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Right Brain - Left Brain


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

The Problem

It seems odd that the right hemisphere of the brain should control the left side of the body and that the left hemisphere of the brain should control the right side - but that’s what happens. Why?

A Crossover

The diagram below contains the clues to one answer.

The lens of the human eye creates an inverted image of the world on the retina. Up and down are inverted, but more important to finding an answer to the question is that right and left are reversed too. Light coming from the right forms an image on the left side of each retina, light coming from the left forms an image on the right side of each retina.

Optic Signal Paths In each hemisphere the hind most part of the brain has a large region for processing images. One might expect the left hemisphere to take only information from the left eye and the right to take only information from the right eye, but that's not what happens.

For stereoscopic vision the information about images from the left and right eye need to be brought together. Nerve trunks from the two eyes carry the signals from each eye to the optic chiasma, a crossing over point located a few centimeters back from the eyes.

In the chiasma 45% of the nerve fibres from each eye cross over to the other side. The two nerve trunks which leave the optic chiasma carry respectively signals from the left of each eye to the left, and from the right of each eye to the right. Because the image on the retina was left-right reversed, the nerve trunk traveling to the left side of the brain carries information about the right field of view, and the nerve trunk to the right carries information about the left field of view.

It is possible to imagine the wiring being a bit different. With a rearrangement inside the optic chiasma the other 55% of the nerves could cross over instead. Alternatively a swap over of the two nerve bundles leaving the optic chiasma would again bring the images to the ‘correct’ sides. Either of these solutions would take up slightly more space. More fibres would have to cross over, and fibres would have further to travel.

Surely though any such increase in wiring to get the signals on the correct sides would be compensated for by reduced wiring later on? With information on 'the wrong side', there remains a need to cross signals back to the correct side for nerve trunks that control muscles.

Whatever the merits of the 'decision', evolution has left it till later to cross things over. At some point before the nerves leave the skull through the Foramen Magnum at the skull's base, left and right have apparently crossed back to their correct sides.

Because the lens crosses images over, there has to be a crossover somewhere. That much is down to optics and the requirements of vision. It isn’t clear though why evolution has made a choice to have the crossover close to exit from the skull rather than, by enlarging the optic chiasma, close to entry of signals at the eyes.

I  don't know the explanation for that, but I can hazard a guess. My guess is that it is connected with balance.

Movements on the right of the body will in general need some compensating movement on the left. This suggests that there may already be a need for some crossing over of muscle-controlling nerve fibres and sense fibres between the left and right - to give a simple automatic level of balance. If this is indeed the case then by reversing left and right, evolution has made the optical signal paths more economical without having to pay for it elsewhere.

James Crook, April 1998.


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Catalase Website IndexJamesTechno •Left/Right Brain •
"Right Brain - Left Brain" page last updated 5-July-2003