cognitive traits include left-right confusion, difficulties with math, and difficulties with writing (Cytowic 2002).

However, synesthetes may be more likely to participate in creative activities (Rich, Bradshaw & Mattingley 2005), and some studies have suggested a correlation between synesthesia and creativity ( Domino 1989; Dailey, Martindale & Borkum 1997). Other research has suggested that synesthesia may contribute to superior memory abilities ( Luria 1968; Smilek et al. 2002a). However, it is unclear whether this is a general feature of synesthesia or whether it is true of only a small minority. This is a major topic of current and future research.

Links with other areas of study

Researchers study synesthesia not only because it is inherently interesting, but also because they hope that studying synesthesia will offer new insights into other questions, such as how the brain combines information from different sensory modalities, referred to as crossmodal perception and multisensory integration.

This picture is used as a test to demonstrate that people may not attach sounds to shapes arbitrarily. Subjects are asked which shape might be called "Kiki" and which might be called "Bouba".

One example of this is the bouba/kiki effect. In a psychological experiment first designed by Wolfgang Köhler, people are asked to choose which of two shapes (pictured right) is named bouba and which is named kiki. 95% to 98% of people choose kiki for the orange angular shape and bouba for the purple rounded shape. With individuals on the island of Tenerife, Kohler showed a similar preference between shapes called "takete" and "maluma". Recent work by Daphne Maurer and colleagues has shown that even children as young as 2.5 (too young to read) show this effect (Maurer, Pathman & Mondloch 2006).

Ramachandran and Hubbard (2001) suggest that the kiki/bouba effect has implications for the evolution of language, because it suggests that the naming of objects is not completely arbitrary. The rounded shape may most commonly be named bouba because the mouth makes a more rounded shape to produce that sound while a more taut, angular mouth shape is needed to make the sound kiki. The sounds of a K are harder and more forceful than those of a B, as well. The presence of these "synesthesia-like mappings" suggest that this effect might be the neurological basis for sound symbolism, in which sounds are non-arbitrarily mapped to objects and events in the world.

Similarly, synesthesia researchers hope that, because of their unusual conscious experiences, the study of synesthesia will provide a window into better understanding consciousness and in particular on the neural correlates of consciousness, or what the brain mechanisms that allow us to be conscious might be. In particular, some researchers have argued that synesthesia is relevant to the philosophical problem of qualia (see, e.g., Gray et al. 2002; Gray et al. 1997; Ramachandran & Hubbard 2001), since synesthetes experience additional qualia evoked through non-typical routes.

Use in art

Vision by Carol Steen; Oil on Paper; 15 x 12-3/4 inches; 1996. A representation of a synesthetic vision the artist experienced during acupuncture treatment.

Main article: Synesthesia in art

The phrase synesthesia in art has historically referred to a wide variety of artistic experiments in order to synthesize different art disciplines (i.e. music and painting) as can be observed in the genres of visual music, abstract film, computer animation, symbolist poetry, multimedia and intermedial art (Berman 1999, Maur 1999, Gage 1994, 1999, Campen 1999). The usage of the term in the arts should, however, be differentiated from "genuine" synesthesia in scientific research. Scientific methods to assess synesthesia have only been developed in the last two decades. To assess synesthesia in artists before that time one has to interpret autobiographical and biographical sources (see also the List of people with synesthesia). In general, it has shown to be extremely difficult to categorize artists as synesthetes without scientific criteria or assessment.

Synesthetic art may refer to either art created by synesthetes or art created to convey the synesthetic experience. It is an attempt to understand the relation between the experiences of congenital synesthetes, the experiences of non-synesthetes, and an appreciation of such art by both synesthetes and non-synesthetes. These distinctions are not mutually exclusive, as, for example, art by a synesthete might also evoke synesthesia-like experiences in the viewer. However, it should not be assumed that all "synesthetic" art accurately reflects the synesthetic experience. This latter category is also sometimes referred to as artificial synesthesia.

Historically, synesthetic art consisted of a number of contrivances, such as color organs, musical painting and more recently, visual music, all of which have been intended to evoke cross-sensory fusions in the audience, although the inventors of such artifices were not necessarily synesthetes themselves, and may not even have been aware of synesthesia as such. Numerous modern synesthete artists, including Carol Steen, Marcia Smilack, and others have described in detail the manner in which they use their synesthesia in the creation of their artworks, demonstrating the complex interplay between their personal experiences and their artistic creations.

Literary depictions

Main article: Synesthesia in literature

In addition to its role in art, synesthesia has often been used as a plot device or as a way of developing a particular character's internal states. In order to better understand the influence of synesthesia in popular culture, and how the condition is viewed by non-synesthetes, it is informative to examine books in which one of the main characters is portrayed as experiencing synesthesia. In addition to these fictional portrayals, the way in which synesthesia is presented in non-fiction books to non-specialist audiences is instructive. Author and synesthete, Patricia Lynne Duffy has described four ways in which synesthete characters have been used in modern fiction.

