Everyone in the room could hear the punchy guitar riff blasting out of the amplifier - but only Poppy Porter could see it. She was on stage at an arts centre in Guildford, England, plunging her hand into a giant box of wax pastels, in a frantic search for just the right colour. Turning to the huge screed of blank white wallpaper set up on an easel behind her, she began to draw the lines, the splodges, the spikes or circles that fizzed in front of her as her guitar-playing friend Steve continued improvising.
Shapes burst into Porter's vision, suddenly blooming, then fading away exactly in sync with the music. She scribbled as fast as possible- and still only caught a fraction of what she was seeing. By the end of the performance, Porter had filled dozens of metres of paper with abstract shapes and colours. Just a tiny glimpse of what it's like to be a synaesthete, someone who experiences 'crossed senses'. For some synaesthetes, words have colours; for others, music can induce the feeling of being touched. There are many different forms.
Although stories about synaesthetes go back centuries, it is only relatively recently that the phenomenon has been taken seriously by scientists. Studies suggest it is rare - perhaps only about 4.4 per cent of people have a form of synaesthesia. But no-one really knows, partly because it's quite common to have synaesthesia and not realise. This is because, for a synaesthete, that's just the way they've always perceived the world - there's potentially no reason to report or register it. During research for this article, I spoke to more than one academic who only found out that they themselves had a form of synaesthesia when they began investigating the topic and writing studies about it.
The 8.28 billion human brains around the world today do not all process information in exactly the same manner. Synaesthesia is just one example of the remarkable diversity living inside our heads - and for some musically gifted synaesthetes, it might even shape the songs they write or the sounds they prefer.
For Porter, a jewellery designer and artist, sound has always had a visual dimension. She describes the "peacock tails" she has seen at Formula 1 races, or the "C-shapes" noisy jackdaws in her garden are making while she speaks to me on the phone. But it was only as an adult that she discovered most people don't see sound. "I never had a name for it until later in life," she says. Being dyslexic, she eventually found a reference to synaesthesia in books about neurodiversity. Today, she leverages her synaesthesia, using it to come up with unique jewellery designs.
Porter's live synaesthetic jam sessions, which took place a few years ago, were an attempt to put her perceptions down on paper in the moment so that she could share them with an audience. "It was amazing," she says. "The music and the art were so intertwined." Synaesthesia is a pleasant experience overall, she says, though the on-stage demonstrations were quite draining. Plus, some sounds that Porter doesn't like bring with them rather off-putting colours. Hand dryers in public bathrooms have a "horrible kind of nondescript, blacky, blurgh, colour", she says. And she has sometimes got up and switched off the radio, perturbed by the "nasty shade of orange" triggered by a certain song. "I don't need that in my life."
For some synaesthetes, the experience, while generally positive, can be overwhelming at times, says Mary Jane Spiller, a cognitive psychologist at the University of East London. And children might find that it can cause occasional difficulties at school. "I've heard people talk about when they've got coloured numbers [...] in school, they can find it quite distracting to focus," says Spiller. "If the sums don't add up to the right colour."
Every form of synaesthesia always has an inducer, the thing that triggers it, and a concurrent, the thing you perceive that someone without synaesthesia would not. In Porter's case, sound is the inducer and shapes and colours are the concurrents. For some people who have visual concurrents, they see them in their mind's eye - like the way you might see a triangle if I say, "imagine a triangle". This is called associator synaesthesia. However, some synaesthetes actually experience the shapes and colours entering their visual field, meaning those forms would partly obscure, say, a white sheet of paper in front of them. This is called projector synaesthesia and is considered especially rare.
For musicians or composers, being able to see the material they're working with, as well as hearing or feeling it, perhaps adds an extra workable dimension. Duncan Carmichael, an experimental psychologist at Edinburgh Napier University, points to the example of 20th Century French composer Olivier Messiaen, whose complicated and challenging music features multi-layered voices and elaborate rhythms. It's "not my cup of tea," admits Carmichael - but Messiaen's compositions are certainly highly regarded by many.
Messiaen was very vocal about the fact that he associated certain musical features, such as chords or intervals, with particular colours - and he even wrote an orchestral work that foregrounded this: Couleurs de la Cité Celeste, (Colours of the Celestial City). In it, he "specifies not only colours and colour combinations, but also their texture and gem-like character", notes one paper about the piece. Some passages were defined as topaz, others crystal-like, and so on. Messiaen seems to have used these music-colour associations to determine what he wrote, and the precise sounds he sought to create.
