Synesthesia is the faculty some people have to entwine the senses in the same perceptive act, so that listening to music can evoke colors, reading a word can suggest a taste or tasting a flavor can cause a tactile sensation in the fingertips. But what at first might seem a hallucination is actually an added perception that arises in the brain, so it becomes a world richer in sensations, but totally normal for the perceiver.

Synesthesia is an involuntary, automatic and timely consistent perceptual experience in which a person experiences feelings in one sensory modality when a different sensory modality is stimulated. Can a sound be red? Can a melody makes us feel cold? Can a smell have a shape?.

The word “synesthesia” comes from the Greek; “syn” (union) and “esthesia” (sensation), that is, binding of sensations.

The synesthetic experience is a condition present in 2-4% of the population, and it can join any two sensory modalities. Although there are two common variants, auditory stimuli and numbers that produce a vivid color perception (50% of cases), at least 60 different forms of synesthesia have been registered.

The synesthetic phenomenon has genetic bases, and 40% of synesthetes have relatives with the same characteristic, suggesting a large percentage of possibility of passing from father to son. It is a dominant X-linked trait, and the question is how the gene (or genes) affects neuroanatomy to produce the sensorial alteration in synesthesia.

There are different theories.

Some authors postulate that a genetic mutation that lead to a deficiency axonal pruning (process that occurs in neural development when a large number of synapses are eliminated) would lead to the persistence of abnormal connections in the adult brain.

Other researchers differ and claim that the cause is an uninhibited feedback in a multimodal cortical region. In the normal brain, connections do not only exist “forward” but also “backwards”, that should be inhibited to prevent malfunctions in communication. The lack of this inhibition could cause the synesthetic experiences.

However, the confirmation of a genetic predisposition for the development of synesthesia does not exclude the possibility that people without synesthesia can develop it with proper training, or may develop a somatic mutation during development.

There are numerous examples throughout history that illustrate the synesthetic experience.

The Russian author Vladimir Nabokov (1899-1977), whose best-known work is “Lolita” (1955), experienced this phenomenon all his life (figure 1). In his case, the inducing stimuli were letters or words to which he attributed a color, always the same, regardless of the tone in which they were printed. The “A” letter would always be red, and the word “phone” would always be yellow.

In his 1967 memoir “Speak, memory” he described this way the colors he saw as he heard the names of the letters:

The long ‘a’ of the English alphabet…has for me the tint of weathered wood, but a French ‘a’ evokes polished ebony. This black group also includes hard ‘g’ (vulcanized rubber) and ‘r’ (a sooty rag bag 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.”

Adjacent tints do not merge, and diphthongs do not have special colors of their own, unless represented by a single character in some other language (thus the fluffy-gray, three-stemmed Russian letter that stands for sh [Ш], a letter as old as the rushes of the Nile, influences its English representation).”

In the green group, there are alder-leaf f, the unripe apple of p, and pistachio t. Dull green, combined somehow with violet, is the best I can do for w. The yellows comprise various e’s and i’s, creamy d, bright-golden y, and u, whose alphabetical value I can express only by ‘brassy with an olive sheen.’ In the brown group, there are the rich rubbery tone of soft g, paler j, and the drab shoelace of h. Finally, among the reds, b has the tone called burnt sienna by painters, m is a fold of pink flannel, and today I have at last perfectly matched v with ‘Rose Quartz’ in Maerz and Paul’s Dictionary of Color. The word for rainbow, a primary, but decidedly muddy, rainbow, is in my private language the hardly pronounceable: kzspygv.”

His son, the opera singer Dimitri Nabokov, was also a synesthete, which confirms the genetic component of this feature.

Another famous case was that of the Hungarian composer, virtuoso pianist, conductor and teacher Franz Liszt (figure 2), who saw colors and shapes when he heard different sounds. In 1842, when he began as Kapellmeister in Weimar, he astonished the orchestra when he said: “Oh please, gentlemen, a little bluer, if you please! This tone type requires it!” Or: “That is a deep violet, please, do not forget it! Not so rose!” First, the orchestra believed Liszt was just joking, but later they got accustomed to the fact that the great musician seemed to see colors where there were only tones (figure 3).

