Does the sound of a drill smell like bleach to you? I don’t mean that in a poetic, “trying to be interesting” kind of way. I mean, when you hear the sound of a drill, can you ˛ąłÜłŮ´Çłľ˛ąłŮľ±ł¦˛ą±ô±ô˛âĚýsmell bleach?
Odds are, you can’t, but for someone out there, that is their reality.
This person, whose uncommon experience has been , is in good company. When classical composer Jean Sibelius would confide to his own experiences, he would reveal that looking at the yellow picture near his fireplace , the same key as Pachelbel’s famous canon. Everything he saw triggered in his ear a particular sound. Kandinsky, a pioneer of abstract art, left a mark on the history of the visual arts by  when painting. Meanwhile, physicist Richard Feynman saw : his mathematical formulas appeared to him possessed with distinct colours, and he wondered how they must look to his students.
These uncanny phenomena all fall under a common name: synaesthesia, meaning “to feel together.” It has often (and wrongly) been conveyed as simply a merging of the senses, where one type of stimulus ends up triggering a different one. Synaesthesia is a type of experience that, because of its quirkiness and—up until recently—our inability to detect it objectively, has often been dismissed over the centuries as a mental illness, an artistic affectation, or a pointless claim unworthy of rigorous investigation.
But an accumulating and occasionally criticized body of evidence in the past two decades has shown us two important things about synaesthesia: that it does exist and that, like a wet fish, it is proving to be very hard to pin down.
The scarlet letter and other wild combinations
Synaesthesia is vast. Depending on whom you read, there are between 60 and 150 documented forms of it. For some, letters and numbers trigger colours. For others, days of the week are automatically arranged in space. Yet other synaesthetes—as they are known—perceive distinct tastes when they hear or see certain words. People often simply develop it at a very early age (“developmental synaesthesia”) or, more rarely, it is acquired later in life due to a change in the body, like becoming blind or receiving trauma to the brain. Certain drugs can also elicit synaesthetic experiences temporarily, although  have specifically tested for this.
Basically, synaesthesia can be described as a combination of two elements. There is the inducer, which is the thing that triggers the synaesthetic experience. And there is the concurrent, which is the thing that is experienced as a response to the trigger. In a common type of synaesthesia, the inducer is a grapheme, like a letter or a digit, and the concurrent is a colour. For many grapheme-colour synaesthetes, the letter A appears red. It is not a choice they are making; rather, their brain is automatically giving the letter “A” a scarlet hue. In fact,  say that their concurrent is a colour, which makes me wonder if some of the people claiming to see coloured auras might actually have synaesthesia.
You can already imagine a woman in the Middle Ages admitting she sees black-inked texts as rainbow-coloured landscapes and the loud accusations of witchcraft that would have followed. Seeing the world differently than the majority is often fraught, and how you will be judged depends on the mores of the era.
Synaesthesia’s first documente case is often said to be that of Dr. George Sachs who, in 1812, wrote in his medical dissertation about the colours he and his sister could see accompanying numbers and letters, but allusions to synaesthesia actually , and it was treated with an open mind by . The phenomenon acquired its name in the decades leading to the 1900s, as its catalogue of subtypes grew and grew, and place-holder names like hyperchromatopsia and pseudochromesthesia were dropped in favour of synaesthesia.
But the 20thcentury was not kind to it. With the rise of behaviourism in psychology, scientists only took seriously that which they could observe. A patient claiming to see the letter A as red was not something that a scientist could testify to. Synaesthesia was trivialized and sometimes attributed to mental illness. It was not until the development of brain imaging technology and of psychometric tests like the modified Stroop test that scientists finally got the data they were looking for.
Yes, synaesthesia was real.
The Stroop test measures a person’s reaction time in two different conditions. Imagine being asked to quickly name the colour a word is printed in when the word itself is the name of a colour. It’s easy when the word is “PURPLE” and it is printed in purple ink. But when the word is “PURPLE” and its colour is green, saying “green” means fighting our reflex to say “purple.” It causes a delay. That is what the Stroop test measures.
This Stroop test has been adapted to test people claiming to have grapheme-colour synaesthesia. People with this type of synaesthesia are simply faster to answer correctly when the colour used is congruent with the colour their brain generates through synaesthesia, and they make fewer mistakes, than when the colour clashes with the synaesthetic halo.
Another useful tool is the test of genuineness, in which synaesthetes are asked to describe their concurrents after being exposed to many potential inducers (“which colour is this letter? And this one? And this one?”). Without warning, they are tested again many months or years later. Their answers are almost always identical, whereas those of non-synaesthetes are not.
While initial self-reports led researchers to believe that many more women had synaesthesia than men, it seems to affect  when less biased means of assessment are used. A on the topic summarized the prevalence of different forms of synaesthesia. Seeing days of the week as having specific colours seems to affect 2.8% of the population. For graphemes eliciting colours, it’s 1.4%. For tastes being associated with particular shapes, it’s 0.2%. These types are not mutually exclusive; most synaesthetes experience more than one form of synaesthesia, and this quirky phenomenon clearly runs in families, which means a genetic predisposition, probably involving more than one gene, is likely to play an important role.
But despite a battery of tests and medical imagery trying to crack the brains of synaesthetes, there is an argument to be made that synaesthesia is just as incomprehensible today as it was a hundred years ago.
Not a merging of the senses
You may have noticed me dancing around the definition of synaesthesia. That is because scientists still cannot agree on how to define it properly.
