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Synesthesia (also spelled synæsthesia or synaesthesia; from the Ancient Greek σύν syn, "together", and αἴσθησις aisthēsis, "sensation") is a neurological phenomenon in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway. People who report such experiences are known as synesthetes.
Difficulties have been recognized in adequately defining synesthesia: many different phenomena have been included in the term synesthesia ("union of the senses"), and in many cases the terminology seems to be inaccurate. A more accurate term may be ideasthesia.
In one common form of synesthesia, known as grapheme → color synesthesia or color-graphemic synesthesia, letters or numbers are perceived as inherently colored. 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 appear as a three-dimensional map (clockwise or counterclockwise).
Although synesthesia was the topic of intensive scientific investigation in the late 19th and early 20th centuries, it was largely abandoned by scientific research in the mid-20th century. Psychological research has demonstrated that synesthetic experiences can have measurable behavioral consequences, and functional neuroimaging studies have identified differences in patterns of brain activation. Many find that synesthesia aids the creative process. 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.
- 1 Characteristics
- 2 Forms
- 3 Diagnostic criteria
- 4 Mechanism
- 5 Epidemiology
- 6 Development
- 7 History
- 8 Society and culture
- 9 Research
- 10 See also
- 11 References
- 12 Further reading
- 13 External links
There are two overall forms of synesthesia: projecting synesthesia and associative synesthesia. People who project will see actual colors and forms when stimulated, as is commonly accepted as synesthesia; associators will feel a very strong and involuntary connection between the stimulus and the sense that it triggers. For example, in the common form chromesthesia (sound to color) a projector may hear a trumpet and see an orange triangle in space while an associator might hear a trumpet and think very strongly that it sounds "orange".
Some synesthetes often report that they were unaware their experiences were unusual until they realized other people did not have them, while others report feeling as if they had been keeping a secret their entire lives. The automatic and ineffable nature of a synesthetic experience means that the pairing may not seem out of the ordinary. This involuntary and consistent nature helps define synesthesia as a real experience. Most synesthetes report that their experiences are pleasant or neutral, although, in rare cases, synesthetes report that their experiences can lead to a degree of sensory overload.
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, 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 definition of the broad phenomenon of synesthesia, individual experiences vary in numerous ways. This variability was first noticed early in synesthesia research. Some synesthetes report that vowels are more strongly colored, while for others consonants are more strongly colored. Self reports, interviews, and autobiographical notes by synesthetes demonstrate a great degree of variety in types of synesthesia, 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 can occur between nearly any two senses or perceptual modes, and at least one synesthete, Solomon Shereshevsky, experienced synesthesia that linked all five senses.[medical citation needed] Types of synesthesia are indicated by using the 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.
In one of the most common forms of synesthesia, individual letters of the alphabet and numbers (collectively referred to as graphemes) are "shaded" or "tinged" with a color. While different individuals usually do not report the same colors for all letters and numbers, studies with large numbers of synesthetes find some commonalities across letters (e.g. A is likely to be red).
As a child, Pat Duffy told her father, "I realized that to make an R all I had to do was first write a P and draw a line down from its loop. And I was so surprised that I could turn a yellow letter into an orange letter just by adding a line." Another grapheme synesthete says, "When I read, about five words around the exact one I'm reading are in color. It's also the only way I can spell. In elementary school I remember knowing how to spell the word 'priority' [with an "i" rather than an "e"] because ... an 'e' was out of place in that word because 'e's were yellow and didn't fit."
Another common form of synesthesia is the association of sounds with colors. For some, everyday sounds such as doors opening, cars honking, or people talking can trigger seeing colors. For others, colors are triggered when musical notes and/or keys are being played. People with synesthesia related to music may also have perfect pitch because their ability to see/hear colors aids them in identifying notes or keys.
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."
Individuals rarely agree on what color a given sound is. B flat might be orange for one person and blue for another. Composers Liszt and Rimsky-Korsakov famously disagreed on the colors of music keys.
Spatial sequence synesthesia
Those with spatial sequence synesthesia (SSS) tend to see numerical sequences as points in space. For instance, the number 1 might be farther away and the number 2 might be closer. People with SSS may have superior memories; in one study, they were able to recall past events and memories far better and in far greater detail than those without the condition. They also see months or dates in the space around them. Some people see time like a clock above and around them.[unreliable medical source?]
A number form is a mental map of numbers that 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".
