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.
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…