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Synesthesia (American English) or synaesthesia (British English) is a perceptual phenomenon in which stimulation of one sensory or cognitive pathway leads to involuntary experiences in a second sensory or cognitive pathway. (1996). 9780631197645, Blackwell Publishing. ISBN 9780631197645 People with synesthesia may experience colors when listening to music, see shapes when smelling certain scents, or perceive tastes when looking at words. People who report a lifelong history of such experiences are known as synesthetes.
Awareness of synesthetic perceptions varies from person to person with the perception of synesthesia differing based on an individual's unique life experiences and the specific type of synesthesia that they have. In one common form of synesthesia, known as grapheme–color synesthesia or color–graphemic synesthesia, letters or numbers are perceived as inherently colour. In spatial-sequence, or number form synesthesia, numbers, months of the year, or days of the week elicit precise locations in space ( e.g., 1980 may be "farther away" than 1990), or may appear as a three-dimensional map (clockwise or counterclockwise). Synesthetic associations can occur in any combination and any number of senses or cognitive pathways.
Little is known about how synesthesia develops. It has been suggested that synesthesia develops during childhood when children are intensively engaged with abstract concepts for the first time. This hypothesis—referred to as semantic vacuum hypothesis—could explain why the most common forms of synesthesia are grapheme-color, spatial sequence, and number form. These are usually the first abstract concepts that educational systems require children to learn.
The earliest recorded case of synesthesia is attributed to the Oxford University academic and philosopher John Locke, who, in 1690, made a report about a blind man who said he experienced the color scarlet when he heard the sound of a trumpet. However, there is disagreement as to whether Locke described an actual instance of synesthesia or was using a metaphor. The first medical account came from German physician Georg Tobias Ludwig Sachs in 1812. The term is from Ancient Greek σύν]] syn 'together' and αἴσθησις]] aisthēsis .Ward, Ossian (10 June 2006). The man who heard his paint box hiss. The Telegraph''. Retrieved 3 December 2018.
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. Types of synesthesia are indicated by using the notation , where x is the "inducer" or trigger experience, and y is the "concurrent" or additional experience. For example, perceiving letters and numbers (collectively called ) 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.
The colors triggered by certain sounds, and any other synesthetic visual experiences, are referred to as photisms.
According to Richard Cytowic, chromesthesia is "something like fireworks": voice, music, and assorted environmental sounds such as clattering dishes or dog barks trigger color and firework shapes that arise, move around, and then fade when the sound ends. Sound often changes the perceived hue, brightness, scintillation, and directional movement. Some individuals see music on a "screen" in front of their faces. For Deni Simon, 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."
Some authors have argued that the term synaesthesia may not be correct when applied to the so-called grapheme-colour synesthesia and similar phenomena in which the inducer is conceptual (e.g. a letter or number) rather than sensory (e.g. sound or color). They have postulated that the term ideasthesia is a more accurate description.
In August 2017 a research article in the journal Social Neuroscience reviewed studies with fMRI to determine if persons who experience autonomous sensory meridian response are experiencing a form of synesthesia. While a determination has not yet been made, there is anecdotal evidence that this may be the case, based on significant and consistent differences from the control group, in terms of functional connectivity within neural pathways. It is unclear whether this will lead to ASMR being included as a form of existing synesthesia, or if a new type will be considered.
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. On the contrary, some report it as a gift an additional "hidden" sense something they would not want to miss. Most synesthetes become aware of their distinctive mode of perception in their childhood. Some have learned how to apply their ability in daily life and work. Synesthetes have used their abilities in memorization of names and telephone numbers, mental arithmetic, and more complex creative activities like producing visual art, music, and theater.
Despite the commonalities which permit the definition of the broad phenomenon of synesthesia, individual experiences vary in numerous ways. This variability was first noticed early in synesthesia research. (2025). 9780543944627, Adamant Media Corporation. ISBN 9780543944627 Some synesthetes report that are more strongly colored, while for others are more strongly colored. Self-reports, interviews, and autobiographical notes by synesthetes demonstrate a great degree of variety in types of synesthesia, the intensity of synesthetic perceptions, awareness of the perceptual discrepancies between synesthetes and non-synesthetes, and the ways synesthesia is used in work, creative processes, and daily life.