In auditory → tactile synesthesia, certain sounds can induce sensations in parts of the body. Auditory → tactile synesthesia may sometimes originate from birth or be acquired sometime later in life. It is one of the least common forms of synesthesia.
Misophonia is a neurological disorder in which negative experiences (anger, flight, hatred, disgust) are triggered by specific sounds. Richard Cytowic suggests that misophonia is related to, or perhaps a variety of, synesthesia. Miren Edelstein and her colleagues have compared misophonia to synesthesia in terms of connectivity between different brain regions as well as specific symptoms. They formed the hypothesis that "a pathological distortion of connections between the auditory cortex and limbic structures could cause a form of sound-emotion synesthesia."
This is a rare form of synesthesia where individuals literally feel the same sensation that another person feels (such as touch). For instance, when such a synesthete observes someone being tapped on their shoulder, the synesthete involuntarily feels a tap on their own shoulder as well. People with this type of synesthesia have been shown to have higher empathy levels compared to the general population. This may be related to the so-called mirror neurons present in the motor areas of the brain, which have also been linked to empathy.
This is another rare form of synesthesia where certain tastes are experienced when hearing words. For example, the word basketball might taste like waffles. The documentary 'Derek tastes like earwax' gets its name from this phenomenon, in references to pub owner James Wannerton who experiences this particular sensation whenever he hears the name spoken. It is estimated that 0.2% of the population has this form of synesthesia.
Other forms of synesthesia have been reported, but little has been done to analyze them scientifically. These include associating people with colors.
Although often termed a "neurological condition," synesthesia is not listed in either the DSM-IV or the ICD since it most often does not interfere with normal daily functioning. Indeed, most synesthetes report that their experiences are neutral or even pleasant. 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 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.
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.
- 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 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 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.
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 are able to identify the color of a grapheme in their peripheral vision even when they cannot consciously identify the shape of the grapheme.
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 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 use of psychedelics 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 alcohol.:137–40
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.
Depending on the study, researchers have suggested 1 in 2,000 people have some form of synesthesia, while others have reported 1 in 300 or even as many as 1 in 23. One problem with statistics is that some individuals will not self-classify as they do not realize that their perceptions are different from those of everyone else.
Grapheme-color, chromesthesia, or anything color-related, appear to be the most common forms of synesthesia. Some of the rarest are reported to be auditory-tactile, mirror-touch, and lexical-gustatory.
There is research to suggest that the likelihood of having synaesthesia is greater in people with autism.
Today little is known how synesthesia develops. The first studies of synesthesia in children and its development are currently undergoing.
Based on the findings that synesthesia is not a phenomenon of crossed senses but has the properties of ideasthesia, it was proposed that synesthesia develops during childhood at the time at which children are for the first time intensively engaged with abstract concepts. This hypothesis—referred to as semantic vacuum hypothesis—explains 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 kids to learn.
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." There is a long history of building color organs such as the clavier à lumières on which to perform colored music in concert halls.
The first medical description of "colored hearing" is in an 1812 German thesis by the "father of psychophysics," Gustav Fechner. Fechner reported the first empirical survey of colored letter photisms among 73 synesthetes in 1876, followed in the 1880s by Francis Galton.C.G.Jung refers to "colour hearing" in his Symbols of Transformation in 1912. 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 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. 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 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 Canadian Synesthesia Association, the German Synesthesia Association, and the Netherlands Synesthesia Web Community.
Society and culture
Synesthetic art historically refers to multi-sensory experiments in the genres of visual music, music visualization, audiovisual art, abstract film, and intermedia. Distinct from neuroscience, the concept of synesthesia in the arts is regarded as the simultaneous perception of multiple stimuli in one gestalt experience.
Neurological synesthesia has been a source of inspiration for artists, composers, poets, novelists, and digital artists. Nabokov writes explicitly about synesthesia in several novels. Kandinsky (a synesthete) and Mondrian (not a synesthete) both experimented with image-music congruence in their paintings. Scriabin composed colored music that was deliberately contrived and based on the circle of fifths, whereas 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.
Contemporary artists with synesthesia, such as Carol Steen and Marcia Smilack (a photographer who waits until she gets a synesthetic response from what she sees and then takes the picture), use their synesthesia to create their artwork. Brandy Gale, a Canadian visual artist, experiences an involuntary joining or crossing of any of her senses - hearing, vision, taste, touch, smell and movement. Gale paints from life rather than from photographs and by exploring the sensory panorama of each locale attempts to capture, select, and transmit these personal experiences.
