Original extended manuscript of article published in Leonardo vol. 32, nr. 1 (1999) 9-14.

Artistic and Psychological Experiments with Synesthesia

Crétien van Campen

Artists and psychologists have been experimenting with synesthesia for centuries. What have they contributed to the knowledge of synesthesia? A historical review shows that artists and psychologists have always had great difficulty to manipulate or control the phenomenon of synesthesia. Within these limits, artistic experiments with color-organs, musical paintings and visual music have mainly uncovered perceptual and emotional aspects of synesthesia. Psychological experiments with Gestalt perceptions, operant conditioning, use of metaphor and semantic differential scales mainly produced a variety of methodologies to the study of synesthesia. Currently, psychologists approach synesthesia foremost as a neurological phenomenon, while generally artists are exploring digital devices to elicit synesthesia.

A modest but crucial province in the empire of the senses is that of synesthesia. Synesthesia refers to the phenomenon that the stimulation of one sense modality gives rise to a sensation in another sense modality too [1]. The term "synesthesia" originates from the Greek syn (together) and aisthesis (perceive). The most prevalent form of synesthesia is known as audition colorée, the phenomenon of hearing of colors in music and vowels. Synesthetes experience these phenomena more strongly - for instance they see light spots in front of their eyes - than others who experience only an inner feeling of color. Other forms of synesthesia are tasting visual shapes and feeling warm and cold colors [2,3].

Synesthesia plays an important role in a recent neuropsychological debate on the organization of the senses [1,2,4,5]. Globally, two positions can be distinguished. The oldest - most accepted and common sense - view holds that humans possess five independently functioning senses (the modularity thesis) [6]. The other view holds that human beings have one integrated sensory organ with five suborgans (the unitary thesis) [2,7].

The history of the arts shows a comparable debate. Are the visual and musical arts distinguished disciplines that work with separately perceived stimuli? Or are these disciplines part of a larger organ that unites the different arts in the ideal of the Gesamtkunstwerk (Total Work of Art)? Since the nineteenth century the Gesamtkunstwerk has been an important motivation for numerous artistic experiments with synesthesia [8]. Specially the last decades, new digital technologies of image and sound production have brought theoretical issues concerning the unity of the arts to the foreground again [9].

It is remarkable that both the symbolist movement's and the early abstract art movement's interest in the Gesamtkunstwerk was synchronic with a boom of psychological publications on synesthesia in the years 1880-1930 (Figure I). After the second world war, the interest of psychologists seemed to gradually disappear almost completely, and was only revived again over the past decades. All the more reason to consider the current debate in the historical perspective of artistic and psychological experiments. In this article, a number of prototypical experiments of important art movements and psychological schools in the late nineteenth century and beginning of the twentieth century are discussed and their contributions to the knowledge of synesthesia are considered.

Artistic experiments

The interest in color hearing goes back to classic Greek philosophy, when it was disputed whether color, like pitch, could be considered as a physical quality of music [10]. Pythagoras discovered the mathematical order of musical harmony by relating the length of strings to the successive octaves. This led to the idea that colors and sounds could be related too, following mathematical rules. However, the first known experiment that tested this hypothesis took place in the sixteenth century by the Milanese artist Arcimboldo, well-known today for his vegetable and fruit portraits. Because he lacked a mathematical system of color harmony, Arcimboldo inferred a scale of grey values from the Pythagorean system of sound intervals. He translated the grey scale to color hues and persuaded a musician at the court of Rudolph II of Prague to install painted paper strips on his gravicembalo [10].

Still, one theoretical problem was not yet solved. One needed not only a system of color harmony next to a system of musical harmony, but also correspondence schemes to connect the two systems. Newton attempted to solve the problem by assuming that musical and color harmonies are related by means of the frequencies of light waves and sound waves. On the basis of Newton's theory the French Jesuit Castel started to develop a color harpsichord (clavecin oculaire) around 1720. In collaboration with the instrument maker Rondet, Castel drafted a harpsichord with colored paper strips that appeared on top of the instrument when a key was pressed. The paper strips were lighted by candlelight [11].

If one compares the descriptions by Arcimboldo and Castel it can be observed that the solving of musicological and mathematical puzzles has contributed to mathematical refinement of theories of color-sound correspondences. However, the practical performance of these refined ideas was hampered by technical problems.

