What Colour is Four? The Perception of Synaesthesia in Art and Science


Marc Tibber
Marc Tibber is currently a postdoctoral research fellow at the University of London. His main professional interest lies in the field of human visual perception. His personal interests should not (and will not) be listed here.

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This article originally appeared in The Skeptic, Volume 19, Issue 4, from 2006.

“BLUE OF COURSE,” was the answer that ZS gave me without hesitation, i.e. the number four is blue in colour. Not in the sense that a particular number four on a particular page of a particular book has been printed in blue, but in the sense that when ZS hears, sees, or imagines the number four, a colour is simultaneously experienced. Similarly, the number one is white “…in a way that would suggest the tiniest, weeniest speck of black paint has been stirred into a glass of milk”; two is a bright green, “..slightly bright in a glowing rather than glaring way”; and three is a grassy green, “..quite healthy, dark grass, like the shiny side of the blade, not the furry side”. What’s more is that were I to ask ZS the same series of questions ten years later, I would get a very similar pattern of answers.

ZS experiences synaesthesia: an intriguing condition in which multiple sensory modalities (e.g. sight and hearing, hearing and smell, or touch and taste) are interconnected. Thus, to a synaesthete, a simple melody may elicit sensations of a deep burgundy, a few words read from a newspaper might conjure the smell of rotten eggs, or perhaps, a discarded object seen on the pavement induce tingling sensations in the spine:

“When I taste something with an intense flavour, the feeling sweeps down my arm to my fingertips”

“I remember at age 2 my father was on a ladder painting the left side of the wall. The paint smelled blue, although he was painting it white.”

“When I listen to music, I see the shapes on an externalized area about 12 inches in front of my face and about one foot high.”

As can be seen from the examples given above (from Cytowic, 2002), the specific pattern of sensory cross-talk differs from individual to individual, reflecting the particular senses that are implicated and, most probably, the specific pattern of crosswiring that characterises the structure of any individual synaesthete’s brain (Rich & Mattingley, 2002). In particular, ZS described the close association that words, numbers, and sounds have with colour: a rather common form of coloured-hearing / coloured grapheme synaesthesia that was also experienced by Rimbaud (see his poem entitled ‘Vowels’). Thus, ZS’s distaste for Bach (as I would later find out) was due to its “turgid brown colour”. Similarly, the number four was ZS’s favourite because of the ethereal blue hue with which it was associated.

Known to medicine for over three centuries, the history of synaesthesia is a fascinating one. In 1690 John Locke wrote in his Essay Concerning Human Understanding:

A studious blind man who had mightily beat his head about a visible object, and made use of  the explications of his books and friends, to understand those names of light and colours, which often came his way, betrayed one day that he now understood what scarlet signified. Upon which, his friend demanded what scarlet was? The blind man answered, it was like the sound of a trumpet.

Over the ensuing centuries synaesthesia drifted in and out of the scientific and medical communities’ attention. By the end of the 19th century there was considerable interest in synaesthesia amongst neurologists and psychologists alike: some 74 articles had been published on the subject between 1881 and 1931. However, with the emergence of behaviourism and its subsequent domination of psychology in the early 20th century, qualitative mental states were no longer deemed a suitable topic for empirical investigation. As a result, only 16 articles were published on synaesthesia between 1932 and 1974 (Baron-Cohen & Harrison, 1997). Behaviourism as a school of thought attempts to examine and describe mental states solely in terms of observable behaviour:

Psychology, as the behaviorist views it, is a purely objective, experimental branch of natural science which needs introspection as little as do the sciences of chemistry and physics. It is granted that the behaviour of animals can be investigated without appeal to consciousness. Watson (1913).

Consequently, the study of consciousness and introspective methods of investigation into the nature of personal experience were largely dropped. Indeed, for a while the scientific community grew relatively sceptical of synaesthesia as a true medical condition, many even doubting its existence beyond a more conceptual association between sounds and images, i.e. as metaphor or analogy.

This perspective of synaesthesia merely as metaphor was reinforced by a general blurring of its definition as a result of the artistic community’s fascination with the condition. Interest in synaesthesia and related phenomena of the mind can be pinned down, roughly speaking, to three main epochs of artistic development, although links connecting them can undeniably be traced.

