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For the woman known as AP, everyday language is like a soap opera. Every letter of the alphabet has a distinct human personality. 鈥淎 is a mother-type, very sensible. I is a little guy, H and G are always fussing over him. M and N are two old ladies who spend all their time together and natter a lot. T is a protective male.鈥�

AP isn鈥檛 making it up. She has the neurological condition called synaesthesia, which somehow causes crosstalk in her brain, tingeing the letters of the alphabet with genders and personality types. AP鈥檚 condition manifests itself in other curious ways too: she sees letters and numbers as coloured, and perceives shapes when she tastes certain flavours. But it is her personification of language that is the most interesting part, because it simply doesn鈥檛 fit psychologists鈥� understanding of synaesthesia.

Ever since it was first described scientifically in 1880, synaesthesia has usually been thought of as a purely sensory phenomenon. For some reason, the normal barriers keeping the sensory modalities apart are absent, allowing spectacular crosstalk between sight, sound, taste, touch and smell. Individual synaesthetes have differing experiences 鈥� some taste sounds, others smell shapes, see music, feel flavours, or experience any other mingling of senses 鈥� but in all cases sensory information entering one channel somehow triggers activity in another.

For AP, though, synaesthesia is more than purely sensory, and according to a small but increasingly influential group of scientists, she is far from alone. 鈥淲e are moving beyond a purely sensory view of synaesthesia,鈥� says Julia Simner, a linguistic psychologist at the University of Edinburgh, UK, who is one of the leading advocates of the new view. 鈥淭here is a crossing of the senses, but there is also so much more going on.鈥� In particular, Simner and her co-workers argue that synaesthesia can have a significant conceptual element, with sensory fireworks being triggered not only by sights, sounds and smells but also by meaning. In fact, conceptual triggers might be more important than sensory ones. If they are right, psychologists may have to revise their understanding of synaesthesia and its causes. In return, though, they may gain a new window on how the human brain handles concepts, and how it connects them to the real world.

Most modern research into synaesthesia starts from the assumption that the condition is purely sensory in nature (see 鈥淪ynaesthesia basics鈥�). This is a sensible starting point, as many reported cases appear to be so. It has also been a useful line of enquiry, helping to confirm that the condition is genuine as well as shedding light on broader questions such as how the brain integrates information from different senses, and how the developing brain builds walls between them. Even so, some researchers have long suspected that sensory crosstalk is not the whole story, and that the condition involves other elements of cognition as well.

For the past 20 years or so this view has been slowly gaining ground among psychologists who study synaesthesia, but more recently it has begun to move into the mainstream. For one thing there are case reports of unusual synaesthetes like AP, whose experiences are hard to explain in purely sensory terms. There is also mounting experimental evidence that is hard to ignore.

In 2005, a team led by Vilayanur Ramachandran of the University of California, San Diego, tested six synaesthetes who had a form of the condition called grapheme-colour synaesthesia. This is a relatively common type of synaesthesia in which letters, numerals and sometimes punctuation marks 鈥� collectively known as graphemes 鈥� are sensed as coloured. Ramachandran wasn鈥檛 searching for non-sensory aspects of synaesthesia: he was more interested in peering into the workings of synaesthetic brains. Nevertheless, he found something that was difficult to explain. As part of the experiment, the researchers asked the subjects to search for specific graphemes in a crowded jumble of letters and numbers. Grapheme-colour synaesthetes are usually better at this 鈥渃rowding鈥� task than non-synaesthetes because they can search for colours as well as shapes. Strangely, not all of the subjects were successful.

Why might that be? It turned out that the synaesthetes in the study fell into two groups. For one, the letters and numbers actually appeared in colour on the page, whereas in the other, colour was present in a more abstract way, in the mind鈥檚 eye. Only the first group, known as 鈥減rojector鈥� synaesthetes, did well on the crowding tests; those who saw colours in their mind鈥檚 eye were no better than non-synaesthetes. The team tentatively suggested that there are two different types of synaesthetes: 鈥渓ower鈥� synaesthetes for whom the act of viewing the grapheme triggers colours, and 鈥渉igher鈥� synaesthetes, who respond not to the visual shape of the grapheme but to the concept behind it. In other words, some common synaesthetic experiences are triggered by concepts, not sensory data. With such a small sample size, though, the researchers cautioned that they couldn鈥檛 draw any firm conclusions.

