November 2001

Assessing the strenght of color synesthesia
in words, music, taste and odor perception

Development of the NeCoSyn Method

Scientific Report

 Crétien van Campen & Clara Froger

Contents:
Abstract
Introduction
    History of synesthesia in art and science
    Theories of synesthesia
    Incidence of types of synesthesia
    Diagnosis of synesthesia
Methods
Results
    Reliability, validity, sensitivity
    Feasibility
    Exactness
    Color dimensions
    Types of synesthesia
Applications
Conclusion
References

abstract

INTRODUCTION: Synesthesia has been widely investigated in the sciences and the arts. Basic questions like 'what is synesthesia? and who is a synesthete? have often been answered on a conceptual level but hardly on a empirical level.

OBJECT: In order to contribute to the assessment of synesthesia we developed a measuring instrument for color-word, color-taste, color-music and color-odor synesthesia. The Netherlands Color Synesthesia (NeCoSyn) instrument is based on the Swedish Natural Color System (NCS cf. www.ncscolor.com) and the British Color-word-synesthesia test (Baron-Cohen et al. 1987, 1993, 1996).

METHODS: In total 248 subjects were tested in three stages to develop the instrument and test its characteristics like reliability, validity, sensitivity/specificity and exactness.

RESULTS: The instrument is characterized by high interrater reliability (r > 0.89), high concurrent validity with the English version and subjective reports of synesthesia. The test showed to be feasible in large groups with low costs. The specifity of the original English test was improved by using abstract terms. The domain of application of the original color-word test was extended to other types of color synesthesia. The NeCoSyn instrument distinguishes four types of synesthesia (color-word, color-taste, color-music and color-odor synesthesia). Scores on each type differed statistically significant (p < 0.05). The instrument specifies dimensions of color experiences (brightness, saturation, tone). Hue scores differed significant (p < 0.05) from brightness and saturation scores in most types.

APPLICATIONS: The instrument can be applied in different fields of the sciences and the arts. For instance in the scientific diagnosis the instrument has advantages to existing methods in specificity (types of synesthesia, color dimensions) and exactness (strength of synesthesia). The instrument is based on a method that can be applied to assess color synesthesia in other sensory domains (e.g. color-tactile synesthesia).

CONCLUSION: The NeCoSyn is a reliable, valid, feasible and exact instrument to assess the strength of different types of color synesthesia on the three dimensions of color. It can be used for diagnosis of synesthesia, estimating incidence of synesthesia in (sub)populations and as a method to assess other types of synesthesia.

Introduction

Synesthesia refers to the phenomenon that the stimulation of one sense modality gives rise to a sensation in another sense modality too (Harrison & Baron-Cohen, 1994). 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 music and/or vowels in color. Day (2001) extended the definition and stresses the multiforms of synesthesia: synaesthesia is the general name for a related set of various cognitive states having in common that the stimulation of one sense, such as smell, is involuntarily simultaneously perceived by one or more other senses, such as sight or/and hearing.

Color synesthesia is one branch within this family of perceptual phenomena and refers to the experience of color as a result of the stimulation of a nonvisual sense modality. From subjective reports we know that this color experience may range from internal (feeling like) to external (photism-like) experiences. Several researchers have stressed the neurological nature of the phenomenon and distinguished the phenomenon from mental association (Cytowic 1989, Harrison 2001, Ramachandran & Hubbard 2001).

History of synesthesia research in art and science

Already in the ancient Greek civilization, color synesthesia was investigated by some philosophers. Aristotle and Pythagoras already studied the correspondences of music and color brightness intervals (Gage 1993, Struycken & Fernwerd 2000). Some artist have explored the phenomena of color music and synesthesia since the seventeenth century and the scientific study started with medical case study followed by psychological experimenting with groups of subjects (Van Campen 1999). Synesthesia research flourished in the Interbellum with hundreds of scientific publications on the subject (cf. Mahling 1926). After a dip in interest after the Second World War the revival of synesthesia started with neuroloigical research in the late eighties (Van Campen 1999).

