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How someone with synesthesia might perceive certain letters and numbers.Synesthesia (also spelled synæsthesia or synaesthesia, plural synesthesiæ or synæsthesiæ), from the ancient Greek σύν (syn), "together," and αἴσθησις (aisthēsis), "sensation," is a neurological condition in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway. People who report such experiences are known as synesthetes. Recently, difficulties have been recognized in finding an adequate definition of synesthesia, as many different phenomena have been covered by this term and in many cases the term synesthesia ("union of senses") seems to be a misnomer. A more accurate term for the phenomenon 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, 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). Yet another recently identified type, visual motion → sound synesthesia, involves hearing sounds in response to visual motion and flicker. Over 60 types of synesthesia have been reported, but only a fraction have been evaluated by scientific research. Even within one type, synesthetic perceptions vary in intensity and people vary in awareness of their synesthetic perceptions.
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 could possibly be as prevalent as 1 in 23 persons across its range of variants. Synesthesia runs strongly in families, but the precise mode of inheritance has yet to be ascertained. Synesthesia is also sometimes reported by individuals under the influence of psychedelic drugs, after a stroke, during a temporal lobe epilepsy seizure, or as a result of blindness or deafness. Synesthesia that arises from events after birth 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 19th century and early 20th century, it was largely abandoned by scientific research 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. 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 synesthetes and non-synesthetes alike.
1 Definitional criteria
3 Various forms
3.1 Grapheme → color synesthesia
3.2 Spatial Sequence Synesthesia
3.3 Sound → color synesthesia
3.4 Number form synesthesia
3.6 Lexical → gustatory synesthesia
3.7 Auditory-tactile synesthesia
3.8 Mirror Touch Synesthesia
4 Research history
5 Prevalence and genetic basis
6 Objective verification
7 Possible neural basis
8 The role of meaning
9 Associated cognitive traits
10 Links with other areas of study
11 Artistic investigations
12 Literary depictions
13 People with synesthesia
14 See also
16 Further reading
17 External links
17.1 Scientific resources
17.2 Synesthesia associations
17.3 On the Web
Definitional criteriaAlthough 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. Indeed, most synesthetes report that their experiences are neutral, or even pleasant. 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 (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 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.,). 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 the majority of synesthetes are completely unaware 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 in his first edition book. However, the criteria are different in the second book:
1.Synesthesia is involuntary and automatic.
2.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.
3.Synesthetic percepts are consistent and generic (i.e., simple rather than pictorial).
4.Synesthesia is highly memorable.
5.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 differentiate between "localizers" and "non-localizers" to distinguish those synesthetes whose perceptions have a definite sense of spatial quality.
 ExperiencesSynesthetes 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, as has been documented in interviews with synesthetes on how they discovered synesthesia in their childhood. 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.
Though often stereotyped in the popular media as a medical condition or neurological aberration, many synesthetes themselves do not perceive their synesthetic experiences as a handicap. To the contrary, most report it as a gift—an additional "hidden" sense—something they would not want to miss. Most synesthetes become aware of their "hidden" and different way of perceiving in their childhood. Some have learned how to apply this gift in daily life and work. Synesthetes have used their gift in memorizing names and telephone numbers, mental arithmetic, but also in more complex creative activities like producing visual art, music, and theater.
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 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 (called "projectors"), while most report that the colors are experienced in their "mind's eye" (called "associators"). It is estimated that approximately one or two per hundred grapheme-color synesthetes are projectors; the rest are associators.
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, 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. In summary, self reports, autobiographical notes by synesthetes and interviews show a large variety in types of synesthesia, intensity of the synesthetic perceptions, awareness of the difference in perceiving the physical world from other people, the way they creatively use their synesthesia in work and daily life. The descriptions below give some examples of synesthetes' experiences, which have been experimentally tested, but do not exhaust their rich variety.
 Various formsSynesthesia can occur between nearly any two senses or perceptual modes, and at least one synesthete, Solomon Shereshevsky, experienced synesthesia that linked all five senses. 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 logically possible combination of experiences can occur, several types are more common than others.
 Grapheme → color synesthesiaMain article: Grapheme-color synesthesia
From Wednesday is Indigo Blue. Note this example's upside-down clock face.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 different individuals usually do not report the same colors for all letters and numbers, studies with large numbers of synesthetes find some commonalities across letters (e.g., A is likely to be red).
