Synaesthesia Revealed (or Why that Word is Yellow)

"The Handbook of Synaesthesia", cover, featuring an image I particularly enjoy.
“The Handbook of Synaesthesia”, cover, featuring an image I particularly enjoy.

A number of years ago I had the good fortune to hear Mr. Sonny Rollins play saxophone in Madison, WI. Rollins, then in his seventh decade, played with an intensity that was unbelievable. The music was amazing, the crowd appreciative. In fact, the entire experience was moving to the point that I saw Rollins, blowing into his saxophone, come out the other end of the horn. That’s right: Rollins appeared to pour into the saxophone, coming out the bell, along with the music.

The experience was ‘a gas’, entirely unaided by alcohol or elicit substance. Of course it was somewhat unsettling as well. And I couldn’t tell anyone about it. That is not until recently, when I met Dr. Edward Hubbard.

Dr. Hubbard is an educational psychology researcher at the University of WI-Madison. In 2001 Hubbard and V.S. Ramachandran introduced the cross-activation model of grapheme-colour synaesthesia in two publications: “Psychophysical investigations into the neural basis of synaethesia” (1) and “Synaesthesia: A window into perception, thought and language” (2).

In 2011, on the tenth anniversary of this work, Hubbard et al. published  “The cross-activation theory at 10”  (3) in which they further  tested and updated the theory with data from functional MRI, anatomical studies (using diffusion tensor imaging and voxel-based morphometry), also EEG and magnetoencephalography (MEG) studies.

Most recently, Hubbard and Dr. Julia Simner have edited “The Oxford Handbook of Synaesthesia”, to be published in 2013 (4).

But a digression is needed. What is synaesthesia?

To quote Ed Hubbard,

“synaesthesia is an experience in which stimulation in one sensory or cognitive stream leads to associated experiences in a second, unstimulated stream.”

There are numerous forms of synaesthesia, grapheme-colour synaesthesia being one of the more common forms, in which letters or numbers are perceived as if seen through or immersed in colour. In another form of synaesthesia, numbers, days of the week or months of the year evoke personalities. Some people experience numbers, days or months as corresponding to precise locations in space, for instance a three-dimensional view of a year as a map.

As well, coloured hearing involving words viewed in colour and music-colour synaesthesia have been reported. According to the Wikipedia page on synesthesia , “Over 60 types of synesthesia have been reported, but only a fraction have been evaluated by scientific research. Even within one type, synesthetic perceptions vary in intensity and people vary in awareness of their synesthetic perceptions.”

Hubbard notes that “additional sensory events” brought on by the influence of psychedelic drugs, a stroke, blindness or deafness are sometimes considered forms of synaesthesia (5).  However, common synaesthesia is known to be congenital, so the forms that arise from psychedelic drugs or the other nongenetic events are referred to as adventitious synaesthesia, to avoid confusion with the hereditary form.

There are other differences between these two types of synaesthesia. Generally, synaesthetic experiences reported by those with congenital synaesthesia are simple, stable and reproducible, while those of the drug-induced  nature are complex, hallucinatory and changing.  I’ll leave adventitious synaesthesia to the pharmacologists and discuss congenital  synaesthesia in this blog.

Congenital synaesthesia is far more common than one might think, occurring in upwards of 4% of the population (1 in 23 persons; 6). Synaesthesia runs strongly in families (5,7) though none of my seven siblings has admitted to (or recognized having) a synaesthetic experience.

Synaesthesia is a neurological condition and although not associated with any negative pathology, it’s not uncommon to be concerned about a neurological diagnosis. It was comforting to hear that recent research has suggested no association between congenital synaesthesia and other neurological or psychiatric conditions (8).  The Wikipedia page states “Rather, like color blindness or perfect pitch, synesthesia is a difference in perceptual experience and the term “neurological” simply reflects the brain basis of this perceptual difference.”

Indeed research performed by Hubbard and others cited here, has proved a precise and logical reason for synaesthetic experiences. Enter the cross-activation theory.