Synesthesia is sometimes used as a plot device or way of developing a character's inner life. Author and synesthete Pat Duffy describes five ways in which synesthetic characters have been used in modern fiction.
- Synesthesia as Romantic ideal: in which the condition illustrates the Romantic ideal of transcending one's experience of the world. Books in this category include The Gift by Vladimir Nabokov.
- Synesthesia as pathology: in which the trait is pathological. Books in this category include The Whole World Over by Julia Glass.
- 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, and A Mango-Shaped Space by Wendy Mass.
- Synesthesia as young adult literature and science fiction: Ultraviolet by R.J. Anderson, and "One Is Not A Lonely Number" by Evelyn Krieger (YM Books, 2010).
Many literary depictions of synesthesia are not accurate. Some say more about an author's interpretation of synesthesia than the phenomenon itself.
Identifying synesthesia in the historical record is fraught with error unless (auto)biographical sources explicitly give convincing details.
There are many famous synesthetes, most of whom are artists, writers, or musicians. David Hockney perceives music as color, shape, and configuration and uses these perceptions when painting opera stage sets (though not while creating his other artworks). Russian painter Wassily Kandinsky combined four senses: color, hearing, touch, and smell. Vladimir Nabokov described his grapheme-color synesthesia at length in his autobiography, Speak, Memory, and portrayed it in some of his characters. Synesthetic composers include Duke Ellington, Nikolai Rimsky-Korsakov, and Olivier Messiaen, whose three types of complex colors are rendered explicitly in musical chord structures that he invented. Physicist Richard Feynman describes his colored equations in his autobiography, What Do You Care What Other People Think?
Other notable synesthetes include musicians Billy Joel,:89, 91 Itzhak Perlman,:53 Ida Maria, Brian Chase and Patrick Stump; actress Stephanie Carswell (credited as Stéphanie Montreux); inventor Nikola Tesla; electronic musician Richard D. James aka Aphex Twin (who claims to be inspired by lucid dreams as well as music); and classical pianist Hélène Grimaud. Drummer Mickey Hart of The Grateful Dead wrote about his experiences with synaesthesia in his autobiography Drumming at the Edge of Magic. Pharrell Williams, of the groups The Neptunes and N.E.R.D., claims to experience synesthesia and used it as the basis of the album Seeing Sounds. Singer/songwriter Marina and the Diamonds experiences music → color synesthesia and reports colored days of the week.
Some artists frequently mentioned as synesthetes did not, in fact, have the neurological condition. Alexander Scriabin's 1911 Prometheus, for example, is a deliberate contrivance whose color choices are based on the circle of fifths and appear to have been taken from Madame Blavatsky. The musical score has a separate staff marked luce whose "notes" are played on a color organ. Technical reviews appear in period volumes of Scientific American. On the other hand, his older colleague Nikolai Rimsky-Korsakov (who was perceived as a fairly conservative composer) was, in fact, a synesthete.
French poets Arthur Rimbaud and Charles Baudelaire wrote of synesthetic 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!).
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. 95% to 98% of people choose kiki for the angular shape 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. Recent research indicated that in the background of this effect may operate a form of ideasthesia.
Researchers hope that the study of synesthesia will provide 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.
Synesthesia also has a number of practical applications, one of which is the use of ‘intentional synesthesia’ in technology.
Synesthesia and virtual reality
One type of application is the pain-reducing virtual reality program. In existing programs, the main purpose is to reduce pain when undergoing a specific treatment by shifting the attention from the experienced pain to the virtual program in which the patient is participating. By using artificial synesthesia and combining various senses, this can help to enhance the control of a person’s attention, which can be used to improve and direct sensory distraction from the perceived pain.
For example, many treatments for burn pain and wounds may increase patients' anxiety, which increases perceived pain. Shifting attention from pain and anxiety is therefore an important part of the treatment process. Virtual reality has proven to be very effective in managing this acute pain in several medical settings by shifting patients' attention from their experienced pain to the program in which they have been introduced. It appears to be far more effective than other distraction techniques, like playing video games. More specifically, the convergence of many sense modalities (e.g. sound, sight, and touch) gives patients the perception of being immersed in the virtual environment, which helps them endure the pain while relying less on pharmacological therapy.