The study of synesthesia is characterized by multidisciplinarity in the sciences and the arts (cf. the bibliography Synesthesia in Art and Science (Van Campen, Berman & Galeyev, 2001). The subject is studied in science for instance in perception psychology (cf. Marks 1978, Baron?Cohen et al., 1987, Ramachandran & Hubbard, 2001), psychology of consciousness (cf. Grossenbacher 2001, Psyche symposium 1995), neonatals studies (cf. Maurer 1993), creativity and cognitive science (cf. Waterworth 1997, Lindauer 1991, Wheeler et al. 1925, Odbert et al. 1942), neurology and brain imaging (cf. Cytowic 1989, Paulesu et al. 1995), genetics (cf. Bailey & Johnson 1997, Baron?Cohen et al. 1996) psychofarmaca (cf. McKellar 1997), art history (Gage 1993, 1999, Dann 1999, Jewanski 1999, Von Maur 1985), musicology (Peacock 1985, Galeyev 1993, 2001, Wellek 1930, Behne 2001). And in the arts: in multimedia and internet studies (cf. Doctor Hugo 2000, Higgins 2001, Galeyev 1994, Ox 1999, Moritz s.n.), linguistics (cf. Marks1984, Johnson 1982), literature (cf. O=Malley 1964), and visual music (cf. Collopy 2001).

Art and science have contributed in different ways to our knowledge of color synesthesia. Artistic experiments have uncovered interesting dynamic and emotional aspects of synesthetic perceptions and contributed to the phenomenology of synesthesia. Psychologists have isolated some aspects of synesthesia and contributed methods for assessing these aspects of synesthesia (Van Campen, 1999).

Theories of synesthesia

Given the multidisciplinarity of synesthesia research, diverging theoretical positions have been proposed. In general, theories can be ordered on two dimensions: 1) synesthesia is a normal versus an abnormal perceptual functioning, and 2) synesthesia is a physical versus a psychological phenomenon. In this two dimensional framework we position the main theories in brief, and we end with a positioning of our research and assumptions.

The oldest ? most accepted and common sense ? view holds that humans possess five independently functioning senses (the modularity thesis). The other view holds that human beings have one integrated sensory organ with five subdivisions (the unitary thesis). Modularistst argue that synesthesia is an anomaly and is explained as abnormal process of neural functioning. Unitarists consider synesthesia as a developed cognitive competence that is been integrated in the daily cognitive functioning of individuals with advantages like creative production and problem solving. In science, synesthesia has been mostly studied by psychologists and neurologists, sometimes in collaboration. Some psychologists have studied the relationships between physical stimulation and sensory experiences (cf. Marks 1975). Some neurologists have looked for disturbances of normal brain functioning (Cytowic 1995, Baron-Cohen et al. 1993, Grossenbacher 2001, Ramachandran & Hubbard 2001).

Positioning our research on the two dimensions, we assume that synesthesia is a normal sensory process. It appears that synesthesia is a perceptual competence that is used in creative problem solving. Individual differences exist in the type, intensity and vividness of these sensations. Differences that probably are the result of individual developments of the senses within a biological and social-cultural environment. However, we do not believe that the development process is determined from birth to adulthood. Developmental synesthesia has plasticity and is influenced by training (e.g. artistic education) that has been given to develop the synesthesia competence. The incidence of color synesthesia among people who professionally work with color (e.g. visual artists and designers, color consultants) is reported to be higher than average (Ramachandran & Hubbard 2001, Root-Bernstein 2001, 1999, Domino 1989, Dailey et al. 1997).

Incidence of types of synesthesia

The incidence figures that have been proposed show large differences. Ranging from early estimations by Galton who thought 1:20 persons was synesthete to 1:20,000 (Cytowic 1995). A population study was done by Baron-Cohen et al. (1996) who found a frequency of 1:2000 for color-word synesthesia. Incidence rates have to be distinguished to types of synesthesia. Day (2001) has gathered information on the basis of 409 subjective reports of synesthetes. He distinguishes 12 types of color synesthesia among others. The incidence of the colored graphemes type is the highest (68.0%), followed by Colored Time Units (20.5%), Colored Musical Sounds (15.9%), Colored General Sounds (11.2%), Colored Musical Notes (10.0%), Colored Phonemes (10.0%), Colored Tastes (5.9%), Colored Odors (5.6%), Colored Pain (4.4%), Colored Personalities (3.9%), Colored Temperatures (2.0%), Colored Touch (2.0%).⁽¹⁾

Diagnosis of synesthesia

Diagnosing synesthesia is complex since it is not a visible trait of persons. Diagnosis has to rely on self-reports and psychological testing. Most scientific studies and artistic explorations start with questions like: what is a synesthetic experience? and 'Who is a synethete? but few have addressed the problem of diagnosing or testing the presence of synesthesia in individuals.

