Synesthesia can involve any of the senses. The most common form, colored letters and numbers, occurs when someone always sees a certain color in response to a certain letter of the alphabet or number. For example, a synesthete (a person with synesthesia) might see the word "plane" as green or the number 4 as brown. There are also synesthetes who hear sounds in response to smell, who smell in response to touch, or who feel something in response to sight. Just about any combination of the senses is possible. There are some people who possess synesthesia involving three or even more senses, but this is extremely rare.
Synesthetic perceptions are specific to each person. Different people with synesthesia almost always disagree on their perceptions. In other words, if one synesthete thinks that the letter "q" is colored blue, another synesthete might see "q" as orange.
Synesthetic perceptions are:
- Involuntary: synesthetes do not actively think about their perceptions; they just happen.
- Projected: rather than experiencing something in the "mind's eye," as might happen when you are asked to imagine a color, a synesthete often actually sees a color projected outside of the body.
- Durable and generic: the perception must be the same every time; for example, if you taste chocolate when you hear Beethoven's Violin Concerto, you must always taste chocolate when you hear it; also, the perception must be generic -- that is, you may see colors or lines or shapes in response to a certain smell, but you would not see something complex such as a room with people and furniture and pictures on the wall.
- Memorable: often, the secondary synesthetic perception is remembered better than the primary perception; for example, a synesthete who always associates the color purple with the name "Laura" will often remember that a woman's name is purple rather than actually remembering "Laura."
- Emotional: the perceptions may cause emotional reactions such as pleasurable feelings.
Estimates for the number of people with synesthesia range from 1 in 200 to 1 in 100,000. There are probably many people who have the condition but do not realize what it is.
Synesthetes tend to be:
- Women: in the U.S., studies show that three times as many women as men have synesthesia; in the U.K., eight times as many women have been reported to have it. The reason for this difference is not known.
- Left-handed: synesthetes are more likely to be left-handed than the general population.
-Neurologically normal: synesthetes are of normal (or possibly above average) intelligence, and standard neurological exams are normal.
- In the same family: synesthesia appears to be inherited in some fashion; it seems to be a dominant trait and it may be on the X-chromosome.
Here is a video I found on Youtube that explains synesthesia very well.