Like many artists, Carol Steen paints what she sees. But judging by the canvases that fill her loft in Manhattan's NoHo neighborhood, her vision is, well, unusual.

This series of canvases, she explains one afternoon, depicts the shapes and colors that appeared to her -- usually in her mind's eye but sometimes suspended before her -- when she underwent acupuncture treatments. In one, a luminous blue orb weeps emerald crescents. Nearby hang paintings whose images she saw while listening to music: flowing shapes in green, teal, gold and violet.

Ms. Steen is a synesthete, someone whose brain is "cross-activated" so that one sensory experience (feeling or hearing, for instance) triggers a wholly different one (seeing). The result is "a world in multimedia," she says. "Synesthesia is a gift."

Brain researchers couldn't agree more. Because the condition promises to shed light on puzzles ranging from the roots of creativity to the origins of language, says V.S. Ramachandran of the University of California, San Diego, "synesthesia is a gold mine for neuroscience."

He estimates that as many as one person in 200 has synesthesia, which can take as many forms as there are sensory pairings. Novelist Vladimir Nabokov wrote that the sound of a long A in English "has for me the tint of weathered wood, but a French A evokes polished ebony." George Gershwin saw notes in color (ever wonder about "Rhapsody in Blue"?), as did Franz Liszt, requesting of musicians, "Gentleman, a little bluer if you please." For Ms. Steen, the radio creates a kaleidoscope so riveting she prefers to turn off the music when she parks her car. In a rarer form, tastes have shapes. One synesthete says a roast chicken in citrus sauce is done to a turn when it is "pointed."

In its most common form, synesthesia makes you always see a particular letter or digit in a particular color. To author Patricia Lynne Duffy, P is invariably pale yellow, R is orange, 5 is purple. "When I think of the alphabet, it's like a sloping scale of brightly colored letters," says Ms. Duffy, whose book "Blue Cats and Chartreuse Kittens" describes her world. One medical professor tells psychologist Thomas Palmeri of Vanderbilt University that although color letters slow down his reading, they help his memory: He breezed through anatomy because the distinct colors of the terms acted as mnemonics.

For decades neurologists figured people like the professor were crazy or lying. Finally, though, brain imaging is establishing the reality of synesthesia. In April, scientists at Goldsmiths College in London reported on fMRI scans of synesthetes who hear spoken words in color. The brain area that processes color when you or I stare at a cerulean sky or an emerald fairway is, in these synesthetes, also activated by the spoken word.

Synesthesia probably strikes when the brain takes E.M. Forster's maxim "only connect" to extremes. Everyone is born with extra connections, or synapses. Most get pruned away in childhood. In synesthetes, the extra synapses seem to remain, producing a rich web of circuitry that connects the cortex's color processor to the numeral area next door, or links touch regions to vision regions. Since synesthesia runs in families, defective pruning might reflect a genetic mutation.

While researchers have fun studying people who see middle C, they're after bigger game. "We hope that synesthesia can give us a window into processes that occur in everyone's brain," says Edward Hubbard of the University of California, San Diego.

Chief among them: creativity (which, after all, is seeing connections that no one before you has) and metaphor (linking seemingly unrelated concepts, as in "Juliet is the sun"). Scientists suspect that crossed wires in the brain's angular gyrus, where information from different senses converges, underlies synesthesia. Not coincidentally, perhaps, when this structure is damaged, your brain can't understand metaphor.

Synesthesia may even explain one of the great mysteries of science -- how language originated. Try this: Draw one spiky shape and one rounded, amoeba-like one. Pretend that, in a lost language, one is a "kiki" and one a "shoosha." Which is which?

Almost everyone says the spiky shape is the kiki. "The spikes mimic the sharp sound of 'kiki,' " says Dr. Ramachandran. If appearances and sounds are really linked in a nonarbitrary way in regular folks just as they are in synesthetes, then early humans could have used sound to represent objects and actions in a way the guy in the next cave would understand. In that case synesthesia, far from being a mere curiosity, offers a window onto the most human of human traits.

Write to me at sciencejournal@wsj.com.