The discovery of the gas lamp provided new possibilities. Around 1870, Kastner developed a gas lamp organ named Pyrophone, which consisted of thirteen gas jets covered with foil that lighted crystal butts. The term "color organ" was first used in a patent application by Rimington in 1893 (Figure II). His three-meter-high color organ resembled a customary house organ with a cabinet of fourteen colored lamps on top. The light of the colored lamps could be adjusted to certain gradations of hue, brightness and saturation. This meant a tremendous progress compared to the paper strip harpsichords of the earlier centuries. Like most color organs, Rimington's instrument did not produce musical sounds. The color organ had to be played simultaneously with an organ that produced musical sound [11,12].

Scriabin's color music

After a number of technical problems were solved, the first synesthetic performances of color organs in concert halls were realized at the end of the nineteenth century. At this level other more psychological questions came into view concerning the effects that synesthetic performances had on the senses of the public. More than anyone else it was the Russian composer Scriabin who was interested in the psychological effects of the simultaneous experience of color and sound. In comparison to contemporary investigation of tone-color correspondences by experimental psychologists - which will be discussed later - Scriabin started at a more complex level of synesthetic experiences. He started from a system of color-key correspondences instead of color-tone correspondences. He investigated the emotional aspects of synesthetic experiences of color during the change of one musical key to another.

According to Scriabin the presentation of the right color corresponding with music works as "a powerful psychological resonator for the listener" [13]. Invited by the experimental psychologist Myers, Scriabin presented his ideas in London. Scriabin explained to Myers that whenever the tonality of a musical piece changes, the color will change too: "color underlines the tonality; it makes the tonality more evident" [14]. Sometimes he perceived a change in color sooner than the change in key. The addition of color to music would intensify both the auditory and visual effect simultaneously, according to Scriabin [15].

In contrast to former performances by color organists, Scriabin based his synesthetic compositions on color-tonality correspondences instead of color-tone correspondences. Scriabin changed the colors only when the tonality changed, like in his composition Prométhée, poème de feu (Prometheus, Poem of Fire), which made the light experience more intense and enduring. For Prometheus he wrote two separate parts for the tastiera per luce, a color organ that was probably based on knowledge of Rimington's color organ [11].

Scriabin's first opportunity to test his synesthetic hypotheses in a concert in Moscow in 1911 failed, due to a defect in the light projector. Unfortunately he did not attend the first successful performance of Prometheus in New York in 1915, just a few weeks before his death. In New York a color organ named chromola was used to play the light parts of the piece. The chromola projected twelve colors on a small white screen. The performance was not appreciated by the public. A critic compared it with "a pretty poppy show" [11,15]. There were also other reasons why this performance became a failure. A number of Scriabin's conditions of a synesthetic performance were not met: first, there was no "flood of light" in the concert hall but a projection on a small screen, and second, the chromola was not equivalent to the orchestra but used as one of the instruments.

A second path, that was explored by Scriabin and contemporary artists like Kandinsky and Schönberg, was the application of synesthetic dissonants next to consonants. Until then color harmonies had been tuned to musical harmonies. Scriabin composed two lines for the tastiera per luce: one consonant line and one dissonant line to the musical lines of the symphony [13]. The result Scriabin aimed at, was an auditive sensation accentuated by consonant and dissonant colors of the tastiera per luce. He concluded that colored light will act as "a powerful psychological resonator for the listener that intensifies musical perceptions [13]. Later, Messiaen continued Scriabin's synesthetic experiments by formulating accurate transposition schemes of color changes in tonalities [16].

Synesthetic dissonance

Symbolistic painters in the late nineteenth century explored synesthesia too [17]. Their interest in synesthesia had two main motives. First, visual artists regarded music as the highest step on the ladder of the arts, because it approached a state of immaterial art. Second, the influential idea of Gesamtkunstwerk (Total Work of Art) proposed by Wagner assumed that visual, auditory and other sensory elements were attuned in one Gestalt experience. The experiments by symbolistic visual artists ranged from whistling during the act of painting (Delacroix and Mengs whistled to catch the right atmosphere for their paintings) to drafts of synesthetic rules of thumb. Van Gogh, for instance, maddened his music teacher by stubbornly testing his ideas on tone-color correspondences during piano lessons [18]. A tradition of musical paintings emerged in the nineteenth century [17].

The artist group Der blaue Reiter executed the first synesthetic experiments in a composite group of painters, composers, dancers and theater producers. The aims of the artist group were focused on: unification of the arts (by means of Gesamtkunstwerke), freedom of expression (abstraction), and spirituality (the ideal of an immaterial art). Kandinsky's theory of synesthesia, as formulated in On the Spiritual in Art [19], shaped the ground for these experiments. He described synesthesia as a phenomenon of transposition of experience from one sense modality to another, like in the sympathy of musical tones. In his Bauhaus lessons, he compared the human nerves to the strings of a piano: if a note is struck on one of two pianos, standing next to each other, the exact same note on the other piano will resonate. Kandinsky was well acquainted with the scientific debates on direct versus associative perceptual processes in the academic psychology at that time, and explicitly defended his theory of direct synesthesia against the arguments of associationist psychologists [20].