In 2011, Professor Julia Simner, now at the University of Sussex and the Science Officer of the UK Synaesthesia Association, published , questioning some of the assumptions her colleagues had been making in trying to fence in synaesthesia.
It was often referred to as a “merging of the senses,” like taste and hearing suddenly melded together in someone’s brain, but most cases of synaesthesia only involve a single sense. Seeing the letter “A” as red is a merging of vision with vision. In fact, in many cases, it’s not a sense at all that triggers the experience, but rather some higher function of the brain. It’s not just “A” that appears red to these synaesthetes, but “a” as well, and a handwritten “a” too. It is not the symbol that triggers the colour but its meaning.
Professor Simner also questioned her field’s obsession with consistency. The synaesthetic experience, researchers agreed, was to be consistent over time in order to qualify as synaesthesia. If Billy saw “1” as green and “2” as purple today, he needed to have this exact experience six months from now, and a year from now. This rule was put in place presumably to rule out malingerers or particularly imaginative artists who might misunderstand what researchers meant when recruiting participants for a study on synaesthesia.
But it became a self-fulfilling prophecy: synaesthesia hadto be consistent over time becausescientists decided that it must be and recruited study subjects accordingly. Likewise, it was often assumed that all synaesthetes could see their experience in the space in front of them, but for some, the experience was intangible. Some see the “A” as red on the page; others see it as red in their mind’s eye; yet others simply and involuntarily know the “A” to be red.
What Simner did was whittle down the pillars of the scientific-sounding definition of synaesthesia—that it was an automatic, consistent, and spatially extended merging of the senses—and concluded that, well, synaesthesia was really hard to define. It was a type of unusual crosstalk between brain functions. There is an inducer and there is a concurrent—a trigger and an experience—and it is spontaneous, effortless, and accepted by the synaesthete as normal for them.
Likewise, all those magnetic resonance imaging studies claiming to find this or that part of the brain being associated with this or that type of synaesthesia may not amount to much. A  by Hupé and Dojat published in 2015 feels like taking one step backward after taking one step forward. “In our view,” the authors conclude, “most published studies to date show, in fact, that the brains of synaesthetes are functionally and structurally similar to the brains of non-synaesthetes.” So why are there so many studies claiming the opposite? Statistical errors, questionable choices of methodology, or low statistical power. If you only test 4 synaesthetes and look at their brains from every conceivable angle, you are bound to find noise that looks interesting.
This reanalysis is pretty damning to the modern hypothesis that synaesthesia is due to wires being crossed in the brain, or to a lack of inhibition in certain parts of the brain, or to a pruning of certain connections that normally takes place early in development but that got skipped. If brains bathed in synaesthesia are the same as those that are not (and future studies may reliably prove otherwise), then why are some people synaesthetes, but not the rest of us?
There is a link between synaesthesia and artistry that deserves to be explored, as it may provide an answer to this question. It seems that , and it is easy to see why. If calendar dates dance around you, if letters come to life in vivid colours, if you can see halos and movements when listening to music, you might dedicate your life to exploring these sensations. But this leap in logic might be premature. What if it’s not synaesthesia that leads to artistry, but an artistic bent that creates synaesthesia?
There is a theory that synaesthesia might be a way that certain children develop when first learning about abstract concepts, like letters, numbers, or music being played over time. Indeed, adults who do not have synaesthesia , to a degree, to develop an automatic association between, let’s say, a particular letter and a colour, although the association fades over time. If forged in the early years of childhood, however, this association could conceivably persevere and might simply be an imaginative tool that a child uses to learn about the world. Indeed, the  about synaesthesia is that it helps with learning, although research on whether synaesthesia helps with memory has produced ambiguous results.
Others have hypothesized that alphabet books or coloured wooden blocks with letters on them, used as children’s toys, might create a grapheme-colour association that persists in adulthood, although  does not lend credence to this idea. It is interesting to note that synaesthetes who see letters as being coloured  that As are red, Bs are blue, Cs are yellow, and Os are white. Why that is, no one is sure.
What we are left with is a very real phenomenon that so far has defied explanation. We have no clear, unambiguous definition for it. We do not know if we are all born with it, bathing in a “sensory bouillabaisse,” until most of us lose the ability. We have no strong evidence that the brain that generates it is distinctly and reproducibly different. And even though it runs in families, related synaesthetes often do not have the same inducers and concurrents. They experience synaesthesia, but its expression is different.
The good news is that synaesthesia is not a medical condition in need of fixing. Most synaesthetes like their synaesthesia. Some even describe the sensation as . The occasional negative feelings often have to do with mismatches: when letters are printed in the “wrong” colour, a synaesthete may be left slightly annoyed. And , like autism spectrum disorder. It is, after all, where our understanding of many brain-related disorders and conditions has led us: away from a strict binary and into the spectrum.
Composer Franz Liszt is reported to have told the orchestra he was conducting, “Oh please, gentlemen, a little bluer, if you please!” The musicians initially thought he was joking. But he was not.
To Liszt, and to other synaesthetes, music has a colour. We may not yet understand why, but we can certainly appreciate its beauty.
Take-home message:
- Synaesthesia is a phenomenon that is hard to describe, where brain functions end up talking to each other in a way that is unusual, such that specific letters, for example, will always be perceived as having specific colours
- Through the use of objective psychometric tests, scientists have shown that synaesthesia is indeed real
- Although studies have reported differences in the brains of people with synaesthesia that might be causing this phenomenon, these studies are of low quality and a recent reanalysis of them shows no apparent difference in the brains of people with and without synaesthesia
