Is Synesthesia A Brain Disorder?

By Neuroskeptic | April 21, 2015 5:08 pm

In a provocative review paper just published, French neuroscientists Jean-Michel Hupé and Michel Dojat question the assumption that synesthesia is a neurological disorder.

synaesthesia

In synesthesia, certain sensory stimuli involuntarily trigger other sensations. For example, in one common form of synesthesia, known as ‘grapheme-color‘, certain letters are perceived as allied with, certain colors. In other cases, musical notes are associated with colors, or smells.

The cause of synesthesia is obscure. Many neuroscientists (including Hupé and Dojat) have searched for its brain basis. One theory is that it’s caused by ‘crossed wires’ – abnormal connections among the sensory processing areas of the brain.

But – according to Hupé and Dojat – the studies to date have failed to find anything, and the only conclusion we can draw from these studies is that “the brains of synesthetes are functionally and structurally similar to the brains of non-synesthetes.”

To reach this conclusion they reviewed 19 studies of brain grey and white matter structure (using MRI and DWI) in synesthetes, comparing them to people without the condition. They conclude

We did not find any clear evidence of structural brain alterations in synesthetes, either local differences or differences in connectivity, at least when considering the data with no a priori

More structural results exist in favor of the role of the parietal cortex in synesthesia. However… there was no consistency across studies about the precise anatomical location of which part of the parietal cortex was supposed to be involved.

Hupé and Dojat then considered 25 studies of brain activity in synesthetes, but, they say, these didn’t paint a consistent picture either. Considering the case of color synesthesia, for instance:

A few significant differences (in six studies) between synesthetes and controls were reported in the frontal and parietal cortex (whole brain analysis). When restricting the analysis to the visual cortex, only a few results (in five studies) were compatible with the involvement of color regions in synesthesia.

In other words, the majority of studies suggest that the experience of synesthetic colors is not caused by neural activity in the brain’s color-detecting cortex, which is inconsistent with the most straightforward version of the crossed wires idea. Hupé et al.’s own 2012 paper is one of the studies that didn’t find evidence of color cortex activity.

So what does this mean? The authors suggest that maybe synesthesia isn’t a brain condition at all:

If none of the proposed structural or functional differences [claimed to exist in synesthesia are] confirmed, this would speak against synesthesia being a neurological condition. But, then, what could be the nature of synesthesia?

This is where it gets a bit more speculative. Hupé and Dojat suggest that the source of synesthesia may lie in childhood memories. On this view,  synesthesia would have a neural basis, but only in the trivial sense that all memories do. The authors suggest that grapheme-color synesthesia, for instance, might represent a kind of vivid memory of colored alphabet blocks or fridge magnets. They admit, however, that there’s not much direct evidence for thus yet.

Childhood toys also can’t easily explain other more abstract associations, e.g. between sounds and tastes. The authors suggest that the “creative mind of children” sometimes constructs these synesthetic patterns of associations. They cite some attempts to “trace back the origin” of these patterns in particular cases of synesthesia. This seems to me a bit close to Freudian dream interpretation, to be honest – with enough effort, you can trace anything back to anything.

Overall, Hupé and Dojat make a convincing case that we’ve yet to discover consistent neural correlates of synesthesia using neuroimaging. It’s worth remembering, though, that neuroimaging is a blunt instrument and can’t tell us anything about the fine scale of brain organization – i.e about individual neurons and circuits. Synesthesia could yet be a neurological disorder, just one we lack the technology to understand. Even some severe forms of epilepsy are not associated with any brain changes visible to MRI.

Incidentally, dedicated Neuroskeptic readers may remember Hupé and Dojat from their 2012 paper on the effects of eye blinks on fMRI signals, that I blogged about as The Blinking Brain.

ResearchBlogging.orgHupé JM, & Dojat M (2015). A critical review of the neuroimaging literature on synesthesia. Frontiers in Human Neuroscience, 9 PMID: 25873873

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  • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

    The Diagnostic and Statistical Manual of Mental Disorders, ver. 5 is an overt Big Pharma shopping list. Psychiatry and its academic co-conspirators are aggressive hunter-gatherers for normal and Gifted (hence aberrant) traits to be labeled pathology, then treated. Everybody must be medicated and periodically examined, for everything! That is how healthcare is made affordable.