The purpose of diagnosis and testing is to distinguish people with synesthetic experiences from people without synesthetic experiences.

In the last decades a few methods have been developed to determine, diagnose, assess or distinguish synesthetes and their synesthetic experiences. We discuss the pros and cons of these methods:

a) Self-report
Self-report is the most often used method. Synesthetic experiences are described to a person and the person is asked if he or she shares these descriptions. The method appeared less reliable in our tests. Beforehand some (modest) synesthetic participants did not believe that they were synesthetic. On other occasions participants who were very convinced of their synesthesia had low scores on the synesthesia test. Self-reports are not always valid or reliable. Some independent measure or standard should be used next to self- reports.

b) Diagnostic criteria
Cytowic has proposed five diagnostic criteria for synesthetic experiences: they are involuntary (but elicited), projected, durable (discrete and generic), memorable, emotional (and noetic) (Cytowic, 1993). This method is more objective (independent of subjective feeling). To state the diagnosis the clinician depends on self-reports of the synesthete. This makes the method less reliable (Korb, 1996). Further it depends on the expertise of the clinician. The interrater judgment reliability of this method is not tested as far as. The method of diagnosis does not provide results on the strength or intensity of the synesthetic experiences.

c) Consistency test
The consistency test was developed by Baron-Cohen et al. 1987, 1993, 1996). In 1964 already, Ostwald formulated the criterion that the type of imagery evoked by specific sensations in another modality is stable over time. The color-word test is based on this criterion (see description of test in Methods section).The results of the study show that subjects with synaesthesia gave exactly the same color correspondence 93% of the time, in spite of being tested over 1.5 years later. Subjects without synaesthesia were only 38% accurate, in spite of being asked to try and remember the colors they gave for words and being tested only one week later. The reliability of the test has been measured by interrater reliability. The test is valid: test scores were in line with subjective reports and interviews of the subjects. Influences of memory trace and possible psychiatric conditions were controlled. The consistency test has been used by Rushton (1998), Svartdal et al (1986) among others.

The reliability is limited to the use of color words (later color chips were used). The color concept was not differentiated: the emphasis is on color tone (hue). Other studies (Marks 1975, Hornbostel, 1931) have shown that the color dimension of brightness is central to color-word associations.

The validity is limited in two respects. First, the validity is limited to one type of synesthesia: color-word synesthesia. Second, the validity is limited to subjects who speak the English language.

d) Brain imaging
Cytowic has recorded the cerebral blood flow of a synesthete during his experiencing of intense synesthesia, revealing an abnormal pattern of brain activity (Cytowic, 1989), studies of blood flow among synesthetes are being continued by Paulesu et al. (1995) and Ramachandran and Hubbard (2001) with PET scans and fMRI.

This method looks like the final brain test, but it has some disadvantages: 1) The studies have not yet resulted in clear criterion or scales or critical values for synesthetic brain activity 2) the research is very expensive, time consuming and not easy for the participants (being locked for sessions of half an hour in small tube).

Starting from these existing assessment methods we set the following requirements for the development of the Netherlands Color Synesthesia (NeCoSyn) instrument:

1. valid and reliable, i.e.
a) rely as little as possible on subjective report.
b) contain clear criterions for the presence of synesthesia
c) be valid for non-english languages too.
2. feasible, i.e.
d) easy and cost-effective test to administer
3. exact, i.e.
e) quantify color experiences
f) be useful for the assessment of several types of color synesthesia
g) differentiating color dimensions (hue, saturation, brightness)
h) accurate in assessing grades of strength of synesthesia.

Methods

We developed the NeCoSyn instrument and tested its characteristics in three stages.

