Some researchers believe synesthesia is an extreme version of what happens in everyone's mind. If so, the result suggests that all abstract thoughts are associated with specific perceptions, says neuropsychologist Julia Simner of the University of Edinburgh, co-author of the report. "The extent to which abstract thought is truly abstract--that's really what the question is."
Simner and her colleague Jamie Ward of University College London tested six synesthetes by showing them pictures of 96 uncommon objects such as a gazebo, sextant, catamaran, artichoke or castanets. Out of 550 trials in total, Simner and Ward induced 89 tip-of-the-tongue states. In 17 of these "um, um" moments, the synesthete reported perceiving a taste while still trying to conjure the word. In short, the word's meaning alone elicited the taste.
To confirm that these reports were truthful the researchers called the participants out of the blue a year later and retested them. The synesthetes consistently associated the same tastes with the same words, the researchers report in the November 22 Nature.
"This looks pretty clever," says neuroscientist David Eagleman of Baylor Medical College, who was not involved in the study. Synesthesia research has blossomed in the last five years, as researchers have gained confidence in the subjective reports of presumed synesthetes, especially those who perceive letters or numbers as being colored, he says. "Essentially all the synesthesia literature is about color just because it's easier to study. This is stepping beyond that." Some experts have estimated that there are more than 150 kinds of synesthesia, based on the possible combinations of subjective sensations, he says.
Prior experiments found that the word-taste associations are locked in during adolescence and have some definite patterns, Simner explains. These synesthetes tend to taste childhood things such as chocolate and lollipops, she says. "Some of these tastes are really strong and some them of them are really unpleasant--some of the them taste of earwax and bodily fluids," she notes. "It starts with words like 'mince' and 'cabbage,' and the taste experience spread to similarly sounding words." "Prince" might also taste of mince, for example. Some of the associations have seemingly obvious roots--"newspaper" might taste like fish and chips, which traditionally comes wrapped in newsprint, she adds.
The brain wiring necessary for synesthesia seems to be present in everyone. Dropping acid or drifting to sleep can both cause synesthetic perceptions, and people who are blindfolded for extended periods may start seeing colors for different sounds, the experts note. "It's possible that we all have that connection but the synesthetes have them to an extreme degree," says Simner.
Some scientists have theorized that we are born synesthetic but lose or block most of the pathways that cause the unusual perceptions, Simner observes. Eagleman disagrees, saying that the condition may be the result of an imbalance in brain signals. He hopes to identify the gene for familial synesthesia in order to learn more, he says.