Synesthesia
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How someone with synesthesia might perceive certain letters and numbers.Synesthesia (also spelled synæsthesia or synaesthesia, plural synesthesiae or synaesthesiae)—from the Ancient Greek σύν (syn), "together," and αἴσθησις (aisthēsis), "sensation"—is a neurologically-based condition in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway.[1][2][3][4] People who report such experiences are known as synesthetes.
In one common form of synesthesia, known as grapheme → color synesthesia or color-graphemic synesthesia, letters or numbers are perceived as inherently colored,[5][6] while in ordinal linguistic personification, numbers, days of the week and months of the year evoke personalities.[7][8] 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).[9][10][11] Yet another recently identified type, visual motion → sound synesthesia, involves hearing sounds in response to visual motion and flicker.[12] Over 60 types of synesthesia have been reported by people,[13] but only a fraction have been evaluated by scientific research.[14] Even within one type, synesthetic perceptions vary in intensity[15] and people vary in awareness of their synesthetic perceptions.[16]
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.[17] 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 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.[18] Psychological research has demonstrated that synesthetic experiences can have measurable behavioral consequences, while functional neuroimaging studies have identified differences in patterns of brain activation.[6] 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.
[edit] Definitional criteria
Although sometimes spoken of as a "neurological condition" synesthesia is not listed in either the DSM-IV or the ICD classifications, since it most often does not interfere with normal daily functioning. It has, however, appeared for many years in both Dorland's and Steadman's medical dictionaries. Indeed most synesthetes report that their experiences are neutral, or even pleasant.[19] Rather, like color blindness or perfect pitch, synesthesia is a difference in perceptual experience and the term "neurological" simply reflects the brain basis of this perceptual difference (see below for associated cognitive traits).
It was once assumed that synesthetic experiences were entirely different from synesthete to synesthete, but recent research has shown that there are underlying similarities that can be observed when large numbers of synesthetes are examined together. For example, sound-color synesthetes, as a group, tend to see lighter colors for higher sounds[20] and grapheme-color synesthetes, as a group, share significant preferences for the color of each letter (e.g., A tends to be red; O tends to be white or black; S tends to be yellow etc.,[19][21][22]). Nonetheless, there are a great number of types of synesthesia, and within each type, individuals can report differing triggers for their sensations, and differing intensities of experiences. This variety means that defining synesthesia in an individual is difficult, and indeed, the majority of synesthetes are completely unaware that their experiences have a name.[19] However, despite the differences between individuals, there are a few common elements that define a true synesthetic experience.
Neurologist Richard Cytowic identifies the following diagnostic criteria of synesthesia:[1][2][3]
- Synesthesia is involuntary and automatic.
- Synesthetic perceptions are spatially extended, meaning they often have a sense of "location." For example, synesthetes speak of "looking at" or "going to" a particular place to attend to the experience.
- Synesthetic percepts are consistent and generic (i.e., simple rather than pictorial).
- Synesthesia is highly memorable.
- Synesthesia is laden with affect.
Cytowic's early cases included individuals whose synesthesia was frankly projected outside the body (e.g., on a "screen" in front of one's face). Later research showed that such stark externalization occurs in a minority of synesthetes. Refining this concept, Cytowic and Eagleman[3] differentiate between "localizers" and "non-localizers" to distinguish those synesthetes whose perceptions have a definite sense of spatial quality.
[edit] 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.[14] 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.[19]
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.[14]
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[23] 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").[24] It is estimated that approximately one or two per hundred grapheme-color synesthetes are projectors; the rest are associators.[24]
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,[15] 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.[19] 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.[14][25] The descriptions below give some examples of synesthetes' experiences, which have been experimentally tested, but do not exhaust their rich variety.
[edit] 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.
[edit] Grapheme → color synesthesia
Main article: Grapheme-color synesthesiaIn 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).[19][21]
As a child, Pat Duffy told her Dad, "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."[26]
[edit] 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 simple shapes that arise, move around, and then fade when the sound stimulus ends.[3] 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."[3]
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.
[edit] Number form synesthesia
Main article: Number form
A number form from one of Francis Galton's subjects.[9] 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".[27] Later research has identified them as a type of synesthesia.[10][11] 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.[28][29] 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.
[edit] Personification
Main article: Ordinal linguistic personificationOrdinal-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.[7][30] Although this form of synesthesia was documented as early as the 1890s[23][31] modern research has, until recently,[1] 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."[31] 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...."[1]
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.[7]
[edit] Lexical → gustatory synesthesia
Main article: Lexical-gustatory synesthesiaIn 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.[32][33] 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."
[edit] Research history
Main article: History of synesthesia researchThe 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.[34] 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[35][35][36]
The first medical description of colored hearing is in a German 1812 thesis.[37] The father of psychophysics, Gustav Fechner reported the first empirical survey of colored letter photisms among 73 synesthetes in 1871,[38][39] followed in the 1880s by Francis Galton.[9][40][41] 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[19][22] 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.
[edit] 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, 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.[17] Colored days of the week and colored graphemes are the most common types.[17][19]
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[42][43] 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.[17]
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[1][43][44] However, the first genome-wide association study failed to find X-linkage,[45] 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,[46][47] and the observation that synesthesia can skip generations within a family.[48] 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.[44] 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).
[edit] Objective verification
Reaction times for answers that are congruent with a synesthete’s automatic colors are faster than those whose answer is incongruent.[3]Synesthesia is hard to fake, and easy to prove as a genuine perception. 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.[1][43]
The automaticity of synesthetic experience. The panel on the left is how a non-synesthete perceives the matrix, while a given synesthete might perceive it like the panel on the right.[28]Modified versions of the Stroop effect 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 synesthetic color of the grapheme. The conflict is strongest when the ink color is the opponent color to the synesthetic one (e.g., red vs. green), indicating that synesthetic color perception uses the same mechanism as the perception of real colors.[49]
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,[50] or a musical key → taste synesthete must identify a bitter taste while hearing a musical interval that tastes sweet .[51] 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, Ramachandran 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.[28] 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.[15][24] It certainly does not happen instantaneously; while synesthesia is evoked very early in perceptual processing, it does not occur prior to attention.[52][53]
[edit] Possible neural basis
Main article: Neural basis of synesthesia
Regions thought to be cross-activated in grapheme-color synesthesia (green=grapheme recognition area, red=V4 color area).[28]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).[28] 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.[54] 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[3] of synesthesia that occur in non-synesthetes under certain conditions: Temporal lobe epilepsy, head trauma, stroke, and brain tumors. It can likewise occur during stages of meditation, deep concentration, sensory deprivation, or with use of psychedelics such as LSD or mescaline, certain prescription medications or even, in some cases, marijuana.
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.[15][55][56] 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.[57] The degree of white matter connectivity in the fusiform gyrus correlates with the intensity of the synesthetic experience.
[edit] Associated cognitive traits
Little 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.[1] These observations await large-scale confirmation. What has been confirmed is elevated, sometimes photographic, memory.[58] When asked, "What good is it?" synesthetes say, "It helps me remember." Indeed, it was reading Alexander Luria'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, 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.[45]
Synesthetes are likely to participate in creative activities.[22][59][60] Individual development of perceptual and cognitive skills, and one's cultural environment likely determine the variety in awareness and practical use of
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