Two essential problems were investigated in the experiments of the Blaue Reiter and later at the Bauhaus academy, i.e. dissonance and temporality. Kandinsky and Schönberg were interested in the problem of synesthetic dissonance, which also intrigued Scriabin. After Schönberg published the atonal theory of dissonant harmony, Kandinsky wanted to make use of these principles in the painting and theater. In his theater piece Der gelbe Klang (The Yellow Sound), he experimented with the opposition of three types of movement: visual movement (film), musical movement and physical movement (dance): "Duplicating the resources of one art (e.g., music), however, by the identical resources of another art (e.g., painting is only one instance, one possibility. If this possibility is used as an internal means also (e.g., in the case of Scriabin), we find within the realm of contrast, of complex composition, first the antithesis of this duplication and later a series of possibilities that lie between collaboration and opposition." [21]. Like Scriabin, Kandinsky wanted to alternate dissonance with consonance in order to intensify synesthetic perceptions, so they would, in his own words, have a "deeper inner impact" [22].

Together with the composer Hartmann and the dancer Sacharoff he experimented with synesthetic relations between the three movements: "I myself had the opportunity of carrying out some small experiments abroad with a young musician and a dancer. From among several of my watercolors the musician would choose one hat appeared to him to have the clearest musical form. In the absence of the dancer, he would play this watercolor. Then the dancer would appear, and having been played this musical composition, he would dance it and hen find the watercolor he had danced." [23] Schönberg experimented with similar relationships in his theater piece Die glückliche Hand of 1913 [10,24].

In comparison with the psychological research into cognitive schemes of color-tone correspondences at that time, these artists cared more for the emotional and dynamic aspects of synesthetic perceptions. In this perspective they were forerunners of the psychological studies of Gestalt processes in synesthetic perceptions [25,26] and of more recent research into emotional aspects of synesthesia [2].

Temporality in synesthesia

The problem of temporality got the attention of most modern artists at that time (e.g. futurists, members of De Stijl and Der Blaue Reiter) [17]. Painting was considered a static art form, and some modern artists wanted to add to the two dimensional painting not only the third dimension of depth but also the fourth dimension of time by means of visual suggestions of movement [27].

Mondrian studied the problem of the visual perception of movement in his apparent static compositions. The first paintings with grids around 1920 were a milestone in his oeuvre. However, these first presentations of universal proportions were lacking the important characteristic of rhythm, according to Mondrian. The perception of reality was chained in forms dictated by the classical art of painting, he wrote [28]. To liberate this perception of its chains, the visual form had to be destroyed and rhythm be given its freedom. Mondrian discussed the attempts of the futurists and the cubists to give space to rhythm in paintings, but concluded that they did not succeed in imagining rhythm [29].

Mondrian's Composition with grey lines (1918) was an early attempt in this direction (Figure III). The rhombus painting contained a grid of squares and planes that crossed each other in the corners. Mondrian systematically varied the thickness and the grey tones of the lines, giving rise to the perceptual effect of a rhythmically moving raster. Since the beholder's eyes were disposed to follow the lines of equal thickness and grey tone, his gaze was lead through the painting in a movement like the quickstep dance. The synesthesia of movement in visual rasters remained a central theme in Mondrian's experiments, culminating in the Boogie Woogie paintings of his last years.

In the first decades of this century, technical limits were a major obstacle in the experiments by Mondrian, Kandinsky and Scriabin. From the fifties the technological innovations in electronic and digital productions of image and sound offered new possibilities for the performance of synesthetic experiments. Another era of synesthetic experiments started as a number of reviews show [9,12,17,22]. The more recent experiments seem mainly involved in the physical or electronic translation of music and sound into images and animation [30]. Lesser attention has been paid to the psychological (i.e. perceptual and emotional) impact of these synesthetic performances. In comparison with the more psychological oriented experiments by Scriabin, Kandinsky and Mondrian, current artistic experiments seem more orientated on the physics of synesthesia (e.g. electronics and computer programming).

