Do You Hear What I See? Synesthesia and Sensory Interactions
We generally think that humans have five senses and that those senses are delegated to a specific organ in our bodies. Anyone would agree that we see with our eyes, taste with our tongues, hear with our ears, feel with our skin and smell with our noses. While this may indeed be the case, it is not the whole story. Our senses are all interconnected. They compliment one another and can even compensate for one another if a sense is weakened or lost (1). However, the interactions between the senses are not uniform from person to person. Synesthesia, the name given to any of a number of conditions involving the experience of a usually unassociated sense in conjunction with the stimulation of another sense, provides insight into the ways our senses can interact.
The most common forms of synesthesia are known as grapheme → color synesthesia, sound → color synesthesia, lexical → gustatory synesthesia, number form synesthesia and ordinal linguistic personification. Individuals with grapheme → color synesthesia perceive colors in association with letters, numbers or words. For instance, the letter N might be blue while the number 4 is red. A person's name might be yellow, and the grapheme → color synesthete may remember the color of a person's name but not the name itself. Similarly, sound → color synesthetes perceive color associated with auditory input, most commonly music. Certain melodies or even individual pitches may elicit particular colors. Lexical → gustatory synesthesia is less common and is characterized by the experience of taste sensations in conjunction with the sound of words or phrases. This response can also be triggered by reading or thinking about certain words. Number form synesthesia and ordinal linguistic personification are somewhat different. Number form synesthetes perceive a distinct spatial arrangement of numbers, dates, months or days of the week. The number 42 might be closer to the person when they encounter it, while the number 887 might be farther away. November 30th might be located in the upper right-hand corner of the person's field of vision. Weeks may be arranged counterclockwise in the shape of a football. Individuals who experience ordinal linguistic personification attribute personalities to letters, numbers, days, or words. Some letters may be male while others are female. Some numbers may be old while others are young. Some words may be friendly while others are rude or anxious or immature (2).
Complex sensory interactions are not as unusual as they might seem. Our sense of sight can greatly influence our sense of sound (3). Scents can conjure up taste sensations. Scent and taste work together to allow us to differentiate between a large number of flavors beyond the basic tastes of sweet, sour, salt, bitter or savory. There is a tactile component to our ability to hear (4,5). From a linguistics standpoint, we have no problem using metaphors that mix senses: a loud shirt, a sour face, a rough voice. We don't tend to think about these interactions between our senses simply because we are so used to them. The same is true for individuals with synesthesia. Many of them do not realize their sensory perception is any different from anyone else's until they talk about it explicitly. They have always experienced coupled sensory stimulation and so their perception is just as "normal" as a non-synesthete's sensory perception. In fact, it is entirely likely that no two people experience sensory input in exactly the same way. It may be the case that synesthesia is diagnosable because the differences can be characterized verbally and thus can be more easily compared with other people's descriptions of the way they perceive things.
What exactly differentiates synesthesia from so-called "normal" perception? Synesthesia is involuntary, automatic, generic and consistent. The responses do not happen in the person's head but instead are perceived as occurring outside of their bodies (2). For instance, a synesthete won't merely think of the color blue when they see the letter N, they will actually see the color blue (often the letter itself will appear blue). Furthermore, the letter N will always appear blue to that individual, no matter what the context. However, the details will differ from person to person. While the letter N might be blue for one person, it could be lighter or darker blue, or red, or brown, or any other color to someone else (2).
It is thought that synesthesia is caused by the "cross-wiring" of the various parts of the brain responsible for processing sensory input (2). It is interesting, in that case, that there is such a wide range of variation between individuals with the same type of synesthesia. For instance, grapheme → color synesthetes have no trouble distinguishing between colors. Two synesthetes would not disagree about the color of a tree, or a car, or the sky, but they might disagree about the color of the letter X or the number 7. This implies that the experience of color is not triggered by external input but instead might come from within the nervous system itself. If that is the case, it is also interesting that an individual synesthete's perception is so consistent throughout their lives. That seems to imply that color is not something a person assigns or learns to attribute to a letter or number. Many people associate scents or words or colors with people or places based on experiences. The smell of baking bread may bring to mind a holiday at a relative’s house because someone had the experience of smelling bread on that holiday at that relative’s house. This does not seem to be the case with synesthesia, but it is possible that synesthetes simply form associations earlier and more effectively than other people and that is the reason their perception is so consistent. Hopefully the differences in perception between synesthetes and non-synesthetes will become clearer with more study; such information could prove valuable in understanding how we perceive the world around us.
1. Lee Dye “Overcoming Blindness: Other Senses Compensate in Just 10 Minutes” ABC News, March 17, 2010
2. Cytowic, R.E. Synesthesia: A Union of the Senses, 2nd ed. Massachusetts: MIT Press, 2002.
3. Bryan Gick & Donald Derrick. “Aero-tactile integration in speech perception” Nature, October 9, 2009
4. Henry Fountain. “People Hear With Skin as Well as Their Ears” The New York Times, November 25, 2009
5. Sandra Blakeslee. “When the Senses Become Confused” The New York Times, December 25, 2007