The word “Monday” is pink to Helen Besgrove – a pink watercolour wash. For her identical twin, Kirsty Neal, it’s red – specifically, red fading to brown. In her mind’s eye, Helen sees the letter A as black with a white line through it. Kirsty sees A as yellow with patches of white.

As youngsters, the twins would play a game where they’d say a word and then call out its colour – they’d never land on a match. Their mother saw colours with words, too, and would ask, “What about red?” The girls would reply: “It’s not actually red.” (For Helen, the word red is green with bits of purple; for Kirsty, it’s brown in a sandpaper texture.) They assumed everyone must conceive of language in colours. “It was never really questioned,” Helen says.

Then, one day, Helen was listening to a radio segment with her university class that described violin music as silver and shiny. “No way – that’s yellow with a stripe of orange through it!” she told the class. The segment went on to mention a neurological trait called synaesthesia. “My whole class had this awakening at the same time as I did,” Helen recalls. “I had it! So I raced to call Mum and Kirsty to tell them what we had was unique.”

Synaesthesia is a crossing of the senses. Some synaesthetes, as they’re known, experience sounds as colours, or they can taste words. In fact, experts have found dozens of types of synaesthesia, depending on which senses are paired, in between 1 and 4 per cent of people.

Some “smell” words or “see” plumes of colour when they walk past a freshly cut lawn or a fragrance counter. Some can “feel” flavour. Most don’t realise this is unusual until someone questions their description of the world. “It’s just a thing that is,” says Kirsty. “And you’ve never known anything different.”

What is synaesthesia? How do synaesthetes experience life? And how do they put this rare trait to use?

How does synaesthesia work?

German physician Georg Sachs noticed something about himself as he sat writing one day in 1812. He saw things differently. The alphabet, for example, conjured very specific colours: A was cinnabar, C was pale ash. “These introduce themselves to the mind as if a series of visible objects in dark space,” he wrote, in the first documented case of synaesthesia. Seventy years later, British polymath Francis Galton, a cousin of Charles Darwin, was fascinated by people who saw sequences of numbers in “long vistas” and curves. There was, Galton wrote, a “tendency among sane and healthy persons to see images flash unaccountably into existence”.

French composer Olivier Messiaen was one of many famous synaesthetes. “When I hear or read music, I always see colour complexes in my mind that go with the sound complexes,” he wrote. Russian-born abstract artist Wassily Kandinsky described hearing colours when listening to Wagner. “They stood before my eyes. Wild, almost crazy lines were sketched in front of me.”

Today, US musician Pharrell Williams says he knows “when something is in key because it either matches the same colour or it doesn’t”. New Zealand singer Lorde describes making music as “getting the actual thing to sound like what I’ve been seeing”. Californian pop star Billie Eilish says every day of the week has a colour, number and shape. “It inspires a bunch of stuff; all of my videos, for the most part, have to do with synaesthesia.”

In his 2007 book, Musicophilia, British neurologist Oliver Sacks tells the story of jazz composer Michael Torke, who associates colours with scales and keys, and distinguishes such perceptions from mere whimsy. “The colours have been constant and fixed since his earliest years, and they appear spontaneously,” Sacks observes. “No effort of will or imagination can change them.” This is why tests for synaesthesia often involve circling back to prompts several times – a synaesthete will usually describe the same association every time. (US neuroscientist David Eagleman designed a collection of standardised tests in 2007.)

He could feel its taste – well-cooked chicken tasted prickly, but this one was flat.

The word itself – from the Greek syn (together) and aisthesis (sensation) – was first used in the 1890s. “Everybody knows the word anaesthesia, which means no sensation,” neurologist Richard Cytowic tells us from Washington, DC. “Synaesthesia is a joined or coupled sensation. Children are born with two or more senses hooked together, so that my voice, for example, is not only something that they hear, but they might also see it, or taste it or feel it as a physical touch.”