It didn鈥檛 take long for more evidence to emerge. Last year, a team led by Mike Dixon of the University of Waterloo in Ontario, Canada, ran tests on a synaesthete called J, who sees graphemes as intensely coloured. They showed her a series of ambiguous shapes that could be either letters or numbers and found that her colour experiences depended on meaning rather than shape: shown a grapheme that could either be an S or a 5, J saw it as green, her colour for S, when it appeared in MU5IC, but as pink, her colour for the number 5, when it appeared in 1234S6 (Cortex, vol 42, p 243).

Around the same time, Simner and her colleague Jamie Ward were also searching for evidence that a synaesthetic experience could be induced by the conceptual meaning of a word alone. They took a group of six 鈥渓exical-gustatory鈥� synaesthetes, who taste words, and showed them pictures of uncommon objects: gazebos, catamarans, metronomes, platypuses, castanets and the like. The aim was to induce that familiar 鈥渢ip-of-the-tongue鈥� state, when you know what an object is but cannot quite dredge its name up from your memory. Simner鈥檚 thinking was that if synaesthetic sensations were triggered exclusively by the sensory elements of a word 鈥� its letters or sounds 鈥� then the subjects should only experience a taste once they remembered the actual word.

That鈥檚 not what she found. On some occasions when she successfully induced the tip-of-the-tongue state, the subject鈥檚 mouths were flooded with flavour. One reported tasting tuna fish as she grappled for the word 鈥渃astanets鈥�; another got Dutch chocolate as she was not quite able to recall the word 鈥減honograph鈥�. Both later confirmed that these were the tastes they normally associated with these words (Nature, vol 444, p 438). What that means, says Simner, is that some synaesthetic experiences are triggered by pure concepts.

It鈥檚 clear, then, that for at least some synaesthetes, experiences are triggered by concepts rather than sensory inputs. But how common is this form of synaesthesia? Last year, Simner came up with the first estimate 鈥� and it came as a surprise.

Historical estimates of the prevalence of synaesthesia have varied widely 鈥� from 1 in 100,000 to as high as 1 in 200 鈥� but the generally accepted figure was about 1 in 2000, with women outnumbering men by about 6 to 1. These figures are not especially reliable, however, as they depend on self-referral 鈥� for example, people coming forward in response to newspaper adverts. So last year Simner鈥檚 team set out to get a more accurate figure.

They randomly tested around 1700 people and found that synaesthesia is significantly more common than previously thought, affecting 1 in 20 people, with a much lower sex bias. 鈥淚 was very surprised,鈥� Simner says. 鈥淥ur estimates were 88 times higher than earlier ones鈥� (Perception, vol 35, p 1024). Most of these closet synaesthetes had never been identified before because they didn鈥檛 realise there was anything unusual about their experiences. 鈥淪ome people never even realise they are synaesthetes. It鈥檚 just how they see the world,鈥� Simner says.

The study produced another surprise. Up to now it had been widely assumed that grapheme-colour synaesthesia was by far the most common form, with some estimates putting it at 68 per cent of cases. Simner expected to find something similar, but she didn鈥檛. In fact, 64 per cent of her synaesthetes had a form in which the days of the week are coloured. She also found cases where colours were triggered by months, certain proper names or specific words.

Coloured days

In other words, around two-thirds of the synaesthetes in Simner鈥檚 study were triggered not by sensory inputs but by concepts: days, months, names and words. If she included grapheme-colour synaesthesia, that figure went up to nearly 90 per cent. Simner鈥檚 tentative conclusion is that the view of synaesthesia as a purely 鈥� or even largely 鈥� sensory phenomenon needs revising.

鈥淭he more I learn about synaesthesia the more convinced I become that you can鈥檛 explain it just by talking about various types of sensation,鈥� Simner says. She now believes that synaesthesia is primarily triggered by concepts, particularly linguistic ones. If so, another long-standing belief about synaesthesia may have to be revised.

Previous descriptions of synaesthesia have tended to agree that each synaesthete has a unique experience. Synaesthetes can indeed be loosely classified into types 鈥� word-tasters, music-smellers, letter-colourers and so on 鈥� but within these groups there were no apparent rules. Just because one lexical-gustatory synaesthete tastes 鈥渃astanets鈥� as tuna fish it doesn鈥檛 mean that others do: they might get porridge, or earwax, or nothing at all. There seemed to be as many different kinds of synaesthesia as there were synaesthetes.

More recent research says this is not strictly true. Once they started looking at grapheme-colour synaesthetes in detail, researchers began to detect previously unnoticed patterns. The letter o, for example, is very often white; a is usually some shade of red, b is blue or brown, while q and j are often purple or pink. These are not hard and fast rules, but they are common enough to allow Simner to draw up a prototype synaesthetic alphabet (see Illustration).

