As other members of my class have posted before me, we are giving presentations on synesthesia. I will be presenting possible mechanisms of grapheme-color synesthesia. Grapheme-color syesthesia is where a person sees numbers or letters as specific colors. One possible mechanism is local crossactivation. In the thalamus is the lateral geniculate nucleus (LGC). The LGC is a sensory relay nucleus. It has six layers, and the fourth layer (V4) is specifically involved in color analysis. It also happens to be next to the visual word form area (VWF). The VWF is responsible for processing strings of letters. In fetal brains 70-90% of the connections made to V4 are from regions of higher brain function including the VWF. Whereas only 20-30% of the connections are made to higher brain function areas in adults. Knowing that, it has been hypothesized that a gene mutation could lead to the persistance of these connections. Including connections between V4 and VWF. Greater connectivity between the area involved in color analysis and the area involved in letter sequence processing could be a mechanism for grapheme-color synesthesia.


  1. MTran says

    When you have completed your research and presentations, I would love to read a synopsis of your findings and those of your classmates. It’s a fascinating topic.

  2. Norris Lurker says

    Was just watching an episode of the BBC comedy quiz show QI with some questions on synesthesia. In case you’re interested:

    Worth it just to hear Stephen Fry say “Listen to this piece of music by the Mamas and the [sic] Papas….”

  3. Hao says

    It’s generally agreed that synaesthesia is caused by some form of cross-activation: neurons in one sensory area cause neurons in a different sensory area to fire, and such an activation is typically much weaker in “normal” individuals. What you describe is actually the “defective neural pruning” hypothesis, where extraneous neural connections that are normally pruned during development fail to do so in certain areas for synaesthetes. An alternative, is of course, that extra neural connections form during development, possibly due to montessori-style teaching. Finally, it’s also possible that the number of connections isn’t the issue so much as the strength of the connection. So synaesthesia could just as easily be a result of strengthening an existing connection rather than forming a new one or preventing them from being pruned.

    Also, lateral geniculate nucleus -> LGN, not LGC.

    You may want to check out Ramachandran’s TED talk: he discusses grapheme->color synaesthesia for the last few minutes.

    And yes, the cool people do spell it with an “a”. :)

  4. synapse says

    I hate to burst your bubble, but you sound really confused. V4, the color-responsive region, is an area of cortex and is not in the LGN. The fourth layer of LGN does carry color information, since it is part of the parvocellular pathway, but so do the 3, 5, and 6th layers of the LGN. All the layers of the LGN, including the fourth layer, then project to layer IV of V1 in neocortex.

    In addition, you should do more homework on V4. There is some controversy about whether V4 specializes on color analysis. You should also do more homework on the “visual word form area” and how much evidence there is for the existence of such an area.

    One other point in your post bothered me. You say “In fetal brains 70-90% of the connections made to V4 are from regions of higher brain function… Whereas only 20-30% of the connections are made to higher brain function areas in adults.” This could be the result of either massive pruning of intracortical connections, as you imply, or the addition of more subcortical connections. You need to clarify your statement to make it clear that it’s pruning.

    That said, your hypothesis that synesthesia is based on a genetically encoded failure to prune connections is interesting, and I’d hate to see you fail on the details.

  5. Hank Fox says

    I occasionally experience certain smells as having textures — smooth, rough, grainy, chunky — and I’ve wondered if that constitutes mild synesthesia, or if everybody else has the same thing.

    Smooth – sweet things; rough – something like fresh-sawn wood; grainy – cigarette smoke; chunky – certain perfumes.


  6. speedwell says

    Hank, I’m not synaesthetic like that, but your associations make sense to me. For all it’s worth.

    Anecdotally on the side of “connections formed during development”: I learned to read really early (I was 2 and a few months when Mom caught me pointing at words in the newspaper and saying them out loud), and to this day it is impossible for me to speak a word or hear a spoken word without “seeing” it in my “mind’s eye” as if printed there. This affects me to such a degree that if I hear an unfamiliar word that’s not clear from context (such as someone’s name), I can’t properly “hear” it or recognize it again until someone tells me how it’s spelled or I (rarely) come up with some spelling on my own.

    Why, yes, I was a spelling bee champ. :)

  7. Mike Fox says

    Great topic. Synapse #6 is spot on.

    I remember reading something about how synesthesia only occurs in the left hemisphere. This does not sound like simple pruning to me, but something much cooler.

    Check out Nunn, JA, et al. (2002) Functional magnetic resonance imaging of synesthesia: activation of V4/V8 Nature Neuroscience 5(4): 571-575

    Bonus round: Compare and contrast the development of language to the development of synesthesia.

  8. Tom says

    I’m a synesthete. I see music (although not as some other “synesthetes” see it) and numbers have inherent colors, fonts, and sometimes personalities. It really is as interesting as it sounds… even more odd when you’ve been doing it all of your life and then one day, while talking about it, you realize other people don’t do that, haha.

  9. The Singing Orange Trapezoid says

    I must be synaesthetic. In my mind, all the posts on Uncommon Descent are “brown”.

  10. Barn Owl says

    Singing Orange Trapezoid, you sound like a West Texas highway construction crew with jackhammers, gravel trucks, asphalt mixers, and road graders. I don’t believe I’m experiencing true synesthesia, though, since construction cones and signage are usually bright orange. Even for similar sound/shape/color associations I’ve experienced with bird songs, I can’t be entirely sure that I haven’t seen a sonogram previously in a journal or field guide, the patterns of which might trigger a false synesthesia.

  11. Hank Fox says

    Speedwell, I have that “gotta have it spelled before I can hear it” thing too! Sometimes even people I know well will say a word I can’t make out, and I have to have them spell it.

    The reverse is that, most of my life, I’ve been weirdly good at spelling people’s names the first time I hear them. A friend of my brother’s once visited when I was about 12, and I asked him how he spelled his name. He playfully told me to guess, and I spelled it without hesitation “Allgeier.” He said nobody had EVER gotten it right. (Ah, past glories.)

    As to the smells with textures, those aren’t associations. Some smells actually FEEL that way to me, in my nose and throat. I can smell certain perfumes, for instance, and they feel like big chunky particles floating in the air, clumping together in my throat. Pfuh, maybe it’s just a mild allergy or something.