The ProblemIt 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 thats what happens. Why?
A CrossoverThe 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.
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
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 isnt 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.|