HAMBURG — In Birgit Böhm's opinion, the "4" she's looking at is balancing on one leg, like in the nursery rhyme "A little man in the forest," and wearing a little red cloak. The "7" next to it looks like it's made of thick gardening wire. And it's green! The rest of the numbers and letters there also seem to have fallen in buckets of paint. Birgit's world is a colorful one.

That's because Birgit, a 60-year-old teacher outside of Hamburg, is a synaesthete. Synaesthesia is a gift, a special quality that affects perception — a kind of constant intoxication of the senses. Letters and numbers have certain colors. Sounds might have a sweet or salty taste to them.

Peter Weiss-Blankenhorn of the Research Centre Jülich in Northrhein-Westphalia describes synaesthesia as a "genetic disposition in a healthy human causing sensory impressions that are not usually linked to one another to be linked."

The most common form is called Graphem-color synaesthesia, which causes letters and numbers to be linked to certain colors in one's brain. This is the case for Birgit Böhm. The colors she sees, when looking at specific letters and numbers, are fixed. A red "4," in other words, does not all of a sudden become yellow.

But Birgit also sees colors when she hears certain sounds or listens to music. If someone were to play a piece on the recorder, Birgit's inner eye would see a black stage with yellow balls bouncing around the place. Other people might associate certain smells or tastes with letters, numbers or sounds.

Markus Zedler, synaesthesia specialist at the Hannover Medical School, calls these manifestations of synaesthesia "genuine," meaning that these are congenital and that the interlinking of these senses is established and permanent.

Unreported cases

Emotions can be another trigger, producing sensory experiences that are not only linked to one but multiple senses. A synaesthete might, for example, associate a person's character with a color. Less is known about this particular form of synaesthesia due to the difficulty of reproducing feelings under laboratory conditions.

Nor is it clear how many people actually are synaesthetes. Birgit Böhm didn't discover her synaesthesia until just a few years ago, when she read a newspaper article on the topic. Until then she simply assumed that all people perceived the world to be as colorful as it is to her. "When you walk out the door and you see a green tree you imagine everyone else thinks that the tree is green too," says Peter Weiss-Blankenhorne.

Simulation of how a synaesthetic person might associate a color to letters and numbers — Source:Mysid

For that reason, the number of unreported cases of people with synaesthesia is probably quite high. Weiss-Blankenhorn estimates that approximately 2% of the population, whether they know it or not, are synaesthetes. Synaesthesia expert Gregor Volberg of the University of Regensburg thinks the incidence could be as high as 5%. He points to research done at the the University of Edinburgh, in Scotland, where scientist Julia Simner and her team tested 500 people and found 4.4% to be synaesthetes.

Working hypotheses

Then there's the question of what causes the condition. Why do some people perceive the world so differently than the rest of us? The most popular thesis is that there is a hyperconnectivity between certain areas of the brain that, usually, have no or very little connection.

This thesis works well where Graphem-color synaesthesia is concerned. "The areas reserved for color and letters within the brain are located in very close proximity to one another," says Volberg. If these two areas are then connected through nerve fibers, sensory experiences that are usually not linked can thus be activated at the same time.

That doesn't, however, explain the more rare cases of people who associate letters with taste, which is handled by a more distant part of the brain. Another unanswered question is why hyperconnectivity exists in some but not in others. One theory states that neuronic connections that formed during the fetal stage and which are usually severed later on, were not removed, says Volberg. Some researchers also believe that these rare neuronic connections are then strengthened during the process of learning how to write.

Three-dimensional datebook

For some everyday activities, according to Weiss-Blankenhorn, synaesthetes have a bit of an advantage. Specifically colored letters or numbers, for example, make it easier to recall stored information.

To remember appointments, Birgit Böhm relies on another form of synaesthesia: so-called time-space-synaesthesia, which enables her to visually map time periods. In her mind, the week resembles an ellipse and every day of the week has a certain color. Saturday, for example, is a deep orange. Sunday is a wonderfully vibrant red. Mondays are dark green. The days resemble little towers made from Lego or can take the form of empty drawers into which Birgit organizes her appointments.

Birgit is also a teacher and organizes her class folders according to color. But her synaesthesia can sometimes cause confusion. "With class 8e, for example, the 8 is blue and the e is yellow, so I have to decide between the two colors for the color of the folder," she says.

Don't try this at home

Those of you who now wish to have synaesthesia may be disappointed to know that many non-synaesthetes have tried to train themselves to become a synaesthete without any success, says Volberg. "It seems that the sensory experiences that synaesthetes encounter every single day and which they cannot control cannot be acquired artificially," he adds.

But who then is able to enjoy the colorful world their brains create? "We know for a fact that synaesthesia is hereditary," says Weiss-Blankenhorn, as synaesthesia seems to be quite common within families where one individual has synaesthesia.

Birgit Böhm, after speaking to her family about the condition, learned that her daughter is also a synaesthete, though with perceptions that are all her own. For the daughter, the week does not have an elliptical shape. Far from it. Instead it looks exactly like a multi-colored xylophone. Naturally.