Singer-songwriter Lorde says that her synaesthesia directly influences her songs. While working on one called Tennis Court, for instance, the song started out a badly textured tan colour - "really dated", she once said. But as she came up with a pre-chorus and a lyric, the colours shifted to "incredible greens". There's a possibility that musicians and composers who have synaesthesia take special delight in music that is more complex than average. "It might make it more interesting," suggests Romke Rouw at the University of Amsterdam, who studies synaesthesia. Perhaps the additional perceptual impact of detailed colours and shapes means that synaesthetes are driven towards more intricate compositions.
People without synaesthesia often marvel at what it would be like to experience it, says Carol Steen, co-founder and president of the American Synaesthesia Association (ASA). "For us, we've had it since babyhood," she says.
But sharing the experience can be important. Steen remembers being seven years old and casually talking to her best friend at the time about how every letter has a colour. This was just a fact, like how the grass is green. But her friend just looked at her and said "You're weird."
Sadly, that ended the friendship and Steen has never forgot it. "Every once in a while, if I thought that it was safe, I would mention synaesthesia to somebody," she says. "I would always hope to hear the echo of, 'Oh yeah, I can do that too"." But she never did until some years later. Eventually, she became motivated to form the ASA, which acts as a kind of nexus for synaesthetes and scientists alike. At conferences and events, and also online, synaesthetes can connect and discuss research into the phenomenon.
Steen has more than one form of synaesthesia. Like Porter, she sees sound, but she also experiences letter-colour associations and visual features triggered by touch - such as acupuncture. Working with an animator, she has made shareable versions of the things she sees. Over Zoom, she shows me short animations of shapes and colours, including fuzzy green, blob-like forms raining slowly downwards, which is something she has experienced during acupuncture.
Steen suggests that certain shapes may be more common than others among the visual elements perceived by synaesthetes like her. She has a list of paintings by artists, some of whom are known synaesthetes, and they seem to share certain similar, abstract forms. There could be some relationship here to the famous hallucinatory shapes described by German-American psychologist and philosopher Heinrich Klüver during the 1920s. These form constants' - fractal-like patterns of spirals or tunnels, for example - are frequently reported by people who hallucinate. At least one study has found it is possible to use sound to induce such visual experiences even in non-synaesthetes after study participants spent around five minutes sitting in a dark room with their eyes closed.
Some researchers stress that hallucinations and life-long synaesthesia are different. But Steen is curious about whether there might be some connection. "Is it possible that people who are not synaesthetic, but who are taking hallucinogenic drugs, get synaesthetic perceptions?" she asks. "Are they the same, are they similar, is there some sort of crossover?"
These questions are certainly thought-provoking. As is an even deeper question: where does synaesthesia come from? Researchers have long known that it tends to run in families, which signals that there must be some genetic basis. Janina Neufeld of the Karolinska Institute in Sweden has investigated how synaesthesia may be associated with psychological features including autism, obsessive-compulsive disorder, anxiety and depression. There could be some link between the genetic factors that increase the chances of someone experiencing such features and also synaesthesia.
Neufeld stresses that synaesthesia, like autism, is not an illness or a disorder but rather a form of neurodiversity.
Environmental factors appear able to influence synaesthesia in those who experience it. Perhaps the best-known indicator of this was picked up by scientists about 20 years ago when they found that, for many synaesthetes who associate particular colours with particular letters, those associations actually match the coloured fridge magnet letters they had in their homes as children. It seems possible that synaesthesia can be shaped by people's development.
Some other associations - for example, between sound and position - are commonly held. Dog owners typically tell their dogs to 'stand' in a relatively high-pitched tone, whereas they instinctively say 'down' in a lower pitch. This is called 'sound symbolism' and researchers in 2025 showed that using the right tone or pitch appears to help even the dogs themselves understand meaning more quickly. A separate 2015 study found that pitch helped people identify the meaning of unfamiliar words - but synaesthetes were noticeably better at it, suggesting that they might have a heightened sensitivity to sound symbolism.
Carmichael says that we all have the ability to learn associations between, say, a word and a colour or a sound and a shape. "This is sometimes where the difficulty comes in- where does synaesthesia end and normal perception begin?"
