Dec 18 2009

The Nature of Synaesthesia

Fans of the show Heroes are familiar with a recently introduced character whose mutant power is to see sounds. She is deaf, making the ability more interesting for her, and discovered that loud sounds, like music, produce beautiful colored lights.

This is a real phenomenon known as synaesthesia – an uncommon neurological condition in which people perceive one kind of sensory input as if it had properties of another – for example numbers have color or shape, or visual stimulation producing the perception of sound. The current interpretation of synaesthesia is that one sensory area in the brain is leaking electrical signals to an adjacent brain area – visual signals are stimulating auditory cortex resulting in the perception of sound.

However recent evidence suggests that synaesthesia may be more complex than just leaking signals between adjacent brain areas. Perhaps it results, at least in some cases, from higher levels of cortical processing involved in attention and concentration.

Jamie Ward and colleagues at the University of Sussex in Brighton, UK, have been studying color-grapheme synesthetes – those who see numbers or letters as colors. Such synesthetes have been shown to perform better on certain kinds of hidden shape tests. If a shape is made out of  the number “2” in a field of “5s” it can be very challenging to perceive the shape. But color-grapheme synesthetes perform much better on such tasks, because the numbers “2” and “5” have different colors, so the shape pops out.

However, when Ward performed this test on a number of color-grapheme synesthetes and normal controls and gave them only 1 second to see the shape, the synesthetes performed better than controls but were only able to see the shape 40% of the time. Some of the subjects reported that the numbers had colors only where they were looking – that portion of the puzzle on which they were focusing their attention. This suggested to Ward that perhaps the phenomenon of synaesthesia is not a simple leaking from one sensory area to another but results from higher-order processing involved in attention.

This could explain the results of this study – if the synesthetes happened to be looking at the portion of the puzzle where the shape was hidden in the 1 second provided to them, the shape popped out – but not if they were not looking directly or mostly at the shape.

There is other evidence to support this view. Another study performed at the Max Plank Institute found that color-grapheme synesthetes could transfer their color associations. They told subjects to associate characters from a Glagolitic language they did not previously know with their existing letters or numbers. They found that the color associations transferred to the new Glagolitic characters.

Transfer of color associations to new languages has been previously described, but this study showed that it could occur after only 10 minutes. The association was confirmed with a Stroop test – which means the researchers were not relying on subjective report. This shows  a high level of plasticity for synaesthesia, but also that synesthetic associations can be made at higher conceptual areas – at the level of abstract meaning, not just simple sensory input.

Still another line of evidence for this view comes from neuroanatomy. Researchers have found that synesthetes have increased gray matter volume, and that they have novel brain connections that are widely distributed – not just in sensory areas.

Although the speed with which some synesthetes can perform the hidden-shape test, argues Edward Hubbard, a synaesthesia researcher, is too fast for complex cortical processing, so simple sensory processing must also be involved.

What all of this means is that synaesthesia is more of a complex neurological phenomenon than simple sensory leakage. It involves a novel pattern of hard wiring in many brain areas, involving attention and abstract concepts in addition to sensory cross-over.

This is interesting on many levels, and researchers hope that it can be exploited to learn more about the anatomy of brain connections.

But also I think this is an excellent example of variation in nature. We tend to think of species as archetypes, but they are much more varied than most people assume – and this includes humans, and specifically the pattern of hard-wiring of the human brain. Nature is always “experimenting” with new patterns, and sometimes those novel patterns produce unexpected results. It seems likely that most of the time these new patterns will be inconsequential or neutral – different but not necessarily better or worse, depending on context. Sometimes they may be detrimental. And on occasional new patterns that emerge by chance may produce a beneficial new ability.

Whether or not synaesthesia is “beneficial” is an interesting question in itself, and likely depends on context. Perhaps if it can be exploited to enhance intellectual performance in useful tasks (not just psychological tests) it can be beneficial. Some color-grapheme synesthetes are math wizards, because they can see the numbers and this helps them manipulate them.

Perhaps the central premise of Heroes is not that far off – new heritable genetic variations can arise in the human population producing new and interesting abilities. Although I doubt this will produce people who can fly anytime soon.

18 responses so far

18 thoughts on “The Nature of Synaesthesia”

  1. banyan says:

    I knew someone in high school who claimed to have a different (sometimes very slightly different) color association for every number. Not just every digit in a base 10 system, but every single number, so 1,430 would have a different color than 14.

    I did a sort of informal test, giving her some large numbers, asking her the color, and then asking the color for one of the same numbers like a week later when I thought she probably would have forgotten. Not really a rigorous test, but not bad either. She seemed to be able to do it, but maybe she just remembered.

    Does that seem plausible?

  2. Justin L. says:

    Great post! It’s a fascinating subject. I think I remember Dr. V. Ramachandran, or someone, talking about the possibility that a facility for metaphors is a kind of synaesthesia. Wouldn’t these studies lend some support to that hypothesis?

  3. CrookedTimber says:

    You mention that synaesthesia is an uncommon neurological condition, I was wondering if there is an idea of the prevalence. Are there sub groups for which it is more common, for instance I’ve read that it is much more common in people with artistic ability?

  4. Eternally Learning says:


    Thanks for a great article! I had never heard of this condition before and find it very interesting. One question though; My mother works for a child psychologist (Dr. Stanley Greenspan, if you’ve heard of him) and ran across a case where a child who is severely autistic would seemingly at random just stop moving and stare at fast moving objects like fans and become basically unreachable until forced away. After years of therapy this child eventually learned to communicate (not facilitated of course) a bit and was able to relate that the reason they were so entranced by fans and the like is that when they stared at them, they heard music. Apparently this is not entirely uncommon and some autistic people have managed to convey that music into some more conventional means (I’ve been looking for examples to link to online, but can’t find anything). Is this related to what you are talking about? It seems similar.

