In the 17th-century fairy tale "Diamonds and Toads," a kind young girl gives a drink of water to an old witch at a well. The grateful witch castes a spell: when the girl returns home, flowers and jewels spill from her lips whenever she speaks. The girl’s evil half-sister then goes to the well, hoping for the same result. But she addresses the old woman rudely, and when the unpleasant sister returns home, toads and snakes wriggle from her lips whenever she speaks.

Such a story sounds fantastic, yet for certain special people an evil tone of voice can actually take visual form. People with synesthesia, or union of the senses [Greek, syn = together + aisthesis = perceive], may see sounds, hear colors, feel the textures of sounds, or experience other "crossovers" among the senses. In fact, one synesthetic man reported that after asking an ice cream vendor what kind of ice cream she had, the vendor replied in such an unpleasant tone that "a whole pile of coals, of black cinders, came bursting out of her mouth." The poor man lost his appetite for ice cream.

Synesthetes, as such people are called, are mentally healthy people who nonetheless sense the world differently from the rest of us. Synesthesia can include the linking of any of the senses. Touching a smooth stone make evoke a sweet taste in the mouth. The flavor of cake might look like pink clouds.

The specific associations differ from person to person. To one synesthete, the sound of a guitar may appear as a jet of purple, the number "5" may appear yellow, or the letter "a," white with red dots. To another, the sound of the guitar is blue and the "5" is green.

But for all synesthetes, the links are consistent over time. A synesthete who hears certain musical notes when she sees something red hears the same music her whole life. Synesthetes may also be more likely to have perfect pitch, the ability to instantly recognize and name a particular note of music.

Synesthesia is rare, affecting perhaps one person in 25,000, and runs in families. But, although its existence has been known for centuries, scientists have only recently had the tools to try to understand it. What about the brain allows synesthesia to happen? Why don’t the rest of us experience such fantastic connections? What do synesthetes’ special brains tell us about our own?

Some researchers have postulated that we are all born synesthetic. Then while we are still babies, we gradually reduce the neural connections in our brains so that the senses are no longer linked. But other evidence suggests that the connections among the senses may still be intact, just unused. People given hallucinogenic drugs such as LSD, for example, experience visions and other sensory experiences similar to what a normal synesthete experiences every day.

In one recent study, researchers used a technique called Positron Emission Tomography (PET) to scan the brains of six synesthetes, all of who linked colors to words, and six nonsynesthetes. As each synesthete listened to a series of spoken words, blood flow in the folded outer surface of the brain, the cortex, increased dramatically.

In humans and most other vertebrates, the cortex processes all the different kinds of sensory information. In both the synesthetes and in the ordinary people in the study, language-processing areas of the cortex lit up, indicating increased blood flow to these areas. But in the synesthetes, other areas of the cortex lit up as well–those that process vision and color. These results seemed to support the idea that synesthesia is caused by unusual connections within the cortex.

In science, we rarely find a neat fairy-tale ending. Like a cascade of jewels, questions spill out, one leading to another. Do the neural connections that allow synesthesia exist in the rest of us? And if that turns out to be the case, how is it that we don’t use those connections? And if we don’t use them, why not? Just as a synesthete makes unexpected connections, perhaps scientists will too, as they continue to study this curious phenomenon.

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