By Holly WilliamsFeatures correspondent

A number of famous artists have experienced synaesthesia – a union of the senses. Holly Williams explores its history and her own experience of the condition.

The experience of colour as we usually understand it is a visual one: objects have colour, artists use colour, and we can recall a colour in our mind’s eye. But for some people, colour is a more multi-sensory affair, linked to sound, texture, taste or shapes. Music has a hue – like the parping of a trumpet that evokes a shower of burnt orange. Numbers, letters and days of the week have their own shade: the number one is white, the letter L is blue and Monday is red.

This neurological phenomenon is called synaesthesia; if you don’t have it, it sounds strange, like the straining of an overactive imagination. But if you're part of the estimated four per cent of the population who are synaesthetes, such descriptions are as obvious and natural as the sky being blue and the grass being green. Synaesthesia is best described as a union of the senses; one sensory experience involuntarily, and consistently, prompts another. There are up to 70 different types – from tasting the time to smelling a symphony – although the most common involve colour.

I have grapheme-colour synaesthesia. Numbers, days, months and letters (and by extension, words) have their own very distinct shades. The examples above are my own: L could only ever be blue. This is the most common form of synaesthesia, and it wasn't until I was in my early twenties that I realised not everyone sees the world this way – indeed, the assertion that words have a certain colour can be met with sneering scepticism. Yet I simply know that Wednesday is dark green, for it always has and always will be. Talk to another synaesthete, however, and they’ll often disagree with you howlingly about the colour, or even the precise shade.

One friend has tastes and textures as well as colours for words. While I find my synaesthesia has little impact on my day to day life, for her, it’s a curse as much as a blessing. There are words she can’t say without physically cringing. For others, however, synaesthesia can actually be a bonus, aiding their creative endeavours. Research on synaesthesia is not as extensive as you might expect: it was disregarded as a phenomenon until the advent of MRI scans in the late 1980s, proving that corresponding areas of the brain really do light up in synaesthetes – but it’s thought it may be more common in artists.

The art of noise

Synaesthesia has been something of a hot topic in music news recently, with the likes of Pharrell Williams, Kanye West, Lady Gaga, Dev Hynes and Frank Ocean suddenly keen to talk up their colourful experiences. Pharrell’s international mega-hit Happy, if you were wondering, is yellow, with accents of mustard and sherbet orange.

The singer-songwriter recently insisted in an interview that synaesthesia “is not a rarity. One of the main things I try to educate the public on is, most artists have it.” That might be overstating it, but there is a rich history of it in the arts. Notable synaesthetes (or suspected ones, given its relatively recent currency and familiarity as a scientific term) include composers Olivier Messiaen, Franz Liszt and Jean Sibelius, Russian author Vladimir Nabokov, artists Vincent van Gogh and David Hockney, jazz legend Duke Ellington and even Marilyn Monroe.

One man helping to educate the public is British composer Nick Ryan. He has grapheme-colour synaesthesia, but also sees corresponding colours, shapes and textures to sound and music. Like Pharrell, he sees his synaesthesia as a boon – “it makes listening to music very enjoyable” – and he has long wanted to recreate it for a non-syn audience.

Working with Synaes, an audio-visual collaboration with digital artists Quayola & Sinigaglia, he did just that: the work recently premiered at London’s Roundhouse as part of Imogen Heap’s Reverb festival. Highly textured, precisely coloured digital imagery was projected on a large screen, morphing to match his music, which blended electronics with sounds made by the London Contemporary Orchestra. “I wanted it to be multi-sensory, because such is the nature of synaesthesia,” he says. “It’s a sensation scrapbook, which the audience is immersed in.”

Electronic booms were matched on screen by crashing waves of monochrome static; sustained, growling strings and deep bass were illustrated with slowly warping spidery nets and grids of deep pink and ice-white. How effective it was for the non-synaesthete, I couldn’t say – but it certainly made me realise my own response to music is more synaesthetic than I’d previously thought. Some graphic matches felt satisfyingly correct; others, downright wrong. That pale grid shape? Far too light and clean and carefully constructed for the sound – it ought to have been altogether darker, heavier, more organic…

Wall of sound

Ryan is far from the only artist to attempt to recreate their synaesthesia, of course. Perhaps best known is Wassily Kandinsky, whose abstract paintings  were his experience of seeing music in colour, line and form. His 1911 work Impression III (Concert) was inspired by a concert in Munich at which he heard the music of Arnold Schoenberg, and while the painting has some representational elements, its vivid wash of yellow describes sound itself. Kandinsky wrote on the subject with a spiritual fervour: “Colour is the keyboard. The eye is the hammer. The soul is the piano with its many strings. The artist is the hand that purposely sets the soul vibrating by means of this or that key.”

Kandinsky played the cello – an instrument that, for him, evoked a deep blue, a colour he also used heavily. Although not commonly cited as a synaesthete, it’s interesting to note that Henri Matisse described the same rich, deep blue – famously used in his Blue Nudes – as affecting the viewer like “a vigorous stroke of a gong."

Writers have also used their craft to recreate the synaesthetic experience. Both Arthur Rimbaud and Charles Baudelaire are thought to have been synaesthetes, and both are notable for actually tackling the subject head-on.

For Baudelaire, there was a relationship between sensation and emotion. We may be in the realm of the metaphorical here – he was a fan the theologian Emanuel Swedenborg's theory that every element of existence corresponds to another in a spiritual sense of universal connection. But his 1857 sonnet Correspondences is still viscerally evocative in describing intertwined sensations: “Perfumes and sounds and colours correspond. / Some scents are cool as children's flesh is cool, / Sweet as are oboes, green as meadowlands”.

Rimbaud, meanwhile, clearly had grapheme-colour synaesthesia, as laid out in his poem Vowels:

Black A, white E, red I, green U, blue O—vowels,

Some day I will open your silent pregnancies:

A, black belt, hairy with bursting flies,

Bumbling and buzzing over stinking cruelties.

Pits of night; E, candor of sand and pavilions,

High glacial spears, white kings, trembling Queen-Anne’s lace;

He catalogues the colours of letters simply, then uses them as a jumping-off point for wild associative flights, both lovely and disturbing. Arguably, that’s what all poetry does, but there seems little doubt those initial colour-coded letters are concrete for him, true to his own synaesthetic experience. There’s only one problem with this poem: he got the colours wrong. A is obviously red.

Holly Williams is a journalist at The Independent.

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