Other several musicians, such as the Russian Rimsky-Korsakov (figure 4), who synesthetically experienced colors for musical keys, the French Olivier Messiaen (figure 5), whose three types of complex colors are rendered explicitly in musical chord structures that he invented, and Jean Sibelius (figure 6), for whom “there existed a strange, mysterious connection between sound and color, between the most secret perceptions of the eye and ear“, experienced this phenomenon.

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.

The American composer, pianist and bandleader of jazz orchestras Duke Ellington (figure 7) commented: “I hear a note by one of the fellows in the band and it’s one color. I hear the same note played by someone else and it’s a different color. When I hear sustained musical tones, I see just about the same colors that you do, but I see them in textures. If Harry Carney is playing, D is dark blue burlap. If Johnny Hodges is playing, G becomes light blue satin.” Miles Davis (figure 8), one of the most important figures in the history of jazz, had synesthetic experiences.

Many other artists experienced synesthetic perceptions. In visual arts, Wassily Kandinsky (figure 9) was a synesthete and with respect to the music of Richard Wagner, he said: “I could see all those wonderful colors in my mind, paraded before my eyes. Wild, wonderful lines that were drawn to me“. So he tried to use this combination of senses to paint box where he expressed the visual equivalent of a symphony of colors.

Some scholars suppose that Vincent van Gogh (figure 10) had a kinesthetic/visual synesthesia because of the way he perceived and expressed the way he physically “felt” about the objects he saw, and the paint he used to express his unique kinesthetic vision.

French poets Charles Baudelaire (figure 11) and Arthur Rimbaud (figure 12) wrote of synesthetic experiences, but there is no a clear 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 in 1871 “Voyelles”, 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!).

In the scientific field, the legendary Serbian-American inventor Nikola Tesla (figure 13) wrote in his book “My Inventions” about his unusual sensory experiences: “When I drop little squares of paper in a dish filled with liquid, I always sense a peculiar and awful taste in my mouth“. His other synesthetic qualities included eidetic memory and prodigious visualization abilities. Richard Feynman (figure 14), who received the Nobel Prize in Physics in 1965, was also a synesthete. “When I see equations, I see the letters in colors”, he once wrote. He describes his colored equations in his autobiography, “What Do You Care What Other People Think?”

Norman Mailer, in his biography of Marilyn Monroe (figure 15), says: “She has that displacement of the senses which others take drugs to find. So she is like a lover of rock who sees vibrations when she hears sounds.” The Australian actor and film producer Geoffrey Rush (figure 16) said: “Synesthesia is in the mind, not the brain“. He experiences colors for the days of the week as well as numbers: “Friday is dark maroon, a type of sienna, and Saturday is definitely white. Monday is a cool blue… Since I was seven, when I first learnt counting, numbers had specific colours.”

But a question arises: Aren’t we all a little synesthetes? How often we relate two different modalities in everyday life, such as a color that is “squeaky” (as yellow), or a song that is “sweet” and “soft” or a color that is “bright,” as white? Do different degrees of synesthesia exist, and are expressed in some people more than in others?

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.

Another relation between sound and colours is found in the Doppler effect, this is the change in frequency of a wave for an observer moving relative to its source. The Austrian physicist Christian Doppler (1803-1853) first proposed this effect in Prague in 1842 in his treatise “On the coloured light of the binary stars and some other stars of the heavens”. The hypothesis was tested for sound waves by the Dutch scientist Buys Ballot (1817-1890) in 1845.

One of the first scientific publications about synesthesia was the one from Sir Francis Galton (figure 17), a British anthropologist, inventor, psychologist and statistician who contributed to different areas of science. He published in 1880 an article about this phenomenon in Nature, although its impact was limited. Before Galton, the Austrian Ludwig Georg Tobias Sachs published in 1812 a medical dissertation regarding his own and her sister’s albinism, and described another phenomenon that involves color synesthesia for music and for simple sequences (numbers, days and letters). But his work, published in Latin and in the third person, had less impact than Galton’s, published 70 years later. However, it seems to have been the first synaesthete described in the literature. But many of the early studies dedicated to synesthesia were cataloged as subjective by behaviorism and were shelved because of the lack of empirical evidence that prevented to give concrete answers to the questions for many years.