As a child, Pat Duffy told her father, "I realized that to make an R all I had to do was first write a P and 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." Another grapheme synesthete says, "When I read, about five words around the exact one I'm reading are in color. It's also the only way I can spell. In elementary school I remember knowing how to spell the word 'priority' [with an "i" rather than an "e"] because ... an 'e' was out of place in that word because 'e's were yellow and didn't fit."
 Spatial Sequence SynesthesiaA special form of the condition, in which people tend to see all numerical sequences they come across as points in space. For instance, the number 1 might be farther away and the number 2 might be closer. A new study shows that those with SSS have superior memories. They were able to recall past events and memories far better, and in far greater detail than those without the condition.
 Sound → color synesthesiaAccording to Richard Cytowic, sound → color synesthesia, or chromesthesia is "something like fireworks": voice, music, and assorted environmental sounds such as clattering dishes or dog barks trigger color and firework shapes that arise, move around, and then fade when the sound ends. For some, the stimulus type is limited (e.g., music only, or even just a specific musical key); for others, a wide variety of sounds triggers synesthesia.
Sound often changes the perceived hue, brightness, scintillation, and directional movement. Some individuals see music on a "screen" in front of their faces. Deni Simon, for whom music produces waving lines "like oscilloscope configurations – lines moving in color, often metallic with height, width and, most importantly, depth. My favorite music has lines that extend horizontally beyond the 'screen' area."
Individuals rarely agree on what color a given sound is (composers Liszt and Rimsky-Korsakov famously disagreed on the colors of music keys); however, synesthetes show the same trends as non-synesthetes do. For example, both groups say that loud tones are brighter than soft tones, and that lower tones are darker than higher tones. Synaesthetes nevertheless choose more precise colours than non-synesthetes and are more consistent in their choice of colours given a set of sounds of varying pitch, timbre and composition.
 Number form synesthesiaMain article: Number form
A number form from one of Francis Galton's subjects. Note 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". Later research has identified them as a type of synesthesia. 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. 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.
 PersonificationMain 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. Although this form of synesthesia was documented as early as the 1890s modern research has, until recently, paid little attention to this form.
For example, one synesthete says, "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." Likewise, Cytowic's subject MT says, "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...."
For some people in addition to numbers and other ordinal sequences, objects are sometimes imbued with a sense of personality. Recent research has begun to show that alphanumeric personification co-varies with other forms of synesthesia, and is consistent and automatic, as required to be considered a form of synesthesia.
 Lexical → gustatory synesthesiaMain article: Lexical-gustatory synesthesia
In the rare lexical → gustatory synesthesia, individual words and the phonemes of spoken language evoke taste sensations in the mouth. According to James Wannerton, "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."
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. For example, James Wannerton has no synesthetic experiences of coffee or curry, even though he consumes 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."
 Auditory-tactile synesthesiaAuditory-tactile synesthesia may originate from birth or acquired sometime in life. It is one of the rarest forms of synesthesia. In a 2008 article, Beauchamp and Ro reported on a patient with a rare infarct restricted to the ventrolateral nucleus of her right thalamus. This infarct had initially resulted in a loss of somatosensory sensation on the contralateral half of the patient's body. Although this deficit completely disappeared within a period of 18 months, the patient developed symptoms of auditory–tactile synesthesia, where certain sounds induced intense and often unpleasant somatosensory tingling sensations in her left hand and arm.
 Mirror Touch SynesthesiaIn this rare form of synesthesia, when you see someone else being touched, you feel a touch as well. This means that you can literally feel the pain of others when you see them get hurt. People with mirror-touch synesthesia, often score high on empathy tests compared to people without the condition.
Research historyMain article: History of synesthesia research
The interest in colored hearing dates back to Greek antiquity, when philosophers asked if the color (chroia, what we now call timbre) of music was a quantifiable quality. Isaac Newton proposed that musical tones and color tones shared common frequencies, as did Goethe in his book, "Theory of Color." Despite this idea being false, there is a long history of building color organs such as the clavier à lumières on which to perform colored music in concert halls.
The first medical description of colored hearing is in a German 1812 thesis. The father of psychophysics, Gustav Fechner reported the first empirical survey of colored letter photisms among 73 synesthetes in 1871, followed in the 1880s by Francis Galton. Research into synesthesia proceeded briskly in several countries, but due to the difficulties in measuring subjective experiences and the rise of behaviorism, which made the study of any subjective experience taboo, synesthesia faded into scientific oblivion between 1930 and 1980.