Sacks, who died in 2015, and Cytowic exchanged emails about synaesthesia; in Musicophilia, Sacks notes that it was Cytowic who renewed interest in the phenomenon, in the ’80s. Cytowic had read about it in old medical books and then was invited to dinner with a neighbour, Michael Watson, a stage lighting designer in New York. Watson apologised that he’d sampled the chicken and it wasn’t ready as it didn’t have “enough points” on it. What he’d meant was he could feel its taste – well-cooked chicken tasted prickly, but this one was flat. Cytowic was gobsmacked. “I said, ‘Oh, you have synaesthesia!’” he recalls. “He said, ‘You mean there’s a name for what I do?’ And I thought, how can he not know?” Watson, who had a rare form of taste-to-touch synaesthesia (fewer than 1 per cent of cases), later described the taste of mint as like reaching out to touch a cold glass column.

For most synaesthetes, the pairing is triggered by reading (or thinking about) letters or words, called grapheme-colour synaesthesia. They also commonly report visualising time in an atypical way: “People might describe their current decades close to their body, the past is behind them and going off into the future like a zigzag,” says Anina Rich, who leads the synaesthesia research group at Macquarie University. Other synaesthetes “see” pain or, stranger still, others’ personalities. “If someone’s got, like, a really good laugh, they might have a bubble in their personality, like a bit of a wobbly line,” says Kirsty.

Someone with one form of synaesthesia has a 50 per cent chance of having another form. As well as words, smells evoke particular images for Kirsty. “You know how some people just have those lingering smells?” she says. “Well, that can kind of change the silhouette of their colour, it might have a bit of a fuzzy edge to it.” Less common is when words produce tastes. James Wannerton famously created a map of the London Underground with the flavours of each stop: Piccadilly Circus was a Picnic chocolate bar and King’s Cross was a fruitcake. One of Anina Rich’s students describes hearing motion, such as a “soft thump” when people blink. “We don’t know yet whether this form is rare or really common.”

‘Mirror-touch synaesthesia is a particularly peculiar phenomenon: when a person sees someone being touched and feels it themselves.’

Synaesthetes are not actually seeing these colours (or smelling these smells, and so on) as if they are real; they appear in the mind, as associations. (Helen likens her visual perceptions to those we form when we read a book. “I don’t see colours on paper, or the road doesn’t turn green when I’m thinking about maths while driving.”) Anina Rich has studied the brains of people with grapheme-colour synaesthesia and found the signal that perceives a word written in black as red, for example, comes a fraction of a second after the person has seen the black writing. “It could be the difference in timing in the brain that tells the system these are actually triggered by different things.”

Mirror-touch synaesthesia is a particularly peculiar phenomenon: when a person sees someone being touched and feels it themselves. While it’s been documented in about 1.6 per cent of the population, Rich says research by one of her PhD students, Sophie Smit, suggests numbers could be as high as 6 per cent. “We have certainly had mirror-touch synaesthetes say they avoid graphic movies,” Rich says. “Even watching TV can be quite difficult because if someone gets punched, they feel it.”

What causes synaesthesia (and can you inherit it)?

As a seven-year-old, Carol Steen was walking home from school one day when she told her best friend the letter A was “the prettiest pink I’d ever seen”. Her friend stopped, gave her a piercing look, and said: “You’re weird.” They never spoke again. At 20, Steen told her father the number five was yellow. “And my father said, ‘No, it’s yellow ochre’.” He was the only other person she knew who saw colours, but he was reticent to speak about it. “I didn’t get another peep out of him for 30 years.”

Steen became a sculptor and, in her 40s, was working on cartoon characters for fast-food chains when she heard a radio interview with neurologist Richard Cytowic describing synaesthesia. She started to cry. “It was like being in a mental jail. I had the questions but I couldn’t find the answers. Cytowic, by giving me knowledge, basically gave me freedom.”

‘It’s similar to having perfect pitch, which is another perceptual trait which runs strongly in families. It appears at an early age without any effort whatsoever.’

Cytowic himself endured withering looks from colleagues over his research. “People would say, ‘Man, this can’t possibly be a real brain phenomenon, this cannot exist’.” He’d thought he took after his father, a doctor fond of offbeat research, but Cytowic, who is gay, now believes he persisted because of the discrimination he experienced growing up. “I wasn’t supposed to exist. So this notion that this thing cannot possibly be real was like, man, I have heard this before.”