Peter Meijer developed a sensory substitution device called The vOICe (the capital letters "O," "I," and "C" in "vOICe" are intended to evoke the expression "Oh I see"). The vOICe is a privately owned research project, running without venture capital, that was first implemented using low-cost hardware in 1991. The vOICe is a visual-to-auditory sensory substitution device (SSD) preserving visual detail at high resolution (up to 25,344 pixels). The device consists of a laptop, head-mounted camera or computer camera, and headphones. The vOICe converts visual stimuli of the surroundings captured by the camera into corresponding aural representations (soundscapes) delivered to the user through headphones at a default rate of one soundscape per second. Each soundscape is a left-to-right scan, with height represented by pitch, and brightness by loudness. Default resolution of the soundscape is 176×64. Therefore, it is roughly comparable to a retinal implant or brain implant with 10,000 electrodes.
The process of converting greyscale camera images into soundscapes works according to three simple rules. The first is 'left and right' in which left-to-right scanning results in hearing the stereo pan from left to right correspondingly. If there is a visual pattern on the left, the user hears a sound on the left, and similarly for the right. The second rule is 'up and down': every scan provides a pitch that indicates elevation. The higher the position of the visual pattern, the higher the pitch. The third and final rule is 'dark and light': loudness corresponds to brightness. The louder the sound, the brighter the visual pattern. Silence indicates no light stimuli, the loudest sounds represent white light, and everything in between is a shade of grey.
For example, a straight bright line on a dark background, running from the top left to the bottom right, would sound like a tone steadily decreasing in pitch; a dot would sound like a short beep; and two dots would sound like two short beeps. Since real-life images are much more complex, there is also much more to hear through this device. While converting the visual pattern into a sound, the device uses a predictable real-time audio and video processing algorithm, allowing users to listen to and then interpret the visual information captured by a digital video camera. The vOICe compensates for the loss of vision by converting information from the lost sensory modality into stimuli in a remaining modality.
This could lead to synthetic vision with truly visual sensations through crossmodal sensory integration through training and education. It requires a certain amount of time and effort to become proficient at differentiating objects, identifying objects, and locating them in space. Users are therefore advised to start training in a safe, familiar home environment in order to integrate the novel stimuli with other senses.
One of the remaining questions in this ongoing research concerning the vOICe is to what extent the use of a sensory substitution system can lead to visual sensations through forms of induced artificial synesthesia.
The Eyeborg is a device developed by Adam Montandan that incorporates the auditory and visual spectra. It makes it possible for people with color-blindness to hear colors. This device was inspired by naturally occurring synesthesia.
- [page needed] Cytowic, Richard E. (2002). Synesthesia: A Union of the Senses (2nd edition). Cambridge, Massachusetts: MIT Press. ISBN 0-262-03296-1. OCLC 49395033.
- [page needed] Cytowic, Richard E. (2003). The Man Who Tasted Shapes. Cambridge, Massachusetts: MIT Press. ISBN 0-262-53255-7. OCLC 53186027.
- [page needed] Cytowic, Richard E; Eagleman, David M (2009). Wednesday is Indigo Blue: Discovering the Brain of Synesthesia (with an afterword by Dmitri Nabokov). Cambridge: MIT Press. ISBN 0-262-01279-0.
- [page needed] Harrison, John E.; Simon Baron-Cohen (1996). Synaesthesia: classic and contemporary readings. Oxford: Blackwell Publishing. ISBN 0-631-19764-8. OCLC 59664610.
- Nikolić D (2009). "Is synaesthesia actually ideaesthesia? An inquiry into the nature of the phenomenon". Proceedings of the Third International Congress on Synaesthesia, Science & Art, Granada, Spain, April 26–29.
- Simner J (2012). "Defining synaesthesia". British Journal of Psychology (Review) 103 (6): 1–15. doi:10.1348/000712610X528305. PMID 22229768.
- Rich AN, Mattingley JB (January 2002). "Anomalous perception in synesthesia: a cognitive neuroscience perspective". Nature Reviews Neuroscience (Review) 3 (1): 43–52. doi:10.1038/nrn702. PMID 11823804.
- Hubbard EM, Ramachandran VS (November 2005). "Neurocognitive mechanisms of synesthesia". Neuron (Review) 48 (3): 509–20. doi:10.1016/j.neuron.2005.10.012. PMID 16269367.
- Galton F (1880). "Visualized Numerals". Nature 21 (543): 494–5. doi:10.1038/021494e0.