He says experiments that appear to "induce" synaesthesia in people who do not typically experience the phenomenon may simply be a form of mimicking. Some stimuli seem to make vaguely synaesthesia-like experiences possible for a much broader swathe of the population. There's a well-known animated GIF, for example, of a slightly bizarre scene - an electricity pylon skipping over bouncing cables. It's completely silent but every time the jumping pylon hits the ground, some people say they can hear a thudding or smashing sound. Carmichael says he experiences this himself. "I'd say this is different to synaesthesia as you're 'hearing' an expected sound, rather than an idiosyncratic and unexpected response to a stimulus," he says. Though it is "quite odd", he adds.
For music-colour synaesthetes, the shapes and colours they see when they hear music, for example, tend to be incredibly varied - but they are also extremely consistent. This is something that you can test, and discovering that consistency actually helped scientists to recognise and define synaesthesia in the first place. "The hallmark of synaesthesia is consistency over time," says Louisa Rinaldi at the University of Sussex. "That's how we verify [that someone has] synaesthesia."
There are online tests for this that you can try yourself. Synaesthetes' brains seem to be built differently. One paper published in 2024 compared the physical features of synaesthete brains to non-synaesthete brains and found some interesting distinctions. The results suggested a range of common differences, including that synaesthete brains were, on average, very slightly smaller and had thinner outer layers. There were also contrasting patterns of myelination - referring to the amount of a fatty substance called myelin that surrounds axons, which transmit electrical signals in the brain. The researchers speculated that synaesthete brains might develop on an unusual schedule, perhaps growing at a slightly different rate, or naturally losing connections in a way that is distinct from non-synaesthete brains. But this still requires more research.
There remain, for example, competing theories over how synaesthesia emerges. We may all have the capacity to some extent, but most people's brains perhaps just tune it out. Or, we might all be born with synaesthesia, but an adult's brain might retain certain key connections that enable those perceptions for the rest of their life.
Brain chemistry does seem to play a role in determining the synaesthetic experience and there are stories about people who have lost, and regained, their synaesthesia. Sean Day, a synaesthesia researcher and instructor at Trident Technical College in South Carolina, US, has multiple forms of synaesthesia. Smells induce shapes and colours in his visual field, for example-even blocking his view. So much so that he once had to pull over for an hour while driving past a recently fertilised field. Different timbres of musical instrument also trigger particular colour and shape projections for Day.
In 1999, he was living in Taiwan on the 17th floor when a massive earthquake struck. The Jiji earthquake had a magnitude of 7.6 and was truly devastating, claiming the lives of more than 2,400 people. Day and his wife were trapped as the building swayed side-to-side. Cracks appeared in the ceiling. Day found the experience terrifying and traumatising - he suffered from post-traumatic stress disorder for years after. In the months folowing the quake, he noticed that his synaesthesia completely disappeared. "I had no synaesthesia at all," he said during a 2023 podcast interview. "I was so concerned with the earthquake and the stress." It wasn't until about four months later that his synaesthesia very gradually began to return.
The experience led Day to concur that synaesthesia could be mediated or affected by the biochemistry of the brain - which is known to change during periods of stress, for example. There is some evidence that levels of serotonin, a brain chemical involved in mood, thinking and bodily functions, can affect how synaesthesia manifests, for instance.
Whatever the factors behind synaesthesia, and however the phenomenon might evolve over the time for a person, it seems that many synaesthetes find ways of using or applying their experiences for practical purposes. Rinaldi says her partner can quickly and easily tell when a word is poorly spelled-because it is the wrong colour. And Poppy Porter's art and design work is directly influenced by the visual features she experiences when listening to music.
In some ways, perhaps synaesthesia forces people to perceive a certain logic in things. The letter that is the correct colour, the song that is just the right shape. For them, for whatever reason, that's what makes most sense.
Someone else who reportedly had synaesthesia was Franz Liszt. He became famous for his astonishing piano melodies that can confound even the most accomplished players. But as Rouw points out, Liszt was also a choirmaster. Apparently, he would conduct while listening intently and watch the marvellous shapes and colours he could see in the music. There are reports that he used to bark directions that must have seemed strange that, if Liszt really was a synaesthete, would have made perfect sense to him. "Gentlemen, a little bluer, if you please!" he would insist, "The key you're singing in demands it!"