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Synesthesia from the ancient Greek (syn), "together," and (aisthēsis), "sensation," is a neurologically based condition in which stimulation

Stimulation

Stimulation is the action of various agents on nerves, muscles, or a sensory end organ, by which activity is evoked; especially, the nervous impulse produced by various agents on nerves, or a sensory end organ, by which the part connected with the nerve is thrown into a state of activity.The word...

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.

In one common form of synesthesia, known as grapheme → color synesthesia

Grapheme-color synesthesia

Grapheme → color synesthesia is a form of synesthesia in which an individual's perception of numbers and letters is associated with the experience of colors. Like all forms of synesthesia, grapheme → color synesthesia is involuntary, consistent, and memorable...

or color-graphemic synesthesia, letter

Letter (alphabet)

A letter is a grapheme in an alphabetic system of writing, such as the Greek alphabet and its descendants. Letters compose phonemes and each phoneme represents a phone in the spoken form of the language....

s or numbers

Numerical digit

A digit is a symbol used in combinations to represent numbers in positional numeral systems. The name "digit" comes from the fact that the 10 digits of the hands correspond to the 10 symbols of the common base 10 number system, i.e...

are perceived as inherently colored, while in ordinal linguistic personification

Ordinal linguistic personification

Ordinal-linguistic personification is a form of synesthesia in which ordered sequences, such as ordinal numbers, days, months and letters are associated with personalities...

, numbers, days of the week and months of the year evoke personalities. In spatial-sequence, or number form

Number form

A number form is a mental map of numbers, which automatically and involuntarily appears whenever someone who experiences number-forms thinks of numbers. Numbers are mapped into distinct spatial locations and the mapping may be different across individuals. Number forms were first documented and...

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 metaphor

Metaphor

A metaphor is a literary figure of speech that uses an image, story or tangible thing to represent a less tangible thing or some intangible quality or idea; e.g., "Her eyes were glistening jewels." Metaphor may also be used for any rhetorical figures of speech that achieve their effects via...

s (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 drug

Psychedelic drug

A psychedelic substance is a psychoactive drug whose primary action is to alter cognition and perception. Psychedelics are part of a wider class of psychoactive drugs known as hallucinogens, a class that also includes related substances such as dissociatives and deliriants...

s, after a stroke

Stroke

A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...

, during a temporal lobe epilepsy

Temporal lobe epilepsy

Temporal lobe epilepsy a.k.a. Psychomotor epilepsy, is a form of focal epilepsy, a chronic neurological condition characterized by recurrent seizures. Over 40 types of epilepsies are known. They fall into two main categories: partial-onset epilepsies and generalized-onset epilepsies...

seizure, or as a result of blindness

Blindness

Blindness is the condition of lacking visual perception due to physiological or neurological factors.Various scales have been developed to describe the extent of vision loss and define 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 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

Psychology

Psychology is the study of the mind and behavior. Its immediate goal is to understand individuals and groups by both establishing general principles and researching specific cases. For many, the ultimate goal of psychology is to benefit society...

research has demonstrated that synesthetic experiences can have measurable behavioral consequences, while functional neuroimaging

Functional neuroimaging

Functional neuroimaging is the use of neuroimaging technology to measure an aspect of brain function, often with a view to understanding the relationship between activity in certain brain areas and specific mental functions...

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.

Definitional criteria

Although sometimes spoken of as a "neurological condition," synesthesia is not listed in either the DSM-IV or the ICD

ICD

The International Statistical Classification of Diseases and Related Health Problems is a medical classification that provides codes to classify diseases and a wide variety of signs, symptoms, abnormal findings, complaints, social circumstances, and external causes of injury or disease...

classifications, since it most often does not interfere with normal daily functioning. It had, however, appeared for many years in both Dorland's and Stedman's medical dictionaries. Indeed, most synesthetes report that their experiences are neutral, or even pleasant. Rather, like color blindness

Color blindness

Color blindness or color vision deficiency is the inability or decreased ability to see color, or perceive color differences, under lighting conditions when color vision is not normally impaired...

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

Richard Cytowic

Richard E. Cytowic is an American neurologist and author who rekindled interest in studying synesthesia in the 1980s. He was nominated for the Pulitzer Prize for his New York Times Magazine cover story about James Brady, the Presidential Press Secretary shot in the brain during the assassination...

identifies the following diagnostic criteria of synesthesia:

Synesthesia is involuntary and automatic
Automaticity
Automaticity is the ability to do things without occupying the mind with the low-level details required, allowing it to become an automatic response pattern or habit. It is usually the result of learning, repetition, and practice....

.
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
Memory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....

.
Synesthesia is laden with affect
Affect (psychology)
Affect refers to the experience of feeling or emotion. Affect is a key part of the process of an organism's interaction with stimuli. The word also refers sometimes to affect display, which is "a facial, vocal, or gestural behavior that serves as an indicator of affect" .The affective domain...

.

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.

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, 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

Sensory overload

Sensory overload , related to Cognitive load in general, is a condition where one or more of the senses are strained and it becomes difficult to focus on the task at hand...

.