- Seron X, Pesenti M, Noël MP, Deloche G, Cornet JA (August 1992). "Images of numbers, or "When 98 is upper left and 6 sky blue"". Cognition 44 (1–2): 159–96. doi:10.1016/0010-0277(92)90053-K. PMID 1511585.
- [page needed] van Campen, Cretien (2007). The Hidden Sense: Synesthesia in Art and Science. Cambridge, Massachusetts: MIT Press. ISBN 0-262-22081-4. OCLC 80179991.
- Campen, Cretien van (2009) "The Hidden Sense: On Becoming Aware of Synesthesia" TECCOGS, vol. 1, pp. 1-13.
- Campen C (1999). "Artistic and psychological experiments with synesthesia". Leonardo 32 (1): 9–14. doi:10.1162/002409499552948.
- [page needed] Sagiv, Noam; Robertson, Lynn C (2005). Synesthesia: perspectives from cognitive neuroscience. Oxford: Oxford University Press. ISBN 0-19-516623-X. OCLC 53020292.
- [page needed] Flournoy, Théodore (2001). Des phénomènes de synopsie (Audition colorée). Adamant Media Corporation. ISBN 0-543-94462-X.
- [broken citation] Dittmar, A. (Ed.) (2007) Synästhesien. Roter Faden durchs Leben? Essen, Verlag Die Blaue Eule.
- Dailey A, Martindale C, Borkum J (1997). "Creativity, synesthesia, and physiognomic perception". Creativity Research Journal 10 (1): 1–8. doi:10.1207/s15326934crj1001_1.
- [unreliable source?] "Slashdot Discussion". 2006-02-19. Retrieved 2006-08-14.
- [unreliable medical source?]Do sequence-space synaesthetes have better spatial imagery skills? Maybe not, The National Center for Biotechnology Information
- [unreliable source?] A Mind That Touches the Past, Sciencemag.org
- Galton F (1881). "The visions of sane persons" (PDF). Fortnightly Review 29: 729–40. Retrieved 2008-06-17.
- Naumer MJ, van den Bosch JJ (July 2009). "Touching sounds: thalamocortical plasticity and the neural basis of multisensory integration". J. Neurophysiol. 102 (1): 7–8. doi:10.1152/jn.00209.2009. PMID 19403745.
- [page needed] Cytowic, Richard E. (2002). Synesthesia: A Union of the Senses (2nd edition). Cambridge, Massachusetts: MIT Press. ISBN 0-262-03296-1. OCLC 49395033
- Edelstein, Miren, David Brang, Romke Rouw, and Vilayanur S. Ramachandran. "Misophonia: Physiological Investigations and Case Descriptions." Frontiers in Human Neuroscience 7 (2013): n. pag. National Center for Biotechnology Information. US National Library of Medicine, 25 June 2013. Web. 5 Dec. 2013.
- Heyes, Cecelia. "Where Do Mirror Neurons Come From?" Neuroscience and Biobehavioral Reviews (2009): 576-77. CognitiveScience. University of Oxford, 7 Nov. 2009. Web. 30 Jan. 2015.
- "Derek Tastes of Ear Wax". Top Documentary Films. Retrieved 2 February 2015.
- "BBC - Science & Nature - Horizon". bbc.co.uk. Retrieved 2 February 2015.
- Simner, Julia (2009). Encyclopedia of Neuroscience. Springer Berlin Heidelberg. ISBN 978-3-540-23735-8.
- Hubbard, Edward (June 2007). "Neurophysiology of synesthesia". NCBI. PubMed. Retrieved 15 Feb 2015.
- [non-primary source needed][dead link]Ramachandran VS and Hubbard EM (2001). "Synaesthesia: A window into perception, thought and language" (PDF). Journal of Consciousness Studies 8 (12). pp. 3–34.
- Grossenbacher PG, Lovelace CT (January 2001). "Mechanisms of synesthesia: cognitive and physiological constraints". Trends Cogn. Sci. 5 (1): 36–41. doi:10.1016/S1364-6613(00)01571-0. PMID 11164734.