What could be behind these correspondences? For one thing, Simner鈥檚 team noticed that common letters tend to be paired with common colours and unusual letters with unusual ones. Other associations reflect the first letter of the colour鈥檚 name: b is often blue or brown, g is green and y is yellow. This priming effect holds true in other languages also. German synaesthetes tend to see g as yellow 鈥� the German word for yellow being gelb. Even more intriguingly, when Simner asked non-synaesthetes what colour a or b would be if they happened to be a colour, they agreed with the synaesthetic alphabet more often than would be expected by chance (Cognitive Neuropsychology, vol 22, p 1069).

Similar commonalities can be found among lexical-gustatory synaesthetes. Words often taste of things they share a speech sound with 鈥� 鈥減rince鈥� tastes of mint, 鈥渇orage鈥� of orange, 鈥淩oger鈥� of sausages. There are conceptual links too: for example, 鈥渂lue鈥� often tastes inky and 鈥渘ewspaper鈥� of fried fish.

What these patterns suggest, says Simner, is that synaesthesia somehow reflects cognitive mechanisms that we all use when processing linguistic concepts. And that has important implications: it means that by studying synaesthetes, psychologists may be able to decode some of the more opaque rules and concepts that make language work.

鈥淚t seems like a strange way to study language, but it actually provides this wonderful window into the implicit rules underlying linguistics,鈥� says Simner. 鈥淗uman beings have this hugely sophisticated knowledge of language, even if they can鈥檛 articulate the knowledge.鈥� Studying synaesthetes allows us to glimpse this knowledge first-hand, she says, because they explicitly experience the linguistic concepts that the rest of us only know implicitly.

This new field of linguistic synaesthesia is in its infancy, but has already started to shine a light into some dark corners of linguistics. For example, linguists have documented a large number of subtle rules of pronunciation. Take the phoneme l, for example, which has three sounds: a 鈥渃lear鈥� l, as in the word 鈥渓ike鈥�, and two 鈥渄ark鈥� ones 鈥� as in 鈥渄eal鈥� and 鈥渂ottle鈥�. Linguists are interested to know whether these different sounds are processed differently by the brain. Synaesthetes can provide an answer.

For one synaesthete, known as JIW, the l sound triggers three different taste sensations 鈥� potatoes, fingernails or Rice Krispies. JIW had no idea why this should be the case, but when Simner analysed his synaesthetic triggers she discovered they exactly matched the linguistic rule. The clear l triggers potatoes, while the two dark ones trigger fingernails and Rice Krispies, respectively. 鈥淗e had no idea that these different l sounds even existed, but that didn鈥檛 matter,鈥� says Simner. 鈥淗is brain was automatically distinguishing between the sounds anyway.鈥�

鈥淭he L sound triggers either potatoes, Rice Krispies or fingernails鈥�

Another area where synaesthesia has proved useful is with words such as 鈥渂lackbird鈥� or 鈥渢ablecloth鈥�. Linguists have long debated whether the brain stores these compound words as one concept or two. So Andreas Kubitza, a linguist at the University of Erfurt, asked a German synaesthete to tell him. For this person, words usually trigger a single colour. Kubitza found that compound words triggered two, unless they were very common. So 贵盲丑谤尘补苍苍 (ferry + man) elicited the two colours usually associated with the words 鈥渇erry鈥� and 鈥渕an鈥� but Bahnhof (train + station) produced just a single colour. What that means, suggests Kubitza, is that compound words are initially stored as two concepts but can be assimilated into one by heavy usage.

The new line of research has impressed other synaesthesia researchers. 鈥淚 think this is striking and elegant work,鈥� says Ramachandran. 鈥淚t demonstrates that you can take this quirky phenomenon into linguistic space and map it out much more precisely, so that you are also learning about how the mind processes language.鈥� Simner believes there are many other linguistic questions you can probe with synaesthesia that you can鈥檛 get at another way. For example, does the brain handle qu as a single grapheme or two? Does it automatically spell out words as we listen to spoken language, like an internal ticker tape?

But perhaps the most important implication is what synaesthesia can tell us about the mysterious inner workings of the human mind. Simner believes that the crosstalk between linguistic concepts and sensory experiences isn鈥檛 confined to synaesthetes. Instead, she thinks it鈥檚 a general principle: in some sense the linguistic concepts we carry around in our heads influence the way we perceive the world, and vice versa. If she鈥檚 right, we鈥檙e all synaesthetes to some extent. It鈥檚 just that we haven鈥檛 noticed yet.