Was it all in the eye's ear?

It is time to re-open the file on patient K. It's an interesting case. It involves psychic cultists and bizarre prophecies of Armageddon. It involves the sinking of the Titanic and the birth of modern abstract art. And it involves a strange neurological disorder called synaesthesia, in which your sensory modalities are all shook up. If patient K were alive today, Oliver Sacks would be writing a book about him. The Case of the Mendacious Synaesthete, perhaps. Or The Artist Who Pretended to See Sound in Order to Invent Abstraction.

Let's give him his full name - Vasily Kandinsky. He's usually regarded as the father of abstraction, beating Mondrian and Malevich by a couple of years. His vivid, lucent and later geometric compositions have made him the most popular and accessible of all abstract artists. These days, of course, you can measure this by the preponderance of his images on ties, jigsaws, birthday cards and crockery. His work is even up on the wall of the Craggy Island parochial house, just to the left of Father Ted's bedroom door.

Kandinsky was born in Moscow on 4 December 1866, the son of a Siberian tea-planter. He trained as a lawyer before deciding to go to Munich to study art, apparently as a result of seeing a production of Lohengrin. Wagner's radical orchestration had a dramatic effect upon him: "I saw all my colours in my mind's eye," he later recalled. "Wild lines verging on the insane formed drawings before my very eyes."

Such passages have added weight to the long-held belief in art-historical circles that Kandinsky was a synaesthete. It's an impression that Kandinsky himself liked to give. His autobiographical writings are full of descriptions that match synaesthetic experiences. "The sun dissolves all of Moscow into a single spot which, like a wild tuba, sets all one's soul vibrating," he wrote in 1913. "But this is only the final chord of a symphony which brings every colour vividly to life, which allows and forces the whole of Moscow to resound like the fortissimo of a great orchestra. Pink, lilac, white, blue, pistachio green. Flame-red houses, each an independent song. The garish green of the grass; the deeper tremolo of the trees; the singing snow with its thousand voices." This psychedelic riff reads like a textbook description of a Russian synaesthete's night on the town. Then again, perhaps he was speaking metaphorically. Or describing some narcotic experience. Or making it all up.

On 14 April, the Royal Academy will open a comprehensive exhibition of Kandinsky's work, which - astonishingly - is the first time a major retrospective on the painter has been mounted in this country. And talk of synaesthesia will haunt the gallery, the catalogue and the reviews.

So what is it, exactly? Well, a synaesthete reading this article would be receiving impressions of colour from every word, and each word would have its own colour-identity. The word "Kandinsky", for instance, might produce the vivid impression of spinach green. There is no obvious logic at work. Correlations like "fire" with orange and "grass" with green are uncommon, and the word "spinach" is as likely to inspire the sensation of translucent aquamarine as any more conventionally spinachy hue.

Though these correspondences vary wildly between individual synaesthetes, each has their own unvarying chromatic grammar - "Kandinsky" will produce the same colour every time. And what is it that they sense? It is hard to be specific, as a vocabulary to describe synaesthetic experiences does not exist. Those with the condition say it is not an obviously visual or mental impression. It is not like the sensation of colour you get when you look at a pillar box in the snow, nor is it like the sensation of colour you get when you shut your eyes and imagine a pillar box in the snow.

The science writer Alison Motluk is a synaesthete, but only realised after reading a description of the condition in an article in The Independent in 1994 : "That letters and numbers should have colours and shapes, strengths and weaknesses, and therefore be the subject of affection or disdain, seems completely normal," she explains. "The astonishing realisation for synaesthetes is not that these characters are imbued with colours but rather a world could exist in which they were colour-free, neutral, characterless. It would be like finding out one day that, while you have been savouring the smells of freshly baked bread, of brandy, of chocolate, all your life, your friends have only been able to taste them."

Recent research into the condition suggests that Kandinsky was not a true synaesthete at all. Dr John Harrison certainly has his doubts. He is a Cambridge-based neuropsychologist and co-editor of an authoritative collection of essays on synaesthesia, but is sceptical about all such retrospective diagnoses. His work has revealed synaesthesia as a condition that is both hereditary and overwhelmingly female. No other members of Kandinsky's family reported synaesthetic symptoms, and nobody has yet suggested that Vasily might have lived a life in drag.

Moreover, synaesthesia was the most attractive perceptual disorder of Kandinsky's day. It was perfectly attuned to early Modernist sensibilities: synaesthetic experience suggested that there was a deeper, more colourful world below the surface of our own; that a realm of vivid abstract forms underpinned grey material reality. Works such as his "Composition II (Two Riders and a Reclining Figure)" (1910), illustrated the possibility of a purer, more colourful dimension. Harrison argues that Kandinsky's supposed synaesthesia is best understood in the context of European decadence. "I think it was down to ennui. The acceptance of esoteric work was part of the mainstream. The desire to be seen as having synaesthesia was quite marked." With its seductive mixture of sound, light and colour, synaesthesia offered the possibility of the mystical reunification of sensory modalities. It was the biological equivalent of Wagner's Gesamtkunstwerk - the `total' work of art which would stimulate every sense.