- Jeffrey A. Gray, David M. Parslow, Michael J. Brammer, Susan Chopping, Goparlen N. Vythelingum, Dominic H. Fytche. “Evidence Against Functionalism from Neuroimaging of the Alien Colour Effect in Synaesthesia.” Cortex. Volume 42, Issue 2, 2006. pg. 317. 15, Feb. 2015
- Baron-Cohen S, Johnson D, Asher J, Wheelwright S, Fisher SE, Gregerson PK, Allison C, "Is synaesthesia more common in autism?", Molecular Autism, Nov 20 2013
- Mroczko-Wąsowicz, A., Nikolić D. (2014) Semantic mechanisms may be responsible for developing synesthesia. Frontiers in Human Neuroscience 8:509. doi: 10.3389/fnhum.2014.00509
- [page needed] Gage, J.Colour and Culture. Practice and Meaning from Antiquity to Abstraction. (London:Thames & Hudson, 1993).
- Peacock, Kenneth. "Instruments to Perform Color-Music: Two Centuries of Technological Experimentation,"Leonardo 21, No. 4 (1988) 397–406.
- [page needed] Jewanski, J. & N. Sidler (Eds.). Farbe – Licht – Musik. Synaesthesie und Farblichtmusik. Bern: Peter Lang, 2006.
- Mahling, F. (1926) Das Problem der `audition colorée': Eine historisch-kritische Untersuchung. Archiv für die gesamte Psychologie, 57, 165–301.
- Fechner, G. (1876) Vorschule der Aesthetik. Leipzig: Breitkopf und Hartel. Website: 
- Campen, Cretien van (1996). De verwarring der zintuigen. Artistieke en psychologische experimenten met synesthesie. Psychologie & Maatschappij, vol. 20, nr. 1, pp. 10–26.
- Galton F (1880). "Visualized Numerals". Nature 21 (533): 252–6. doi:10.1038/021252a0.
- [page needed] Galton F (1883). Inquiries into Human Faculty and Its Development. Macmillan. Retrieved 2008-06-17.
- Jung, C.G. The Tansformation of Libido in "Symbols of Transformation", CW5, London 1912/1956, Routledge & Kegan Paul, para.237.
- Berman G (1999). "Synesthesia and the Arts". Leonardo 32 (1): 15–22. doi:10.1162/002409499552957.
- [page needed] Maur, Karin von (1999). The Sound of Painting: Music in Modern Art (Pegasus Library). Munich: Prestel. ISBN 3-7913-2082-3.
- [page needed] Gage, John D. (1993). Colour and culture: practice and meaning from antiquity to abstraction. London: Thames and Hudson. ISBN 0-500-27818-0.
- [page needed] Gage, John D. (1999). Color and meaning: art, science, and symbolism. Berkeley: University of California Press. ISBN 0-520-22611-9.
- [page needed] Campen, Cretien van (2009) Visual Music and Musical Paintings. The Quest for Synesthesia in the Arts. In: F. Bacci & D. Melcher. Making Sense of Art, making Art of Sense. Oxford: Oxford University Press.
- Steen, C. (2001). Visions Shared: A Firsthand Look into Synesthesia and Art, Leonardo, Vol. 34, No. 3, Pages 203–208 doi:10.1162/002409401750286949
- Marcia Smilack Website Accessed 20 Aug 2006.
- "Coastal Synaesthesia: Paintings and Photographs of Hawaii, Fiji and California by Brandy Gale - Gualala Arts Center exhibit: January, 2015". gualalaarts.org. Retrieved 2 February 2015.
- EG. "The Wondrous Sensory Spectrum of Brandy Gale". FORA.tv. Retrieved 2 February 2015.
- Duffy, P.L. (2006). Images of Synesthetes and their Perceptions of Language in Fiction. 6th Annual Meeting of the American Synesthesia Association. University of South Florida.
- Duffy PL, Simner J (2010). "Synaesthesia in fiction". Cortex 46 (2): 277–278. doi:10.1016/j.cortex.2008.11.003. PMID 19081086.
- [page needed] Nabokov, Vladimir. 1966. Speak, Memory: An Autobiography Revisited. New York: Putnam.
- [page needed] Ellington, as quoted in George, Don. 1981. Sweet man: The real Duke Ellington. New York: G.P. Putnam’s Sons. Page 226.
- according to the Russian press: Yastrebtsev V. "On N.A.Rimsky-Korsakov's color sound- contemplation." Russkaya muzykalnaya gazeta, 1908, N 39–40, p. 842–845 (in Russian), cited by Bulat Galeyev (1999).
- see Samuel, Claude. 1994 (1986). Olivier Messiaen: Music and Color. Conversations with Claude Samuel. Translated by E. Thomas Glasow. Portland, Oregon: Amadeus Press.