    If I mentally manipulate haversines in lavender, what business is it of yours? And who are you to stipulate I should be seeing them in nomadic taupe?

    http://www.dunnedwards.com/colors/browser/de6192

    Yeah, really. What kind of mental aberration does not see this as beige?

    • Mark Fergerson

      Everybody but the neurotypicals have to be medicated, don’t you know. How do you identify neurotypicals? They own drug companies.

      Also, that’s the schwa of colors.

  • Jeffrey Kline

    I have to wonder if the sample of synestheics are self selected when initially identified. Are they primarily people who have read an article on BuzzFeed and “discovered” their synesthesia? It’s not like we can confirm their internal experience.

    • Nacho Sanguinetti

      Aren’t there some type of test that can classify grapheme-color synaesthesia? For example present a cloud of letters or numbers(2’s and 5) in black font, with the distribution of 2’s making a certain macro pattern. I thought synaesthetes are able to distinguis this pattern with a shorter RT and with more discriminability than nonSyn.

      • Nacho Sanguinetti

        You can google image search ” Synaesthesia test” and see what i mean.

        • http://blogs.discovermagazine.com/neuroskeptic/ Neuroskeptic

          Yes. e.g. here!

    • Mark b

      I was not even aware of it until late teens (currently late 50s). I thought everyone saw the first letter of words and names as colors. I was astounded to find that not everyone did.

  • Nacho Sanguinetti

    I’ve always found this topic fascinating. I agree this crosswiring idea is geting a bit old. Specially considering that we now know that pure primary sensory cortices are active even when an animal is confronted with a different sensory modality. For example V1 in rats being activated and somewhat coding for somatosensory info (probably viceversa).

    http://www.pnas.org/cgi/pmidlookup?view=long&pmid=21876148

    I have the feeling we are slowly leaving the view of the brain as areas responsible for a certain functionality which are then connected, and more as a whole network interacting to achieve certain functions. Of course there are certain areas which tend to have more significance for a certain function, but this function is probably more distributed than we though.

    So, Just speculating.

    This makes me think that we haven’t found a difference in connectivity or activity, because the connectivity is there also at a high level in the control subject. Which probably means that synaesthesia could be in the pattern and strengths of the specific connections themselves. So, It does make sense that it is not hard wired at birth stuff but that is some sort of connectivity biased achieved during the strong plasticity window in our childhood.

  • Mark b

    I have always been a little frustrated that it is labeled as a disorder. I do not know how I could produce the music I do without it. When I am composing and playing music all my senses are engaged. I see the sounds I am making, feel the texture of each note, and in addition it is all taking place in a sea of wave forms and mathematics. I would not know how to compose any other way. It is not just music but it colors my entire world this way. I do not consider that a disorder!

    • SkyDriver

      Me too…I am also frustrated that it is being considered a thing to “have”, that it sets you apart from society, makes you “different” or “unique”. I feel like I’m the only one who feels this way, but I don’t think that synesthesia is that big of a deal. If it was, where would I fit in? I do associate colors with numbers, but not automatically, only if I really concentrate on it: would I be a part-synesthete then? See, this is where putting people into boxes and labels comes in, and it looks strikingly similar to the stigma that sometimes surrounds mental disorders…

      Edit: Also, I think that everyone has a little bit of synesthesia in a way…so why should it be considered that special or unnatural?

  • rsanchez1

    I wonder, for the example of synesthesia relating to musical notes, how synesthetes react when they listen to musical notes from alternate musical scales, like microtonal scales or non-western scales.

    • Mark b

      The new scales, for me anyway, take on new wave form and mathematical structure, and a slightly different color and tactile feel.

  • Matt Spaunhorst

    I’m skeptical of calling it a disorder as well. I’d be more inclined to strictly believe it as perception and nothing more. Color affects your perception of taste, why cant color affect your perception of numbers and letters? Also, synesthesia is shown in many 6 and 7 year olds. Some loose it as they grow, others have a stronger experience of it. So the speculative claim in the article of alphabet magnets or colored blocks could hold some merit if there is a way to dig deeper into it as a sort of perceptual learning experience.

  • Matt (82)

    Perhaps worth pointing out that the Wikipedia entry makes no mention of it being a disorder (http://en.wikipedia.org/wiki/Synesthesia ).

