||This article cites its sources but does not provide page references. (January 2014)|
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 coming up with an adequate definition of synesthesia: many different phenomena have been included in the term synesthesia ("union of senses"), and in many cases it seems to be an inaccurate one. 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 have a (three-dimensional) view of a year as a 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, while functional neuroimaging studies have identified differences in patterns of brain activation. People with synesthesia use their experiences to aid in their creative process, and many non-synesthetes have attempted to create works of art that may capture what it is like to experience synesthesia. Psychologists and neuroscientists study synesthesia not only for its inherent interest, but also for the insights it may give into cognitive and perceptual processes that occur in synesthetes and non-synesthetes alike.
- 1 Characteristics
- 2 Diagnostic Criteria
- 3 Mechanism
- 4 Epidemiology
- 5 History
- 6 Society and culture
- 7 Research
- 8 See also
- 9 References
- 10 Further reading
- 11 External links
Synesthetes often report that they were unaware their experiences were unusual until they realized other people did not have them, while others report feeling as if they had been keeping a secret their entire lives, as has been documented in interviews with synesthetes on how they discovered synesthesia in their childhood. The automatic and ineffable nature of a synesthetic experience means that the pairing may not seem out of the ordinary. This involuntary and consistent nature helps define synesthesia as a real experience. Most synesthetes report that their experiences are pleasant or neutral, although, in rare cases, synesthetes report that their experiences can lead to a degree of sensory overload.
Though often stereotyped in the popular media as a medical condition or neurological aberration, many synesthetes themselves do not perceive their synesthetic experiences as a handicap. To the contrary, most report it as a gift—an additional "hidden" sense—something they would not want to miss. Most synesthetes become aware of their "hidden" and different way of perceiving in their childhood. Some have learned how to apply this gift in daily life and work. Synesthetes have used their gift in memorizing names and telephone numbers, mental arithmetic, but also in more complex creative activities like producing visual art, music, and theater.
Despite the commonalities which permit definition of the broad phenomenon of synesthesia, individual experiences vary in numerous ways. This variability was first noticed early on in synesthesia research.
Some synesthetes report that vowels are more strongly colored, while for others consonants are more strongly colored. In summary, self reports, autobiographical notes by synesthetes and interviews show a large variety in types of synesthesia, intensity of the synesthetic perceptions, awareness of the difference in perceiving the physical world from other people, the way they creatively use their synesthesia in work and daily life.
Synesthetes are likely to participate in creative activities. Individual development of perceptual and cognitive skills, and one's cultural environment likely determine the variety in awareness and practical use of synesthetic skills
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] The type of synesthesia is 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, grapheme → color synesthesia, individual letters of the alphabet and numbers (collectively referred to as graphemes), are "shaded" or "tinged" with a color. While different individuals usually do not report the same colors for all letters and numbers, studies with large numbers of synesthetes find some commonalities across letters (e.g., A is likely to be red).
As a child, Pat Duffy told her father, "I realized that to make an R all I had to do was first write a P and draw a line down from its loop. And I was so surprised that I could turn a yellow letter into an orange letter just by adding a line." Another grapheme synesthete says, "When I read, about five words around the exact one I'm reading are in color. It's also the only way I can spell. In elementary school I remember knowing how to spell the word 'priority' [with an "i" rather than an "e"] because ... an 'e' was out of place in that word because 'e's were yellow and didn't fit."
With spatial sequence synesthesia (SSS), people 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 even see time like a clock above and around them.[unreliable medical source?]
According to Richard Cytowic, sound → color synesthesia, or chromesthesia is "something like fireworks": voice, music, and assorted environmental sounds such as clattering dishes or dog barks trigger color and firework shapes that arise, move around, and then fade when the sound ends. For some, the stimulus type is limited (e.g., music only, or even just a specific musical key); for others, a wide variety of sounds triggers synesthesia.
Sound often changes the perceived hue, brightness, scintillation, and directional movement. Some individuals see music on a "screen" in front of their faces. Deni Simon, for whom music produces waving lines "like oscilloscope configurations – lines moving in color, often metallic with height, width and, most importantly, depth. My favorite music has lines that extend horizontally beyond the 'screen' area."
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".
Auditory-tactile synesthesia may originate from birth or be acquired sometime in life. It is one of the rarest forms of synesthesia.
Richard Cytowic suggests that the disorder misophonia is related to, or perhaps a variety of, synesthesia. He theorizes that, in the way certain synesthetes have an altered perception of sounds, misophonics exhibit a similar pathology, with certain sounds or types of sound resulting in a specific negative emotional response. 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"
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.[medical citation needed] Indeed, most synesthetes report that their experiences are neutral, or even pleasant. Rather, like color blindness or perfect pitch, synesthesia is a difference in perceptual experience and the term "neurological" simply reflects the brain basis of this perceptual difference.[medical citation needed]
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.
Grapheme-color synesthetes, as a group, share significant preferences for the color of each letter (e.g., A tends to be red; O tends to be white or black; S tends to be yellow etc., Nonetheless, there are a great number of types of synesthesia, and within each type, individuals can report differing triggers for their sensations, and differing intensities of experiences. This variety means that defining synesthesia in an individual is difficult, and the majority of synesthetes are completely unaware that their experiences have a name.
- 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 differentiate between "localizers" and "non-localizers" to distinguish those synesthetes whose perceptions have a definite sense of spatial quality.
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).
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, or even, in some cases, marijuana. However, synesthetes report that common stimulants, like caffeine and cigarettes do not affect the strength of their synesthesia, nor do alcoholic beverages.:137–40
Colored days of the week and colored graphemes are the most common types.
