The term synesthesia (Greek: syn = at the same time, aesthesia = perception) indicates the simultaneous, involuntary perception of different, unrelated sensory impressions and is therefore also referred to as a blending of the senses. Thus, language, for example, can arouse the perception of color, or odors may generate geometric figures.
If one regards synesthesia as a topic of research, two discrete approaches need to be distinguished. In a neurological sense synesthesia differs fundamentally from efforts to expose similarities between sense modalities, in particular between sounds and colors in terms of a hidden correspondence or a higher formula (Johann Wolfgang von Goethe) as well as from the stylistic device primarily found in romantic and symbolist literature.
Humankind has occupied itself with models of correspondence and color-sound analogies since Aristotle, while in the neurological sense synesthesia did not become an object of scientific interest until the mid to late 19th century. The following entry deals with the history of synesthesia from a neuroscientific perspective.
The frequency of the incidence of synesthetic perceptions cannot be unequivocally determined. The results of various field studies vary between 0.05% and 4%, women being more frequently affected (estimations vary here from 1:1.1 to 1:6). It is presumed that synesthesia has a genetic component as it often appears within a single family.
Each type of synesthesia can be formally characterized by a set of activating features (e.g., music), called inducers, and activated features (e.g., colors), called concurrents, that are automatically coupled, which means they cannot be suppressed (Bergfeld-Mills, 1999). The inducing perceptions can be either complex (such as, e.g., music) or simple (such as, e.g., sounds), whereas the concurrent perceptions are for the most part simple (e.g., colors, simple geometric figures, individual sounds). An exception to this are so-called ordinal spatial sequences (Sagiv et al. 2005) and an only recently investigated form of synesthesia in which numbers, units of time, or letters are perceived as personalities, so-called ordinal-linguistic personification (OLP).
The most important criterion for determining synesthesia is the individual consistency of a coupling over extended periods of time. For example, one synesthete always perceives the number 1 as green, while for another it is always yellow. Thus, every synesthete has his or her individual constant couplings of activating and activated perceptions, or his or her own synesthetic reality, which clearly complicates the study of the synesthetic phenomenon.
According to Peter G. Grossenbacher, three types of synesthesia can generally be distinguished with respect to their origin: 1) For people with genuine synesthesia, there are couplings of inducer and concurrent that are constant over time and were always present. 2) In the case of the acquired form, consistent couplings occur due to a neuropathological state. 3) In drug-induced synesthesia, there are temporary, variable couplings caused by psychoactive substances (such as, e.g., LSD or mescaline). This article will only deal with genuine synesthesia.
In the area of genuine synesthesia, the scientific literature most frequently describes colored hearing, in which spoken words, letters, numbers, voices, or sounds activate visual impressions like colors and/or figures, so-called photisms. During the course of Synaesthesia and Sound, a research and development project by Jamie Ward and Samantha Moore, it was shown that visualizations of synesthetic photisms in combination with sounds were perceived as particularly aesthetic even by non-synesthetes.
The perception of photisms is less frequently triggered by taste, smell, or when a person feels pain. A little-researched phenomenon is so-called emotional synesthesia. This term is used in two respects: It describes a form of synesthesia in which a sensory modality, such as, e.g., the sense of touch, elicits a feeling depending on the stimulus. Vilayanur S. Ramachandran, for example, describes a woman who felt inferior when she touched denim with her hand, while she felt calm and strong when she touched smooth metal. In its other meaning, emotional synesthesia can also describe synesthetic perceptions that are caused by an emotional state (Emrich 2002).
However, there are not only many different forms of synesthesia, there also can be major differences between perceptions by synesthetes within the same form of synesthesia. While, for instance, some grapheme-color synesthetes report they see colors on the letters themselves, or projected around them, others report that the colors cannot really be seen visually, but only in their mind’s eye.
A synesthesia-like phenomenon was first mentioned in 1690 by John Locke in his text An Essay Concerning Human Understanding. He described a blind man who associated the color scarlet red with the sound of a trumpet. In 1720, the ophthalmologist T. Woolhouse also described a blind man who could hear and feel colors (Wellek 1937).
In 1812, the physician G. T. L. Sachs published a report on two albinos who perceived sounds and numbers in color. However, to this day it has not been possible to establish a connection between synesthesia and albinism (Ione and Tyler 2004).
In 1873, J. A. Nussbaumer announced through the medical weekly Wiener Medizinische Wochenschrift that he was a color-sound synesthete, hoping that this would perhaps call forth people who had made similar observations, as his self-description fundamentally contradicted the prevailing theory of a specific energy of the sensory nerves (according to Johannes P. Müller 1826). Encouraged by this description, in 1881 the psychiatrist Eugen Bleuler, himself a synesthete, published a first quantitative study of synesthesia in which he classified 12% of the 600 participants as synesthetes (Bleuler and Lehmann 1881) and provided detailed descriptions of these seventy-seven persons. This also enabled him to compare synesthetic perceptions with one another. He determined that inter-individually, photisms differ widely and yet share certain similarities. High notes, for example, tend to activate bright colors. Bleuler also ruled out that these perceptions could be learned, and he identified the brain as the original source. Further scientific papers were published in 1880 (Visualised Numerals) and in 1883 (Inquiries into Human Faculty) by Sir Francis Galton, a cousin of Charles Darwin.
