Question of the Week

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

Sat, 26th Jun 2010

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Perng, Texas asked:

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


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

Roger -    Well thatís a Optic chiasmatough question.  I mean, itís a "why" question and itís 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 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 thatís 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, itís 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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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, Thu, 24th Jun 2010

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, Thu, 24th Jun 2010

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, Thu, 24th Jun 2010

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, Thu, 24th Jun 2010

Peppercorn - it's true even in flies, let alone higher organisms!

Chris chris, Thu, 24th Jun 2010

If the neurological crossover is to correct reversed images it should only occur in insects with apposition eyes, not superposition eyes ...

The brain of this eyeless shrimp is a half-turn different compared with its sighted brethren ...

If crossover is to correct reversed images then the extra half twist in the neurology is not necessary for the sightless.

RD, Thu, 24th Jun 2010

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, Fri, 25th Jun 2010

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, Fri, 25th Jun 2010

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, Sat, 26th Jun 2010

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, Sat, 26th Jun 2010

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 180į 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, Sun, 1st Aug 2010

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, Sun, 1st Aug 2010

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. Michael Berg, Thu, 2nd Dec 2010

The discrepancy between contralaterality of nervous system and ipsilaterality of most of our cranial nerves may suggest some explanations for this obscurity. Sunny, Tue, 1st Jan 2013

A reason for the brain crossover The most important sense organ of the human body is the eye. Thanks to the eye, we have a general picture of what lies ahead of us, crucial for survival. Thanks to the vision we can lead the movements in to the space and have a sense of where our body is collocated in the space. Visual information is perceived by the two retinas: the left and right retinas of the two eyes. The retina can be represented as a hemisphere with the convexity facing the back. So the upper hemiretina receives the image of the lower hemispace and the lower hemiretina receives the image of the upper hemispace (so the retina receives the image UPSIDE DOWN). Farther, nasal hemiretina (or medial) receives a portion of the ipsilateral temporal visual field and instead temporal hemiretina (or lateral) receives a portion of the controlateral nasal visual field. But in the optic chiasm the nasal fibers of left and right crossing them, so the optic tract of the left brings information of the visual hemifield of right and the right optic tract brings the information of the visual hemifield of the left. Then the information is carried to the medial two sides (right and left) of the occipital lobes which are divided into two parts by a horizontal fissure (fissure calcarine): the upper and lower lips of the calcarine fissure. The upper lip receives information from the lower half of the controlateral visual hemifield and instead the lower lip from the upper half of the controlateral visual hemifield (the visual image is UPSIDE DOWN just like in the retina), and all of the primary visual cortex is organized in an orderly and retinotopic way, and all areas of the brain are organized in this way. Indeed both the motorius and sensitivus homunculus are UPSIDE DOWN (their heads are down and their feet are up !!)!!!!!!! So since the right hemisphere of the brain has the information of the visual field of the LEFT then the same hemisphere must have the motor and sensory control of LEFT. Infact in this way the cerebral hemisphere connects in a orderly and precise way the different cerebral areas having the same somatotopic organization. In this manner in the brain everything is arranged in an orderly and precise way with the possibility of association between different brain areas with no possibility of confusion. This is the reason of the pyramidal tract motility and the streets of the sensitivity and the others are crossver. Giacomo Cariani, italian medical doctor, Sat, 1st Jun 2013

I think you mistaken. I believe the left half of the brain controls the left half of the body but its because the eye works like a lens and light from the left is captured on the right of the retina that we have our left and right perspectives mixed up. Pity I have no way to prove it. Furyan5, Tue, 5th May 2015

And yes I realise what that means. Our perception of reality is reversed. The sun rises in the west. When we want to raise our left hand the body raises the right hand. Lol. We see the world from inside the looking glass. Furyan5, Tue, 5th May 2015

I realized this question after I started to have cramps on my left leg. Then I thought that it may be my overweight pressing on my spine and resulting in stimuli in terms of cramps on my left leg. I noticed obviously that I was trying to shift my weight to right side and starting to lean on my right leg to get some relief while walking. Then I suddenly thought that this might be one of the reasons of intrinsic way of evolution to safeguard the integrity of the organism. There might be a damage to the left of the body including a trauma on the left hemisphere of brain where the right hemisphere takes over the control to initiate reflexes via motor neurons to avoid pain or further damage or pain being incurred to the left side limbs etc. Without the nerve reversal at the neck it could be difficult for the organism to initiate a reflex on the side where a potential total damage is imminent. I dont know if this is an overstreched idea but sounds in lime with evolutionary reasoning and does not require optical or eye related explanations. I would be glad if anyone could comment on this. Mech Eng, Sun, 15th Nov 2015 Anthony Barker, Sun, 21st Feb 2016

I've been wondering about this for a while now. My theory is the brain being crossed is a result of the entire organism turning inside out... Martin, Fri, 29th Apr 2016

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