Intended for healthcare professionals



BMJ 2010; 340 doi: (Published 08 January 2010) Cite this as: BMJ 2010;340:b4616
  1. David M Eagleman, assistant professor
  1. 1Department of Neuroscience and Department of Psychiatry, Baylor College of Medicine, Houston, TX 77030, USA
  1. eagleman{at}

    Is a common and harmless perceptual condition

    Imagine a world of magenta Tuesdays, tastes that have shapes, and wavy green symphonies. At least 1% of otherwise normal people experience the world this way—in a harmless neurological condition called synaesthesia. In synaesthesia, stimulation of one sense triggers anomalous perceptual experiences.1 2 For example, a voice or music may be not only heard but also seen, tasted, or felt as a touch. Synaesthesia is a fusion of different sensory perceptions: the feel of sandpaper might evoke an F sharp, a symphony might be experienced in blues and golds, or the concept of February might be experienced above the right shoulder. Synaesthetes are typically unaware that their experiences are unusual. In the linked article (doi:10.1136/bmj.b3191), Logsdail reports one patient’s journey with synaesthesia.3

    Synaesthesia comes in many varieties, and a person can have several different types. Experiencing letters and numbers with colours or textures is an especially prevalent form (fig a)4 that affects at least 1% of the population.5 Logsdail reports a woman’s first hand experience with this form of the condition,3 known as ”grapheme-colour” synaesthesia. Other common varieties include experiencing colours in response to sounds, or tastes in response to words.1 Another very common form is spatial-sequence synaesthesia, in which a person perceives sequences (such as numberlines, years, or weekdays) as having a spatial three dimensional form.6 For example, someone with this form of the condition may say that Monday is in front of them to the right, next to that is Tuesday, and so on, with specific locations to which they can point.


    (a) Representation of the colours evoked by individual letters in a word for a grapheme-colour synaesthete. (b) Synaesthesia seems to result from higher than normal crosstalk between neighbouring areas in the brain—in this case the nearby brain areas involved in graphemes (orange) and those involved in colour, texture, or form (shades of blue). Adapted from Eagleman and Goodale4

    Synaesthetic perceptions are typically basic: people sense things like simple colours, shapes, or textures, rather than something pictorial or specific (for example, synaesthetes do not say, “This music evokes a vase of flowers on a restaurant table”). Moreover, the particular synaesthetic pairings (for example, number 3 is purple) are unique to each person. Synaesthetic perceptions are involuntary, automatic, and consistent over time.

    Synaesthetic experiences are not hallucinations. Synaesthetes do not think that their synaesthetic perceptions exist in the outside world—instead they are internal experiences (in “the mind’s eye”) and recognised as such.

    Although synaesthesia was first described in Nature 126 years ago,7 its study was hindered for almost a century from a lack of tests to verify the phenomenon. Synaesthesia can now be rigorously phenotyped thanks to simple diagnostic tests ( Such tests use the fact that synaesthetes are consistent in their letter-colour matches over years, a feat that cannot be imitated by controls. In recent years, the authenticity and automaticity of synaesthesia have been confirmed by functional magnetic resonance imaging.9

    Synaesthetic brains reflect crosstalk between normally separated brain areas, such that activity in one area kindles activity in another (fig 1b). Whether this crosstalk results from increased physical connectivity between areas or a slight imbalance of inhibition and excitation is unknown. Interestingly, synaesthesia clusters in families, and the patterns of inheritance suggest the possibility of a single dominant gene.10 11 A large scale genetic study (a family linkage analysis) is currently mapping the gene(s) that correlate with coloured sequences (such as letters and numbers).12 Understanding the genetic basis of synaesthesia should clarify the different neural hypotheses.

    Synaesthetes do not seek medical help—nor should they—and they do not need support groups. They accept the reality presented to them, as we all do. (Analogously, we would not expect a colour blind person to suggest a support group for those with normal vision under the assumption that “seeing all those colours” must be troubling.)

    Doctors, parents, and educators should all be aware of this condition to avoid showing misplaced concern when hearing a synaesthete’s unusual description of the world. It is far too common for synaesthetes to be stigmatised as saying something “crazy” when they describe their perceptual experience—a point germane to why the patient described by Logsdail stopped talking about her synaesthesia for 25 years.

    Given the high prevalence of synesthesia, doctors need to know about this phenomenon in case they mistake it for a peculiar type of cognitive fragmentation.


    Cite this as: BMJ 2010;340:b4616



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