Psychological experiments

Psychological studies into synesthesia have their roots in medical research. The physician Georg Sachs published the first study into color hearing (audition colorée) in 1812 [31]. At that time synesthesia was considered as a medical pathology. The clinical case study was often used as a method of research. Sachs described photisms (the visual perception of colored spots in front of the eyes, like in after-images) that were perceived by his sister and himself with numbers, days of the week, letters and musical tones [31]. Halfway through the nineteenth century the first scientific hypotheses on the physiological nature of audition colorée were formulated. It was assumed that synesthesia was a normal quality of the brain, that was more developed in some individuals than in others. Gradually, the pathological approach of synesthesia as an abnormal individual functioning was abandoned in favor of physiological and psychological approaches of normal cognitive functioning. Psychologists opposed the physiological approach and considered synesthesia as the product of a mental association of sense-data. The debate between naturalists and mentalists laid the foundation for a century of psychological research.

Should synesthetic perceptions be explained by direct sensory-physiological mechanisms or by mental operations of sense-data? This question gave rise to Fechner's first aesthetic-psychological experiments with groups of subjects [32]. Fechner surveyed 73 persons on their letter photisms. This type of social scientific experiment was adopted by a number of experimental psychologists and marked an important turn in synesthetic research, which was limited to individual case studies before that time. The perceived correspondences between colors and consonants, vowels and numbers, were studied extensively. The psychologist Marks published a meta-analysis of 35 studies with in total 400 persons [3]. It appeared difficult to infer any systematic rules of color-letter or color-number correspondences. However, Marks notified that the observations can be ordered on the dimension of brightness. Bright consonants would correspond to light colors and dark consonants to dark colors.

The increasing significance that was attributed to synesthesia was expressed in the International Congress of Physiological Psychology, held in Paris in 1890. The subject of synesthesia was scheduled as the opening session in the program, and was devoted to a debate between the naturalists and the mentalists. On the basis of a small survey, a special committee proposed a taxonomy of synesthetic experiences and a definition of the concept of audition colorée [31]. The first laboratory experiments with synesthetic subjects emerged in the late twenties of the twentieth century (Figure V). The debates between the naturalists and the mentalists developed into a dispute of two theses that are still influential today: the unity-of-the senses thesis and the modularity thesis.

Unity of the senses

The thesis of the unity of the senses was developed by Gestalt-psychological oriented researchers like Von Hornbostel and Werner [7]. Hornbostel elaborated on the Aristotelian idea of common sense. For instance, brightness, warmth and roughness supposedly originated from more than one sense modality and had logically to be perceived by the common sense. Hornbostel assumed brightness to be the central dimension that appeared in each sense modality (light-dark, hard-soft, sharp-dull, light-heavy, cold-warm) [3,25]. In Hornbostel's view the unity of the senses was the basis of the unity of the arts. Hornbostel designed laboratory experiments in which he asked subjects to match different stimuli. First he gave them an odor and asked them to match it to a corresponding grey card. Then he asked them to match the odor to a corresponding pitch. Finally, the subjects matched the pitch with a grey card. Comparisons of the chosen grey cards produced fits that were almost identical. Hornbostel interpreted the results as proof that the unity thesis was valid [3,25]. But in fact he only tested the cross-modal transfer of the brightness dimension.

Werner investigated a developmental variant of the unity thesis: Although adults differentiate the perceptions of different senses, babies would perceive the environment undifferentiated [7]. The emphasis of psychological research shifted from studies of static correspondence rules to the dynamics of the process of synesthetic perception. According to Kouwer, experiments with color-tone correspondences had failed to show consistent results [33]. Zietz was one of the first psychologists who designed experiments to investigate the process of synesthetic experiences [26]. In an obscured room in a psychological laboratory, subjects were shown strong visual stimuli that produced clear after-images. While the subjects perceived after-images auditory stimuli were given. It was investigated how the auditory stimuli affected the visual after-images. The main result was that extremely low and high tones influenced the visual perception of after-images. A high tone made the perceived color more intense and clearer. Moreover, high tones made colors look harder and colder, and the rhythm of the after-images appeared to accelerate. Zietz concluded that sensations of color and sound are part of an indivisible Gestalt perception in which the auditory and visual perceptions mutually influence each other.