1. validity and reliability
First we tested if a Dutch translation of the existing consistency test was valid and reliable for application in Dutch speaking subjects, as an example of a non-english language.⁽²⁾ Nine self-reported synesthetes responded on an announce in the Journal of Dutch Color Society. Their mean age was 48 years, ranging from 25 to 71. The group consisted of 6 females and 3 males, most had professions related to color (artists, chemical analyst, designers).

The test we used was based on the English Color-Word Synesthesia test (Baron-Cohen et al. 1987, 1993), that consists of 117 words: 40 meaningful words from four semantic categories (animals, towns, objects, professions), seven days of the week, 20 forenames, 26 letters of the alphabet, eight emotional neutral abstract terms, and eight nonsense words. The words are read in random order to the participants. Participants note color words for each word item. Some weeks later the test is repeated with a selection of 13 words from the collection of 117 words. The responses, i.e. the color names, on test1 and test2 of the 13 identical words are compared and judged by independent raters.

For our test we translated and shortened the English version. The Dutch version consisted of 50 words, containing words from four semantic categories (animals, towns, objects, professions), letters of the alphabet, days of the week, nonsense words, and abstract words.⁽³⁾ The test was repeated with 50 words one year later. Four raters compared the responses of the first and second test independently of each other. The interrater agreement was tested.

2. Feasibility
In the second stage, an improved version of the test was applied to a large group of 223 art students by paper instruction. The subjects were a random sample of all students in one year of a preliminary study of art and design. The responses of 42 students (16%) were not used since they did not attend both two sessions. The mean age was 17,5 years, ranging from 16 to 22 years, 51% of the participants are female.

On the basis of the results of the first test with nine subjects (see results), the test was adapted into a more sensitive and shorter list of 20 abstract words (see Results section). The participants used paper and pencil. They noted color words. After a two week interval the test was repeated. Because the interrater reliability was very high in the first stage (see Results section), one rater was used in this stage.

3. Exactness
In the third stage, the instrument was refined with respect to accurateness, distinction of types of color synesthesia and distinction of color dimensions. On the basis of experiences and results from the earlier stages that the use of color words appeared too little refined to assess the color experiences, we decided to use color chips and give subjects the possibility too choose up to three color chips to indicate the experienced colors. We asked them to order the three colors in number of dominance. We used color chips from official NCS books to ensure standardization.

NCS Natural Color System is a logical color system built on how humans sees color. With NCS, all imaginable surface colors can be described and given unambiguous NCS notations. The six pure colors are the basis of the built-in ability of humans to characterize different colors are white W, black S, yellow Y, red R, blue B, and green G - the six elementary colors. NCS color notations are based on how much a given color seems to resemble these six elementary colors. In the NCS notation S 2030-Y90R, for example, 2030 indicates the nuance, i.e. the degree of resemblance to black and to the maximum chromaticness; in this case, 20% blackness (s) and 30% chromaticness (c). The hue Y90R indicates the percentage resemblance of the color to two chromatic elementary colors, here Y and R. Y90R means yellow with 90% redness. In this three-dimensional model, called the NCS color space, all imaginable surface colors can be placed and thus be given an exact NCS notation. (see illustration below) (www.ncscolor.com).

The Natural Color System has been chosen because it has several advantages over other color systems. First, The NCS uses a notation of colors that is simple to read and understand. Even for laymen the code for hue indicates what type of color it is, for instance Y50R indicates that the hue in the middle between the yellow and red hue. The user understands immendiately this must be a kind of orange. The code for brightness shows what is the percentage of blackness in the color and the code for saturation how much percent 'pure' hue is in the color. Second, the differences between two colors are immediately visible on the basis of their NCS code, even for non-experts. These two advantages go with the advantages of color systems in general: the codes are numeric and like other color systems can be stored in large databases and colors can be reproduced.

Sixteen participants were recruited partly from announcements and partly from earlier experiments, offering us the possibility to compare their scores on different stages of the NeCoSyn test. The mean age was ca. 35 years, ranging from 18 to 75 years. Five men and eleven woman participated. The stimuli were chosen to cover a range of well-known dimensions. The taste stimuli were varied on the dimensions salt, sweet, bitter and sour. The odors were varied on the same dimensions plus the dimension light-heavy. The musical instruments were chosen from a variety of wind and stringed instruments.⁽⁴⁾ In the taste session and the odor sessions the participants were blindfolded during stimulus exposure.