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 vowel

Vowel

In phonetics, a vowel is a sound in spoken language, such as English ah! or oh! , pronounced with an open vocal tract so that there is no build-up of air pressure at any point above the glottis. This contrasts with consonants, such as English sh! , where there is a constriction or closure at some...

s are more strongly colored, while for others consonant

Consonant

In articulatory phonetics, a consonant is a speech sound that is articulated with complete or partial closure of the vocal tract. Examples are , pronounced with the lips; , pronounced with the front of the tongue; , pronounced with the back of the tongue; , pronounced in the throat; and ,...

s 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 forms

Synesthesia 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 synesthesia

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

Color

Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...

. 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

Patricia Lynne Duffy

Patricia Lynne Duffy is an instructor in the UN Language and Communications Programme. She has an M.A. from Teachers College, Columbia University...

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."

Sound → color synesthesia

According to Richard Cytowic, sound → color synesthesia 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 face. 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

Franz Liszt

Franz Liszt ; ), was a 19th-century Hungarian composer, pianist, conductor, and teacher.Liszt became renowned in Europe during the nineteenth century for his virtuosic skill as a pianist. He was said by his contemporaries to have been the most technically advanced pianist of his age...

and Rimsky-Korsakov

Nikolai Rimsky-Korsakov

Nikolai Andreyevich Rimsky-Korsakov was a Russian composer, and a member of the group of composers known as The Five.The Five, also known as The Mighty Handful or The Mighty Coterie, refers to a circle of composers who met in Saint Petersburg, Russia, in the years 1856–1870: Mily Balakirev , César...

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

Loudness

Loudness is the quality of a sound that is primarily a psychological correlate of physical strength . More formally, it is defined as "that attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud."Loudness, a subjective measure, is often...

tones are brighter than soft tones, and that lower tones are darker than higher tones.

Number form synesthesia

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

Francis Galton

Sir Francis Galton /ˈfrɑːnsɪs ˈgɔːltn̩/ FRS , cousin of Douglas Strutt Galton, half-cousin of Charles Darwin, was an English Victorian polymath: anthropologist, eugenicist, tropical explorer, geographer, inventor, meteorologist, proto-geneticist, psychometrician, and statistician...

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

Parietal lobe

The parietal lobe is a part of the Brain positioned above the occipital lobe and behind the frontal lobe.The parietal lobe integrates sensory information from different modalities, particularly determining spatial sense and navigation. For example, it comprises somatosensory cortex and the...

that are involved in numerical

Numerical cognition

Numerical cognition is a subdiscipline of cognitive science that studies the cognitive, developmental and neural bases of numbers and mathematics. As with many cognitive science endeavors, this is a highly interdisciplinary topic, and includes researchers in cognitive psychology, developmental...

cognition

Cognition

In science, cognition refers to mental processes. These processes include attention, remembering, producing and understanding language, solving problems, and making decisions. Cognition is studied in various disciplines such as psychology, philosophy, linguistics, and computer science...

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.

Personification

Ordinal-linguistic personification (OLP, or personification for short) is a form of synesthesia in which ordered sequences, such as ordinal number

Ordinal number (linguistics)

In linguistics, ordinal numbers are the words representing the rank of a number with respect to some order, in particular order or position . Its use may refer to size, importance, chronology, etc...

s, days

Week-day names

The names of the days of the week from the Roman period have been both named after the seven planets of classical astronomy and numbered, beginning with Monday. In Slavic languages, a numbering system was adopted, but beginning with Monday. There was an even older tradition of names in Ancient...

, months and letters

Letter (alphabet)

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 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

James Wannerton

James Wannerton from Blackpool, England, experiences Lexical-Gustatory Synaesthesia - he "tastes" words or word sounds.A committee member of the UK Synaesthesia Association, Wannerton has been the subject of detailed research carried out by the University College London and the University of...

, "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."

Research history

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

Clavier à lumières

The clavier à lumières , or tastiéra per luce, as it appears in the score, was a musical instrument invented by Alexander Scriabin for use in his work Prometheus: Poem of Fire. However, only one version of this instrument was constructed, for the performance of Prometheus: Poem of Fire in New York...

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

Psychophysics

Psychophysics quantitatively investigates the relationship between physical stimuli and the sensations and perceptions they effect. Psychophysics has been described as "the scientific study of the relation between stimulus and sensation" or, more completely, as "the analysis of perceptual...

, Gustav Fechner

Gustav Fechner

Gustav Theodor Fechner , was a German experimental psychologist. An early pioneer in experimental psychology and founder of psychophysics, he inspired many 20th century scientists and philosophers...

reported the first empirical survey of colored letter photisms among 73 synesthetes in 1871, followed in the 1880s by Francis Galton

Francis Galton

. Research into synesthesia proceeded briskly in several countries, but due to the difficulties in measuring subjective experiences and the rise of behaviorism

Behaviorism

Behaviorism , also called the learning perspective , is a philosophy of psychology based on the proposition that all things that organisms do—including acting, thinking, and feeling—can and should be regarded as behaviors, and that psychological disorders are best treated by altering behavior...

, which made the study of any subjective experience taboo, synesthesia faded into scientific oblivion between 1930 and 1980.

As the 1980s cognitive revolution

Cognitive revolution

The cognitive revolution is the name for an intellectual movement in the 1950s that began what are known collectively as the cognitive sciences. It began in the modern context of greater interdisciplinary communication and research...