Conditioning synesthesia

Opposite the unitary thesis stood the modularity thesis that was rooted in the European associationistic psychology and the American behaviorism of the beginning of this century. The modularity thesis claimed that synesthetic perception was the product of the conditioning of mental associations of sensory elements. Howells, for instance, designed laboratory experiments in which subjects were reinforced to distinguish two complementary colors with help of a low organ tone that was played before the presentation of a carmine red stimulus and a high organ tone before a blue green one [34]. Eight subjects got presented 5000 stimuli sessions. After a while, Howells sometimes played the wrong tone before a color and investigated how it affected the responses of the subjects. Playing the wrong tone disturbed the color perception. Good tones made the color more clearly distinguishable. Howells concluded that synesthetic perception is a product of conditioning. According to Kouwer, most other experiments into the conditioning of synesthesia produced inconsistent results [33].

In social psychology, attempts to measure consistencies in synesthetic experiences led to the development of the famous Semantic Differential Scale [35]. Karwoski and his associates carried out a series of surveys with subjects, asking to mark their mood on an emotion circle while listening to pieces of classical music that were chosen by the experimentators [36]. These experiments were repeated with color circles. According to the authors, the correspondences between the scores on different scales were consistent. And the results of normal subjects resembled those of synesthetes, suggesting that synesthesia and metaphor were only gradually different [35].

The assumption that synesthesia and metaphor were on one and the same gradual scale gave rise to linguistic and literary studies into poetic metaphor [3,37,38]. Marks assumed universal meaningful correspondences between sensory modules, which in principle could be understood by everyone. Marks tested Hornbostel's brightness thesis in a series of experiments with cross-modal transfer [39]. Subjects had to distinguish brief stimuli. The results showed that subjects performed the distinction task faster and more accurate when the experimental stimulus was primed by a corresponding stimulus of a different sense modality. Marks concluded that the effects could be attributed to cross-modal transfer.

The neuropsychological turn

The dip in the number of publications concerning synesthesia (Figure I) after the second world war may be explained by the fact that synesthesia was no longer considered as an isolated phenomenon but as an example of metaphor. The revival of synesthesia studies from the 1980s (Figure I) can be attributed to the rise of neuropsychological interest in synesthesia.

The conceptualization of synesthesia as an example of metaphor elicited the reaction that research had to be redirected at pure synesthetic phenomena [2]. After a psychological committee had clearly defined the concept of synesthesia (i.e. audition colorée) in 1890, it had been progressively diffused in research during the twentieth century. The diffusion of the concept had neither led to conceptual clarifications nor to consistent results [2,33]. In order to produce more valid and reliable results on synesthesia, neuropsychologists started with the development of well-defined criteria [2] and clinical tests to distinguish synesthetes from non-synesthetes [1,6].

In comparison with former psychological studies, the new neuropsychological studies have been focused on a few subjects [2]. After almost a century of psychological studies with groups of subjects, the individual medical case study was reintroduced in the study of synesthesia by neurologists; only preceded by Luria's study of the synesthetic mnemonist S. [40]. Baron-Cohen and his associates developed a reliability test of synesthetes in order to test the consistency of synesthetic responses over a long-term period [6]. The latest studies have started to locate synesthetic processes in the brain by means of PET and MRI scans [5]. The results of neuropsychological research so far is that a very small group of synesthetes (1: 2,000 according to Motluk) have been distinguished [4]. The brain scan studies have not yet succeeded in locating abnormal brain activity that would explain synesthetic perceptions. The debate between the modularists and unitarists continues. The modularists consider synesthesia as the product of neural miswiring between for instance the visual and auditory areas [6]. The unitarists consider synesthesia as a natural emotional process of integral sensory perception and locate the phenomenon in the limbic system, coined as the "emotional brain" [2].

Conclusion

The current attempts by artists and psychologists to discover formal correspondence schemes of synesthesia have a long history of experiments in art and science, and it may be concluded that the phenomenon of synesthesia is difficult to control or manipulate. Synesthesia cannot be understood in simple correspondence schemes as at least the innumerable experiments with tone-color correspondences have shown.

Artistic experiments, specially by the early modern movement, have uncovered interesting dynamic and emotional aspects of synesthetic perceptions and contributed to the phenomenology of synesthesia. Psychologists have restricted themselves to a the accurately formalizing of a few isolated aspects of synesthesia and contributed methods for assessing synesthesia.

Current psychological studies into synesthesia have catched up with the booming brain research and are mainly involved in a search for organic substrates. Current artistic experimentation seems mostly involved with digital devices and a search for algorythms to translate music into images (e.g. visual music, animation). The present situation in artistic synesthesia research may be compared to the late nineteenth-century period of experimentation with color-organ technology. After that an era followed of fruitful artistic explorations and scientific testing of the psychological (perceptual and emotional) impact of sound-image devices and performances.

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SENSE MODALITY: A sensory system, usually qualified to specify the sense intended; e.g. the visual modality.