What color is the letter F? What does a musical note taste like? What color is a stubbed toe? These questions may seem nonsensical to you, but they make perfect sense to as many as one out of every few thousand of your fellow citizens.

Shabana Tajwar, an environmental engineer, sees the color green when the letter “F'” is printed. Another person tastes pickles whenever a certain musical note is sounded. And artist Carol Steen saw a vivid orange when she stubbed her toe. In fact, she yelled “orange,” rather than the words many of us might use when painful accidents take place.

These folks are among the countless people around the world who have a condition known as synesthesia, which causes them to see colors, experience tastes or even hear sounds with certain letters, words or shapes. Susan Hornik tells of this strange phenomenon in the February issue of Smithsonian magazine.

Richard Cytowic, a neurologist who wrote a book on synesthesia titled “The Man Who Tasted Shapes,” said the condition has been known to medicine for almost 300 years, with the first medical reference being a comment by a Dr. Thomas Woolhouse in 1710 that he had a patient who said, “The sound of a trumpet was like the color scarlet.” Cytowic found sporadic mentions of similar experiences in medical literature up to about 1860, when interest in synesthesia peaked, particularly among artists, musicians and others seeking to tie the five senses together.

As the 20th century dawned, interest in “sensory fusion” gradually died out as the arts communities adopted other intellectual fashions, and physicians, who never considered it as being any more than an individual patient’s peculiarity, based their diagnoses on new technologies such as the X-ray machine.

In recent years, synesthetes once again have started to use their “gift” in composing works of art. Hornik writes that composer Michael Torke wrote a series of works in the 1980s relating colors to musical notes. Among these were “Bright Blue Music” in D major and “Green” in E major.

Cytowic said his interest in synesthesia began with a chance remark by a neighbor, who was testing a sauce for roasted chicken and said, “There aren’t enough points on the chicken.” When pressed, he admitted that he experienced tastes as shapes, such as round, boxlike, pointed or some combination of all three. By accident, Cytowic found a related condition in one of his own medical colleagues, who said she saw the noise of a beeper as brilliant red flashes.

Cytowic went into the archives of the medical library, found the few past references to synesthesia, and set out on a course of study that has made him an international authority on the subject. Over time, he developed a set of five criteria that allows physicians to diagnose synesthesia and has achieved a skeptical acceptance for the condition, even though most doctors today still will never have heard of it.

No one knows the cause of synesthesia, but Peter Grossenbacher, a neuroscientist at the National Institutes of Health who has studied the condition for years, has a theory. He thinks it may be that the wiring for all five senses are joined together in the brains of infants until 6 months of age, when they begin to disconnect. This separation may not fully happen in synesthetes, letting them see sounds as colors, tastes as shapes, etc., depending on what connections remain in place. It may even be possible that hallucinogenic drugs, such as LSD, whose users often report vivid color sensations, may reconnect the brain’s sensory circuits, causing a temporary synesthesia.

Recent studies using positron-emission tomography and magnetic resonance imaging scans tend to support this theory. Dr. Simon Baron-Cohen, a psychologist at Cambridge University, used these tools to study cerebral blood flow in both synesthetes and a control group. When the groups were blindfolded and read a list of words, both showed neural activity in the language areas of their brains, but the synesthetes also showed strong activity in the visual cortex linked with the seeing of colors. The implication is that the synesthetes were seeing colors associated with each word, even though they could not, in actuality, see anything.

Estimates of synesthetes range from one in 25,000 persons by Cytowic, to one in 2,000 persons by Baron-Cohen. Females are thought to outnumber males 8 to 1. Most do not find the condition a handicap, and even enjoy their unique and colorful existence.

Tajwar did say she has a real problem with the colored A,B,C routes on Prague’s subway maps. “Their colors don’t match what I see for the letters,” she said, “so I keep getting on the wrong line.”

Clair Wood taught chemistry and physics for more than 10 years at Eastern Maine Technical College in Bangor.