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

Richard Cytowic

, and later in England by Simon Baron-Cohen

Simon Baron-Cohen

Simon Baron-Cohen FBA is professor of Developmental Psychopathology in the Departments of Psychiatry and Experimental Psychology at the University of Cambridge in the United Kingdom. He is the Director of the University's Autism Research Centre, and a Fellow of Trinity College...

and Jeffrey Gray

Jeffrey Alan Gray

Jeffrey Alan Gray was a British psychologist. He was born in the East End of London. His father was a tailor, but died when Jeffrey was only seven. His mother, who ran a haberdashery, brought him up alone....

, 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

American Synesthesia Association

The American Synesthesia Association is a not-for-profit academic and public society whose mission is to foster and promote the education and the advancement of knowledge of the phenomena of synesthesia, a neurological condition in which stimulation in one sensory modality leads to experiences in...

, the UK Synaesthesia Association

UK Synaesthesia Association

The UK Synaesthesia Association was originally founded by Professor Simon Baron-Cohen, Professor of Developmental Psychopathology at Cambridge University and a leading researcher into synaesthesia.-Activities:...

, the Belgian Synaesthesia Association, the German Synesthesia Association and the Netherlands Synesthesia Web Community.

Prevalence and genetic basis

Early 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

Self-selection

In statistics, self-selection bias arises in any situation in which individuals select themselves into a group, causing a biased sample with nonprobability sampling...

, 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

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

Biased sample

In statistics, sampling bias is when a sample is collected in such a way that some members of the intended population are less likely to be included than others. It results in a biased sample, a non-random sample of a population in which all individuals, or instances, were not equally likely to...

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

Sex linkage

Sex linkage is the phenotypic expression of an allele related to the chromosomal sex of the individual. This mode of inheritance is in contrast to the inheritance of traits on autosomal chromosomes, where both sexes have the same probability of inheritance...

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

Genome-wide association study

In genetic epidemiology, a genome-wide association study , also known as whole genome association study , is an examination of many common genetic variants in different individuals to see if any variant is associated with a trait...

failed to find X-linkage, and furthermore verified two cases of father-to-son transmission.

Suggestive of incomplete gene penetrance

Penetrance

Penetrance in genetics is the proportion of individuals carrying a particular variant of a gene that also express an associated trait . In medical genetics, the penetrance of a disease-causing mutation is the proportion of individuals with the mutation who exhibit clinical symptoms...

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

Gene expression

Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...

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).

Objective verification

The simplest approach is test-retest reliability over long periods of time, where synesthetes consistently score much higher—around 90% after years, compared to 30–40% after just a month in non-synesthetes even when they are warned they will be retested—using stimuli of color names, color chips, or a computer-screen color picker providing 16.7 million choices.

Modified versions of the Stroop effect

Stroop effect

Purple Blue Purple----Blue Purple RedGreen Purple Green----the Stroop effect refers to the fact that naming the color of the first set of words is easier and quicker than the second....

are popular. In the standard paradigm, it is harder to name the ink color of the word "red," for example, when it is printed in blue ink than when the ink is red. Similarly, if a grapheme → color synesthete is shown the digit 4 (which he sees as red, say) in blue ink, he is slower to name the ink color than when it is printed in red. He sees the blue ink, but the same sort of conflict responsible for the standard Stroop effect occurs between the ink color and the automatic

Automaticity

Automaticity is the ability to do things without occupying the mind with the low-level details required, allowing it to become an automatic response pattern or habit. It is usually the result of learning, repetition, and practice....

synesthetic color of the grapheme. The conflict is strongest when the ink color is the opponent color

Opponent process

The color opponent process is a color theory that states that the human visual system interprets information about color by processing signals from cones and rods in an antagonistic manner...

to the synesthetic one (e.g., red vs. green), indicating that synesthetic color perception uses the same mechanism as the perception of real colors.

Cross-sensory Stroop tests are possible: for example, a music → color synesthete must name a red swatch while listening to a sound that produces a blue sensation, or a musical key → taste synesthete must identify a bitter taste while hearing a musical interval that tastes sweet . Likewise, Stroop tests work even in those for whom merely thinking about a numeral elicits color. Take a person who sees 7 as yellow and 9 as blue, and make the task one of having to say a math solution out loud followed by naming a color square. In the illustration, having to answer “7” and then “yellow” is congruent with the subject’s synesthesia, which unconsciously primes him to respond faster than controls. The automatic blueness of 9, however, interferes with naming the green square, slowing him down compared to controls.

Synesthetic colors can also improve performance for some synesthetes. Inspired by tests for color blindness

Color blindness

Color blindness or color vision deficiency is the inability or decreased ability to see color, or perceive color differences, under lighting conditions when color vision is not normally impaired...

, Ramachandran

Vilayanur S. Ramachandran

Vilayanur Subramanian "Rama" Ramachandran, born 1951, is a neuroscientist known for his work in the fields of behavioral neurology and visual psychophysics...

and Hubbard presented synesthetes and non-synesthetes with a matrix of 5s in which embedded 2s formed a hidden pattern such as a square, diamond, rectangle or triangle. For someone who sees 2s as red and 5s as green, for example, synesthetic colors help zero in on the embedded figure. Subsequent careful studies have found substantial variability among synesthetes in their ability to do this. It certainly does not happen instantaneously; while synesthesia is evoked very early in perceptual processing, it does not occur prior to attention

Attention

Attention is the cognitive process of paying attention to one aspect of the environment while ignoring others. Attention is one of the most intensely studied topics within psychology and cognitive neuroscience....

