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Why is the right side of our brain in control of the left side of our body?Why is the right side of our brain in control of the left side of our body, and vice versa? Perng, Texas |
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We put this question to Roger Keynes, Professor of Neuroscience at the University of Cambridge... Roger - Well that’s a 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 that 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. But 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. |
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June 2010 |
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The physical reason is that the nerves cross over each other inside the brain, and nerves on the right side of the brain cross to run down the left side of the body, vice versa.
- The Scientist - 24th Jun 10
Sure - but why has the body - very early in evolution, as evidenced by even very simple creatures having the same crossed neurological architecture - become wired up like this? Chris - chris - 24th Jun 10
Is the left-to-right thing true of all vertebrate brains or just the higher forms? Does the reptile brain (that mammal brains are a sort of bolt-on modification of) have this architecture? Maybe it's like a cable trunking issue - sometimes it's easier to go the long way round - peppercorn - 24th Jun 10
I did read that the image-reversal in pupil-type eye may be the reason. A reflection is necessary to correct this reversal and can be achieved by crossing the optic nerves. The motor control could then have conformed to this crossover necessary for vision: more efficient to have the vision and motor control of one side close together, thus causing motor control to be contralateral. However the crossover may just be a consequence of the way the neurology is packed, (I can't think of any external advantage of ipsilateral over contralateral motor control). - RD - 24th Jun 10
Peppercorn - it's true even in flies, let alone higher organisms! Chris - chris - 24th Jun 10
If the neurological crossover is to correct reversed images it should only occur in insects with apposition eyes, not superposition eyes ... http://en.wikipedia.org/wiki/Compound_eye#Compound_eyes The brain of this eyeless shrimp is a half-turn different compared with its sighted brethren ... http://icb.oxfordjournals.org/cgi/content/full/46/2/162 If crossover is to correct reversed images then the extra half twist in the neurology is not necessary for the sightless. - RD - 24th Jun 10
Ooops. Motor centres are low down in the brain & have (obviously really) been there from year dot for animal-evolution - Yes? So there must be another mechanism that favoured crossover. In fact, what evolutionary advantage was there to split left-right operation at all? - peppercorn - 25th Jun 10
I understand that the nerves crossover due to a quirk in development that has nothing to do with survival, and that we simply adapted to this condition. Other than this I can provide no evidence showing an advantage to having this odd feature. - diverjohn - 25th Jun 10
The big question is how this got wired up this way in the first place? We assume that small, primitive species with very few neurones to direct acquired it first and this led subsequently to the increasingly complicated systems we see emerging later. Chris - chris - 26th Jun 10
I think it's a consequence of the way organisms grow by cell division. The organism's growth radiates from a single cell. Imagine a very immature organism that only has four cells. Two of the cells are destined to be on the "brainy" end of things. The other two will be on the "less brainy" end. If the two "brainy" cells were diagonally opposite, the organism would tend to develop with each half of its brain at opposite ends of its body. Selecting adjacent cells for the "brainy end" overcomes this problem, but, as "more brainy" cell development has to keep pace and be connected through the nervous system with "less brainy" cells as they develop, the consequence is a diagonal connection. - Geezer - 26th Jun 10
This new paper talks about brain rotation as a result of selective breeding in dogs Roberts T, McGreevy P, Valenzuela M, 2010 Human Induced Rotation and Reorganization of the Brain of Domestic Dogs. PLoS ONE 5(7): e11946. doi:10.1371/journal.pone.0011946 It made me think -- perhaps the cross wiring was produced by a much more dramatic 180° rotation of a primitive brain in relation to the rest of the body. Maybe certain parts of the brain would have been better protected in this new position (?) - JackassPenguin - 1st Aug 10
A Straight line saves time, promotes symmetry and stronger construction. If the nerves on the right side needed to make a downward path, would it get bunched up and messy if they went straight down and took a turn to the right? Same with the left, front and back. Lots a things in nature displays symmetry why stop there. - tommya300 - 1st Aug 10
I seem to remember watching a documentary which said that way earlier in our evolutionary past someone had to have done a 180 with their head, and then of course evolved this way. The reasoning had to do with the fact that there is no conceivable advantage crossing the wires so to speak. I pictured maybe a rat or something who had to fend off air-borne preditors, but the fact that insects also have this, kinda suggests that it was WAY back.. Anyway, the documentary basically said that this was the only conceivable way it could have happened.. Either way, fascinating.
See the whole discussion | Make a comment- Michael Berg - 2nd Dec 10
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tough question. I mean, it’s a "why" question and it’s an evolutionary question, and answering "why" questions in biology is very difficult.
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