Consciousness and how it got to be that way

Sunday, December 29, 2013

Reading Comprehension and Synesthesia

Literacy has strong similarities to synesthesia, and indeed is really a form of learned synesthesia. If you can read, you look at visual marks and automatically, involuntarily experience sound and meaning. Yes, these sense- and meaning-associations are initially learned, but they then become automatic. Even when you encounter marks that resemble characters in your language that are the chance result of natural processes - for example, a rock-arch that looks like the letter A - you can't look at it, and not think "A". (To understand what I mean by "automatic", go learn 10 characters from a writing system you don't currently know, and then find a website using that writing system. Understanding the characters is effortful, and by not concentrating, you can look at it, and not think of the sound or meaning.)

Since the serious study of synesthesia began with Francis Galton, it has also been noted that synesthesia runs in families, and that these families are enriched for artists and poets. This has led to the idea that the basis of synesthesia is some genetic influence resulting in insufficient cortical "pruning" in early life; extra fibers are left in areas like the fusiform or superior temporal gyrus, and this leads to color-grapheme or color-sound synesthesia. (An interesting implication is that infants and toddlers may actually all be synesthetes, prior to pruning.)

It stands to reason that if synesthetes are able to more highly associate sensory and meaning experiences, rates of dyslexia (if writing is a form of learned synesthesia) should be lower than in the general population. Doing a web search for this, I inadvertently found a synesthesia discussion forum where participants reported a higher-than-average rate of dyslexia. (Note, you won't have to rely on this dangerous foundation of anecdotal internet discussions for long; but in any event it was interesting that the possible correlation was the opposite of my expectations.)

Now along comes a new Ramachandran paper with David Brang (previously at UCSD, now at Northwestern) using made-up characters in varying colors. Grapheme-color synesthetes have a harder time learning new color-character associations than the rest of us. Extending to dyslexia, it's as if synesthetes' neuronal connections are richer but less trainable. Color-grapheme synesthetes report that it's unpleasant when real characters are printed in colors other than their "normal" synesthetic ones, much like they're constantly taking a Stroop test.


  1. This is a really fascinating area of study, and since I live near a research facility that does a lot of research into dyslexia, I'm highly tempted to run this by some of the researchers there.

    Anyway, I ended up perusing the ASA's recent abstracts and one of them seems to be potentially relevant to the synaesthesia-dyslexia link: Julian Ghloum1, Laura Gibson1, & Daphne Maurer - Less Perceptual Narrowing in Synaesthesia?
    The gist of it being that synaesthetes don't seem to have undergone as much perceptual narrowing, which would plausibly lead to them having worse ability to distinguish phonemes of their native language compared to everyone else, which would then feed into any tendency towards dyslexia.

  2. And a bit more digging got me this paper:
    Relevant quote: "Interesting candidate genes for neurological and psychiatric disorders and CNS development are also located in the regions of suggestive linkage. The region on chromosome 6 detected by both NPL and HLOD analyses has been strongly linked to dyslexia and specifically to difficulties in phonological decoding and orthographic processing,54,55 which is of particular interest given the importance of linguistic stimuli in synesthesia. Two genes in this region, KIAA0319 (MIM 609269)56 and DCDC2 (MIM 605755),57 have been proposed as candidate genes for dyslexia; both have been shown to play roles in neuronal migration, which has important implications for synesthesia given the evidence for altered neural architecture in the disorder"

  3. Thanks a bunch for being a more thorough researcher! I'll definitely check these out. The only dyslexia-associated gene I was aware of below was (interestingly) an axon-guidance receptor, ROBO1, that supports axon growth to and across the midline. It also has a pretty exciting connection to verbal working memory:

    Galaburda AM, LoTurco J, Ramus F, Fitch RH, Rosen GD (October 2006). "From genes to behavior in developmental dyslexia". Nat. Neurosci. 9 (10): 1213–7.

    Bates TC, Luciano M, Medland SE, Montgomery GW, Wright MJ, Martin NG (January 2011). "Genetic variance in a component of the language acquisition device: ROBO1 polymorphisms associated with phonological buffer deficits". Behav. Genet. 41 (1): 50–7.