Possible neural basis

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

Synaptic pruning

In neuroscience, synaptic pruning, neuronal pruning or axon pruning refer to neurological regulatory processes, which facilitate a change in neural structure by reducing the overall number of neurons or connections, leaving more efficient synaptic configurations. Pruning is a neurological...

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

Temporal lobe epilepsy

, head trauma, stroke, and brain tumors. It can likewise occur during stages of meditation, deep concentration, sensory deprivation

Sensory deprivation

Sensory deprivation or perceptual isolation is the deliberate reduction or removal of stimuli from one or more of the senses. Simple devices such as blindfolds or hoods and earmuffs can cut off sight and hearing respectively, while more complex devices can also cut off the sense of smell, touch,...

, or with use of psychedelics

Psychedelics, dissociatives and deliriants

This general group of pharmacological agents can be divided into three broad categories: psychedelics, dissociatives, and deliriants. These classes of psychoactive drugs have in common that they can cause subjective changes in perception, thought, emotion and consciousness...

such as LSD

LSD

Lysergic acid diethylamide, abbreviated LSD or LSD-25, also known as lysergide and colloquially as acid, is a semisynthetic psychedelic drug of the ergoline family, well known for its psychological effects which can include altered thinking processes, closed and open eye visuals, synaesthesia, an...

or mescaline

Mescaline

Mescaline or 3,4,5-trimethoxyphenethylamine is a naturally occurring psychedelic alkaloid of the phenethylamine class used mainly as an entheogen....

, certain prescription medications or even, in some cases, marijuana.

Functional neuroimaging

Functional neuroimaging

studies using PET

Positron emission tomography

Positron emission tomography is nuclear medicine imaging technique that produces a three-dimensional image or picture of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide , which is introduced into the body on a...

and fMRI

Functional magnetic resonance imaging

Functional magnetic resonance imaging or functional MRI is a type of specialized MRI scan used to measure the hemodynamic response related to neural activity in the brain or spinal cord of humans or other animals. It is one of the most recently developed forms of neuroimaging...

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

White matter

White matter is one of the two components of the central nervous system and consists mostly of myelinated axons. White matter tissue of the freshly cut brain appears pinkish white to the naked eye because myelin is composed largely of lipid tissue veined with capillaries. Its white color is due to...

fiber pathways in the intact brain. This method demonstrates increased connectivity in fusiform gyrus

Fusiform gyrus

The fusiform gyrus is part of the temporal lobe in Brodmann Area 37. It is also known as the occipitotemporal gyrus. Other sources have the fusiform gyrus above the occipitotemporal gyrus and underneath the parahippocampal gyrus....

, intraparietal sulcus

Intraparietal sulcus

The intraparietal sulcus is located on the lateral surface of the parietal lobe, and consists of an oblique and a horizontal portion. The IPS contains a series of functionally distinct subregions that have been intensively investigated using both single cell neurophysiology in primates and human...

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.

Associated cognitive traits

Little is known about what, if any, cognitive traits might be associated with synesthesia. As early as 1980, Richard Cytowic

Richard Cytowic

first noted mild difficulties in left-right confusion, arithmetic

Dyscalculia

Dyscalculia is a specific learning disability involving innate difficulty in learning or comprehending simple arithmetic. It is akin to dyslexia and includes difficulty in understanding numbers, learning how to manipulate numbers, learning maths facts, and a number of other related symptoms...

, and sense of direction. These observations await large-scale confirmation. What has been confirmed is elevated, sometimes photographic

Eidetic memory

Eidetic , commonly referred to as photographic memory, is a medical term, popularly defined as the ability to recall images, sounds, or objects in memory with extreme precision and in abundant volume. The word eidetic, referring to extraordinarily detailed and vivid recall not limited to, but...

, memory. When asked, "What good is it?" synesthetes say, "It helps me remember." Indeed, it was reading Alexander Luria

Alexander Luria

Alexander Romanovich Luria was a famous Soviet neuropsychologist and developmental psychologist. He was one of the founders of neuropsychology and the jointly led the Vygotsky Circle.- Biography :...

's 1968 book The Mind of a Mnemonist that alerted Cytowic to the link between synesthesia and elevated 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

Daniel Tammet

Daniel Tammet is a British writer. His best selling 2006 memoir, Born On A Blue Day, about his life with high-functioning autism and savant syndrome, was named a "Best Book for Young Adults" in 2008 by the American Library Association.Tammet's second book, Embracing the Wide Sky, was named one of...

, the savant who set a European record for reciting the digits of pi

' is a mathematical constant that is the ratio of any circle's circumference to its diameter. is approximately equal to 3.14. Many formulae in mathematics, science, and engineering involve , which makes it one of the most important mathematical constants...

, 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

Creativity

Creativity refers to the phenomenon whereby a person creates something new that has some kind of value. What counts as "new" may be in reference to the individual creator, or to the society or domain within which the novelty occurs...

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.