Genes are thought to influence synaesthesia but the exact number involved and how they become expressed remain a mystery. Some people with synaesthesia do not know of anyone in their family tree with it. Still, studies show people whose parents or siblings have synaesthesia have about a 40 per cent chance of having the trait themselves. Says Cytowic: “If you do have it, it’s similar to having perfect pitch, which is another perceptual trait which runs strongly in families. It appears at an early age without any effort whatsoever. You cannot practise it and develop it. It’s just there, and it’s lifelong.”

According to one school of thought, people with synaesthesia have different connections than most people between parts of the brain responsible for, say, processing sound or colour perception. If that were so, some researchers believe synaesthesia might be linked to a process called synaptic pruning: babies have more brain connections than adults, which, as they experience the world, are “pruned”. Theoretically, if this process was disrupted in people with synaesthesia, they might retain additional connections.

Another school of thought says synaesthetes have the same but stronger brain connections as other people. “We know that, essentially, neurons that fire together, wire together,” says Rich. “That’s how learning occurs. So our brains have evolved to make connections. If every time I play a sound to you, I show you a colour, then the connections between those areas of your brain are going to get stronger.

“Maybe what happens is that if you’ve got a genetic predisposition to develop synaesthesia, and then you’re exposed at the right time to particular coloured letters as you’re learning your letters, then that becomes a very strong link.” However, scientists have found that a colour-blind person with synaesthesia reported “Martian colours” that he could not have seen with his eyes. Either way, Cytowic says, synaesthesia is a reminder that “making a human being is ungodly complicated”. “Everybody has a different roll of the dice.”

What are the benefits of synaesthesia?

“If you ask people what good is the trait, without fail, they say it helps you remember,” says Cytowic. Synaesthetes he’s tested have “memory quotients” much higher than the “standard deviation”. Twins Helen and Kirsty say they have excellent memories. Kirsty, a specialist doctor in Alice Springs, found colours particularly helpful in medical school. “I feel like I cheated,” she says. She can recall almost verbatim what she reads in books, particularly if she uses highlighters or patterns to mark text while studying. “Literally, I can just read off the page,” she says. “I got mostly high distinctions.”

She still taps into the colours her mind has assigned to medical conditions, such as when she surprised her colleagues by recalling the name of the rare Creutzfeldt-Jakob disease. “The disease itself is these little prions, and they’re kind of grey in my head, and the name of it is green and brown because of the letters in the word.”

Helen put the trait to use in a contest for coffee company Nespresso, where she worked for 10 years, which tested her ability to blind-taste coffees. She was so accurate the company flew her to Switzerland to compete in the finals. Her secret: recalling taste and aroma by colour. “I didn’t train myself; I was just genuinely able to attach more to a smell than just the smell – it had an attached feeling, colour, texture that gave me more stimuli.” There’s no particular overall logic to how her mind makes these connections. The only trend she notices is with sounds or pain. “Lower notes or duller, heavier pains are darker in colour.”

Dan Smith, a project manager at the University of Sydney, realised he had synaesthesia when he came across a poster asking people if they saw numbers in colours. “My first thought when I looked at it was that the numbers were the wrong colours. The two should be blue,” he laughs.

When Smith sees a phone number, he experiences strong colours; triples of the number six, for example, will evoke a lot of purple. He codes documents accordingly. “For me, a number has to correspond to the colour that gets assigned. Otherwise, it’s just a bit distressing to look at this list and see all these twos and have it not be blue.” Mostly, though, he’s sanguine about his sensory crossover: “It’s a natural variation, it’s a trait. It doesn’t cause any problems.”

A colour-coded memory does occasionally trip up some synaesthetes. Kirsty often mixes up names that have the same colour and texture. “Michael and William are the same colour,” she explains. Helen has this issue with directions. “When someone says left or right, both of those words are green for me,” she says. “So it takes me probably three or four seconds to process left and right.”

Why is synaesthesia associated with artists?