  5. superdave says:

    For anyone interested, Oliver Sacks’ book Musicophilia has a chapter devoted to this phenomena.

  6. El Guerrero del Interfaz says:


    It’s something that I remember from when I was a kid: numbers for me had a color, a precise and distinct one. I never told anybody because that seemed silly to me, even then.

    I don’t know if it has anything to do with that but I was also dyslexic.

    Anyway, thanks for the tip. I’ll certainly try to read more about this.

  7. Fifi says:

    Eternally Learning – Thanks for sharing the story about the autistic boy and the fan. It’s always fascinating to hear what people are actually experiencing.

    In this spirit, I’d highly recommend checking out Daniel Tammet’s blog and books if you’re interested synaesthesia as explained by someone with it rather than someone outside looking in (who may have no personal experience of synaesthesia). His theories about language and synaesthesia are fascinating, and grounded in a very intimate knowledge that’s combined with Tammet’s remarkable intelligence.

  8. lizkat says:

    I have synesthesia for numbers, letters, words, dates, music, etc. Until I heard about synesthesia I just assumed everyone had it. I was really surprised to find out people can see letters and numbers in black and white! How boring!

  9. lizkat says:

    And I don’t know if this is related to synesthesia in any way, but I have always had very strong preferences for some letters, numbers, words, and names over others. Maybe everyone has that, I don’t know. For example, I have always liked even numbers much much better than odd numbers. The number 8 is just so much more likable, to me, than the number 5, for example.

    Another thing I have noticed is that their are classes of letters and numbers that share a similar color range, and this seems to be somehow connected to their sound and their shape. For example, B, C, D and G are all similar in color, and so are M, N and W.

  10. stargazer9915 says:

    To CrookedTimber:

    I have no artistic ability beyond that of stick figures or mary had a little lamb on the piano but when I make a mental image of numbers, days, months, countries, or other such groups, I see them all in vivid colors. I do not think there is any correlation between synaesthesia and artistic ability.

  11. stargazer9915 says:

    I honestly don’t think synaesthesia is a phenomenon. Until recently I did not know that it was a “condition” or that it even had a name. Nor did I know that not everyone had color associations with numbers or letters or any other thing for that matter. Based on my own experience, it would seem that people who have this (for lack of a better word) condition do not know that it is not a normal experience and therefore never mention it to anyone. Just a thought provoked by this interesting article. Thank you Dr. Steve.

  12. tmac57 says:

    Like Fifi, I would recommend looking at the life of Daniel Tammet as chronicled in this biography ‘Born On A Blue Day’. He has done some fascinating paintings that represent the way he sees sequences of numbers, to give us non-synesthetes a tantalizing look into the way he experiences numbers.

  13. becca says:

    My synaesthesia is sight/sound, that is, I see sound as colors. They’re vague and insubstantial, but I can’t see very well if the noise is too loud. I was 13 before I realized that I was the only one in my family who could do this, and in my 20s before my parents believed me. Oddly, my children’s birth mother is mildly synaesthetic too (hers relates to colors and textures). Alas, neither of the children seem to have inherited that trait.

  14. kevinf says:

    Neat. I think that conversations of mechanism might benefit from considering LSD and other similar compounds. Compelling evidence tells me that this compound seems to grey the boundaries of various sensory inputs in something that sounds a lot like synaesthesia.

  15. Old Coyote says:

    I had a personal experience with synaesthesia when I was about 7 years old. Tastes would evoke colors. I wouldn’t see colors when I was eating but I would sometimes get this sense of a color and the urge to describe the flavor of something as a color. I would put something in my mouth and say, “this tastes blue.” Remembering the experience, now more than thirty years later, it was as like having the cognitive experience of looking at something blue without the sensory experience of it. The experience only lasted about a month and never came back.

  16. Watcher says:

    The Heroes chick can also manipulate the sound to create some type of physical distortion in the world around her too, one of the more interesting “powers” they’ve come up with … not to hijack the thread to much 🙂

  17. Tosh says:

    I find it to be something of a paradox that you in the most recent podcast talk about and make a point of how science is hyping itself in a way it can not possibly live up to, while you’re writing that perhaps we’re going to evolve heroe-like abilities. But hey, nobodys perfect. 🙂 (and yes I realize you’re not totally serious)

    Furthermore, it is confusing to write that nature is “experimenting” with new ways to do stuff. It suggest that there is a thought behind it.

    Well, I love your show and I don’t mean to be critical, altough sometimes I sense, in my opinion, that you are slightly off at the evolutionary speculations in your podcasts. Probably because I study biology myself. And it is absolutely not a big issue, most of the time you’re spot on, but I do hold you to a very high standard so the ever so slight error puts me off.

    But yeah, fascinating subject and keep up the good work!

    Greetings from Sweden

  18. Shamrock says:

    Great article Steve.

    I am currently studying under one of the authors of that published some interesting findings in Nature ( Mike Dixon. At Waterloo we actually have a discrete research centre for Synaesthesia.

    Another body of research I found interesting was looking at what stimuli are necessary to evoke the extra sensory response. The paper I mentioned presented stimuli at a ratio of 1 congruent trial to 3 incongruent trials in an attempt to isolate automaticity. They found that the Stroop effect is present even if the actual digit, 7 for example, is not presented but alluded to with the string 3 + 4 =?. The extra photism was acting upon the synaesthete even without the traditional stimulus being presented.

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