  • Catherine Bailey

    Well as a triplet, and with all 3 of us being synaesthetes, I have long since abandonned the nursery colour-blocks theory that colours are learned, interesting and compelling though it may be. My two (triplet) sisters and I were all brought up together, all have colour-grapheme synaesthesia, and yet all have different colours for our alphabet, numbers, days of the week, months of the year. I am also convinced that this is a genetic thing, since two out of three of my children and half my nephews and nieces report colour grapheme synaesthsia., not to mention the older generations. And this is not something that any of us picked up from the press or Buzzfeed etc by the way. This is something we were all conscious of feeling from early childhood (1960s in my case) – we ony learned that it had a name much later in life (the 1990s if I recall). We all took the synaesthesia battery test (http://www.synesthete.org/pretest_start.php) and scored in the high 90s in terms of percent, whereas apparently a non-synaesthete could never replicate even if they made great efforts to learn by rote their previous responses.

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  • Don Crawford

    Ever manifested form has color, sound, a soul and purpose. It is only those who are more spiritually advanced than others who can recognize these things. Most of us are too underdeveloped spiritually to be able to cognize the more subtle things in Nature

  • Franck Ramus

    Finding a brain basis has nothing to do with defining something as a disorder. The two questions are just completely independent.

    If synesthetes are cognitively different to some extent, they must be functionally different to some extent as well. It may not have been proven yet, but it must be the case unless one is a dualist.
    They may also be anatomically different, and still it may not have been proven, just as previous anatomical differences associated with autistic traits and dyslexia seem to be dubious (as exemplified in this other post and my comment: http://blogs.discovermagazine.com/neuroskeptic/2015/04/15/autistic-traits-arent-linked-to-brain-anatomy/#.VVHdzZNqVoE). So maybe the reason is the same, that people have used too rough analysis methods and have not looked at the right kind of anatomical traits.

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  • Christina Lobo

    I hear so many stories about color and aural synesthesia. Mine is Form Synesthesia. I see numbers, months, etc. in three-dimensional patterns.
    This must be more rare. Anyone else?

  • André Borges

    Its not a disorder for sure, the title gives that way like there’s a chance of Synesthesia being one! Its a thing just like being left handed! The best blog i’ve seen until now even with synesthete’s interviews is this one http://www.synesthesiaworld.com/synesthetes-explain-synesthesia-videos-youtube-compilation/

  • Jerry Mills

    I am inclined to say synesthesia isn’t a disorder; as it can be a learned trait. I have learned to experience a few types of synesthesia and taught some senior citizens to do the same with the use of mnemonic links. I just published a book teaching non-synesthetes synesthesia. The book is Synesthize and can be found on Amazon.com.

    • pandora

      I don’t think synaesthesia can be taught or learnt. Pairing two things together and learning them doesn’t encompass the full synaesthetic experience. I had what I would term real synaesthesia until I was 14. I actually saw vivid neon colours and shapes. There was also an emotional feeling of awe and wonder. I still remember the pairings 43 years later and am therefore classed as an associator. although I would disagree. The two conditions are very different, one is an actual experience (sensation) and one is a memory of an experience.
      I wonder if the Fisher Price magnet theory is bringing a lot of associators into research. Not a lot about the emotional content is mentioned theses days. In the 60s when I was a synaesthete I doubt there were many neon colours in the my environment, we didn’t even have a colour telly.

      Personally I think including associators in synaesthetic research without differentiating between the two will lead everyone down a blind alley.

      • Jerry Mills

        It sounds like it really depends on how much neuroplasticity can affect these parts of the brain at a later age. I have developed color grapheme synaesthesia / synesthesia in myself. The description of your synesthesia sounds very specific to you; as per the term suggests it is unique to everyone. The placeholder in my mind for the colors of my numbers and letters are very strong and have become a piece of knowledge which is my own. So we are talking semantics here as to whose synesthesia is acceptable.

  • Olivia

    I’ve had synesthesia for as long as I can remember. As a child, I would answer questions with colors or taste and I thought that was perfectly normal. Apparently not. Now that I’m in my 20s I understand synesthesia better. I’ve used my synesthesia with my work as a chef and musician. I can look at colors and smell and taste them. I can also hear music, although it’s only classical, and see notes as colors: I’ve been reading some of these comments and was wondering if it’s normal to have two forms of synesthesia at the same time like I do.

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Neuroskeptic is a British neuroscientist who takes a skeptical look at his own field, and beyond. His blog offers a look at the latest developments in neuroscience, psychiatry and psychology through a critical lens.

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