Synesthetes are very likely to participate in creative activities. It has been suggested that individual development of perceptual and cognitive skills, in addition to one's cultural environment, produces the variety in awareness and practical use of synesthetic phenomena. Synesthesia may also give a memory advantage. In one study, conducted by Julia Simner of the University of Edinburgh, it was found that spatial sequence synesthetes have a built-in and automatic mnemonic reference. Whereas a non-synesthete will need to create a mnemonic device to remember a sequence (like dates in a diary), a synesthete can simply reference their spatial visualizations.
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). This is supported by the fact that grapheme–color synesthetes can identify the color of a grapheme in their peripheral vision even when they cannot consciously identify the shape of the grapheme.
An alternative 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 was 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 and Eagleman find support for the disinhibition idea in the so-called acquired forms of synesthesia that occur in non-synesthetes under certain conditions: temporal lobe epilepsy, head trauma, stroke, and brain tumors. They also note that it can likewise occur during stages of meditation, deep concentration, sensory deprivation, or with the use of hallucinogen such as LSD or mescaline, and even, in some cases, marijuana. However, synesthetes report that common stimulants, like caffeine and cigarettes do not affect the strength of their synesthesia, nor does ethanol.
A very different theoretical approach to synesthesia is that based on ideasthesia. According to this account, synesthesia is a phenomenon mediated by the extraction of the meaning of the inducing stimulus. Thus, synesthesia may be fundamentally a semantic phenomenon. Therefore, to understand neural mechanisms of synesthesia the mechanisms of semantics and the extraction of meaning need to be understood better. This is a non-trivial issue because it is not only a question of a location in the brain at which meaning is "processed" but pertains also to the question of understanding epitomized in e.g., the Chinese room problem. Thus, the question of the neural basis of synesthesia is deeply entrenched into the general mind–body problem and the problem of the explanatory gap.
Although often termed a "neurological condition," synesthesia is not listed in either the DSM-IV or the ICD since it usually does not interfere with normal daily functioning. Indeed, most synesthetes report that their experiences are neutral or even pleasant. (2025). 9780195166231, Oxford University Press. ISBN 9780195166231 Like perfect pitch, synesthesia is simply a difference in perceptual experience.
The simplest approach is test-retest reliability over long periods of time, using stimuli of color names, color chips, or a computer-screen color picker providing 16.7 million choices. Synesthetes consistently score around 90% on the reliability of associations, even with years between tests. In contrast, non-synesthetes score just 30–40%, even with only a few weeks between tests and a warning that they would be retested.
Many tests exist for synesthesia. Each common type has a specific test. When testing for grapheme–color synesthesia, a visual test is given. The person is shown a picture that includes black letters and numbers. A synesthete will associate the letters and numbers with a specific color. An auditory test is another way to test for synesthesia. A sound is turned on and one will either identify it with a taste or envision shapes. The audio test correlates with chromesthesia (sounds with colors). Since people question whether or not synesthesia is tied to memory, the "retest" is given. One is given a set of objects and is asked to assign colors, tastes, personalities, or more. After some time, the same objects are presented and the person is asked again to do the same task. The synesthete can assign the same characteristics because that person has permanent neural associations in the brain, rather than memories of a certain object.
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 is a great variety in types of synesthesia, and within each type, individuals 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.
Neurologist Richard Cytowic identifies the following diagnostic criteria for synesthesia in his first edition book. However, the criteria are different in the second book:
Cytowic's early cases mainly 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 David Eagleman differentiated between "localizers" and "non-localizers" to distinguish those synesthetes whose perceptions have a definite sense of spatial quality from those whose perceptions do not.