Results

1. Reliability, validity and sensitivity

Reliability refers to the accuracy of the test. Is the measured score close to the real score? Validity test are designed to test if the instrument measures what is intended to measure. For instance does a synesthesia test not measure memory competence or eidetic memory. Baron-Cohen et al (1987, 1993) have already ruled this possibility out for the test. Another question concerns the validity of the test in other languages. Sensitivity is an indication how capable the instrument is of detecting synesthetes and excluding the non-synesthetes.

To test the reliability we asked four raters to judge the responses of the nine subjects in the first stage testing. For each item they had to decide whether the two responses (color words) were equal or not. The Pearson moment correlation between the four judges ranged from 0.85 to 0.91, indicating very high agreement.

We tested if an Dutch translation of the original English test is valid for Dutch speaking by comparing the scores of synesthetes and non-synesthetes on both language versions. In the English test the average score of synesthetes was 93% against 38% for non-synesthetes (Baron-Cohen et al.1993). In the Dutch version, the average for the synesthetes was 90% against 43% for the the non-synesthetes. The scores show almost an exact identical pattern. The slight differences are attributed to the small number of subjects in the study.

A first step to enhance the sensitivity of the instrument was to investigate which words were most useful in distinguishing synesthetes form others. On the basis of the individual test scores we formed a synesthetes group and a non-synesthetes group. Which words did discriminate most between synesthetes and non-synesthetes? The group of abstract words turned out to hav ethe most distinguishing power. In contrast words of animals were less sensitive since non-synesthetes were able to produce high consistency scores on animal names items on basis of their knowledge of the skincolor of these animals. The extra power of the abstract words is shown in figure 1. The figure shows the scores (100% is maximum) of the nine participants. The participants are ordered from the person with the lowest score on the left and the person with the highest score on the right. The blue line links the scores on the total test of 50 items and the pink line connects the scores on a selection of abstract words. The pink line is steeper, having lower (than normal) scores for the non-synesthetes and higher (than normal) scores for the synesthetes. The test of 20 abstract words turns out to be more sensitive for detecting synesthetes.

Figure 1: strength (consistency) of synesthesia scores (in 0-100%) of nine subjects on the total test and a selection of abstract words.

2. Feasibility

Feasibility of instruments refers to practical requirements in their application. Do participants understand the instructions, do they perform well, is the test a burden for participants, are the costs in relation to the goals of the assessment etcetera?

In the second stage we tested the feasiblity of the more sensitive version of 20 abstract words⁽⁵⁾ in a large group of 223 students. The test was adminstered by a teacher, who was instructed by the researchers. The test appeared to be understood well. Less than one percent did not fill two forms regularly. The reason was disinterest. The test consumed little time. It took ten minutes of two lessons with a time interval of two weeks. The test was administered in 10 classes. Copied A4 forms were used, keeping the costs low.

3. Exactness

In the third stage we continued with the calibration of the instrument. In stage one the reliability or accuracy and sensitivity of the color-word instrument was tested. In this stage we tried to improve the specifity of the instrument. The test showed some disadvantages in the former stages: 1) color words are too little specific to assess the fine color sensations of synesthetes, 2) scores on color-word test are limited to one type of synesthesia. Therefore we added other stimuli (taste, odor and music) and used color chips of the Natural Color System instead of color words.

First we compared the assessment by paper and pencil based on color names and the NeCoSyn test that is based on the color chips that contain information about the three dimensions of color: hue, saturation and brightness. Table 1 shows the results of five subjects that did the paper and pencil test and the color chip test on color-word synesthesia. The first column shows their score on the paper and pencil test and the last three columns their differentiated scores on hue, saturation and brightness on the color chip test. As was expected the differentiated color chip test offers more detailed information. Not only the scores are more exact (on NCS scales) but individual patterns are more explicit. Subject S3 has higher scores on hue and brightness than on saturation. For subjects S4 en S5 the dimension of brightness has lower scores than hue and saturation dimensions. This indicates that a general average score covers individual differences with respect to color dimensions.

