The Man With Uncrossed Eyes

By Neuroskeptic | April 14, 2013 8:56 am

“GB” is a 28 year old man with a curious condition: his optic nerves are in the wrong place.

Most people have an optic chiasm, a crossroads where half of the signals from each eye cross over the midline, in such a way that each half of the brain gets information from one side of space. GB, however, was born with achiasma – the absence of this crossover. It’s an extremely rare disorder in humans, although it’s more common in some breeds of animals, such as Belgian sheepdogs.

Here’s GB and a normal brain for comparison:

Canadian neurologists Davies-Thompson and colleagues describe GB in a new paper using functional neuroimaging to work out how his brain is organized.

In the absence of a left-right crossover, all of the signals from GB’s left eye end up in his left visual cortex, and vice versa. But the question was, how does the brain make sense of it? Normally, remember, each half of the cortex corresponds to half our visual field. But in GB’s brain, each half has to cope with the whole visual field – twice as much space (even though it’s getting no more signals than normal.)

It turns out that the two halves of space overlap in GB’s visual cortex, as these fMRI results show:

The four colours represent the four quarters of the visual field, and the brain blobs that light up in response to them. Although the bottom and the top of space are separately represented, as they normally are, there’s complete overlap between the areas that respond to bottom-left and bottom-right stimulation, and likewise top-left and top-right. It’s possible that they are separately represented at a smaller scale, however.

Despite this, GB’s vision was remarkably good – he scored around 20/80 vision, one quarter as accurate as a typical person.

This is a fascinating case report, and vision neuroscientists will find much to ponder here. Still, what I’d love to know is how does it feel to have overlapping representations of the two sides of space? Does everything seem to be mirrored vertically? Does GB find it easier to tell objects apart when they’re above and below the other, compared to side-to-side?

ResearchBlogging.orgDavies-Thompson, J., Scheel, M., Jane Lanyon, L., & Sinclair Barton, J. (2013). Functional organisation of visual pathways in a patient with no optic chiasm Neuropsychologia DOI: 10.1016/j.neuropsychologia.2013.03.014

CATEGORIZED UNDER: fMRI, papers, select, Top Posts
  • http://twitter.com/zim2918 Lee Zimmerman

    I wonder what other neurological issues this person has. What brought them in to be scanned?

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

      The interesting thing was that GB was pretty much healthy otherwise. He had no brain abnormalities other than achiasma. He suffered from nystagmus (abnormal eye movements) however.

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  • petrossa

    I guess it feels normal. Same as being colorblind, you don’t know it till it gets pointed out to you

    • Ray Dillon

      I knew in 5th grade that I was color blind. i didnt know the word but when my mom bought me cheap crayons that didnt have the color name written on the side I had to ask the student beside me what color was the crayon in my hand. Just because someone hasnt pointed something out to you, or you havnt been given a diagnosis doesnt mean that you dont know that something isnt right with you. I knew I was attracted to men and not women at the age of 4 years old, long before someone pointed it out or even knew that there were other people like me. Just because its his normal does not mean that it feel right or normal to him.

      • petrossa

        At the age of 4 i didn’t realize there was a difference between boys and girls. You were some early bloomer.

  • JonFrum

    From an article in Neuron last year: “. Even in the presence of these large functional abnormalities, the
    effect on visual perception and daily life is not easily detected.”

  • Buddy199

    What does the subject say about it?

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

      I don’t know – the paper doesn’t say. Maybe GB will tell us in a follow-up paper…

  • Christel Platt

    well considering how many people do NOT get brain scans I would bet there are others just like “GB”-plenty of others….but with the disgusting cost of healthcare, insurances as well as Dr.’s rarely ask for this to be done. Ive suffered from migraines my entire life, seen many Dr’s and NEVER have I had this done,….smfh

    • m12345

      Just multiply two 3 digit numbers in your head every time you get a headache and it should disappear quickly.

      By articulating the neural activity into a difficult mental problem, you need to visualise mentally, answer spatially, and calculate mathematically, it should fire the brain and make the headache disappear. Must do it in your head though.

      Dont just sit there and go ouch that hurts…

      • http://twitter.com/whitefaerie Jennifer

        You do get that migraines aren’t just headaches, right?

      • http://twitter.com/whitefaerie Jennifer

        But congratulations on finding the cure for migraines! Math! Why has no one thought of it before!

        • m12345

          :) Yes I discovered this cure a long time ago. It will help everyone that has headache problems, assuming they can visualise problems and concentrate on the problem, it will in moments remove the headache completely.

    • JonFrum

      The vast majority of people who suffer from headaches have no reason to be given brain scans. it is highly probably that your doctors have done the right thing. If you really want a brain scan, save your money and ask for an appointment – anyone who can pay for one can get one.

      • http://twitter.com/whitefaerie Jennifer

        Migraines are a NEUROLOGICAL CONDITION. They aren’t just headaches. And no, actually, not anyone who can pay for an appointment can get one for something as specialized as a brain scan. Do you have no idea how our medical system works? Or I guess you’ve just never had to deal with a health problem more complex than a simple headache.

      • http://twitter.com/whitefaerie Jennifer

        Also, when someone specifically says they can’t afford something and your answer is just “save your money”? You do understand that not everyone HAS money to save, right?

    • http://www.facebook.com/magicsparks.hedgehogs Magicsparks Hedgehogs

      Have you seen a neurologist? I have something called Idiopathic intracranial hypertension which i have had for 25 years that causes varying severe headaches most days, i suspect my situation was kicked off by a car accident when i was 4 as memory of that day is clear, yet nothing else until i was 5/6 and thats memory of being in pain with headaches. People often ask me if its “migraine” that i have but its not. Due to local neurologist incompetance im still trying to get treated, but most neuro’s are good.

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  • sedeer

    Interesting post about an interesting patient. It’s hard to imagine what the world looks like to someone whose senses work so differently. It reminds me of something I’ve often wondered about: how creatures with lateral eyes — like chameleons or whales — see their surroundings.

  • jason barton

    Hi you all. GB has something called congenital seesaw nystagmus, which you can see with this link: http://www.neuroophthalmology.ca/cases/case94/case94.html. This is how most of these subjects come to attention. He only notes reduced vision, as do most subjects with congenital nystagmus of any sort.

    We are very interested in how vision is organized in GB. And yes, we are in the midst of designing some interesting visual experiments to find out how the left and right eye interact. So stay tuned!

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

      Thanks very much for commenting. I look forward to hearing more about GB – perhaps he could be the ‘HM’ of vision…

  • V. Versa

    “all of the signals from GB’s left eye end up in his left visual cortex, and vice versa.”

    So all the signals from his left visual cortex end up in his left eye?

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

      Oh, you know what I mean. Unless a pedant is you, and vice versa.

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About Neuroskeptic

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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