Synesthesia is an involuntary joining of two different senses. A person might see the color red, for example, whenever they hear a bell ring. Or, the sound of music played on a piano might evoke the taste of coffee for another. An important aspect of synesthesia is consistency — the linked perception is the same every time so that the number "5" will always be red, while the number "2" will always be green.

What is it like to experience synesthesia? A common theme in stories from synesthetes is the moment they discover they are different. "I nodded meekly but inside, I was reeling," wrote one synesthete when he first discovered others did not perceive the world as he did. "No colors? Everyone lived in a white word where numbers and letters were simply black? And music didn’t fill the air with sparkles and waves of colored light? Did that mean they didn’t see auras around people either? Their worlds were colorless?"

It's also important to understand synesthetes do not regard the additional perception as imaginary — the experience of a blend of senses is just as vivid and real as a single sense is for others. Just as you would not doubt you are seeing red, for instance, a synesthete does not doubt that a song by Sonic Youth is pale yellow. The pairing of senses, individual and varied though they may be, are not infinite just as the senses are not infinite. Some researchers estimate records of 35 or so different subtypes, such as taste-hearing (hearing a sound produces a taste) and sound-touch (feeling an object produces a sound). The most common are color-graphemic, in which number, letters, and/or shapes produce colors (or sometimes simple patterns) and color-auditory, in which voices, music, and/or random noise produces colors, textures, and shapes.

Should synesthetes consider themselves lucky?

Many do, though sometimes they may feel isolated and worry about what others will think. “The oddest thing about all this is that I thought this was how everyone perceived the world until about 8th grade when I asked a childhood friend about how he liked the purple tone a certain song had and he looked at me like I had 4 heads,” wrote a male synesthete. “I had something similar happen in a college music room where I was tired and not thinking and the teacher asked what I thought of a particular piece and I blurted out ‘I hate it it is too pale yellow’ which of course caused half the room (including the teacher) to freak out or just start laughing.”

Disharmony may sometimes bother other synesthetes, who prefer the regularity of their own visions. Seeing a poster with the numbers in color, for instance, one synesthete commented, “If a 4 is purple instead of green it makes me mad.” A synesthete who tastes language said, “I taste the words whether I read them in a book or hear them in a conversation. The downside of this is when I am eating something and the taste of the word spoken does not go very well with the taste of whatever I am eating.”

One synesthete who routinely sees colors when she hears music also reports a more abstract linking between personality and color: “This is good, it helps me choose good friends,” she notes, adding “purple people are snobbish.”

Friendship is a necessary requirement for confidence with many who possess this unusual ability. “I usually don't share with many people because it takes way too much time to explain,” wrote another synesthete. Yet he also strongly believes this trait is a gift. “I could not imagine a life without it.”

Synesthesia occurs in about four percent of the population. In particular, women have greater odds of being born with the condition; in the U.S., studies show that three times as many women as men, while in the UK, the ratio is six to one. This difference in prevalance may be tied to the fact that the condition also runs in families. For this reason, researchers believe there is a genetic component even though the specific genes underlying synesthesia are still unknown. Oddly, synesthetes are more likely to be left-handed than the general population.

Neuroimaging studies show that there are unusual connections in the brains of synesthetes; regions not usually wired together are linked and this is what causes a sensation along one channel to automatically trigger a perception in another.  A recent study found that there is a link between synesthesia and autism spectrum conditions. Autism occurs in one percent of the population. Yet when researchers tested 164 adults with autism and 97 adults without the condition, synesthesia occurred in nearly one in every five people (20 percent) with autism — much greater than the four percent found in the population at large. The similarities between the two conditions are striking; as with synesthesia, neuroimaging studies have also discovered greater connectivity in the brains of people with autism as compared to those without the condition.