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 a German 1812 thesis. The father of psychophysics, Gustav Fechner reported the first empirical survey of colored letter photisms among 73 synesthetes in 1871, followed in the 1880s by Francis Galton. Research into synesthesia proceeded briskly in several countries, but due to the difficulties in measuring subjective experiences and the rise of behaviorism, which made the study of any subjective experience taboo, synesthesia faded into scientific oblivion between 1930 and 1980.
As the 1980s cognitive revolution 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 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 onegestalt experience.
Contemporary synesthetic artists such as Carol Steen and Marcia Smilack (a synesthetic photographer who waits until she gets a synesthetic response from what she sees, and then takes the picture), have described how they use their synesthesia to create their artworks. They demonstrate the complex interplay between personal experience and artistic creation.
Synesthesia has been a source of inspiration for artists, composers, poets, novelists, and digital artists. Nabokov writes explicitly about synesthesia in several novels. Kandinsky (a synesthete) and Mondrian (not a synesthete) both experimented with image-music correspondences in their paintings. Scriabin composed color music that was deliberately contrived and based on thecircle of fifths, whereas Messiaen invented a new method of composition (the modes of limited transposition) to specifically 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.
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
Many literary depictions of synesthesia are not accurate. Some say more about an author's interpretation of synesthesia than the phenomenon itself.
Determining synesthesia from the historical record is fraught with error unless (auto)biographical sources explicitly give convincing details.
Famous synesthetes include David Hockney, who perceives music as color, shape, and configuration, and who uses these perceptions when painting opera stage sets but not while creating his other artworks. Russian painter Wassily Kandinsky combined four senses: color, hearing, touch, and smell. Vladimir Nabokov describes his grapheme-color synesthesia at length in his autobiography, Speak, Memory, and portrays it in some of his characters. Composers include Duke Ellington, 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. 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 condition. Alexander Scriabin's 1911 Prometheus, for example, is a deliberate contrivance whose color choices are based on the circle of fifths and appear to have been taken from Madame Blavatsky. The musical score has a separate staff marked luce whose "notes" are played on a color organ. Technical reviews appear in period volumes of Scientific American. On the other hand, his older colleague Nikolai Rimsky-Korsakov (who was perceived as a fairly conservative composer) was in fact a synesthete.
French poets Arthur Rimbaud and Charles Baudelaire wrote of synesthetic experience but there is no evidence they were synesthetes themselves. Baudelaire's 1857 Correspondances introduced the notion that the senses can and should intermingle. Baudelaire participated in a hashish experiment by psychiatrist Jacques-Joseph Moreau, and became interested in how the senses might correspond. Rimbaud later wrote Voyelles (1871), which was perhaps more important than Correspondances in popularizing synesthesia, although he later boasted "J'inventais la couleur des voyelles!" (I invented the colors of the vowels!).
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.
Given synesthetes' conscious experiences, researchers hope that their study will provide better understanding of consciousness and its neural correlates, meaning what the brain mechanisms that make us conscious might be. In particular, synesthesia might be relevant to the philosophical problem of qualia, given that synesthetes experience extra qualia (e.g., a colored sound).
Synesthesia also has a number of practical applications, one of which is by using ‘intentional synesthesia’ in technology.
Synesthesia and virtual reality
One type of applications are pain reducing virtual reality programs. 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 wound treatment may increase patients' anxiety, which has a negative effect on pain perception. 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 compared to other distraction techniques, e.g. the use of video games for example. More specifically, the convergence of many sensors like sound, sight and touch, gives patients the idea of being immersed into the virtual environment, which helps to endure the pain while relying less on pharmacological therapy.
Peter Meijer developed a sensory substitution device called The vOICe, spelled with capitals O I C in "vOICe", which stand for ”Oh I see”. The vOICe is a privately owned research project running without venture capital, which 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 the right scan, while associating height with pitch and brightness with loudness. Default resolution of the soundscape is 176×64. Thereby 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. A first is 'left and right' in which left to the right scanning results in hearing the stereo pan from left to the right correspondingly. If there is a visual pattern on the left, user hears a sound on the left and vice versa. A second is 'up and down': every scan provides a pitch, which means elevation. The higher the position of the visual pattern, the higher the pitch. A third and final rule is 'dark and light in which loudness means brightness. The louder the sound, the brighter the visual pattern. Silence stands for black, loud sounds represent white and everything in between is a shade of grey. The functional basis of this visuo-auditory transformation lies in sound synthesis from any input image. This is then further perceptually enhanced through stereo panning representing the horizontal axis in the soundscape, tone frequency creates the vertical axis, and loudness correspondents to pixel brightness. For example, a straight bright line on a dark background, running from the top left to the bottom right, sounds as a tone steadily decreasing in pitch. A dot sounds like a short beep, two dots are 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, by using brain plasticity through training and education. It requires a certain amount of time and effort to get on a proficient level of differentiating objects, identifying actual objects and also locating them in space. It is therefore advised to start training in a safe, familiar home environment, because the interpretation for extended situational awareness first needs to become a second nature integrated 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 that incorporates the auditory and visual spectrums and has been developed by Adam Montandan. It makes it possible for people with colour-blindness to hear colours. The idea to overlap these senses comes from the natural condition synaesthesia.
- [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 ideaestesia? 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 synaesthesia: 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
- Book- Wednesday is Indigo Blue: Discovering the Brain of Synesthesia, Page- 93, By Richard E. Cytowic, David Eagleman
- 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.
- [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.
- [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, Th. (1871) Vorschule der Aesthetik. Leipzig: Breitkopf und Hartel.
- 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.
- 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.
- 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
- 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.
- 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.
- 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.
- 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.