As the 1980s cognitive revolution began to make inquiry into internal subjective states respectable again, scientists once again looked to synesthesia. Led in the United States by Larry Marks and Richard Cytowic, and later in England by Simon Baron-Cohen and Jeffrey Gray, research explored the reality, consistency, and frequency of synesthetic experiences. In the late 1990s, the focus settled on grapheme → color synesthesia, one of the most common and easily studied types. Synesthesia is now the topic of scientific books and papers, Ph.D. theses, documentary films, and even novels.
Since the rise of the Internet in the 1990, synesthetes began contacting one another and creating Web sites devoted to the condition. These early grew into international organizations such as the American Synesthesia Association, the UK Synaesthesia Association, the Belgian Synaesthesia Association, the German Synesthesia Association and the Netherlands Synesthesia Web Community.
Prevalence and genetic basisEarly estimates of prevalence varied widely (from 1 in 20 to 1 in 20,000). These studies all had the methodological shortcoming of relying on self-selection, meaning individuals reporting their experience to investigators. Random population studies later determined that 1 in 23 individuals have some kind of synesthesia, while 1 in 90 have colored graphemes. Colored days of the week and colored graphemes are the most common types.
Many studies noted that synesthesia runs in families, consistent with a genetic origin for the condition. Francis Galton's 1880 report noted a familial component. Studies from the 1990s that noted a much higher prevalence in women than 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 random samples find an equal sex ratio of 1.1:1.
At first, the observed patterns of inheritance were consistent with an X-linked mode of inheritance because there had been no verified reports of father-to-son transmission, whereas father-to-daughter, mother-to-son and mother-to-daughter transmission were readily observed However, the first genome-wide association study failed to find X-linkage, and furthermore verified two cases of father-to-son transmission.
Suggestive of incomplete gene penetrance is the situation of identical twins in which only one member of the pair is synesthetic, and the observation that synesthesia can skip generations within a family. It is furthermore common for family members to experience different types of synesthesia, suggesting that the gene(s) involved do not lead to invariably specific types of synesthesia. Developmental factors such as gene expression and environment must also play a role in determining which types of synesthesia an individual has (for example, children must interact with culturally learned artifacts such as alphabets and food names)
Possible neural basisMain article: Neural basis of synesthesia
Regions thought to be cross-activated in grapheme-color synesthesia (green=grapheme recognition area, red=V4 color area).Dedicated regions of the brain are specialized for given functions. Increased cross-talk between regions specialized for different functions may account for the many types of synesthesia. For example, the additive experience of seeing color when looking at graphemes might be due to cross-activation of the grapheme-recognition area and the color area called V4 (see figure). One line of thinking is that a failure to prune synapses that are normally formed in great excess during the first few years of life may cause such cross-activation.
An alternate possibility is disinhibited feedback, or a reduction in the amount of inhibition along normally existing feedback pathways. Normally, excitation and inhibition are balanced. However, if normal feedback were not inhibited as usual, then signals feeding back from late stages of multi-sensory processing might influence earlier stages such that tones could activate vision. Cytowic & Eagleman find support for the disinhibition idea in the so-called acquired forms of synesthesia that occur in non-synesthetes under certain conditions: Temporal lobe epilepsy, head trauma, stroke, and brain tumors. They also note that it can likewise occur during stages of meditation, deep concentration, sensory deprivation, or with use of psychedelics such as LSD or mescaline, or even, in some cases, marijuana. However, synesthetes report that common stimulants, like caffeine and cigarettes do not affect the strength of their synesthesia, nor do alcoholic beverages.p. 137–140
Functional neuroimaging studies using PET and fMRI demonstrate significant differences between the brains of synesthetes and non-synesthetes. fMRI shows V4 activation in both word → color and grapheme → color synesthetes. Diffusion tensor imaging allows visualization of white matter fiber pathways in the intact brain. This method demonstrates increased connectivity in fusiform gyrus, intraparietal sulcus and frontal cortex in grapheme-color synesthetes. The degree of white matter connectivity in the fusiform gyrus correlates with the intensity of the synesthetic experience.
The role of meaningMain article: Ideasthesia
Evidence has shown that concurrents in synesthesia may be operating at the level of the meaning of the stimulus (i.e. semantic representations), not at the level of the sensory inputs. For example, if presented with letter A, a synesthete would associated concurrent experiences only once the letter has been recognized and the meaning of the stimulus has been extracted. Hence, the basics for understanding synesthesia may be in the semantic structures that, uniquely for synesthetes, associate sensory-like experiences. It has been proposed that a more accurate definition of the phenomenon is within the context of ideasthesia.
