Seeing music, tasting colors

Green Mondays, pointy chickens and chocolate-flavored metronomes may sound like children’s-book fantasies, but they’re a mundane reality for some people. To them, words can have tastes, tastes can take on shapes, and numbers and letters can have a spectrum of hues.

In this small subset of the population, stimulation of one sense triggers a seemingly unrelated sensory experience. This neurological condition, called synesthesia, can involve two senses or, less commonly, up to all five. English philosopher John Locke was probably the first to document a synesthete’s experience when, in 1690, he wrote of a blind man who “saw” the color scarlet on hearing the bleat of a trumpet.

Not much else was said or written on the topic until the 1880s, when Charles Darwin’s cousin, geographer and psychologist Francis Galton, described a “peculiar habit of mind” in some of his fellow Brits: They saw colors when they looked at numbers, heard or read the days of the week or listened to single notes or pieces of music.

Then, around the turn of the 20th century, artists, writers, composers and other creative types suddenly began boasting that they saw azure when they heard B flats or tasted curdled milk when they saw their next-door neighbors.

But many probably didn’t. And in the absence of scientific validation, skeptics dismissed such claims.

By the middle of the century, synesthesia had been largely written off as the product of overly active imaginations, the consequence of psychological associations left over from childhood, or an artifact of drug use. (LSD, mescaline and hallucinogens used by rain forest shamans can induce temporary synesthesia.)

But in 1968, Russian neuropsychologist Alexander Luria published a book, “Mind of a Mnemonist,” about a patient who remembered everything -- in large part thanks to his multi-fold synesthesia (all five of his senses were crossed).

Twelve years later, American neuropsychologist Richard Cytowic was eating dinner at a friend’s house when the friend exclaimed the chicken wasn’t “pointy” enough. Cytowic, having read Luria’s book, was inspired to turn his scientific attention to synesthesia, ultimately reigniting interest in the topic.

Researchers began employing clever tests to distinguish true synesthetes from fakes. In one, subjects look at a page printed with twos and fives. The twos form the shape of a triangle, visible only to the synesthetes, who see the numbers in two different hues.

But word-color or number-color synesthesia isn’t the only form of the condition. Taste-shape, word-taste and music-color synesthesia can be trickier to diagnose. A common test for word-taste (lexical-gustatory) synesthesia presents subjects with pictures of unusual objects, such as sextants and metronomes. The subjects are asked to identify the object’s taste -- which often comes to the synesthete’s mind before the object’s hard-to-remember name does.

Scientists still aren’t sure what causes synesthesia, but recent brain imaging studies have helped spawn theories. Genetic defects or chemical imbalances may be responsible for more communication than normal between distinct sensory regions of the brain. Some neurologists think that the human brain begins with countless connections between regions and that only in synesthetes do these not get “pruned” back.

Synesthesia was once thought to affect about 1 in every 2,000 people and far more women than men. Experts now estimate that it’s far less gender-biased and quite a bit more prevalent.

It’s also, according to one study, about seven times more common among artists and writers than among the population-at-large. If scholars are right, many of the most creative minds in history -- including Russian writer Vladimir Nabokov and composer Duke Ellington -- had synesthesia of one form or another.