Imagine that when you see a city's skyline, you taste blackberries. Or maybe when you hear a violin, you feel a tickle on your left knee. Perhaps you are completely convinced that Wednesdays are light red. If you have experiences like these, you might have synesthesia.

What is synesthesia?

Synesthesia is a condition in which one sense (for example, hearing) is simultaneously perceived as if by one or more additional senses such as sight. Another form of synesthesia joins objects such as letters, shapes, numbers or people's names with a sensory perception such as smell, color or flavor. The word synesthesia comes from two Greek words, syn (together) and aisthesis (perception). Therefore, synesthesia literally means "joined perception."

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 mint green or the number "4" as dark 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.

Diagnosis

Although there is no officially established method of diagnosing synesthesia, some guidelines have been developed by Richard Cytowic, MD, a leading synesthesia researcher. Not everyone agrees on these standards, but they provide a starting point for diagnosis. According to Cytowic, 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.

Who has it?

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.

Famous People

Some celebrated people who may have had synesthesia include:

Vasily Kandinsky (painter, 1866-1944)
Olivier Messiaen (composer, 1908-1992)
Charles Baudelaire (poet, 1821-1867)
Franz Liszt (composer, 1811-1886)
Arthur Rimbaud (poet, 1854-1891)
Richard Phillips Feynman (physicist, 1918-1988)

It is possible that some of these people merely expressed synesthetic ideas in their arts, although some of them undoubtedly did have synesthesia.

The Biological Basis of Synesthesia

Some scientists believe that synesthesia results from "crossed-wiring" in the brain. They hypothesize that in synesthetes, neurons and synapses that are "supposed" to be contained within one sensory system cross to another sensory system. It is unclear why this might happen but some researchers believe that these crossed connections are present in everyone at birth, and only later are the connections refined. In some studies, infants respond to sensory stimuli in a way that researchers think may involve synesthetic perceptions. It is hypothesized by these researchers that many children have crossed connections and later lose them. Adult synesthetes may have simply retained these crossed connections.

It is unclear which parts of the brain are involved in synesthesia. Richard Cytowic's research has led him to believe that the limbic system is primarily responsible for synesthetic experiences. The limbic system includes several brain structures primarily responsible for regulating our emotional responses. Other research, however, has shown significant activity in the cerebral cortex during synesthetic experiences. In fact, studies have shown a particularly interesting effect in the cortex: colored-hearing synesthetes have been shown to display activity in several areas of the visual cortex when they hear certain words. In particular, areas of the visual cortex associated with processing color are activated when the synesthetes hear words. Non-synesthetes do not show activity in these areas, even when asked to imagine colors or to associate certain colors with certain words.

Synesthesia and the Study of Consciousness

Many researchers are interested in synesthesia because it may reveal something about human consciousness. One of the biggest mysteries in the study of consciousness is what is called the "binding problem." No one knows how we bind all of our perceptions together into one complete whole. For example, when you hold a flower, you see the colors, you see its shape, you smell its scent, and you feel its texture. Your brain manages to bind all of these perceptions together into one concept of a flower. Synesthetes might have additional perceptions that add to their concept of a flower. Studying these perceptions may someday help us understand how we perceive our world.

Hear IT! Synesthesia Limbic

Synesthesia Experiment

  1. Read a list of random numbers between 0 and 9 at a rate of about one every 3 seconds. For example: 7, 9, 4, 0, 3, 8, 2, 5, 1, 6.

  2. After each number is read, ask people to write down the number and what COLOR that they associate with each number.

  3. Collect the answers. These will be called "Answers #1".

  4. Two to three weeks later, repeat the experiment, but change the order of the numbers. For example: 3, 6, 5, 9, 4, 1, 7, 0, 5, 2, 8.

  5. Collect the answers. These will be called "Answers #2".

  6. Compare Answers #1 with Answers #2. A person with synesthesia will have all or most of the same number-color pairs on both Answers #1 and Answers #2.

    This experiment can also be done using letters instead of numbers.

References and more information on synesthesia, see:

  1. Belgian Synaesthesia Association
  2. Cytowic, R., Synesthesia: Phenomenology and Neuropsychology, A Review of Current Knowledge. Psyche: An interdisciplinary journal of research on consciousness. Website: Synesthesia: Phenomenology And Neuropsychology
  3. Mixed Signals
  4. Synesthesia
  5. Synesthesia Links
  6. American Synesthesia Association
  7. International Synaesthesia Association
  8. Richard E. Cytowic Web Site
  9. Nunn, J.A., Gregory, L.J., Brammer, M., Williams, S.C.R., Parslow, D.M., Morgan, M.J., Morris, R.G., Bullmore, E.T., Baron-Cohen, S., and Gray, J.A. Functional magnetic resonance imaging of synesthesia: activation of V4/V8 by spoken words, Nature Neuroscience, 5:371-375, 2002.
  10. Palmer, T.J., Blake, R., Marois, R., Flanery, M.A., and Whetsell, Jr., W., The perceptual reality of synesthetic colors, Proc. Nat. Acad. Sci., 99:4127-4131, 2002.
  11. Lemley, B., Do You See What They See? Discover, Vol 20, No. 12, December 1999.
  12. Cytowic, R., The Man Who Tasted Shapes, Cambridge: MIT Press, 1998.
  13. Duffy, P.L., Blue Cats and Chartreuse Kittens: How Synesthetes Color Their Worlds, New York: WH Freeman & Co, 2001.

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Page prepared by Melissa Lee Phillips
Neuroscience for Kids Consultant