In the 1980s the American neurologist Richard Cytowic was the first to predict something that advances in electro-physiological recordings and positron emission tomography (PET) have confirm later: the simultaneous activation or “co-activation” of two sensory areas or more of the cerebral cortex in synesthetes. Later in England, Simon Baron-Cohen and Jeffrey Gray explored the reality, consistency, and frequency of synesthetic experiences. Vilayanur Ramachandran and Edward Hubbard raised the local interconnection model (“crossover cable”). Weiss (2001) and Nunn (2002) used functional magnetic resonance imaging (fMRI) in different synesthetes and showed the activation of area V4 when listening names of relatives or different tones. They, among others, made of synesthesia a scientific entity empirically demonstrable.

The specificity of synesthesia remains stable throughout the life of the individual, and is not the result of imagination: it has psycho-physiological basis and is described by most neurologists as an “abnormal communication between brain areas,” or “crossover cables”. The investigations showed that in the brains of synesthetes, the graphemes (the minimum unit of writing that corresponds to the letters) activate brain regions of that grapheme but also related regions, for example, the one of colors, as V4 region (a region of the visual cortex that shows a response to colors). These results support the theory that synesthesia is a result of existing brain connections, which connect different cortex regions, possibly due to the lack of disconnection between regions that are interconnected in the fetus.

The stimuli that would be designed to be decoded by a single sensory system generate an activity in other systems that evoke the synesthetic sensation. So, this theory asserts that the difference between a synesthetic brain and a non-synesthetic brain is merely functional.

Synesthetes do not speak in terms of imaging a color or remembering an odor when exposed to the inducing stimulus, but refer to a specific halo of color, a particular tactile sensation or a taste on the tongue. It is a truly sensory phenomenon.

So we can say that synesthesia is a disorder of perception, but not one that remove capabilities, such as blindness or deafness. On the contrary, it adds new perceptions, enhancing the original. This phenomenon also enhances the creative activity of those who have it, so is common a close relationship between art and synesthesia (most of the characters mentioned are part of the world of art).

However, it was not always easy for people with synesthesia to talk about their different ways of perceiving the world, mainly because of hostility toward the bizarre. In past times, these people were probably cataloged as drug addicts, schizophrenics, or mentally ill, even sorcerers. Those who claimed to see auras, smells and sounds, or taste in music were considered insane, and many ended up in the psychiatric hospital.

So, what happens to non-synesthetes? Why do we use synesthetic metaphors in everyday life? Could such metaphors we use as part of our vocabulary be considered “cultural synesthesia”? Probably the yellow colour does not sound like a sharp cry in our heads, nor the song provoke us a sweet taste in our mouth. We use a word with a syntactic function that does not belong to itself.

People with synesthesia enjoy this phenomenon and not allude to their perceptions as metaphors, they experience them truly. They don’t need any kind of hallucinogenic drugs like LSD or mescaline to generate their experiences.

There is a psychological experiment called “Bouba and Kiki”, first designed by Wolfgang Köhler (figure 18), that demonstrates that people do not attach sounds to visual shapes arbitrarily. People are asked to choose which of these shapes (figure 19) is named Bouba and which is named Kiki. 95% to 98% of people choose Kiki for the angular shape and Booba for the rounded shape.

It is thought that this has implications for language development, in that the naming of objects is not completely arbitrary. The rounded shape may most commonly be named Booba because the mouth makes a more rounded shape to produce that sound. Similarly, a more taut, angular mouth shape is needed to make the sound Kiki. The sounds of a K are harder and more forceful than those of a B, as well. Note also that, in the Roman alphabet, the angular shape mimics the angular letters K and I, while the rounded shape mimics the rounded letters B and O.

In people with synesthesia exist, then, a higher number of connections between two areas of the sensory cortex of the brain, so that at least two directions are inseparable. Could be seen as an evolutionary trait?.

Cytowic has extensively discussed the evolutionary purpose of synesthesia and has come to ask to what extent this feature is an adaptive product of natural selection or a kind of product of adaptation. It is a matter of further research and further deepening and advance in the study of this incredible and amazing feature.


María Andrea Buzzi

National Collaboration
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abril 2020
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