Links with other areas of study

Researchers study synesthesia not only because it is inherently interesting, but also because studying it can offer insights into other questions, such as how the brain combines information from different sensory modalities, referred to as crossmodal

Crossmodal

Crossmodal perception or cross-modal perception is perception that involves interactions between two or more different sensory modalities. Examples include synesthesia, sensory substitution, and the McGurk effect, in which vision and hearing interact in speech perception.Crossmodal perception,...

perception and multisensory integration.

An example of this is the bouba/kiki effect

Bouba/kiki effect

The Bouba/Kiki Effect is a non-arbitrary mapping between speech sounds and the visual shape of objects. This effect was first observed by German-American psychologist Wolfgang Köhler in 1929...

. In an experiment first designed by Wolfgang Köhler

Wolfgang Köhler

Wolfgang Köhler was a German psychologist and phenomenologist who, like Max Wertheimer, and Kurt Koffka, contributed to the creation of Gestalt psychology.-Early life:...

, people are asked to choose which of two shapes is named bouba and which kiki. 95% to 98% of people choose kiki for the angular shape and bouba for the rounded one. Individuals on the island of Tenerife

Tenerife

Tenerife is the largest and most populous island of the seven Canary Islands, it is also the most populated island of Spain, with a land area of 2,034.38 km² and 906,854 inhabitants, 43% of the total population of the Canary Islands. About five million tourists visit Tenerife each year, the...

showed a similar preference between shapes called takete and maluma. Even 2.5 year-old children (too young to read) show this effect.

Ramachandran and Hubbard suggest the kiki/bouba effect has implications for the evolution of language, because the naming of objects is not completely arbitrary. The rounded shape may intuitively 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 articulate kiki. The sound of K is also harder and more forceful than that of B. Such "synesthesia-like mappings" suggest that this effect might be the neurological basis for sound symbolism

Sound symbolism

Sound symbolism or phonosemantics is a branch of linguistics and refers to the idea that vocal sounds have meaning. In particular, sound symbolism is the idea that phonemes carry meaning in and of themselves.-Origin:...

, in which sounds are non-arbitrarily mapped to objects and actions in the world.

Given synesthetes' extraordinary conscious experiences, researchers hope that their study will provide better understanding of consciousness

Consciousness

Consciousness is a term that refers to the relationship between the mind and the world with which it interacts. It has been defined as: subjectivity, awareness, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind...

and its neural correlate

Neural correlate

A neural correlate of a content of experience is any bodily component, such as an electro-neuro-biological state or the state assumed by some biophysical subsystem of the brain, whose presence necessarily and regularly correlates with such a specific content of experience.When the full ontological...

s, meaning what the brain mechanisms that make us conscious might be. In particular, synesthesia might be relevant to the philosophical

Philosophy

Philosophy is the study of general and fundamental problems, such as those connected with existence, knowledge, values, reason, mind, and language. Philosophy is distinguished from other ways of addressing such problems by its critical, generally systematic approach and its reliance on rational...

problem of qualia

Qualia

Qualia , singular "quale" , from a Latin word meaning for "what sort" or "what kind," is a term used in philosophy to refer to subjective conscious experiences as 'raw feels'. Examples of qualia are the pain of a headache, the taste of wine, the experience of taking a recreational drug, or the...

, given that synesthetes experience extra qualia (e.g., a colored sound).

Artistic investigations

The word "synesthesia" has been used for 300 years to describe very different things, from poetry and metaphor to deliberately contrived mixed-media applications such as son et lumière

Son et lumière

Son et lumière may refer to:*Son et lumière , a sound and light show*"Son et Lumiere", song by The Mars Volta on the album De-Loused in the Comatorium...

shows or odorama

Polyester (film)

Polyester is a 1981 comedy film directed, produced, and written by John Waters, and starring Divine, Tab Hunter, Edith Massey, and Mink Stole...

. It is crucial to separate artists using synesthesia as an intellectual idea—pseudo-synesthetes such as Georgia O'Keeffe

Georgia O'Keeffe

Georgia Totto O'Keeffe was an American artist.Born near Sun Prairie, Wisconsin, O'Keeffe first came to the attention of the New York art community in 1916, several decades before women had gained access to art training in America’s colleges and universities, and before any of its women artists...

who used such titles as "Music-Pink and Blue"—from those who had the genuine perceptual variety, such as Wassily Kandinsky

Wassily Kandinsky

Wassily Wassilyevich Kandinsky was an influential Russian painter and art theorist. He is credited with painting the first purely-abstract works. Born in Moscow, Kandinsky spent his childhood in Odessa. He enrolled at the University of Moscow, studying law and economics...

or Olivier Messiaen

Olivier Messiaen

Olivier Messiaen was a French composer, organist and ornithologist, one of the major composers of the 20th century. His music is rhythmically complex ; harmonically and melodically it is based on modes of limited transposition, which he abstracted from his early compositions and improvisations...

.

Synesthetic art historically refers to multi-sensory experiments in the genres of visual music

Visual music

Visual music, sometimes called "colour music," refers to the use of musical structures in visual imagery, which can also include silent films or silent Lumia work. It also refers to methods or devices which can translate sounds or music into a related visual presentation...

, music visualization

Music visualization

Music visualization, a feature found in electronic music visualizers and media player software, generates animated imagery based on a piece of music...

