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Detail from a painting made by Carol Steen to illustrate her synesthetic vision.

To Carol Steen, the number five is yellow, Thursdays are dark burgundy and the elevator bell of her apartment rings in an "amazingly bright magenta."

As someone with synesthesia, a neurological condition that causes certain stimuli to trigger multiple senses, the New York artist experiences the world differently – a realization she first made at the age of seven. She and a classmate had been walking home from school, when Steen mentioned the letter "A" was "the prettiest pink" she had ever seen.

"While I expected her to completely agree with me, that's not what happened," Steen recalls. "Her reaction was to stop, stare at me, wrinkle up her face and say, 'You're weird.' And then we continued to walk home, but it was now silent."

Like many fellow synesthetes, Steen kept quiet about her sensory associations for years to avoid further embarrassment. These days, however, synesthetes are gaining greater acceptance, as the public becomes more aware of the condition – thanks, in part, to prominent synesthetes like rap artist Kanye West, who last year publicly discussed his ability to see sounds – and as scientists explore the how's and why's of the phenomenon. At the heart of their research is the quest to understand the individual differences in how people experience the world.

University of Waterloo cognitive neuroscientist Mike Dixon recalls that as recently as 2000, he had trouble convincing fellow scientists that synesthesia was worthy of study. Subsequent research he conducted with his colleagues confirmed the condition is, in fact, real and not the imaginings of those who experience it. Much about synesthesia is still unknown, but some scientists believe synesthetes have more connections in the brain, so that a scent, for instance, may set off visual signals as well as olfactory responses. While it tends to run in families and is believed to affect less than 5 per cent of the population, it is unclear what causes it.

A new Stanford University study released last month has opened a tiny window into how some synesthetes may develop their sensory associations. The study involved 11 participants with grapheme-colour synesthesia, which means they experience colour when reading printed letters and numbers. The findings revealed the subjects experienced surprisingly similar letter-to-colour pairings. With few exceptions, they perceived the letter "A" as red, "B" as orange, and "C" as yellow – a pattern that could be traced to mass-produced, coloured alphabet magnets, with which they played as children.

"It says the associations can develop as a result of experience with stuff in the environment," co-author Nathan Witthoft said of the study's findings.

The ability to experience things that others cannot often lends a sense of mysticism to those who have synesthesia. Indeed, in a study published in the journal Consciousness and Cognition, researchers in Spain found that many individuals traditionally identified as "healers" who claimed to see auras and energy fields around people were actually synesthetes. The fact that prominent creative types like West, violinist Itzhak Perlman and the late author Vladimir Nabokov are reported to have synesthesia add to the mystique. But some researchers believe the brains of synesthetes may be more similar to those of non-synesthetes than most people think.

Daphne Maurer, a developmental psychologist at McMaster University, suggests all human brains produce a multitude of connections. But over time, depending on how much those connections are used, some are strengthened and others are pruned. "What seems to happen in synesthesia is less pruning, so you end up with a brain with extra connections," she says. Another possibility is that these extra connections are inhibited in individuals without synesthesia. Dixon notes that people using the drug LSD commonly report seeing colours when listening to music – a common experience of synethetes. "Something in the LSD is turning on a neural connection between sound and colour that is normally turned off, whereas synesthetes have these turned on all the time," he says.

Beyond teasing out the oddities of the human brain, scientists believe there are practical applications of studying synesthesia. Synesthetes, for example, tend to have heightened memory, and Dixon says the way they make memory associations could teach the rest of the population how to improve memory.

But having synesthesia can have its downside. Gaby Loughry, based in Bedford, N.S., experiences colours and shapes when she hears music and when she has intense emotional experiences. Because of this, unpleasant experiences can sometimes be difficult to forget, and pain is much more vivid. "I get the pain two ways: I get the pain physically, but I actually see it," she says. "Nausea is like a metal mesh that's gradually tightening all over my body; it's a silver colour, a grey silver colour. An injection is a long light of electric white."

In spite of this, Loughry says she would never wish to be "cured" of her synesthesia, even if it were possible. "I couldn't imagine my life without it," she says. "Because of the beauty in it – the music and the art, the words and the patterns, all of that – I would keep it."

To share information and encourage further collaboration between researchers and synesthetes, the non-profit American Synesthesia Association, co-founded by Steen, is holding its annual conference this May at OCAD University in Toronto.

Types of synesthesia

There are numerous forms of synesthesia, involving various stimuli and senses, and some individuals may have multiple forms. University of Waterloo cognitive neuroscientist Mike Dixon, for instance, says he was once told by a synesthete that his voice was brown and "tasted like maple syrup."

While Dixon says it is hard to pin down the exact number of types out there, some are more common than others:

Time-space synesthesia:The most common form of synesthesia, in which units of time, such as days of the week or months of the year, are arranged mentally in space. (For example, January may be perceived as being located straight ahead, February to the right, March on the left, and April behind.) People with time-space synesthesia may also perceive coloured days of the week.

Grapheme-colour synesthesia:Letters and numbers are associated with colours. Some grapheme-colour synesthetes have a knack for spelling because they can identify misspelled words if their colour patterns are wrong.

Sound-colour synesthesia:Sounds are associated with colours. Some sound-colour synesthetes have perfect pitch.

Taste-touch synesthesia: A rare form in which tastes are associated with shapes. Neurologist Richard Cytowic, author of The Man Who Tasted Shapes, famously described one synesthete's perception of the taste of chicken as pointy.

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