Kandinsky was not the only one to feel the power of such ideas. The affinity between music and painting was a commonplace in 19th-century thought, and not everyone who championed the idea claimed to be a neurological embodiment of its efficacy. Charles Baudelaire's Correspondances (1860) proposed a natural unity between different sensory experiences, and Arthur Rimbaud's Le sonnet de voyelles (1871) traversed the same territory by ascribing a colour to each of the vowels.

In 1871, John Abbot McNeill Whistler declared: "Art should stand alone, and appeal to the artistic sense of eye or ear ... that is why I insist on calling my works `arrangements' and `harmonies'." (The famous painting of his down-in-the-mouth mother is more properly referred to as Arrangement in Black and Grey No 1.) Paul Gauguin spoke of the "musical part" of his work in 1892. A decade or so later, Arnold Schoenberg (who was an intimate of Kandinsky) was creating musical pieces accompanied by psychedelic light-shows. Aleksander Scriabin's Prometheus (1914) employed a "clavier a lumiere", whose keyboard produced coloured light rather than sound. (An unfinished work, Mystery, was planned to add smell to the composer's sensory arsenal.) Kandinsky himself wrote a series of strange operatic pieces which achieved their effects using coloured lights, an orchestra and actors' voices. He also conceived a ballet-like work based on Mussorgsky's Pictures at an Exhibition - the costume and set designs for which feature in the RA show.

Posing as a synaesthete was as integral to the European avant garde as (in the Marquess of Queensbury's words), "posing as a somdomite [sic]" was to British aesthetes such as Oscar Wilde. Scriabin, for instance, also claimed to have the condition and gave an interview on the subject to Charles Myers, a British psychologist who was something of a groupie when it came to European avant-garde musicians. The interview, says Harrison, is proof that Scriabin's imagination was working overtime. "He describes Beethoven as being `too intellectual to need colours' - which is something a true synaesthete would never say."

This scepticism is news to Frank Whitford, who curated the RA show, but he is very ready to accept that Kandinsky was engaging in a bit of wishful thinking. "He was by no means unique," Whitford argues, "but the degree to which he used this phenomenon in his work - either because he was a genuine synaesthete or because he was doing his best to pretend to be one - isn't matched by anyone else at the time. It's a major motivation for his decision to become an abstract artist."

There were other influences pushing Kan- dinsky in this direction. He devoured occultist texts that preached the destruction of the material world. He read his Rudolf Steiner and his Madame Blavatsky. Blavatsky, who claimed the quasi-synaesthetic ability to sense the coloured auras of individuals, was an enthusiastic believer in the imminent end of civilisation. She envisioned that a global catastrophe would usher in the Golden Dawn, after which the world would be ruled by a psychic elite (Madame B and her mates, presumably). Such views were not as eccentric as they now seem. The years leading up to the First World War were happy days for apocalypticists of every colour. Kipling enthused about the imminent "big smash". Yeats formed his theory of the Gyres, gleefully anticipating that "a terrible joy" would soon "overturn governments, and all settled order". H G Wells scripted a dozen different versions of the end of the world, involving wars, comets, disease and alien invasion.

Three portentous events, argues Whitford, convinced Kandinsky that the world was about to be shaken to pieces, and that art would need to address the process. There was the devastating earthquake at Messina in 1908, the arrival of Halley's Comet in 1910 and the sinking of the Titanic in 1912. "He'd already decided, on a theoretical level, that abstraction was both necessary and inevitable. He thought it was the language of the future. And the future wasn't just some date a bit farther ahead, it was what he described as `the Great Epoch of the Spiritual' - when all material values would be swept away and the world peopled by individuals of `pure spirit'." The cataclysm was punctual: Planck and Einstein's theories ripped the Newtonian universe apart, and the war in Europe did the same to millions of young men. The Spiritual Epoch, however, refused to dawn.

Gradually, synaesthetic notions became a byword for artistic pretension. You might remember the episode of Hancock's Half Hour in which the lad himself joins a poetry society run by a lisping bohemian named Gregory, and introduces its members to his housemates at 23, Railway Cuttings. Gregory: "I'm getting definite turquoise vibrations." Sid: "That's the trains going by." Kandinsky would never have tolerated such flippancy. But if no one had taken his synaesthetic affectations seriously, 20th- century art would have been the poorer for it.

`Kandinsky: Watercolours and Other Works on Paper': Royal Academy, W1 (0171 300 8000), 14 April to 4 July