, audiovisual art

Audiovisual art

The exploration of kinetic abstract art and music set in relation to each other.Visual music, abstract film, audiovisual performances and installations.- See also :* Abstract film* Color organ* Experimental film* Sound art* Sound installation* Sound sculpture...

, abstract film

Abstract film

Abstract film is a subgenre of experimental film. Its history often overlaps with the concerns and history of visual music. Some of the earliest abstract motion pictures known to survive are those produced by a group of German artists working in the early 1920s, a movement referred to as Absolute...

, and intermedia

Intermedia

Intermedia was a concept employed in the mid-sixties by Fluxus artist Dick Higgins to describe the ineffable, often confusing, inter-disciplinary activities that occur between genres that became prevalent in the 1960s. Thus, the areas such as those between drawing and poetry, or between painting...

. Distinct from neuroscience, the concept of synesthesia in the arts is regarded as the simultaneous perception of multiple stimuli in one gestalt

Gestalt psychology

Gestalt psychology or gestaltism is a theory of mind and brain of the Berlin School; the operational principle of gestalt psychology is that the brain is holistic, parallel, and analog, with self-organizing tendencies...

experience. Only recently can science verify and study synesthesia in artists; for deceased artists, one must interpret (auto)biographical information.

Synesthetic art can refer to either art created by synesthetes or art that attempts to convey the synesthetic experience. It is an attempt to understand the relation between the experiences of born synesthetes, non-synesthetes, and an appreciation of such art by both groups. These distinctions are not mutually exclusive given that art by a synesthete might also evoke synesthesia-like experiences in the viewer.

Contemporary synesthetic artists such as Carol Steen

Carol Steen

Carol Steen is an artist, writer, curator, and synesthete who lives and works in New York. She has had over 20 solo gallery exhibitions, her first solo exhibition was at the Detroit Institute of Arts, and her work has been in over 50 group exhibitions including shows at the Philadelphia Museum of...

and Marcia Smilack have described in detail how they use their synesthesia to create their artworks. They demonstrate the complex interplay between personal experience and artistic creation.

Synesthesia has been a source of inspiration for artists, composers, poets, novelists, and digital artists. Nabokov

Vladimir Nabokov

Vladimir Vladimirovich Nabokov was a multilingual Russian novelist and short story writer. Nabokov wrote his first nine novels in Russian, then rose to international prominence as a master English prose stylist...

writes explicitly about synesthesia in several novels. Kandinsky

Wassily Kandinsky

(a synesthete) and Mondrian (not a synesthete) both experimented with image-music correspondences in their paintings. Scriabin

Alexander Scriabin

Alexander Nikolayevich Scriabin was a Russian composer and pianist who initially developed a lyrical and idiosyncratic tonal language inspired by the music of Frédéric Chopin. Quite independent of the innovations of Arnold Schoenberg, Scriabin developed an increasingly atonal musical system,...

composed color music that was deliberately contrived and based on the circle of fifths

Circle of fifths

In music theory, the circle of fifths shows the relationships among the 12 tones of the chromatic scale, their corresponding key signatures, and the associated major and minor keys...

, whereas Messiaen

Olivier Messiaen

invented a new method of composition (the modes of limited transposition

Modes of limited transposition

Modes of limited transposition are musical modes or scales that fulfill specific criteria relating to their symmetry and the repetition of their interval groups...

) to specifically render his bi-directional sound-color synesthesia. For example, the red rocks of Bryce Canyon

Bryce Canyon National Park

Bryce Canyon National Park is a national park located in southwestern Utah in the United States. The major feature of the park is Bryce Canyon which, despite its name, is not a canyon but a giant natural amphitheater created by erosion along the eastern side of the Paunsaugunt Plateau...

are depicted in his symphony Des canyons aux étoiles ("From the Canyons to the Stars"). New art movements such as literary symbolism, non-figurative art, and visual music have profited from experiments with synesthetic perception and contributed to the public awareness of synesthetic and multi-sensory ways of perceiving.

Literary depictions

Synesthesia is sometimes used as a plot device or way of developing a character's inner life. Author and synesthete Pat Duffy

Patricia Lynne Duffy

Patricia Lynne Duffy is an instructor in the UN Language and Communications Programme. She has an M.A. from Teachers College, Columbia University...

describes four ways in which synesthetic characters have been used in modern fiction.

Synesthesia as Romantic
Romanticism
Romanticism was an artistic, literary and intellectual movement that originated in the second half of the 18th century in Europe, and gained strength in reaction to the Industrial Revolution...

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
Vladimir Nabokov
Vladimir Vladimirovich Nabokov was a multilingual Russian novelist and short story writer. Nabokov wrote his first nine novels in Russian, then rose to international prominence as a master English prose stylist...

.
Synesthesia as pathology: in which the trait is pathological. Books in this category include The Whole World Over by Julia Glass
Julia Glass
Julia Glass is an American novelist. Her debut novel, Three Junes, won the National Book Award in 2002. Glass followed this with a second novel, The Whole World Over, in 2006, which was also set in the Bank Street, Greenwich Village universe with three interwoven stories featuring several...

.
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.
Synesthesia as psychological health and balance: Painting Ruby Tuesday by Jane Yardley
Jane Yardley
Jane Yardley is an English author, raised in a village in 1960s Essex, . She went to university in London and gained a Ph.D. degree from Charing Cross Hospital Medical School...

