• Question: Why does the left part of our brain control the right part of our body

    Asked by Alice to Áine, Ciarán, Eoin, Lydia, Victoria on 11 Nov 2014.
    • Photo: Lydia Bach

      Lydia Bach answered on 11 Nov 2014:

      Hiya,

      That’s a difficult question and scientists are still not 100% sure about it.
      Whatever we see with our left eye gets processed in the right side of our brain. Some scientists think that by sending or wiring information across either side of the brain, there are less errors than just wiring it straight to the same side of the brain!

      Other scientists think it has to do with how the nervous system developed over millions of years when animals first got binocular vision (2 eyes) and how when an image hits your eye it gets reversed in the retina. That can be corrected by crossing the optic nerves. Over millions of years everything else – like motor control – conformed to that arrangement.

      And others again think its just an odd quirk with no reason behind it!

    • Photo: Ciarán O'Brien

      Ciarán O'Brien answered on 13 Nov 2014:

      Nobody seems to know for sure why this should be, but I did read an interesting paper from the journal Medical Hypotheses (basically thought experiments as oppose to “real” ones, but scientifically thought out and with a lot of sense to them) that might shed some light on this question. There’s a lot of complicated neuroscience stuff that I don’t understand, but the basic gist of the paper seems to go like this:

      It’s mostly vertebrates (all animals with a backbone) that have the left brain control the right side of the body, right? And you have to have a left and a right side for this to matter (it’s called bilateral symmetry by zoologists). Bilateral symmetry was knocking around long before vertebrates evolved, and a bunch of creatures before vertebrates had some version of eyes to see with, and these guys were fine with the right sides of their brains controlling the right sides of their bodies.

      The thing is though, the eyes of vertebrates do this weird thing where what we see is actually inverted as the light passes through the lens in our pupils. So the picture that hits the nerves in the back of our eyes is both upside down and back to front. So the brain and nervous system could have developed “back to front” in order to best make sense of the inverted picture our eyes see. If the right side of the brain mostly processes images from the left eye, then it would be most efficient for the right side of the brain to connect to the nerves and muscles on the left side of the body and control them too, and the same goes for the other half of your brain. Information wouldn’t have to bounce around your brain as it gets sorted as much, and so your response times and reflexes would be quicker, and your brain could process the most information for its size.

      There’s no real way to prove it, but it makes a lot of sense, and it’s a pretty good explanation for what we see in brains. 🙂

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