A modern day concert - lights in pinks and purples shine from the stage. The image is taken from the perspective of an audience member looking through a sea of silhouetted hands against the pinky purple light

(1) The late 19th/early 20th century: during this period synaesthesia had become a highly fashionable topic to an art movement that idealised a fusion of the senses (see Campen, 1997, for a critical review of synaesthesia and artistic experimentation). Concerts that combined music, light (and occasionally even odour) abounded, and became typified by Vasilly Kandinsky’s opera Der Gelbe Klang (“The Yellow Sound”) of 1912, which incorporated the use of colour, light, dance and sound. Kandinsky, himself a well-documented synaesthete, described his paintings using terminology borrowed from the world of music, referring to them as “compositions” and “improvisations” (Cytowic, 1995).

(2) The post-war period: in the early 20th century interest in consciousness and the individual’s perspective flourished within the arts with the emergence of movements such as Dadaism and surrealism. Although the emphasis was not on synaesthesia per se, these groups turned away from classic reductionist views of the mind and adopted introspective methods of investigation that explored the unconscious mind and altered states of consciousness. This shift in perspective was largely a result of two major forces of this period: Sigmund Freud (and the psychoanalytical approach), and the First World War. With the publication of Freud’s major works, it became apparent that the mind held a wealth of secrets that could not be explained or explored within the framework of previous psychological methods that had focused solely on externally directed observable behaviour. In parallel, many of the period felt that the horrors of the First World War were a reflection and consequence of a utilitarian way of thinking that had denied the spiritual, artistic life of man. Andre Breton, a founding member of surrealism who served during WWI in a neurological ward, wrote in his Surrealist Manifesto of 1924:

…a part of our mental world which we pretended not to be concerned with any longer […] has been brought back to light […] thanks to the discoveries of Sigmund Freud. On the basis of these discoveries a current of opinion is finally forming by means of which the human explorer will be able to carry his investigation much further, authorized as he will henceforth be not to confine himself solely to the most summary realities. (Breton, 1924).

(3) The 1960s: an explosion of interest in synaesthesia and related phenomena of the mind can be traced back to this decade, largely as a consequence of the increased use and availability of psychotropic substances, most notably LSD. Many of these chemicals have been reported to induce ‘synaesthesia-like’ states of sensory fusion. The resulting influence on the artwork of the period is undeniable, from the intricate interplay of light and music (as typified by the early performances of Pink Floyd) to the work of psychedelic pop artists like Martin Sharp (see Cream’s Disraeli Gears album cover).

Terence McKenna, a philosopher and writer of the period, went so far as to hypothesise that a pharmacologically triggered experience of synaesthesia was the catalyst for the development of spoken language in humans. According to his ‘Stoned Ape Theory’, in some age far back in man’s ancestral past, the experience of synaesthesia induced by the ingestion of psilocybin (‘magic mushrooms’) provided a crucial link between vocalised sound and the formation of an abstract image in the mind that facilitated the emergence of language. Although this idea is appealing, no empirical evidence (to my knowledge) has been found to support the theory and, indeed, several reasonable objections have been put forward. Irrespective, an underlying neurochemical connection has been sought between the pharmacological effects of various psychotropic drugs, the associated experience of sensory fusion and true synaesthesia.

Artistic exploration of sensory fusion over the last 150 or so years can thus be seen as both a catalyst and a hindrance to the study of synaesthesia as a neurological condition (though I do not question its merits on an aesthetic level). In one sense, scientific exploration of this condition has shamefully lagged behind the artistic community’s initiative, which can be seen to have opened the way for empirical research in the field. However, in parallel, methods of sensory fusion in the world of art and popular culture, and its use of the term synaesthesia to describe such works, have undoubtedly blurred the definition of synaesthesia as a very real neurological condition. In parallel, the scientific community is often reluctant to address novel territory that lies outside the ambit of existing scientific orthodoxy. Thus, as late as the 1970s, Richard E. Cytowic (a leading researcher in the field) remembers how synaesthesia was still not deemed a suitable field for empirical research:

…no one was studying synaesthesia and no one was interested in doing so […] Synaesthesia just didn’t fit their tidy worldview. “Stay away from it. It’s too New Age,” they advised. ”It will ruin your career. (Cytowic, 2002).

Over the last few decades the scientific community’s perception of synaesthesia has changed drastically and the field has rid itself of its ‘new age’ reputation. Consequently, research into synaesthesia has finally been absorbed into the scientific orthodoxy. This is due, at least in part, to the development of objective methods of investigating psychological phenomena, particularly functional neuroimaging [e.g. functional magnetic resonance imaging (fMRI) and positron emission tomography (PET)]. fMRI and PET track changes in cerebral blood flow to determine which regions of the brain are preferentially activated during a particular task or experience. The critical studies of synaesthesia have thus demonstrated that in a synaesthete, the experience of synaesthesia involves similar physiological processes and anatomical structures that underlie common sensory experience. Thus, fMRI studies of a coloured-hearing synaesthete have shown that spoken words elicit activity in areas of the brain normally associated with colour perception (Nunn et al., 2002). This pattern of activity is only found in a non-synaesthete when they are exposed to a coloured visual stimulus, and in both cases, it is an involuntary, automatic response. It is little wonder therefore that to a synaesthete, the experience of synaesthesia tends to be as vivid as any other.