- [page needed] Feynman, Richard. 1988. What Do You Care What Other People Think? New York: Norton. P. 59.
- Seaberg, M. (2011). Tasting the Universe. New Page Books. ISBN 978-1-60163-159-6.
- Cairns, Dan (2008-02-24). "Times Online interview". The Times (London). Retrieved 2008-07-24.
- Forrest, Emma (March 30, 2009). "Emma Forrest meets New York's favourite art-punk rockers Yeah Yeah Yeahs". guardian.co.uk (London: The Guardian). Retrieved 2009-05-07.
- Chase, Brian. "Brian Chase's blog". yeahyeahyeahs.com. Retrieved 2009-05-07.[dead link]
- Tesla, Nikola. "The Strange Life of Nikola Tesla". pitt.edu. Retrieved 4 September 2012.
- [unreliable source?] It just always stuck out in my mind, and I could always see it. I don't know if that makes sense, but I could always visualize what I was hearing... Yeah, it was always like weird colors." From a Nightline interview with Pharrell
- "Synesthetes: "People of the Future"". Psychology Today. March 3, 2012. Retrieved May 15, 2014.
- Loose Women | Marina and the Diamonds – ITV Lifestyle ITV – 27 April 2010 – Retrieved 28 April 2010.
- [page needed] Dann, Kevin T. (1998). Bright colors falsely seen: synaesthesia and the search for transcendental knowledge. New Haven, Conn: Yale University Press. ISBN 0-300-06619-8.
- This is according to an article in the Russian press, Yastrebtsev V. "On N.A.Rimsky-Korsakov's color sound- contemplation." Russkaya muzykalnaya gazeta, 1908, N 39-40, pp. 842–845 (in Russian), cited by Bulat Galeyev (1999).
- [page needed] Tammet, Daniel (2007). Born on a Blue Day. Free Press. ISBN 978-1416535072.
- "Chocolat author Joanne Harris talks about her latest novel Blue Eyed Boy". Metro. 7 Apr 2010.
- [non-primary source needed] Maurer D, Pathman T, Mondloch CJ (May 2006). "The shape of boubas: sound-shape correspondences in toddlers and adults". Dev Sci 9 (3): 316–22. doi:10.1111/j.1467-7687.2006.00495.x. PMID 16669803.
- Gómez Milán E., Iborra O., de Córdoba M.J., Juárez-Ramos V., Artacho Rodríguez, Rubio J.L. (2013). "The Kiki-Bouba effect: A case of personification and ideaesthesia". The Journal of Consciousness Studies 20 (1-2): 84–102.
- Gray JA, Chopping S, Nunn J et al. (2002). "Implications of synaesthesia for functionalism: Theory and experiments". Journal of Consciousness 9 (12): 5–31.
- Suslick, Kenneth S (December 2012). "Synesthesia in science and technology: more than making the unseen visible". Current Opinion in Chemical Biology 16 (5-6): 557–563. doi:10.1016/j.cbpa.2012.10.030. PMC 3606019. PMID 23183411.
- Reif, John. "Advancing Attention Control Using VR-Induced Artificial Synesthesia". Retrieved 4 February 2014.
- Hoffman, Hunter G.; Doctor, Jason N.; Patterson, David R.; Carrougher, Gretchen J.; Furness III, Thomas A. (March 2000). "Virtual reality as an adjunctive pain control during burn wound care in adolescent patients". Pain 85 (1-2): 305–309. doi:10.1016/S0304-3959(99)00275-4.
- Gold, Jeffrey I.; Belmont, Katharine A.; Thomas, David A. (August 2007). "The Neurobiology of Virtual Reality Pain Attenuation". CyberPsychology & Behavior 10 (4): 536–544. doi:10.1089/cpb.2007.9993.
- Meijer, Peter. "Augmented Reality for the Totally Blind". Retrieved 4 February 2014.
- Striem-Amit, Ella; Guendelman, Miriam; Amedi, Amir; Serino, Andrea (16 March 2012). "‘Visual’ Acuity of the Congenitally Blind Using Visual-to-Auditory Sensory Substitution". PLoS ONE 7 (3): e33136. doi:10.1371/journal.pone.0033136.
- Carmichael, Joey. "Device Trains Blind People To 'See' By Listening". Retrieved 4 February 2014.
