For one person, the taste of mint is described as the shape of "cool glass columns." For another, hearing music conjures up images of gold balls, flashing lines and oscillating waves.
These individuals exhibit synesthesia, a strange and uncommon condition that scientists have yet to unravel.
The word synesthesia is derived from Greek words meaning "joined sensation." The condition broadly is defined as the inability to differentiate between assorted sensory stimuli.
"There are people who feel shapes when they taste. Listening to music, A sharp is one color and A flat is another," Vanderbilt University Psychology Prof. Thomas Palmeri said. "Synesthesia is a condition characterized by anomalous perceptual experiences."
Even though the first documented evidence of synesthesia appeared more than three centuries ago, little is known about the condition and few scientists have researched it.
Last month Palmeri and fellow Vanderbilt psychology professors Randolph Blake and René Marois published a paper detailing their study of a synthesthetic adult male who perceives black-and-white words and numbers in specific colors.
According to Palmeri, the experience of synesthetes differs from the experience of individuals who use hallucinogenic drugs.
"It's not like a hallucination, rather they are consistent experiences," Palmeri said. "Hallucinations are not tied to particular stimuli, whereas synesthesia is tied to precise stimuli."
Psychologists classify synesthesia into two categories. Developmental synesthesia begins in early childhood. According to Palmeri, it seems to have a genetic basis because it runs in families.
Acquired synesthesia begins after development and results from brain injury.
The developmental variety accounts for the bulk of synesthetes, affecting as many as 1 in every 200 people. Synesthetes often are shocked to realize they are different and that most individuals do not experience mixed sensory perception.
Furthermore, when synesthetes divulge their strange perceptions, their friends and family often question their sanity. For this reason, many synesthetes praised the recent study.
"People have been calling and saying, 'Thank you, I've had this all my life, my family and girlfriend thought I was nuts,'" Palmeri said.
One potential explanation for synesthesia hints that we all might experience the phenomenon during early development.
"One of the ideas is that maybe everyone was born with synesthesia," Palmeri said. "Most of us have the connections that presumably cause synesthesia pruned away, and synesthetes have not had those connections pruned."
The experiences reported among different synesthetes vary widely.
"The colors that people see are all over the place. Some see vivid colors for all letters while others do not see as much," Palmeri said.
With the exception of blurred sensory readouts, synesthetes otherwise function normally.
Edward Hubbard, a psychology professor at the University of California-San Diego, said in an MSNBC interview last month that synesthetes often experience a great deal of pleasure from their altered perception.
The bizarre cornucopia of colors that some synesthetes view on a seemingly black and white page of words, however, can divert their attention from reading.
Some synesthetes report their condition makes it easier for them to remember certain kinds of information, often having an uncanny ability to remember movie lines, conversations and prose passages.
Palmeri studied a synesthete who associates colors with the digits in telephone numbers, referring to the synesthetic adult male simply as "WO" to protect his identity.
WO, a professor of medicine, found studying complex scientific words easier as a result of his synesthesia. To study the reliability of WO's color associations, Palmeri and his colleagues showed him a list of 100 words and recorded the colors that WO associated with the words.
After one month, WO repeated the test with an identical list of words, identifying words associated with the same colors.
WO rarely made mistakes, such as when he substituted beige for off-white. The stability of synesthetes' perceptions typically last throughout life.
An important aspect of future synesthesia research will focus on understanding what elicits the unique brain function that causes this condition.
Palmeri plans to perform brain-imaging studies to see what parts of WO's brain are active during synesthesia.
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