The term synesthesia was probably first used in 1866 by the French physiologist and neurologist Alfred Vulpian, though with a different meaning than is common today.
Following Francis Galton’s publications, a wave of synesthesia research ensued in the late 19th and early 20th centuries in which individual cases were in part meticulously described. Between 1880 and 1920, approximately one hundred pieces were published every decade. As had been the case with Eugen Bleuler, research interest focused on identifying shared characteristics between color perceptions, since despite obvious differences there were evidently certain regularities: a connection between pitch and brightness was proposed frequently. There were also scattered reports of a connection between volume and size, volume and brightness, and pitch and size. There were various explanatory approaches: on one side, certain scientists were of the opinion that synesthesia had a pathological origin (Claviere 1889); it was assumed that there was either a faulty circuit between the respective sensory centers (Pedrono 1882) or a kind of insufficient differentiation between sensory centers (Coriat 1913). Another theory postulated that different sensory impressions shared characteristics through which synesthetic perception was conveyed (Bleuler 1881, Féré 1892).
Synesthesia disappeared again from the focus of scientific interest in the late 1940s because it had not been possible to objectively measure the phenomenon, and introspection (i.e. self-description) was no longer recognized as a method of collecting data in experimental psychology. The number of publications consequently decreased. Between 1930 and 1940, only 30 pieces of writing were published; in the ensuring years it was even less (Marks 1975).
It was not until 1975 that science again began to increasingly focus on synesthesia research. The pioneers were Lawrence E. Marks, Richard E. Cytowic, and Simon Baron-Cohen. In the 1970s, Marks was interested in the similarities shared by the senses, in particular seeing and hearing, as well as in synesthesia. He wrote On colored-hearing synesthesia: Cross-modal translations of sensory dimensions, a very comprehensive historical summary of sound-color synesthesia research. In contrast, Cytowic devoted himself primarily to synesthesia and in 1989 published the book Synesthesia: A Union of the Senses, in which he both described individual cases and developed a theory on the causes of the phenomenon. He assumed that the limbic system took on the role of a mediator between the senses and that it was hyperactive among synesthetes, whereby more sensory impressions were interconnected than normal. Finally, Simon Baron-Cohen successfully developed a test of genuineness, an objective measure of the temporal consistency of synesthetic perception and thus made synesthesia an object of scientific research again. In contrast to Cytowic, he regarded the neocortex as the locus of origin for synesthetic perceptions, and not the limbic system.
While early synesthesia research dealt primarily with the question of similarities and the description of the phenomenon, recent research concentrates increasingly on the neurological foundations of synesthesia.
Motivated by the invention of imaging systems, such as fMRI (functional magnetic resonance imaging) and PET (positron emission tomography), synesthesia research has recently gained in importance (Ione and Tyler 2004). Up to now, research has mainly focused on the neurological foundations of grapheme-color synesthesia. Apart from several analogies, however, the results of these studies and the theories derived from them vary widely.
Synesthesia is not regarded as a neurological disease, because as a rule it does not seriously impact people’s everyday lives and is therefore also not described in neurological classification systems (ICD-10 or DSM-IV). Furthermore, there does not appear to be a connection between synesthesia and psychiatric illnesses, with the exception of the fact that synesthesia may be triggered by epilepsy.
There is a good deal of evidence that the occurrence of synesthesia is substantially influenced by early childhood experiences. Witthoft and colleagues discovered, for example, that in the case of one female synesthete there was a 100% agreement between the colors she associated with individual letters, and the colors of magnetic letters her parents had used to decorate the refrigerator in her childhood (Witthoft et al. 2006). However, most synesthetes report that their colors have been there since they began to think and provide no clues with respect to concrete situations to which the associations could be traced back.
It has still not been sufficiently explained what gives rise to synesthetic perceptions. By means of psychophysical experiments, it has at least been proven that synesthesia is a perceptual phenomenon, as synesthetic colors can generate certain perceptual effects. If one, for example, arranges twos in a triangle among fives, a color-grapheme synesthete immediately recognizes the triangle, because it stands out in terms of color (pop-out effect), whereas non-synesthetes only recognize a tangle of digits (see fig. 1). It has not yet been conclusively clarified whether the geometric forms actually pop out or whether synesthetes are more competent in solving a task like this. If one has synesthetes search for a digit between other digits (e.g., a two among fives), the time they require to find it depends on the total number of digits presented (Edquist at al. 2006). This indicates that the color of the number does not pop out, but that synesthetes like non-synesthetes have to visually sift through all the digits before they can identify the ones they are searching for.