- Haigh, Alastair; Brown, David J.; Meijer, Peter; Proulx, Michael J. (2013). "How well do you see what you hear? The acuity of visual-to-auditory sensory substitution". Frontiers in Psychology 4. doi:10.3389/fpsyg.2013.00330.
- Montandon, Adam. "Colourblind Eyeborg Colours to Sound". Retrieved 4 February 2014.
- de Broucker T (April 2013). "Synaesthesia, an augmented sensory world: phenomenology and literature review". Rev. Neurol. (Paris) (Review) (in French) 169 (4): 328–34. doi:10.1016/j.neurol.2012.09.016. PMID 23434143.
- Cohen Kadosh R, Terhune DB (February 2012). "Redefining synaesthesia?". Br J Psychol (Review) 103 (1): 20–3. doi:10.1111/j.2044-8295.2010.02003.x. PMID 22229770.
- De Cordoba, M.J.; Riccò, D.; Day, Sean (July 2014). Synaesthesia: Theoretical, artistic and scientific foundations. Granada, Spain. p. 372. ISBN 978-84-939054-9-1.
- Dael N, Sierro G, Mohr C (2013). "Affect-related synesthesias: a prospective view on their existence, expression and underlying mechanisms". Front Psychol (Review) 4: 754. doi:10.3389/fpsyg.2013.00754. PMC 3798864. PMID 24151478.
- Fitzgibbon BM, Enticott PG, Rich AN, Giummarra MJ, Georgiou-Karistianis N, Bradshaw JL (January 2012). "Mirror-sensory synaesthesia: exploring 'shared' sensory experiences as synaesthesia". Neurosci Biobehav Rev (Review) 36 (1): 645–57. doi:10.1016/j.neubiorev.2011.09.006. PMID 21986634.
- Luke DP, Terhune DB (2013). "The induction of synaesthesia with chemical agents: a systematic review". Front Psychol (Review) 4: 753. doi:10.3389/fpsyg.2013.00753. PMC 3797969. PMID 24146659.
- Mylopoulos MI, Ro T (2013). "Synesthesia: a colorful word with a touching sound?". Front Psychol (Review) 4: 763. doi:10.3389/fpsyg.2013.00763. PMC 3804765. PMID 24155733.
- Price MC, Mattingley JB (May 2013). "Automaticity in sequence-space synaesthesia: a critical appraisal of the evidence". Cortex (Review) 49 (5): 1165–86. doi:10.1016/j.cortex.2012.10.013. PMID 23237480.
- Rothen N, Meier B, Ward J (September 2012). "Enhanced memory ability: Insights from synaesthesia". Neurosci Biobehav Rev (Review) 36 (8): 1952–63. doi:10.1016/j.neubiorev.2012.05.004. PMID 22634573.
- Sinke C, Halpern JH, Zedler M, Neufeld J, Emrich HM, Passie T (September 2012). "Genuine and drug-induced synesthesia: a comparison". Conscious Cogn (Review) 21 (3): 1419–34. doi:10.1016/j.concog.2012.03.009. PMID 22521474.
- Simner J (February 2012). "Defining synaesthesia". Br J Psychol (Review) 103 (1): 1–15. doi:10.1348/000712610X528305. PMID 22229768.
- Suslick KS (December 2012). "Synesthesia in science and technology: more than making the unseen visible". Curr Opin Chem Biol (Review) 16 (5-6): 557–63. doi:10.1016/j.cbpa.2012.10.030. PMC 3606019. PMID 23183411.
- Ward J (2013). "Synesthesia". Annu Rev Psychol (Review) 64: 49–75. doi:10.1146/annurev-psych-113011-143840. PMID 22747246.
- Red Mondays and Gemstone Jalapeños: The Synesthetic World a documentary short featuring, featuring David Eagleman and four synesthetes, from ResearchChannel.
- Lawrence Marks, a synesthesia researcher, interviewed by Anton Dorso
- Danis, Alex. "Grapheme → colour synesthesia". Numberphile. Brady Haran.
- Mailman, Joshua B. "Improvising Synesthesia: Comprovisation of Generative Graphics and Music" in Leonardo Electronic Almanac v.19 no.3, Live Visuals, 2013, pp. 352–84.
- Synaesthesia Research Group at the University of Sussex Information and article links.
- Synaesthesia genetic research group at the Max Planck Institute for Psycholinguistics
- Sinestesie.it: Italian Books on synesthesia