Introduction
Synaesthesia is a phenomenon in which the stimulation of a sense can activate other senses. Synaesthetes perceive their environment a little differently, than other people: Music can be colored, letters and figures can be associated with genders and personality types, and forms can have a taste. There are a number of different synaesthesias; every sense can theoretically be coupled with any other sense. Synaesthesia is not the product of imagination or hallucination. It is the result of enhanced neural connections[1].
The incoming stimulus, e.g, the letter A, is called the "inducer"; and the synaesthetic perception originating from it, e.g., the color blue, is described as "concurrent". This is sometimes so described : Letter -> Color; The letter activates the color
Graphemic synaesthesia
One of the most common forms of synaesthesia is grapheme-color synaesthesia [2], in which signs, figures and words induce colors. Only 2% of synaesthetes questioned indicated they had never experienced this type of synaesthesia[3]. This is also the most widely studied form of synaesthesia to-date[4]. Such synaesthetes perceive a color when reading, hearing or imagining a letter or number.
Synaesthetes often report that reading, hearing or imagining a letter or a number leads to the perception of a specific color. These colors can differ in "form, spatial arrangement, transparency, covering degree, intensity, and nuance"[5] Some describe it as "Halo"[6] or fog which surround the letter or number. The color correlation is different for each synaesthete: one thinks A is red, another thinks it is blue. The colors are stronger when the printed letters shows a high contrast to their background[7]. In color distribution some regularities were found: taller figures seem to be darker, in general, than shorter ones. Zero and 1 are often white or black[8].
Some synaesthetes state that they see the colors of letters with their "mind's eye". These people are known as "association-synaesthetes". However, about 10% of synaesthestes, state that they perceive colors as if they are projected directly onto the letter on the page. Such synesthetes are known as "projector-synaesthetes" [9]. A trend towards the distribution of colors on a single grapheme has also been seen. Thus, for instance, 43% of synaesthetes see "A" as red [10]. A survey of Australian synaesthetes showed that the colors they perceived were not related to the letter colors used in Australian school books [3].
For synaesthetes, a grapheme can also have a gender or a character [11, 12]. For example, for me, "A" is a wise, blue lady, and "3" is a green, cheeky boy. If a synaesthete is supposed to decide whether a name is male, he will answer more slowly if the name is composed of female letters[11].
We are thus presented with the question, whether the color of the grapheme is activated by the grapheme's a) form, b) sound, or c) semantic meaning. One study has shown that it is not the forms of letters which produce the color, but their meanings. For example, let's take the letter "S" and the number "5" (see Figure 1). Even if the S and the 5 are shown as the same symbol, synaesthetes perceive the symbol in different colors, depending on whether they recognize it as a 5 or as a 2 [13] (see image left).
Newer studies have shown that the perception of a grapheme is followed by activation of the human brain center V4, which seems to be responsible for colors [1, 14]. This fact leads to the presumption that synasthesia is a real phenomenon and not a product of the imagination. This point is further emphasized by the experiences of synaesthetes who lose their synaesthetic perceptions due to brain damage [15].
It has been proven that grapheme-color synaesthetes differ slightly in their brain anatomy from non-synaesthetes: Synaesthetes seem to show thicker areas of gray matter in certain parts of the brain [16]. By means of a newer technology, called "diffusion tensor imaging", stronger neural connections could be ascertained between different brain regions [17].
General
The word synaesthesia derives from the Greek meaning "perceiving together". This word was first used by Mary Calkins at the end of the nineteenth century[18] Synaesthesia is also found in arts, literature and in reports of drug experiences, such as LSD consumption. Please do not confuse genuine synaesthesia (as previously described) with metaphoric or drug-induced synaesthesias.
There are different characteristics that define what is genuine synaesthesia and what is not [15, 19]. Synaesthetic perception a) is considered to be stable over time; b) is experienced differently by each individual; and c) can be remembered. Furthermore, it is supposed that synaesthesia d) seems to be normal for synaesthetes and e) cannot be learned.
Prevalence and familiarity
The prevalence of synaesthesia within populations seems to be much higher than previously assumed [20]: one out of twenty-three individuals in the general population (4.4 %) seems to have at least one type of synaesthesia [21]. Studies conducted in a Fine Art School [22] and in a secondary school [23] both revealed a prevalence of 23% for grapheme-synaesthesia.
It has been said that synaesthetes are likely to be creative. A survey conducted in Australia found that 24% of questioned synaesthetes are employed in an artistic profession, whereas in the non-synaesthete population this applies to only 2% [3]. In another study, synaesthetes and control subjects had to take tests which measured creativity. In some of these tests the synaesthetes performed significantly better, in others they did not. A broader analysis revealed that there may be a correlation between the number of synaesthesias and the outcome of the creativity test [24]. Ward et al. (2008) stated that creativity is not only a feature of art. He claimed that creativity is also needed in fields other than art, for example, science. Thus, not only artists can profit from the partially enhanced creativity that may be influenced by synaesthesia.
A study about the familial patterns of synaesthesia was conducted in 2008. It showed that 42% of the probands have a first-degree relative with synaesthesia [25]. Some researchers claim that there is a gender bias in favor for women [26] whereas others found no difference in the ratio of males and females[21].
It was long supposed that synaesthesia could be inherited through the x-chromosome. This was based on the earlier assumption that women were more frequently synaesthetes, and because, at that time, there was no known case of a son inheriting synaeshesia from his father. A new study in which several families were genetically scanned shows that several regions related to synaeshesia lie on chromosomes 2, 5, 6, and 12. The process by which synaesthesia is inherited seems to be more complicated than had been assumed. [27].
Studies run on monozygotic twins with different types of synaesthesia also came to the conclusion that other factors besides genetics may have an influence on the development of synaesthesia [28, 29]. However, one theory about the mechanisms leading to synaesthesia proposes that in the period after birth - also called neonatal phase of a child - some pruning processes in the brain might be inhibited. This could cause increased neuro-anatomical structures [30]. Neurologists think that the world of a newborn is quite different from that of an adult. Some Researcher go a step further by claiming that a newborn child only has one sense. They argue that the life of everybody begins with a form of synaesthesia and therefore this ability may remain in some form in synaesthetes[31].
Multisensory Senses
Jamie Ward formulates an interesting approach: Our 5 senses do not work in isolation from each other. To a certain extent, every person lives with multisensory modalities; i.e. every person lives to a certain degree with coupled senses: High tones are perceived brightly, deep, however, rather darkly[15]. (see following image)
This is also reflected in our language usage. Typical examples are concepts like "jangling cold" and "sharp sauce". The fact that phonetics are not only acoustically perceived, but are also visually perceived, is shown in the following famous example: Which of these forms is called "Kiki", and which is called "Buba"?
Most people call the pointed figure Kiki[32]. So, we see that the ability to tie different senses to each other exists within every person. For some, however, namely synaesthetes, this ability appears to be more pronounced. It has been reported that non- synaesthetic people who take LSD experience similar synaesthetic states. It is cannot be assumed that new neural structures are generated within two hours of ingesting this drug. Rather, it seems, as if this psychoactive substance temporarily lifts certain blockades, and thus allows an increased multisensory perception[15].
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Translated by Tiffany Ebner
Images by MJM