Also, a synesthetic Stroop effect can be provoked when synesthetes are confronted with words in color (Bergfeld-Mills et al. 1999). Due to the conflict between the actual color of the words and the color of the synesthetic reaction, they require more time to select and name the color that is being shown. Thus one can draw the conclusion that the synesthetic perception of color occurs automatically and involuntarily; in other words, it cannot be suppressed.
However, Mattingley was able to demonstrate that a presentation of colors in such quick succession (28 or 56 milliseconds) that they become imperceptible (priming) does not elicit color photisms (Mattingley et al. 2006). This indicates that conscious perception is necessary for the synesthetic generation of colors.
Whether or not synesthetic perception requires visual attention is still disputed.
By distracting synesthetes with difficult tasks, Mattingley was able to prove that attention modifies synesthetic perception. Ramachandran, on the other hand, established that the generation of color is also possible without visual attention. He used the so-called crowding task, in which numbers surrounded by other numbers appear in the peripheral field of vision and have to be named by the test persons. Because of their restricted resources of attention in the peripheral field of vision, non-synesthetes were not in a position to identify the number in the center, whereas synesthetes were able to recognize the numbers through their synesthetic color, although the number was identified unconsciously, without visual attention (Ramachandran and Hubbard 2003). It was furthermore ascertained that color perception is context-dependent. In the presence of other letters, for example, the symbol I is the same color as the letter I, whereas in the presence of other numbers it has the color of the number 1.
Fig. 1: Modified after Ramachandran et al. 2003: Figure consisting of twos in randomly assembled fives A) from the perspective of a non-synesthete, and B) from the possible perspective of a synesthete.
There are also newer insights with respect to similarities among synesthetic perceptions of color. The common denominator between colors and letters appears to be the relative frequency with which they occur in language. Thus, for example, the color red is often bound to the letter A, and both of them are used more often in language than, for instance, the letter Z or the color violet (Simner 2005). In addition, a connection was observed between the relative frequency of letters, brightness, and saturation of photisms. Letters that occur more frequently are linked to colors that are brighter and more highly saturated (Beeli 2007).
The fact that the results of imaging techniques seem to be very heterogeneous, constitutes one of the problems of current synesthesia research. On the one hand, this is because the number of test persons is often not representative, and the test designs can be very different. On the other hand, because the intensity of perception varies according to the individual, it is hardly possible to investigate a homogenous group. Despite the large discrepancy among the results, there is evidence that the color region (V4), the posterior interior temporal cortex (PIT), and the transitional region between the parietal and the occipital cortex are fundamentally involved in synesthetic perception (see, e.g., Nunn et al. 2002, Rouw and Scolte 2007, Sperling et al. 2006). There is also evidence for the involvement of the prefrontal cortex (see, e.g., Paulesu et al. 1995, Beeli et al. 2007, Sperling et al. 2006), to which general functions such as working memory, attention, and personality are ascribed. The occipital cortex primarily processes visual information, and the parietal cortex is responsible for, among other things, spatial perception, orientation, as well as somatosensory functions. There are various association fields in the parieto-occipital transitional region which, among other things, carry out the integration of visual information. The PIT cortex is also known as the integrative region of visual information, in particular of color and form. Because up to now mainly grapheme-color synesthetes have been examined using imaging techniques, it cannot be ruled out that other regions of the brain are involved in other forms of synesthesia.
Four different models for explaining synesthesia have been developed based on previous results:
These models are not necessarily mutually exclusive, so that a combination of the different theories is conceivable. Thus it is possible that the re-entrance of activity in presynaptic regions could occur via the disinhibition of feedback projections, or the pruning model could be the basis for hyperbinding or re-entrant processes.
In summary one can say: it is accepted that synesthesia is not an illusion in the minds of those concerned but a genuine neurophysiological phenomenon that can provide science with unique insights into human perception.
 Locke’s essay does not deal with synesthesia in a neurological sense. For this, cf. Kevin T. Dann, Bright Colors Falsely Seen: Synaesthesia and the Search for Transcendental Knowledge (New Haven: Yale University Press, 1998), 8–9. The example of the blind man, which comes up several times in Locke’s essay, is used within the scope of a question related to perception theory. Following an empirical approach, Locke argues that mental pictures are necessarily based on sensory-specific perceptions. A blind man who has never experienced the sensory quality of scarlet red has no notion of this color whatsoever and accordingly attempts to understand it by forming a linguistic analogy (sound of the trumpet). John Locke, An Essay Concerning Human Understanding, Book 3, Chapter 4 (London, 1690), available online at http://oregonstate.edu/instruct/phl302/texts/locke/locke1/Essay_contents.html.
 Cf. Alfred Vulpian, Leçons sur la physiologie générale et comparée du système nerveux faites au Museum d’Histoire Naturelle: Rédigés par Ernest Brémond (Paris, 1866), 464f. Vulpian referred to the photic sneeze reflex as synesthésie. Jules Millet referred to Vulpian in Audition Colorée (Paris, 1892), 14. Cf. also Emilie Noulet, Le Premier Visage de Rimbaud (Brussels, 1953), 122, and Dann 1998, p.188.
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