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Uncommon Sense

Synesthesia helps the brain luxuriate in metaphor.

Sir Francis Galton, the Victorian anthropologist, performed arithmetic by smell. In an experiment to prove he could banish all visual and auditory associations with numbers, he taught himself to add and subtract small sums by imagining aromas. Two whiffs of peppermint, for example, equaled one whiff of camphor, while three whiffs of peppermint amounted to one hit of carbolic acid. Encouraged by the results, he began tasting sums, with salt, sugar, and quinine in place of integers.

© Andrzej Burak<br/>iStockphoto.com Some people experience a more natural—and neurally based—commingling of the senses through synesthesia, a phenomenon in which one modality, such as vision, produces an automatic, involuntary perception in another, such as hearing. An estimated 1 percent of people have some form of synesthesia, which runs in families and appears more often in women.

My own form is the most common: grapheme-color, in which letters of the alphabet evoke specific hues. For me, the letter f is pine green, while a is a dark manila. For years I assumed that such mental images stemmed from my play with letter-shaped refrigerator magnets as a child. Only after I learned about synesthesia in a college lecture did I dig up my old magnet set and realize that the colors didn’t match the ones lodged in my brain.

Synesthesia comes in many forms. Spatial-sequence synesthetes experience three-dimensional perceptions; the 1950s, for instance, may appear near the ground. Numbers and letters gain personalities in ordinal-linguistic personification synesthesia. And in lexical-gustatory synesthesia, words cause taste sensations. One synesthete reports that she loves to chant the word because; to her, it tastes like a Mars bar.

Out of Your Senses

Most people seem to have mild synesthesia, perceiving, for example, high-pitched sounds as bright and low-pitched sounds as dark. Similarly, we all tend to hear certain speech sounds as sharp and others as rounded. In a 1947 study, the Gestalt psychologist Wolfgang Köhler showed people jagged and smooth shapes, then asked which were called takete and which maluma. Participants consistently—and across languages—paired the jagged image with the otherwise meaningless sound takete and the smooth image with maluma.

Nearly a century earlier, poet and philosopher Benjamin Blood had described how certain words conjure the objects they describe. The sound tub suggests “short and stubby,” he declared, while icicle conjures up “spindling and slim.”

Blood then went on to offer an exuberant description of generalized synesthesia. “Consider,” he said, “the use of the words entrails, reins, bowels—all good in scientific and social discourse, but for some unmentionable reason classic culture draws the line at guts!” When asked what the trouble with guts was, Blood replied that it was vulgarized by the absurd genius of the guttural u, which he described as “a huge, lubberly, blundering dunderhead, a blubbering numskull and a dunce, ugly, sullen, dull, clumsy, rugged, gullible, glum, dumpish, lugubrious...a musty, fussy, crusty, disgusting brute....”

Apart from such broad associations as sound with luminosity and syllables with shape, synesthetic perceptions are idiosyncratic: One grapheme-color synesthete’s m is mauve while another’s is chartreuse. Yet links remain remarkably consistent. A true synesthete will retain the same color–sound associations across decades.

Key to Your Art

Synesthesia is an example of what psychiatrists call hyperassociativeness, often seen in creative people. Vivid synesthesia, in fact, is disportionately likely among artists. The painter Wassily Kandinsky, poets Arthur Rimbaud and Charles Baudelaire, and composers Leonard Bernstein, Nikolai Rimsky-Korsakov, and Franz Liszt were all synesthetes. For the novelist Vladimir Nabokov, a long a had “the tint of weathered wood” while a hard g evoked “vulcanized rubber” and l was “noodle-limp.”

People without synesthesia sometimes envy those with it. But hyperassociative thinking can also appear in psychosis, and synesthetes are more susceptible to mental illness.

Not surprisingly, synesthetes have better memories in the modalities for which they are synesthetic, presumably because those sensations are multimodal and vivid. But their memories may become full of irrelevant flashes of color and sound, as jangled as Times Square at night. In one poignant case study, the Russian neuropsychologist Alexander Romanovich Luria described the mind of “a little man with a perfect memory” whose mnemonic powers stemmed from multimodal synesthesia. Nothing was forgotten; everything was connected. The man, who had started life as a writer, became overwhelmed by the tangle in his head and finished his life doing memory tricks in a circus.

Living in Metaphor

At first glance, synesthesia may seem merely vivid metaphor, elaborate figures of speech rather than direct experiences or brain states. But a closer look suggests the opposite: Many metaphors are directly synesthetic, such as “her voice was sharp” and “his name leaves a bad taste in my mouth.”

Synesthesia appears to have a clear neurological basis, as, in fact, does metaphor. When synesthetes with colored hearing undergo functional magnetic resonance imaging while listening to letters being spoken, they show more diffuse patterns of brain activity than nonsynesthetes. Like nonsynesthetes, their primary auditory cortex is active while their primary visual cortex is not. Yet synesthetes also have activity in several higher-level visual areas in the temporal and parietal cortices—despite the absence of visual stimuli.

Additional functional imaging studies show that synesthetes have greater white matter connectivity, the anatomical basis of their hyperassociativeness. Other research suggests that synesthesia may be the result of overwiring in the brain, in which some neurons that should connect to just one sensory center instead connect to two or more. This increased connectivity can foster delusions, such as when a racehorse’s name smells like money. But the overbranched neurons may also foster creative thinking by integrating apparently unrelated sensory inputs, memories, ideas, and actions.

No doubt much about synesthesia remains to be discovered. In the meantime, its very existence stimulates one of our most enduring senses: our sense of wonder.

Alice Flaherty ’94, PhD, is an HMS assistant professor of neurology at Massachusetts General Hospital.