Coloured liquids in laboratory glassware

In the last 10-20 years research into synaesthesia has undergone an explosion of activity, with laboratories approaching the subject from a diversity of different directions and disciplines. Whilst psychologists attempt to unravel the precise nature of associations between the different implicated senses, the physiologists make headway in tracing potential pathways of communication between the associated brain regions. In parallel, geneticists have begun to determine the extent to which the condition is inherited. Indeed, a great deal more is now known about synaesthesia than ever before. However, irrespective of this significant advance in knowledge, we nonsynaesthetes are still no closer to understanding what it actually feels like to see the colour of a vowel. To quote and further exploit an already much abused line: “a rose by any other name would smell as sweet”. We may know which areas of the brain are activated during the experience of synaesthesia, but the gulf between experience and knowledge stubbornly persists. This is where the arts step back in: to convey a dimension of reality that cannot be reduced to the level of the neurone, an aspect of reality that nonetheless dominates the vast majority of our experience.


Finally, I would like to suggest that the history of synaesthesia can provide a lesson for research in other fields that lie on the fringes of scientific investigation. Thus, in common with phenomena such as extra-sensory perception (ESP) or kinaesthesia, synaesthesia was exposed to a great deal of scepticism over the years, and perhaps consequently, research in this field has been slow in its progress. The validity of synaesthesia as an area of research is unquestionable, both in terms of its inherent interest per se as a neurological condition, but equally, because of what it might tell us about normal development and function in the adult brain. Likewise, if ESP were shown to exist, the value of this knowledge would be immense. However, the important point to bear in mind is that despite the scepticism initially accorded to synaesthesia, and in part directly as a result of it, synaesthesia eventually stood up to rigorous empirical investigation.

Thus, scepticism can be seen as both a necessary tool and a hindrance to the scientific researcher, indeed to the researcher within any discipline. Scepticism is a necessary tool as it enables the individual to question and examine existing paradigms and accepted systems of belief. Without scepticism we would still be living on a flat world, hemmed in by our horizon for fear of falling off the edge of the earth. In parallel, scepticism can be a hindrance, as all too quickly it will drift into dogma. Once this occurs, ideas are rejected outright without further investigation and thought grinds to a halt. Healthy scepticism merely questions that which cannot be demonstrated within the context of existing knowledge. However, the key word is ‘questions’, as opposed to ‘denies’ or ‘rejects.’ Whilst a 17th century sceptic may have been justified in questioning whether sounds can trigger a sensation of colour, the evidence today is overwhelming. To paraphrase ZS’s words: “what do you mean you don’t see colour when you hear a number? So what’s actually there then? You must see something!” ZS was blind to the limitations of others’ everyday sensory experience. In the absence of further information, her model of their reality was shaped by projections of her own introspective experience.


  • Baron-Cohen, S., & Harrison, J. E. (1997). Synaesthesia: Classic and Contemporary Readings. Cambridge: Blackwell Publishers.
  • Breton, A. (1924). Manifestoes of Surrealism. Michigan: Michigan University Press.
  • Campen, C. (1997). Synesthesia and artistic experimentation. Psyche, 3(6). November 1997.
  • Cytowic, R. E. (1995). Synesthesia: Phenomenology and neuropsychology: A review of current knowledge. Psyche, 2(10). July 1995.
  • Cytowic, R. E. (2002). Synesthesia: A Union of the Senses. 2nd ed. London: The MIT Press.
  • Locke, J. (1690). An Essay Concerning Human Understanding: Book 3. London: Basset; reprinted Oxford: Clarendon Press, 1984.
  • Nunn, J. A., Gregory, L. J., Brammer, M., Williams, S. C. R., Parslow, D. M., & Morgan, M. J. (2002). Functional magnetic resonance imaging of synaesthesia: activation of V4/V8 by spoken words. Nature Neuroscience, 5, 371-375.
  • Rich, A. N., & Mattingley, J. B. (2002). Anomalous perception in synaesthesia: a cognitive neuroscience perspective. Nature Reviews Neuroscience, 3, 43-52.
  • Watson, J. B. (1913). Psychology as the behaviourist views it. Psychological Review, 20,158-177.
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