Note: short paper and pencil test (of abstract words and weekdays), last two subjects in step 1 the full word test and between brackets the score on the abstract word list.

Color dimensions

Is synesthesia different on color dimensions? We tested the hypothesis that no difference exist with respect to color dimensions. We used the data of 12 subjects that had a score of ninety percent or more on one of the color dimensions of one of the sensory domains. The scores of these 12 subjects are presented in table 2 for the four types of sensory stimuli by the three color dimensions .

The hypothesis that differences between color dimensions are absent or accidentally was tested rowwise by means of paired-samples T test that compares the means of two variables for a single group.⁽⁶⁾ The hypothesis of no statistical difference was rejected in most cases as is indicated by the notes a-h. In these cases the difference was statistically significant (p < 0.05). For instance the individual scores on the brightness of words differ significantly from individual scores on the hue of words (note a).

Notes: paired samples t test rows
a-h: significant differences (t-test, p<.05)

The results table 2 show some interesting patterns. Comparing the scores on brightness, saturation and hue, it appears that brightness synesthesia is always significant stronger than color tone synesthesia;

saturation synesthesia is significantly stronger than color tone synesthesia except for words; and brightness synesthesia is only significantly stronger than saturation synesthesia for odors.

This implies that experiences on the brightness dimensions are different from experiences on the color tone dimensions. The experiences on the saturation dimension differ more from hue dimension than from the brightness dimension.

Types of synesthesia

Day (2001) and others have suggested that types of synesthesia are distinct and not related. A person is not a synesthete in general but has one (or more) types of synesthesia, e.g. color-word synesthesia of color-taste synesthesia. We took the same group of 12 synesthetes with 90-plus scores on one type of synesthesia and tested the hypothesis that no difference exists between types of synesthesia. The hypothesis assumes that only a general type of synesthesia exists and that differences on different sensory stimuli are random. Table 3 represents the same scores as table 2 but to test the hypothesis the results are analyzed columnwise by paired samples t test.

Notes: paired samples t test rows
a-h: significant differences (t-test, p<.05)

The hypothesis of no differences between types was rejected in a number of cases. Colorbrightness-word synesthesia differs significantly from colorbrightness-music synesthesia en colorbrightness-ordor synesthesia. Only the difference with taste is not significant. Colorsaturation-word synesthesia differs significantly from the other three types concerning the color saturation of music, tastes and odors.

Colortone-word synesthesia differs significantly from the other three types of synesthesia.

The results show that color-word synestetes are significantly different from the other three types synesthesia. Among the other types of synesthesia no significant differences were found. The latter result can be explained by the fact that the number of these synesthetes were few. Most synesthetes in this group were of the color-word type.

Applications

In general the NeCoSyn profiles are maps of someone's individual color synesthesia. Since they are scaled the profiles can be compared to profiles of other synesthetes and group means. In this paragraph we look for possible applications of this young and new method in the fields of art an science.

Art

In the fields of arts, the NeCosyn profiles can be used as an analytic tool by individual artists for understanding and gaining more detailed insight in their personal synesthetic experiences. Compare for instance how the synesthetic artists Jack Ox (2000) and Carol Steen (2002) profited from more insight in their synesthetic experiences. The results of the NeCoSyn method can be used as an input for the creative process. Collopy (2001) has shown how knowledge of synesthetic correspondences is put into musical visualizers and other multimedia instruments. In art schools and art education the NeCoSyn method and profiles can be used to learn students about their personal perceptual functioning and sensibilities. In the process of developing our method we tested a group of more than 200 art students. The testing revealed synesthesia to some students and stimulated discussion of synesthesia and perception.

Diagnosis

In addition to existing methodologies, the NeCoSyn instrument provides the opportunity to differentiate types of sensory domain synesthesia and types of color synesthesia.

In comparison with the method of self-report we can report from our interviews with participants that self-reports are not always in line with the test results. Another point is that the subjective intuition of participants were objectified and specified in numbers by the NeCoSyn instrument. In particular the extra information on color dimensions of synesthetic experciences was new to participants.

In comparison to the clinician method of diagnosis on basis of criteria, the NeCoSyn offers more standardized results, not dependent on the raters individuals judgment. The instrument is robust and not prone to different interpretations.