The most common forms of synesthesia are those that trigger colors, and the most prevalent of all is day–color. Also relatively common is grapheme–color synesthesia. Prevalence refers to both how common is synesthesia (or different forms of synesthesia) within the population, or how common are different forms of synesthesia within synesthetes. So within synesthetes, forms of synesthesia that trigger color also appear to be the most common forms of synesthesia with a prevalence rate of 86% within synesthetes. In another study, music–color is also prevalent at 18–41%. Some of the rarest are reported to be auditory–tactile, mirror-touch, and lexical–gustatory.
Some studies suggest that the likelihood of having synesthesia is greater in autism.Baron-Cohen S, Johnson D, Asher J, Wheelwright S, Fisher SE, Gregerson PK, Allison C, "Is synaesthesia more common in autism?", Molecular Autism, 20 November 2013 Another study claims that synesthesia is specifically more common in the subset of autistic people who self-report unusual skills not ordinarily found in the general population, which study authors describe in terms of savant syndrome. The relationship between autistic traits and synesthesia appears to be primarily driven by shared genetic underpinnings.
Prior research also reports that sensory hyperresponsiveness, and according to one study also hyposensitivity, are more common in people with synesthesia (as well as autistic people) than in control participants. Researchers have also suggested that synesthesia may be related to misophonia, a specific type of heightened responsiveness to sensory stimulation in which certain trigger sounds can evoke distressing emotional responses. A study exploring whether individuals with misophonia reported chromesthesia, grapheme-colour synesthesia, number form synesthesia, or other kinds of synesthesia reported 9-17% experienced each type; although the study did not include a comparison group of people without misophonia, the study authors noted this prevalence would be higher than previously found in the general population. However, research exploring whether synesthesia and misophonia are linked remains very limited.
The first medical description of "colored hearing" is in an 1812 thesis by the German physician Georg Tobias Ludwig Sachs. The "father of psychophysics," Gustav Fechner, reported the first empirical survey of colored letter photisms among 73 synesthetes in 1876, followed in the 1880s by Francis Galton. Carl Jung refers to "color hearing" in his Symbols of Transformation in 1912.
In the early 1920s, the Bauhaus teacher and musician Gertrud Grunow researched the relationships between sound, color, and movement and developed a 'twelve-tone circle of colour' which was analogous with the twelve-tone music of the Austrian composer Arnold Schönberg (1874–1951). Bauhaus100.Curriculum. Harmonisation theory 1919-1924 . Retrieved 2 December 2018 She was a participant in at least one of the Congresses for Colour-Sound Research (German: Kongreß für Farbe-Ton-Forschung) held in Hamburg in the late 1920s and early 1930s. Farbe-Ton-Forschungen. III. Band. Bericht über den II. Kongreß für Farbe-Ton-Forschung (Hamburg 1. - 5. Oktober 1930). Published 1931.
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 made inquiry into internal subjective states respectable again, scientists returned to studying synesthesia. Led in the United States by Larry Marks and Richard Cytowic, and later in England by Simon Baron-Cohen and Jeffrey Gray, researchers 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. Psychologists and neuroscientists study synesthesia not only for its inherent appeal but also for the insights it may give into cognitive and perceptual processes that occur in synesthetes and non-synesthetes alike. 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 1990s, synesthetes began contacting one another and creating websites devoted to the condition. These rapidly grew into international organizations such as the American Synesthesia Association, the UK Synaesthesia Association, the Belgian Synesthesia Association, the Canadian Synesthesia Association, the German Synesthesia Association, and the Netherlands Synesthesia Web Community.
Neuroscientist and author V.S. Ramachandran studied the case of a grapheme–color synesthete who was also color blind. While he couldn't see certain colors with his eyes, he could still "see" those colors when looking at certain letters. Because he didn't have a name for those colors, he called them "Martian colors."