, and A Mango-Shaped Space
A Mango-Shaped Space
A Mango-Shaped Space is a novel by Wendy Mass. It is about Mia Winchell, a thirteen-year-old girl living with synesthesia. Her synesthesia causes her problems in school, with friends, and winning the understanding of her parents and peers. The book received the American Library Association...

by Wendy Mass
Wendy Mass
Wendy Mass , is an award-winning author of young-adult novels and children's books. Her most successful book was A Mango-Shaped Space which won the American Library Association Schneider Family Book Award for Middle School in 2004...

.
Synesthesia as Young Adult Literature 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 synesthesia

Determining synesthesia from the historical record is fraught with error unless (auto)biographical sources explicitly give convincing details.

Famous synesthetes include David Hockney

David Hockney

David Hockney, CH, RA, is an English painter, draughtsman, printmaker, stage designer and photographer, who is based in Bridlington, Yorkshire and Kensington, London....

, 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

Wassily Kandinsky

combined four senses: color, hearing, touch, and smell. Vladimir Nabokov

Vladimir Nabokov

describes his grapheme-color synesthesia at length in his autobiography, Speak, Memory
Speak, Memory
Speak, Memory is an autobiographical memoir by writer Vladimir Nabokov.-Scope:The book is dedicated to his wife, Véra, and covers his life from 1903 until his emigration to America in 1940. The first twelve chapters describe Nabokov's remembrance of his youth in an aristocratic family living in...

and portrays it in some of his characters. Composers include Duke Ellington

Duke Ellington

Edward Kennedy "Duke" Ellington was an American composer, pianist, and big band leader. Ellington wrote over 1,000 compositions...

, Franz Liszt

Franz Liszt

, Nikolai Rimsky-Korsakov

Nikolai Rimsky-Korsakov

, and Olivier Messiaen

Olivier Messiaen

, whose three types of complex colors are rendered explicitly in musical chord structures that he invented. Physicist Richard Feynman

Richard Feynman

Richard Phillips Feynman was an American physicist known for his work in the path integral formulation of quantum mechanics, the theory of quantum electrodynamics and the physics of the superfluidity of supercooled liquid helium, as well as in particle physics...

describes his colored equations in his autobiography, What Do You Care What Other People Think?

Other notable synesthetes include musicians Billy Joel

Billy Joel

William Martin "Billy" Joel is an American musician and pianist, singer-songwriter, and classical composer. Since releasing his first hit song, "Piano Man", in 1973, Joel has become the sixth best-selling recording artist and the third best-selling solo artist in the United States, according to...

,^{p. 89, 91} Itzhak Perlman

Itzhak Perlman

Itzhak Perlman is an Israeli-born violinist, conductor, and instructor of master classes. He is regarded as one of the pre-eminent violinists of the 20th and early-21st centuries.-Early life:...

,^p. 53 Ida Maria

Ida Maria

Ida Maria Børli Sivertsen is a Norwegian rock musician known primarily by the stage name Ida Maria. Her first album; Fortress Round My Heart was released in 2008...

, Brian Chase

Brian Chase

Brian Chase is an American drummer playing in the New York rock band Yeah Yeah Yeahs. He was ranked at #50 in Gigwise's list of The Greatest Drummers of All Time. He plays drums with traditional grip.-Early life:...

and Patrick Stump

Patrick Stump

Patrick Vaughn Stump is an American singer-songwriter, composer, record producer, and music critic. He is the composer, lead singer, and multi-instrumentalist of Fall Out Boy, an American rock band from Wilmette, Illinois, and is also a solo artist...

; actress Stephanie Carswell

Stephanie Carswell

Stephanie Carswell is an Australian actress and lyric soprano, with a vocal range of over 3½ octaves. Commonly credited as Stéphanie Montreux. She is the elder sister of Jennifer Carswell.-Life and career:...

(credited as Stéphanie Montreux

Stéphanie Montreux

Stéphanie Montreux was born Stéphanie Shoshanna May Montreux Carswell on 19 June 1985 in Melbourne, Australia. Sometimes credited as Stephanie Carswell / Stéphanie May Carswell / Shoshanna May. She is the older sister of Jennifer Carswell.-Life and career:...

); inventor Nikola Tesla

Nikola Tesla

Nikola Tesla was a Serbian-American inventor, mechanical engineer, and electrical engineer...

; electronic musician Richard D. James aka Aphex Twin

Aphex Twin

Richard David James , best known under the pseudonym Aphex Twin, is an Irish-born electronic musician and composer described as "the most inventive and influential figure in contemporary electronic music"...

(who claims to be inspired by lucid dreams as well as music); and classical pianist Hélène Grimaud

Hélène Grimaud

Hélène Grimaud is a French pianist.-Biography:Grimaud was born in Aix-en-Provence, France. Although her autobiography Variations Sauvages suggests a...

. Founder of Pink Floyd

Pink Floyd

Pink Floyd were an English rock band that achieved worldwide success with their progressive and psychedelic rock music. Their work is marked by the use of philosophical lyrics, sonic experimentation, innovative album art, and elaborate live shows. Pink Floyd are one of the most commercially...