Melburnian Zinia Chan, who grew up playing saxophone and the piano, can see music. It can look a bit like Gustav Klimt’s famous painting The Kiss. “I will start seeing textures and movements and colours in front of me. When that happens I just try to stare at it because it’s just such an incredible experience.”

When she was 27, Chan learned she had at least 16 forms of synaesthesia. “When I’m listening to music, I would naturally be drawn towards certain textures or colours, and then I would just select those colours and see where it goes.” More often than not, her main impulse is to associate particular feelings with sounds, giving another layer to the mental images she conjures up. And occasionally, if music moves her emotionally, she sees projected colours in her vision and not just in her mind’s eye.

‘For [Hockney], it wasn’t the pitch itself, the sound of the note, but rather the sequence of them that determined the colour.’

Chan has turned her own composition, Gaze Upon the Liquid Sky, into a virtual-reality scene to give people a sense of her inner world. She has created a night sky with bright pink stars sparking from a burning bright mass. Pastel blue stars shimmer within arm’s length. Soft oboe and piano play alongside birdsong and crickets. It’s like walking in a planetarium, only this is her colourful universe.

The scene, which she shows us at a television studio next to the Melbourne Conservatorium of Music, is not an exact representation of what’s in her head. That would be nearly impossible, she says, as the colours in her mind flash with the notes then vanish. “I get a lot of imagery.”

Studies have found between 7 and 24 per cent of artists have synaesthesia, well above the general population. Cytowic says a higher number of synaesthetes are drawn to art, but whether the trait assists creativity or not is “a chicken-and-egg thing”. He determined that painter David Hockney had synaesthesia in the 1980s after reading that the artist was painting stage scenery to music. Cytowic tested Hockney at his Los Angeles home, playing random sounds and asking the artist to choose a coloured chip. “For him, it wasn’t the pitch itself, the sound of the note, but rather the sequence of them that determined the colour.” He believes synaesthetes do have a talent with metaphors, or “seeing the similar in the dissimilar. Basically, synaesthesia is the ability to link seemingly unrelated things.”

For synaesthetes, a lifelong association with, say, C-sharp and a particular shade of green can be powerful. It’s what Solange Glasser, a researcher at the University of Melbourne, calls “the real relationship” synaesthetes have with what they see. “They recall the colours and the musical keys as their friends. It [is] very strongly tied to their sense of identity.”

Chan says synaesthesia helps her write music because “it allows you to compare different sound palettes and timbre that you won’t naturally access”. But it can limit her, too. “It can definitely get in the way of writing the most easily played music, or music that is technically correct.” Glasser says it can be “a bit of a curse” if the way the music sounds becomes “less important than what they’re visually trying to recreate”.

Sometimes, she replays a trumpet note hundreds of times to get an exact red. ‘It’s trying to take the essence of what one sees in a split second and put it onto a single canvas.’

When Carol Steen started out as an artist, she worked with metal to avoid colour – she didn’t know if seeing colour “was a good thing, or a bad thing”. “Having lost my best friend at age seven, I truly believed that silence was safer.” But she later found that her “non-colour” sculptures followed specific geometric patterns called “form constants”. In 1926, psychologist Heinrich Kluver discovered these patterns in people who took the hallucinogen mescaline and reported seeing lattices, cobwebs, tunnels and spirals. Many people with synaesthesia also see these patterns. “I had no idea I was working with my synaesthesia, but I was,” Steen says.

It wasn’t until she was in her 40s that she used colour to paint sound. Sometimes, she replays a trumpet note hundreds of times to get an exact red. “It’s trying to take the essence of what one sees in a split second and put it onto a single canvas.”

The desire to share her vision – she’s had more than 20 solo shows – is what drives her art. “I see something that is so gorgeous [and think], is there a way I can show that to other people, share that with them? ” And when she meets other kindred spirits, she’ll ask, “What colour is Tuesday?” Yellow, they might say. “Tuesday is not yellow. Friday, however, is yellow,” she’ll reply, playfully. “I mean, you’ve had this since you can remember, and you’ve never had a chance to play with it,” she says. “So let’s play.”

Get fascinating insights and explanations on the world’s most perplexing topics. Sign up for our weekly Explainer newsletter.