 Associated cognitive traitsLittle is known about what, if any, cognitive traits might be associated with synesthesia. As early as 1980, Richard Cytowic first noted mild difficulties in left-right confusion, arithmetic, and sense of direction. These observations await large-scale confirmation. What has been confirmed is elevated, sometimes photographic, memory. It was reading Alexander Luria's 1968 book The Mind of a Mnemonist that alerted Cytowic to the link between synesthesia and enhanced memory: Luria's subject had a 5-fold synesthesia that gave him extra hooks on which to hang and remember numerous facts.
Autism and epilepsy occur with synesthesia more often than chance predicts. Daniel Tammet, the savant who set a European record for reciting the digits of pi, has all three conditions indicating that they might share an underlying genetic cause. Synesthesia has so far been linked to a region on chromosome 2 that is associated with autism and epilepsy.
Synesthetes are likely to participate in creative activities. Individual development of perceptual and cognitive skills, and one's cultural environment likely determine the variety in awareness and practical use of synesthetic skills These are major topics of ongoing research.
Literary depictionsMain article: Synesthesia in literature
Main article: Synesthesia in fiction
Synesthesia is sometimes used as a plot device or way of developing a character's inner life. Author and synesthete Pat Duffy describes five ways in which synesthetic characters have been used in modern fiction.
1.Synesthesia as romantic ideal: in which the condition illustrates the Romantic ideal of transcending one's experience of the world. Books in this category include The Gift by Vladimir Nabokov.
2.Synesthesia as pathology: in which the trait is pathological. Books in this category include The Whole World Over by Julia Glass.
3.Synesthesia as romantic pathology: in which synesthesia is pathological but also provides an avenue to the Romantic ideal of transcending quotidian experience. Books in this category include Holly Payne’s The Sound of Blue.
4.Synesthesia as psychological health and balance: Painting Ruby Tuesday by Jane Yardley, and A Mango-Shaped Space by Wendy Mass.
5.Synesthesia as young adult lterature and science fiction: Ultraviolet by R.J. Anderson
Many literary depictions of synesthesia are not accurate. Some say more about an author's interpretation of synesthesia than the phenomenon itself.
People with synesthesiaMain article: List of people with synesthesia
Determining synesthesia from the historical record is fraught with error unless (auto)biographical sources explicitly give convincing details.
Famous synesthetes include David Hockney, who perceives music as color, shape, and configuration, and who uses these perceptions when painting opera stage sets but not while creating his other artworks. Russian painter Wassily Kandinsky combined four senses: color, hearing, touch, and smell. Vladimir Nabokov describes his grapheme-color synesthesia at length in his autobiography, Speak, Memory and portrays it in some of his characters. Composers include Duke Ellington, Franz Liszt, Nikolai Rimsky-Korsakov, and Olivier Messiaen, whose three types of complex colors are rendered explicitly in musical chord structures that he invented. Physicist Richard Feynman describes his colored equations in his autobiography, What Do You Care What Other People Think?
Other notable synesthetes include musicians Billy Joel,p. 89, 91 Itzhak Perlman,p. 53 Ida Maria, Brian Chase and Patrick Stump; actress Stephanie Carswell (credited as Stéphanie Montreux); inventor Nikola Tesla; electronic musician Richard D. James aka Aphex Twin (who claims to be inspired by lucid dreams as well as music); and classical pianist Hélène Grimaud. Although it has not been verified, Pharrell Williams, of the groups The Neptunes and N.E.R.D., claims to experience synesthesia, and to have used it as the basis of the album Seeing Sounds. Singer/songwriter Marina and the Diamonds experiences music → color synesthesia, and reports colored days of the week.
Some artists frequently mentioned as synesthetes did not in fact have the condition. Alexander Scriabin's 1911 Prometheus, for example, is a deliberate contrivance whose color choices are based on the circle of fifths and appear to have been taken from Madame Blavatsky. The musical score has a separate staff marked luce whose "notes" are played on a color organ. Technical reviews appear in period volumes of Scientific American. On the other hand, his older colleague Nikolai Rimsky-Korsakov (who was perceived as a fairly conservative composer), was in fact a synesthete.