, Syd Barrett

Syd Barrett

Syd Barrett , born Roger Keith Barrett, was an English singer-songwriter, guitarist, and painter, best remembered as a founding member of the band Pink Floyd. He was the lead vocalist, guitarist and primary songwriter during the band's psychedelic years, providing major musical and stylistic...

, is thought to have had synesthesia. Harpist and fiddler Tina Larkin experiences Music/color synesthesia. Although it has not been verified, Pharrell Williams

Pharrell Williams

Pharrell Williams , commonly known simply as Pharrell, is an American rapper, singer, record producer, composer, and fashion designer. Williams and Chad Hugo make up the record production duo The Neptunes, producing hip hop and R&B music...

, of the groups The Neptunes

The Neptunes

The Neptunes are a record production duo consisting of Pharrell Williams and Chad Hugo, who are credited with contributing the sound for many successful hip hop, R&B and pop artists in the late-1990s and 2000s...

and N.E.R.D., claims to experience synesthesia, and to have used it as the basis of the album Seeing Sounds

Seeing Sounds

Seeing Sounds is the third studio album by American funk rock band N.E.R.D released June 10, 2008 on Star Trak Entertainment and Interscope Records in the United States. After ending their contract with Virgin Records in 2005, the band felt their previous album Fly or Die was too consistent. Using...

. 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

Alexander Scriabin

's 1911 Prometheus

Prometheus: Poem of Fire

Prometheus: The Poem of Fire, Op. 60 , is a symphonic work by the Russian composer Alexander Scriabin for piano, orchestra, optional choir, and clavier à lumières or "Chromola" . However, the clavier à lumières is rarely featured in the performance of the piece, including performances during...

, for example, is a deliberate contrivance whose color choices are based on the circle of fifths

Circle of fifths

In music theory, the circle of fifths shows the relationships among the 12 tones of the chromatic scale, their corresponding key signatures, and the associated major and minor keys...

and appear to have been taken from Madame Blavatsky

Madame Blavatsky

Helena Petrovna Blavatsky , was a theosophist, writer and traveler. Between 1848 and 1875 Blavatsky had gone around the world three times. In 1875, Blavatsky together with Colonel H. S. Olcott established the Theosophical Society...

. The musical score has a separate staff marked luce whose "notes" are played on a color organ

Color organ

The term color organ refers to a tradition of mechanical , then electromechanical, devices built to represent sound or to accompany music in a visual medium—by any number of means. In the early 20th century, a silent color organ tradition developed...

. 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

Arthur Rimbaud

Jean Nicolas Arthur Rimbaud was a French poet. Born in Charleville, Ardennes, he produced his best known works while still in his late teens—Victor Hugo described him at the time as "an infant Shakespeare"—and he gave up creative writing altogether before the age of 21. As part of the decadent...

and Charles Baudelaire

Charles Baudelaire

Charles Baudelaire was a French poet who produced notable work as an essayist, art critic, and pioneering translator of Edgar Allan Poe. His most famous work, Les Fleurs du mal expresses the changing nature of beauty in modern, industrializing Paris during the nineteenth century...

wrote of synesthetic experience but there is no evidence they were synesthetes themselves. Baudelaire's 1857 (text available here) introduced the notion that the senses can and should intermingle. Baudelaire participated in a hashish experiment by psychiatrist Jacques-Joseph Moreau

Jacques-Joseph Moreau

Dr. Jacques-Joseph Moreau was a French psychiatrist and member of the Club des Hashischins. Moreau was the first physician to do systematic work on drugs' effects on the central nervous system, and to catalogue, analyze, and record his observations. His 1845 book, Hashish and Mental Alienation,...

, and became interested in how the senses might correspond. Rimbaud later wrote Voyelles (1871) (text available here), which was perhaps more important than in popularizing synesthesia, although he later boasted "J'inventais la couleur des voyelles!" [I invented the colors of the vowels!].

Sean Day, synesthete and the President of the American Synesthesia Association

American Synesthesia Association

, maintains a list of famous synesthetes, pseudosynesthetes, and non-synesthetes who used synesthesia in their art or music.

Synthetic synesthesia

In the late 1970s and early 1980s Steve Mann

Steve Mann

Steven Mann , is a tenured professor at the Department of Electrical and Computer Engineering at the University of Toronto.-Education:...

began to experiment with the use of wearable computing to deliberately create useful forms of synesthesia, by, for example, mapping certain senses to other senses (e.g. sights to sounds or feelings, etc., to help the visually impaired). Additionally, Mann regarded the Internet as what he called a "SixthSense" that can be mapped to the other five senses by way of such synthetic synesthesia (e.g. neckworn instrumentation mapping cyberspace to one or more of the other five senses). He named this system "SyntheticSynesthesia of the SixthSense", which is often abbreviated as simply SixthSense.

Synesthesia

Definitional criteria

Experiences

Various forms

Grapheme → color synesthesia

Sound → color synesthesia

Number form synesthesia

Personification

Lexical → gustatory synesthesia

Research history

Prevalence and genetic basis

Objective verification

Possible neural basis

Associated cognitive traits

Links with other areas of study

Artistic investigations

Literary depictions

People with synesthesia

Synthetic synesthesia

Further reading

Scientific resources

Synesthesia associations

On the Web