There is, of course, some debate about what causes the synaesthetic experience, debate on what is occuring in the brain to cause such experience. Hubbard et al. (1–3) proposed and subsequently tested the cross-activation theory, that during brain development, pruning  of brain connections usually occurs. However, in the synaesthete, there is a failure of pruning, such that “adjacent brain regions in the fusiform gyrus remain connected, even in adults, leading to cross-activation between these regions.

During our talk, Hubbard drew an image of the brain on the blackboard, noting that in the left hemisphere, the VWFA (visual word form area) responsible for recognizing letters and numbers is adjacent to the V4 area, responsible for colour recognition. The cross-activation theory posits that in persons where pruning failed, these regions would remain connected, resulting for instance, in grapheme-colour synaesthesia (3).

Brain image showing the proximity of word forming and colour regions. Image originally provided by E.M. Hubbard, used with his permission and that of Wikipedia.
Brain image showing the proximity of word forming and colour regions. Image originally provided by E.M. Hubbard, used with his permission and that of Wikipedia.

This is the very lightest of treatises of synaesthesia. However, for the sake of brevity, I’ll leave the details to the references cited here for those wishing to do more in-depth study.  What Hubbard et al. have done in the past 12+ years, is provide real data supporting the neurological basis of synaesthesia. They have, through data from fMRI and PET (to name a few), shown neurological differences between synaesthetes and nonsynaesthetes, as well as having identified the brain regions responsible for some synaesthetic experiences (1-3).

During our interview, Dr. Hubbard told a story that you might enjoy if wondering whether you are a synaesthete or not. Hubbard was at an American Synesthesia Association meeting, sitting next to a well-known synesthete, during a performance. They were watching a woman who was demonstrating the colours she  experienced with certain music. The synesthete seated next to Hubbard turned to him and said, “I don’t have that type of synesthesia. Besides, those colours are all wrong.”

While research into mechanisms of the synaesthetic experience is helping the neuroscience community understand and identify the phenomena, it is also proving valuable to synaesthetes in recognizing, identifying  and understanding their experiences.


  1. Ramachandran, V.S & Hubbard, E.M. (2001). Psychophysical investigations into the neural basis of synesthesia. Proceedings of the Royal Society, B. 268, 979–83.
  2. Ramachandran, V.S. & Hubbard, E.M. (2001) Synaesthesia: A window into perception, thought and language. Journal of Consciousness Studies 8, 3–34.
  3. Hubbard, E.M., Brang, D. and Ramachandran, V.S. (2011). The cross-activation theory at 10. Journal of Neuropsychology 5, 152–77.
  4. Simner, J. & Hubbard, E.M. (Eds; Expected 2013). The Oxford Handbook of Synaesthesia. Oxford, UK: Oxford University Press. ISBN 9780199603329 (HB).
  5. Hubbard, E.M. (2007). Neurophysiology of synesthesia. Current Psychiatry Reports 9, 193–9.
  6. Simner, J. et al. (2006). The prevalence of atypical cross-modal experiences. Perception 35, 1024–33.
  7. Galton F. (1880) Visualized numbers. Nature 22, 494–5.
  8. Rich, A.N. et al. (2005) A systemic, large scale study of synaesthesia: Implications for the role or early experience in lexical-colour association. Cognition, 98, 53–84).
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Kari Kenefick

Kari Kenefick

Kari has been a science writer/editor for Promega since 1996. Prior to that she enjoyed working in veterinary microbiology/immunology, and has an M.S. in Bacteriology, U of WI-Madison. Favorite topics include infectious disease, inflammation, aging, exercise, nutrition and personality traits. When not writing, she enjoys training her dogs in agility and obedience. About the practice of writing, as we say for cell-based assays, "add-mix-measure".

One thoughtful comment

  1. Interesting proposition, neurons bleeding together, except for the lack of blood in the brain…right? Ok, head wounds bleed, but vasularization is external to the brain, largely, right? But your comment asks of the hows…so these two brain regions next to each other cross-wire. Or rather, fail to uncross. There are so many things in development, that close and compartmentalize, either before birth or after, and during childhood. The liver shunt is one of my favorite examples.
    Not familiar with D. Hebb, except for what I just read on Wikipedia:

    Thanks for pointing him out to us! -Kari

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