French poets Arthur Rimbaud and Charles Baudelaire wrote of synesthetic experience but there is no evidence they were synesthetes themselves. Baudelaire's 1857 Correspondances (text available here) introduced the notion that the senses can and should intermingle. Baudelaire participated in a hashish experiment by psychiatrist Jacques-Joseph Moreau, and became interested in how the senses might correspond. Rimbaud later wrote Voyelles (1871) (text available here), which was perhaps more important than Correspondances in popularizing synesthesia, although he later boasted "J'inventais la couleur des voyelles!" [I invented the colors of the vowels!].
In addition to being a natural mimic and polyglot, the lauded Cartoon-network voice actor, Brian Hamilton, is a self-ascribed synesthete.
Daniel Tammet wrote a book on his experiences with synesthesia, called Born on a Blue Day.
Joanne Harris, author of Chocolat, is a synesthete who says she experiences colours as scents. Her novel Blueeyedboy features various aspects of synesthesia.
Sean Day, synesthete and the President of the American Synesthesia Association, maintains a list of famous synesthetes, pseudosynesthetes, and non-synesthetes who used synesthesia in their art or music.
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Bosch, P. (2007) The Name of This Book is Secret Little, Brown Young Readers. ISBN 978-0-316-11366-3.
Campen, Cretien van. (2007) The Hidden Sense. Synesthesia in Art and Science. Cambridge, MA: MIT Press/Leonardo Books. ISBN 0-262-22081-4
Cytowic, R.E. (2003)The Man Who Tasted Shapes. Cambridge: MIT Press ISBN 978-0-907845-43-0.
Cytowic, R.E. (2002) Synesthesia: A Union of The Senses, second edition. Cambridge: MIT Press ISBN 978-0-262-03296-4.
Cytowic, R.E. & Eagleman, D.M. (2009) Wednesday is Indigo Blue: Discovering the Brain of Synesthesia, with an afterword by Dmitri Nabokov. Cambridge: MIT Press ISBN 978-0-262-01279-9.
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.
Jay, C. (2009) Breathing in Colour. Little, Brown. ISBN 978-0-7499-2978-7.
Marks L.E., The Unity of the Senses. Interrelations among the modalities, Academic Press, New York, 1978.
Robertson, L. and Sagiv, N. (Eds., 2005). Synesthesia: Perspectives from Cognitive Neuroscience. Oxford: Oxford University Press. ISBN 0-19-516623-X.
Tammet, D. (2006) Born on a Blue Day: A Memoir of Aspergers and an Extraordinary Mind. Hodder & Stoughton Ltd. ISBN 978-0-340-89974-8.
Mass, W. (2003) A Mango-Shaped Space. Little, Brown and Company. ISBN 0-316-52388-7
Ward, J. (2008) The Frog who croaked Blue: Synesthesia and the Mixing of the Senses. Routledge. ISBN 978-0-415-43014-2.
External links Scientific resourcesRichard E. Cytowic Downloads, videos, and information.
David Eagleman's Synesthesia Battery: take the test to see if you are synesthetic.
Houston synesthesia study: Click here for more information.
Synesthetics by Cretien van Campen Artistic and scientific experiments, historical background.
Synaesthesia Research Group at the University of Sussex Information and article links.
Synesthesia in Art and Science Bibliography compiled by Cretien van Campen for Leonardo/ISAST
Blue Cats Resource Center by Patricia Lynne Duffy
 Synesthesia associationsAmerican Synesthesia Association
Australian Synaesthesia Association
Belgian Synesthesia Association
UK Synaesthesia Association
synaesthesia.com: international synaesthesia community (synaesthesia-tests, workshops, Infos)
Russian Synaesthesia Web-community
 On the WebTED talk: "I listen to color"
Synesthesia: What it is and how to diagnose it.
TED Blog, including video links to V. S. Ramachandran's TED talk.
Cytowic's video lecture at the Smithsonian's Hirshhorn Museum Visual Music exhibit. Four-part YouTube version .
Scientific American article Hearing Colors, Tasting Shapes (PDF version) by Ramachandran & Hubbard, May 2003.
Campen, Cretien van (2009), The Hidden Sense: On Becoming Aware of Synesthesia, TECCOGS, vol. 1, pp. 1–13.
Synaisthesis Publishers, a Luxembourgish publishing house with focus on synaesthesia
Red Mondays and Gemstone Jalapeños: The Synesthetic World a documentary short featuring, featuring David Eagleman and four synesthetes, from ResearchChannel.
Lawrence Marks, a pioneering synesthesia researcher, interviewed by Anton Dorso
Danis, Alex. "Grapheme → colour synesthesia". Numberphile. Brady Haran.