I present a fine case of colored hearing. Perhaps "hearing" is not quite accurate, since the color sensations seem to be produced by the very act of my orally forming a given letter while I imagine its outline. The long a of the English alphabet (and it is this alphabet I have in mind farther on unless otherwise stated) has for me the tint of weathered wood, but the French a evokes polished ebony. This black group also includes hard g (vulcanized rubber) and r (a sooty rag being ripped). Oatmeal n, noodle-limp l, and the ivory-backed hand mirror of o take care of the whites. I am puzzled by my French on which I see as the brimming tension-surface of alcohol in a small glass. Passing on to the blue group, there is steely x, thundercloud z, and huckleberry k. Since a subtle interaction exists between sound and shape, I see q as browner than k, while s is not the light blue of c, but a curious mixture of azure and mother-of-pearl.
Daniel Tammet wrote a book on his experiences with synesthesia called Born on a Blue Day. Joanne Harris, author of Chocolat, is a synesthete who says she experiences colors as scents. Her novel Blueeyedboy features various characters with synesthesia.
Mikalojus Konstantinas Čiurlionis, a Lithuanian painter, composer, and writer, perceived colors and music simultaneously. Many of his paintings bear the names of matching musical pieces: sonatas, fugues, and preludes.
Alexander Scriabin composed colored music that was deliberately contrived and based on the circle of fifths, whereas Olivier Messiaen invented a new method of composition (the modes of limited transposition) specifically to render his bi-directional sound–color synesthesia. For example, the red rocks of Bryce Canyon 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. Other composers who reported synesthesia include Duke Ellington,Duke Ellington as quoted in Nikolay Rimsky-Korsakov, cited by Bulat Galeyev (1999). and Jean Sibelius.
Several contemporary composers with synesthesia are Michael Torke, and Ramin Djawadi, best known for his work on composing the theme songs and scores for such TV series as Game of Thrones, Westworld and for the Iron Man movie. He says he tends to "associate colors with music, or music with colors."
British composer Daniel Liam Glyn created the classical-contemporary music project Changing Stations using grapheme-colour synesthesia. Based on the 11 main lines of the London Underground, the eleven tracks featured on the album represent the eleven main tube line colours. Each track focuses heavily on the different speeds, sounds, and mood of each line, and are composed in the key signature synaesthetically assigned by Glyn with reference to the colour of the tube line on the Tube map.
French poets Arthur Rimbaud and Charles Baudelaire wrote of synesthetic experiences, but there is no evidence they were synesthetes themselves. Baudelaire's 1857 Correspondances introduced the notion that the senses can and should intermingle. Baudelaire participated in a hashish experiment by psychiatrist Jacques-Joseph Moreau and became interested in how the senses might affect each other. Rimbaud later wrote Voyelles (1871), which was perhaps more important than Correspondances in popularizing synesthesia. He later boasted "J'inventais la couleur des voyelles!" (I invented the colors of the vowels!). (2025). 9783211997505, Springer Vienna. ISBN 9783211997505
Literary depictions of synesthesia are criticized as often being more of a reflection of an author's interpretation of synesthesia than of the phenomenon itself.
An example of this is the bouba/kiki effect. In an experiment first designed by Wolfgang Köhler, people are asked to choose which of two shapes is named bouba and which kiki. The angular shape, kiki, is chosen by 95–98% and bouba for the rounded one. Individuals on the island of Tenerife showed a similar preference between shapes called takete and maluma. Even 2.5-year-old children (too young to read) show this effect. Research indicated that in the background of this effect may operate a form of ideasthesia.
Researchers hope that the study of synesthesia will provide a better understanding of consciousness and its neural correlates. In particular, synesthesia might be relevant to the philosophical problem of qualia, given that synesthetes experience extra qualia (e.g., colored sound). An important insight for qualia research may come from the findings that synesthesia has the properties of ideasthesia, which then suggest a crucial role of conceptualization processes in generating qualia. Some philosophers argue that synesthesia creates problems for theories that say our conscious experiences are determined by what our minds represent about the world. For example, when both a synesthete and a non-synesthete feel pain in the same way, they might be representing the same bodily damage, but the synesthete also experiences colors while the non-synesthete does not. This suggests that identical mental representations can lead to different conscious experiences.
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