A question arose the other day that I’m researching. We’ve all heard that the left hemisphere of the brain controls the right side of the body, and the right brain controls the left side of the body.
Somewhere there is a crossover mechanism.
Assuming the crossover is located somewhere in the brain, where in the brain does this crossover take place?
I wondered if it specifically affected our eyes and ears. Is the left eye connected to the right hemisphere or the left hemisphere? Eyes and ears are so close to the brain and intimately connected to brain centers for processing images and sounds, I wondered if they were affected. Perhaps the crossover occurred beneath these brain centers…
So I googled left brain right brain crossover.
I found a site that explains how the eyes cross over. James Crook points out that when looking straight ahead, light from the right side of the visual field hits the left side of the retina in both eyes.
So it’s not like each eye corresponds to one hemisphere, either left or right. Both eyes feed visual information to both hemispheres. Our eyes are on the front of our heads, and we see stereoscopically.
The information from each eye comes together in the optic chiasma a few centimeters behind the eyes, where nerve bundles from each eye converge and then separate, going to the occipital lobe, so named because it’s near the occiput, the plate in the skull with a large hole through which the brain narrows into the spinal cord.
That bone at the back of your head that sticks out the most? That’s your occiput, and your occipital lobe is just inside.
In the optic chiasma, 45 percent of the nerve fibers from each eye cross over to the other side.
Crook states:
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.
Click the link above to view the image, and all will be clear!
Elsewhere I read that the crossover from the body to the brain occurs where the nerves enter the brain. In other words, at or near the occiput.
So there’s a crossover for the eyes (the optic chiasma) and a crossover for the body (somewhere near the occiput). I hadn’t suspected there were multiple crossovers!
Now my left brain is tired! To be continued…
The Well: bodymindheartspirit 
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there are multiple crossovers, which vary depending on which sensation or information they are conveying.
motor fibres (control movement) decussate (cross sides) in the brainstem (medulla), wheras some sensory fibres (touch, temperature, pain etc) do so much lower down in the spinal cord.
As for the brain itself, there are many brain centres which control the opposite side. and there are many which are found only on one side of the brain and not the other (such as the language centres). your example of vision was a good one although in this case it is not really the nerves which cross sides but the light as it passes through the front of the eye. For example light on the left side of the visual field shines onto the right side of both retinas and then the information is conveyed to the right side of the brain.. they nerves themselves dont cross. well.. most of them dont .. !
just to let you know that you could study this for years and it would still remain an enigma. such is the complexity of the human brain – even at a macroscopic level!
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Thank you so much for sharing this. It makes sense that there would be multiple crossovers.
I’m curious about the evolutionary purpose of crossovers. Is it about redundancy, so if there’s an injury, the nervous system can rewire?
I assume it occurs in all mammals. I wonder if it happens with other vertebrates, like birds and reptiles.
Fascinating topic!
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Our prof gave us a homework: ” In CVA patients, an impairment in the left brain is manifested as the paralysis/paresis in the opposite side and vice versa. what is the rationale?” I’ve been reading a lot of info about cross overs and I’ve just read the site by James Crook, and still I couldn’t find how the light passing through the retina has to do with movement control?? i mean, i still couldn’t find how it is related as to how the left brain controls the right side of the body and vice versa..though I read the question again, my prof didn’t specify WHAT opposite side, when it could be the body or the other brain hemisphere..IDK..my eyes are tired and I still have to study for my neuro exam tom.
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I think i found the answer..i think so =))
“Although the brain acts as a complete system, it is divided into two complex halves known as the right and left hemispheres. The right hemisphere specializes in controlling the movements of the left side of our bodies, while the left hemisphere controls the right side of our bodies. This occurs because the complex nerves that attach our brains to the rest of our bodies cross over as they enter the brain. In addition, research has shown that certain areas in each of the two hemispheres are responsible for controlling specific functions. The right sides of our brains contain centers that are associated with creativity, while the left sides control logic and organization. Our right-brains are artistic and imaginative. They are visual in nature and see things as a whole. They like music, colors, shapes, and help us with directionality and location. They allow us to recognize faces and are the center for our emotions and feelings. Our left-brains, on the other hand, control the centers for our speech, reading, and writing skills. They see the parts of a problem and analyze them in an orderly step-by-step manner. They categorize, reason, and are responsible for our mathematical abilities. The left-brains is our verbal and analytical side.”
from: http://www.highbeam.com/doc/1G1-17781828.html
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Thank you for sharing this, Kristina!
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Good luck on your exam, Kristina. Wish I could help…
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