The comparison with the color-word test has been discussed extensively in this article. To summarize, the main advantages are the assessment on color dimensions instead of hue and the extensions to other types of color synesthesia.

In comparison with brain imaging the NeCoSyn is far less cheaper to administer, is less burden to participants and can therefore be used in larger scale testing in populations. One of our subjects was tested in the medical lab of Utrecht University. The results were general and less specified than the results of the test with the NeCoSyn instrument.

Incidence

Reported incidence figures have a wide range: 1:20 (Galton) 1:200 (Hubbard & Ramachandran), 1:2000 (Baron-Cohen cs) and 1:20,000 (Cytowic). The most elaborate epidemiologic study by Baron-Cohen et al. restricted to colortone-word synesthesia. The NeCoSyn offers a more differentiated measure for assessing the incidence of types of color synesthesia. This will be subject of a separate article.

Method to assess synesthesia in other sensory domains

The current methods claim to assess synesthesia in a global sense (self-report) or very specific (Baron-Cohen). This study has its limitations, since it was restricted to color synesthesia with four types of stimuli.

The consistency method that was developed by Baron-Cohen cs. and refined /elaborated by us with respect to color dimensions can be applied to more sensory domains. We give the example of assessing color-tactile synesthesia. Black little sacs with textures inside. Let persons choose colors after touching the textures inside. One can vary with cold and warm textures, fine versus coarse etcetera.

Conclusion

Current methods assess synesthesia in a global sense (self-report) or on one type (color-letter) The scientifically most used consistency method , developed by Baron-Cohen et al., is refined and elaborated by us with respect to color dimensions and other sensory domains. In the present study we restricted ourselves to four types of color synesthesia, but the NeCoSyn method can be applied to more sensory domains. We give the example of assessing color-tactile synesthesia. Take little black sacs with substances of different textures inside. Let persons choose colors after touching the textures inside. One can vary with cold and warm textures, fine versus coarse etcetera. That means that the NeCoSyn method is not finished. The artistic and scientific study of synesthesia has grown along separated lines, more by tradition than by logic. The NeCoSyn is an example that critical interaction of both disciplines can deepen our insight in synesthesia and probably multisensory sensibility in general.

Acknowledgements

We would like to thank the Netherlands Society of Color Study (Nederlandse Verenging voor Kleurenstudie, Florence Husen, art teacher at the Lucas College in Boxtel, the members of the Rotterdam Philharmonic Orchestra, Fenna and Rosa Boekhorst, all participants, and the Natural Color System Sweden for their generous help.


Notes

1. 40% of the subjects have multiple synesthesies

2. A relation between language and synesthesia is suggested by John Gage (1999), who argues that the Russian language lents itself to color-hearing. Not coincidentally, three well known 'synesthetes' were from Russian origin: Nabokov, Kandinsky and Scriabin.

3. The Dutch words were: Woensdag, hij, Vlissingen, olifant, b, asbak, vaas, loodgieter, lamp, a, Assen, wethouder, voor, zij, Maandag, tussen, m, kapstok, s, roermond, Dinsdag, onder, suval, bord, Groningen, Vrijdag, Zaterdag, k, wij, kangaroe, slager, taal, o, Alkmaar, stoel, l, vos, analyse, olko, raaf, amba, aap, i, Donderdag, naast, p, blad, zin, Zondag, u.

4. The tastes were: rice, sea salt, peanut butter, honey, chinese noodles, olive oil, gherkin, pure chocalate, lemon juice, coffee, peppermint. The odors: coffee, anise, marmite, whey, eucalyptus oil, madeira, plum vinegar, soft soap, jasmine incense, sprat, arnica oil. And the musical instruments: violin I, piano, viola, violin II, oboe, harp, bassoon, trombone, tuba, trumpet, double bass. The music was specially taped for the experiment by instrumentalists of the Rotterdam Philharmonic.

5. The 20 abstract Dutch word were: woensdag, duur, lenen, wethouder, voor, maandag, tussen, dinsdag, onder, auto, vrijdag, zaterdag, taal, list, jou, boer, donderdag, haast, pin, zondag.

6. The paired samples t test computes the differences between values of two variables for each case and tests whether the average differs from 0.

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