How Synesthesia Works

As a general rule, synesthesia can occur between just about any combination of senses or cognitive pathways.

The most common type of synesthesia is grapheme-color. Graphemes (letters or numbers) take on certain colors to the synesthete. This is almost always uniform for each person, but not among synesthetes in general. For instance, one person with synesthesia could always see the letter A as red, but that doesn't mean all synesthetes will experience the same color. (Although there are some studies that show certain combinations appearing more among synesthetes -- including A as inherently red [source: Ramachandran and Hubbard].)

Another synesthesia that is reported is sound-to-color synesthesia. This could indicate a few different things; it might mean that something as simple as a noise (a car horn, flushing toilet) takes on a color or shape in the mind's eye when it's heard. You might experience some sounds as a texture. Some lucky folks get to "see" music and associate a wide variety of musical notes with a rainbow of colors. Interestingly, a 2006 study did show that sound-to-color synesthetes associated a higher pitch to a lighter color -- which non-synesthetes also agreed on [source: Ward]. This led researchers to conclude that synesthesia might be using the same cross-modal pathways that most people use, rather than pathways special to the condition.

Next up is one of the rarer (and more delicious) forms of synesthesia, which is lexical-gustatory (or olfactory). This kind of synesthete might taste certain flavors (or even whole dishes or meals) based on a visual picture or word or sound. They might also associate smells with certain colors, or shapes -- let's say the smell of toast might be ochre. Some synesthetes even claim to taste the emotions of other people [source: Carlsen].

Some less common forms include mirror-touch synesthesia, which causes some people to experience a physical feeling when other people are touched (read Can people feel the pain of others? for more info on this particular twist). Ordinal-linguistic personification is where a person might give personality traits to individual items in ordinal lists. For instance, saying that the letter A appears egotistical, the letter B nurturing and the letter C brave -- it can happen with numbers, days of the week and so on. Number-form is where a person sees numbers as a very distinct map -- often, lower numbers (more commonly seen ones) are bunched up at one end (or the first 12 might imitate a clock face). They're not necessarily linear, but might curve in and out, or move up and down. However the map "looks," it stays uniform for the number-form synesthete.

So we got it: There are lots of synesthesia forms out there. But why in the world do they occur?

So let's say you really want to taste purple or find a way to associate colors with numbers. How the heck does one get synesthesia? First, let's delve into the nitty-gritty of how scientists think (smell? hear?) how synesthesia works.

We don't know a ton about synesthesia, although it has become a popular field for study. Researchers do think the condition tends to be somewhat inherited or genetic, as about 40 percent of synesthetes report a first degree relative with synesthesia [source: Brang and Ramachandran]. Many synesthetes recall synesthesia as long as they can remember, and it can be discerned at an early age.

If you don't have synesthesia, you might be trying hard to make associations with colors or numbers related to memories. (You might decide the letter G is blue because you can recall a teal "G" on a poster in your childhood bedroom.) But synesthesia has been shown to be a sensory phenomenon, unrelated to memory. For example, if you are given a matrix mostly consisting of the number five all over the paper -- scattered with a few twos, which form the shape of a triangle -- you would have a hard time discerning the triangle. You'd have to look closely to search for the twos, and then discern the shape from there. A grapheme-color synesthete can see the triangle almost instantly. This has led researchers to conclude that synesthesia is quite real, and that it happens early in the perception process; your mind, in other words, doesn't spend time searching for a memory association [source: Ramachandran and Hubbard].

The more global theory of how it works is pretty straightforward: Researchers think that synesthesia is a kind of cross-wiring in the brain. In grapheme-color synesthetes, seeing a number simply stimulates your grapheme region and the area of your visual cortex that responds to color stimuli [source: Brang and Ramachandran]. One theory is that there are increased neural connections in the brain of synesthetes that could've been the result of less "neural pruning" while in utero [source: Brang and Ramachandran]. Even cooler is that there might be actual anatomical differences in the brains of synesthetes, like increased white and gray matter in the brain [source: Brang and Ramachandran].

Although one study did find that some exposure to color-letters built up their association, the effect didn't stick around [source: Colizoli]. So far, it doesn't appear one can "get" synesthesia.

Most discussions of synesthesia will include famous creative types. Vladimir Nabakov wrote about his synesthesia in his biography "Speak, Memory," and the composers Franz Liszt and Nikolai Rimsky-Korsakov apparently clashed over what colors certain musical keys were [source: Cytowic and Eagleman]. But if you're a non-synesthete and feeling bummed about how mundane your worldview appears, take heart: Although much has been said about synesthesia among artists, there is no evidence that having synesthesia makes one more creative [source: Brang and Ramachandran].

However, there just may be some secondary cognitive benefits for synesthetes. Memorization appears to be one of them. Some synesthetes can memorize a substantial list of numbers by recalling color associations, for example. And some color synesthetes have also demonstrated an impressive sensitivity to discerning extremely similar colors [source: Brang and Ramachandran]. (It remains to be seen whether synesthesia is the cause of this, or if it can be attributed to the broad and frequent experience synesthetes have with color.)

Alas, it's not all idyllic. Some synesthetes report their condition can be a little grating at times. It can be uncomfortable, for instance, for a grapheme-color synesthete to see a number in the "wrong" color. One lexical-gustatory synesthete also made the point that if a certain name doesn't taste right to him, he has a hard time liking the person it's attached to [source: Carlsen]. (One can only assume that's where nicknames come in handy.)

And while not everyone has synesthesia, we should point out that hallucinogenic drugs might be one way that synesthesia could be "manufactured." Several drugs can produce vivid synesthesia in otherwise non-synesthetes, which might be a key to understanding the condition. One researcher has posited that in non-synesthetes, information in a multisensory area travels back easily to its single-sense area, but in synesthetes it gets a bit mixed up along the way. Taking something like LSD might mean that the subsequent change in neurochemistry causes those existing connections to get confused for a while [source: Carpenter].