1. Synesthesia as Romantic ideal: in which the synesthetic experience illustrates the Romantic ideal of transcending our experience of the world. Books in this category include The Gift by Vladimir Nabokov.
2. Synesthesia as pathology: in which synesthesia is portrayed as pathological. Books in this category include The Whole World Over by Julia Glass.
3. Synesthesia as Romantic pathology: in which synesthesia is portrayed as pathological, but also as providing an avenue into the Romantic ideal of transcending normal experience. Duffy selects Holly Payne's novel, The Sound of Blue as an example of this category.
4. Synesthesia as health and balance for some individuals: in which synesthesia is portrayed as indicating psychological health and well being. In particular, Duffy selects two novels, Painting Ruby Tuesday by Jane Yardley and A Mango-Shaped Space by Wendy Mass to illustrate this usage of synesthesia as a plot or character device.

Note that not all of the depictions of synesthesia in the fictional works are accurate. Some are highly inaccurate and reflect more about the author's interpretation of synesthesia than about the phenomenon itself.

People with synesthesia

Main article: List of people with synesthesia

There is a great deal of debate about whether or not synesthesia can be identified through historical sources. A small number of famous people have been labeled as synesthetes on the basis of at least two historical sources. This includes individuals of many different talents, such as artists, novelists, composers, musicians, and scientists.

Artists with synesthesia include the painter David Hockney, who perceives music synesthetically as colors, and who used these synesthetic colors when painting stage sets, but not in creating his other artworks. Also, Russian painter Wassily Kandinsky had the same type of synaesthesia (sound and colour). Perhaps the most famous synesthete author was Vladimir Nabokov, who had grapheme ? color synesthesia, one of the most common types, which he described at length in his autobiography, Speak Memory, and which he sometimes portrays in giving his characters synesthesia. Composers include Duke Ellington (timbre ? color), Franz Liszt (music ? color), Nikolai Rimsky-Korsakov, and Olivier Messiaen, who had a complex form of synesthesia in which chord structures produced synesthetic colors. Notable synesthete scientists include Nikola Tesla and Richard Feynman. Feynman describes in his autobiography, What Do You Care What Other People Think?, that he had the grapheme ? color type. Currently, one of the most popular synesthetes is perhaps hip-hop producer and musician Pharrell Williams (music ? color) and musician John Mayer. Other notable synesthetes include Justin Chancellor (music ? color), bassist for the prog-metal band Tool, and electronic musician Aphex Twin, who borrows inspiration from lucid dreams as well as synesthesia (music ? color). The classical pianist Hélène Grimaud has the condition also.

Some of the most frequently mentioned artists in connection with synesthesia probably were not synesthetes. Despite compositions such as Prometheus: The Poem of Fire and Mysterium, the Russian composer Alexander Scriabin was most likely not a synesthete. He was particularly interested in the psychological effects on the audience when they experienced sound and color simultaneously. His theory was that when the correct color was perceived with the correct sound, 'a powerful psychological resonator for the listener' would be created. On the score of Prometheus Scriabin wrote next to the instruments separate parts for the color organ (Galeyev 2001, Gleich 1963).

The French Romantic poets Arthur Rimbaud and Charles Baudelaire wrote poems which focused on synesthetic experience, but were evidently not synesthetes themselves. Baudelaire's Correspondances (1857) (full text available here) introduced the Romantic notion that the senses can and should intermingle. Kevin Dann (Dann 1998) argues that Baudelaire probably learned of synesthesia from reading medical textbooks that were available in his home. Rimbaud, following Baudelaire, wrote Voyelles (1871) (full text available here) which was perhaps more important than Correspondances in popularizing synesthesia, although he later admitted ""J'inventais la couleur des voyelles!" [I invented the colors of the vowels!].

Sean A. Day, a synesthete, and the President of the American Synesthesia Association, maintains a list of people with synesthesia, "pseudosynesthetes," and individuals who are most likely not synesthetic, but who used synesthesia in their art or music.

 Further reading

• Baron-Cohen, S. and Harrison, J. (Eds., 1997). Synaesthesia: Classic and Contemporary Readings. Oxford: Blackwell Publishers. ISBN 0-631-19764-8.
• Cytowic, R. (2003). The Man Who Tasted Shapes. New York: Tarcher/Putman. ISBN 0-262-53255-7.
• Dann, K. (1998). Bright Colors Falsely Seen. Cambridge: Harvard University Press. ISBN 0-300-06619-8.
• Duffy, P. L. (2001). Blue Cats and Chartreuse Kittens: How Synesthetes Color their Worlds. New York: Henry Holt & Company. ISBN 0-7167-4088-5.
• Harrison, J. (2001). Synaesthesia: The Strangest Thing. Oxford: Oxford University Press. ISBN 0-19-263245-0.
• Robertson, L. and Sagiv, N. (Eds., 2005). Synesthesia: Perspectives from Cognitive Neuroscience. Oxford: Oxford University Press ISBN 0-19-516623-X.
• Daniel Tammet "Born on a Blue Day: A Memoir of Aspergers and an Extraordinary Mind " Hodder & Stoughton Ltd (13 Jul 2006) ISBN 978-0-34-089974-8