For most of us, the boundaries between our bodily senses are clear-cut and rigid.
But for a few rare individuals, the demarcation between vision and hearing, or between taste and touch, are less solid, with one bleeding into the other.
These people have a condition called synesthesia, in which two or more of the senses are crossed. Some see colors when listening to music, while others associate tastes with shapes or words with colors.
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A very small number of synesthetes can "taste" words.
A new study finds that individuals with this last form of synesthesia — called lexical-gustatory synesthesia — can taste a word before they ever speak it, and that the word's meaning, not its sound or spelling, is what triggers this taste sensation.
The finding, detailed in the Nov. 23 issue of the journal Nature, could help scientists unravel how perception works in the rest of us.
In the experiment, the researchers showed six lexical-gustatory synesthetes images of objects they were familiar with, but which they didn't normally encounter.
The images included a platypus, a gazebo, an artichoke, a metronome and a sextant.
Doing this induced a "tip-of-tongue" state in the participants, during which they recognized the object but couldn't immediately identify it.
"At the moment they're trying to find the word, we ask them two things: whether they knew any part of the word at all, and what it tasted of," said study team member Julia Simner of the University of Edinburgh in Scotland. "I remember one participant — we showed her a phonograph, and she said 'I know what that is ... um ... um ... Oh! I'm tasting Dutch chocolate and I don't know why!'"
To ensure that the synesthetes' word-taste associations weren't arbitrarily chosen, Simner and her colleague Jamie Ward asked them to repeat the associations after the trial.
They also cold-called participants up to two years later and asked them the same questions.
"We phone the synesthetes completely out of the blue," Simner said. "We say ''Hello, we did this study on you ... Can you tell me what 'phonograph' tastes of?' and they say 'Yeah, it tastes of Dutch chocolate.'"
Simner said that most non-synesthetes, if asked to remember a list of word-taste associations, might accurately recall about a quarter of them two weeks later.
"Synesthetes are accurate 100 percent over many, many, many years — over decades even," she said.
The researchers also found that many of the six synesthetes studied associated similar tastes for the same words.
"You can predict the nature of the taste if you know how the word sounds," Simner said. "It seems like it's not really words that are related to tastes, but certain sounds within words."
For example, many of the synesthetes reported words with the sounds "eh" or "mmm" tasted of mint, and that those containing the sound "aye" tended to taste of bacon.
Because of this, Simner said she could pick any word, and hazard a guess about what a lexical-gustatory synesthete would taste.
"For example, for me, it's not a surprise at all that for lots and lots of these synesthetes, the name 'Tony' taste of macaroni," she said.
The two words rhyme.
The researchers think the synesthetes develop their word-taste associations at a young age, and that the associations persist into adulthood.
Also, since a word's sound determines its taste, synesthetes speaking non-English languages likely have completely different word-taste associations.
Simner thinks her findings could help explain how human perception works in general.
"We know that synesthetes and nonsynesthetes make the same types of associations — it's just that synesthetes experience them perceptually," she said.
For example, synesthetes who see colors when hearing sounds tend to see light colors for high-pitched sounds and dark colors for low-pitched ones.
"This is exactly the same type of association that we all make if we're forced to make a judgment," Simner told LiveScience. "If I play the top note of a piano, and ask if that's a light yellow or a deep, dark purple, you're probably going to say it's a light yellow sound."
The same parallel probably exists for word and taste associations. Simner notes that for lexical-gustatory synesthetes, food names tend to taste of themselves.
For example, the word "cabbage" tastes like cabbage and "mint" tastes like mint.
"Although nonsynesthetes don't have a taste experience when they read food names, it's still likely that the same associations exist — that the word 'cabbage' is linked to the taste of cabbage in all people [who know what cabbage is and have eaten it]," Simner said.
While estimates of the prevalence of synesthesia vary, one of the most commonly cited studies pegs it at about 1-in-2,000, with a heavy skew toward females.
But in another recent study, published in the August issue of the journal Perception, Simner and colleagues found that 1 out of 23 people in Britain — or about 4 percent of the population — have at least one form of synesthesia.
The exact cause of synesthesia is still unknown, but one popular hypothesis, put forth by Daphne Maurer and Catherine Mondloch at McMaster University in Hamilton, Ontario, suggests that all of us begin life as synesthetes.
The researchers suggest our infant brains once contained connections between different sensory areas, and that these connections became pruned or blocked as we matured.
"There's some suggestion that for synesthetes, this process doesn't take place fully, and that some of those connections are left active," Simner said.
Some of the lexical-gustatory synesthetes examined found the condition disruptive.
"One of our participants found it interfered when he's having a conversation or trying to read," Simner said. "Or when he's driving and trying to read the street signs, he'll have a really intense sensation of something really unpleasant ... like earwax."
But the majority of synesthetes say they wouldn't trade their abilities for anything.
"I think if you took a straw poll of 100 synesthetes, 96 would say they would never ever lose their synesthesia, that they like it and are glad to have it," said Simner. "Some say it is like having a nose or a little finger — it's just there."
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