Why is the right side of our brain in control of the left side of our body?

27 June 2010

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Question

Why is the right side of our brain in control of the left side of our body, and vice versa?

Answer

We put this question to Roger Keynes, Professor of Neuroscience at the University of Cambridge...

Roger - Well that's a tough question. I mean, it's a "why" question and it's an evolutionary question, and answering "why" questions in biology is very difficult.

In this case, I think we have to go back several hundred million years to see the very first evolving animals with complicated nervous systems that are getting more and more sophisticated.

At some point, vision evolved and probably started becoming more important when mammals developed binocular vision, with their two eyes looking forwards from the front of the animal.

This arrangement means that when and animal is looking at something, each of the two retinas - one in each eye - are seeing the same thing, providing binocular 3D vision.

But the point there is that the lens of the eye inverts the image that forms on the retina, so in binocular animals like us, things seen to our left are sensed by the right half of our left eye, and by the left half of our right eye, and that's a product of the physics of the situation.

If each eye were to send all its nerves to just one half of the brain, the picture of the world on one side of the body would be split between the two halves of the brain. Instead, it makes much more sense for this picture to be fused in just one half of the brain by the crossing over of some of the nerves of the two eyes.

But more recently, people have been thinking theoretically about how can you wire up a clever brain just on first principles and they decided that, probably, it's useful to wire things crossed over simply because - for reasons we don't need to go into - it prevents or reduces wiring errors compared with if you wire things up just on the same side of the brain.

Diana - So wiring might be better off crossed, and binocular vision could be the cause of our brain alignments; and some theoretical work has indicated that a brain is actually more likely to wire itself up correctly during development if one half controls the opposite side of the body. On the forum, RD pointed out that the crossover is still present in eyeless animals and insects, while Diver John said that it could just be a quirk that all subsequent life forms have adapted to.

Comments

I had brain surgery to stop my seizures in 2011 & the 3 doctors messed up and took out my short-term memory lob.(and the Dr's that did it was only suppose to take out the size of a quarter, but he didn't it was the half a size of my left head.) And the seizures never stopped. so each time I have another seizure im loosing more, and more of my memory.. So I have been looking for someone to help me please!! and fighting to keep what memory I have left..Cause no one understands what im going through!..:((( so please help me ????

I wish i could help. Man, i really do. I see you reaching out, and that's a good thing. All I can think of is to keep trying; talking to your local doctors, new doctors... Sometimes it's like hockey: Success comes from "shots on goal", not necessarily a first, perfect shot. Shoot a lot. Good luck. I hope health gets better, i really do.

Get in touch with Citizens Advice Bureau. Tell them what's happened and you want to know your rights in getting this fixed surgically and/or compensation. Ask them to help you get a social worker to sort you through this. Through the citizens advice bureau (I would advise tip go into your local office) and your social worker, wow to the local MP in your area and explain what, where. When, why, who, and how; this happened. Address it with your details included and sign I hastily wait your reply.
Regards, your name.

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