Why does the left brain control the right side of the body?

Posted by David

I’m pretty excited right now. I just found a minor confirmation of something I’ve mulled over recently. I’ve thought that the reason the right brain controls our left-side motor function and vice versa is due to the fact that we have eyes. See Why are vertebrate nervous systems crossed? 

To understand this idea think about how a pinhole camera works. The light comes through the pinhole and displays the exterior image flipped horizontally and vertically on the film. What was on the right is now projected on the left. It works the same way with our eyes. The pupil in the eye causes the field of view to be flipped when the image is projected on the retina. The left side of the body ‘thinks’ it is controlling the left side, but it actually controls the right, and vice versa. Here’s a nice synopsis of the theory.

So, why is this important? It says to me that feedback mechanisms drive the development of an organisms physiology. And as one might expect, crossed nervous systems are not specific to humans. Fish work the same way. When the early proto fish first developed eyes, it was visual feedback that caused its nervous system to develop in reverse. The evolution of physiology is a direct effect of sensory feedback. It’s not just some random gene mutation that caused the nervous system to cross. Sensory feedback caused the reversal. I expect that this is how all of evolution works. Species develop and emerge based on feedback.

One must then ask, how does this feedback get back into the genetics of subsequent generations?  I conclude that the individual’s genes itself are affected by the sensory feedback. They pass the trait to the next generation, because their genes have been modified by sensory feedback. In the case of a crossed nervous system, why would a gene mutation be preferred for development? There would seem to be no special reason why a species would be better equipped to have a reversed nervous system. 

Another interesting implication has to do with awareness, but as of yet, I’m not sure what to make of that!



Filed under evolution

7 responses to “Why does the left brain control the right side of the body?

  1. Wayne

    I have been wondering about this too and I don’t think it is because of vision. Lower vertibrates do not have binocular vision and what they see with their left eye is what is on the left side of their body. I think it has to do with some accident of evolution and embryonic development but I haven’t found more details yet.

  2. Which lower vertebrates are you speaking of?

  3. makayla

    Y did only 2 people reply

  4. Lionel Abril

    “It says to me that feedback mechanisms drive the development of an organisms physiology.”

    I believe so too, it ensures balance. The evolutionary process is not a product of accident.

  5. Yep. That’s what I get out of it also. In my mind, this particular issue shows that organisms develop purposefully.

    It seems that recently there has been renewed interest in Lamarkian thought on the subject, which is a breath of fresh air.


  6. manoj kanna

    why the right brain control the left side of the body and left brain the right what is the scientific reason for that

  7. robert moore

    this is Robert Moore and i have had a TB IaccidentI wou ld like more information on the left side controling the right side of the bosy thank you

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