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Mokele

Why the right/left hemisphere/body crossover?

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As everyone probably knows, in the human body, the right hemisphere controls the left side, and vice versa. A little while ago I was struck by a question: Why is this the case?

 

Is there some sort of efficiency gain, in spite of what I'd think? Or is it a retained primitive trait? If so, what other groups of animals have this, and why did it occur when it did? Is it the product of some sort of developmental/evolutionary constraint (if so, which), or is it genuinely adaptive?

 

In short, why does this crossover exist?

 

Mokele

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Its a question thats been bothering me as well, I'm right handed but left footed.

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Has that actually been proven????

 

I vaguely remember reading somewhere that that theory was / has been discarded....

 

Correct me if i'm wrong....If it's true i'de also like to know why the crossover exists.... ;)

 

I would like to think that the corpus collosum was put there for a purpose ;)

 

::edit:: Rakuenso you think you're screwed up?? I'm left eye dominant, left foot dominant, right handed while writing, and left handed when using a rifle or shotgun (as if eye dominance didn't tell you that already).

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I am left handed, left footed, but have a right dominating eye, which makes me close the right eye when shooting with a shotgun (Most people don't close any).

But anyway, the motions of the left side are still commanded by the right side. Speech centers are only on the left hemisphere, although in some cases the right can learn. Also, this crossover is not 100%, some senses are not crossed; unfortunately I missplaced my book "Maping the Mind" and can not check which are not crossed.

The real amazing thing though, is the case of "split brain" lessions, where right and left hemisphere have lost the comunication. The left arm, many times undoes what the right does; the right arm pulls your pants up and the left pushes them down.

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It is true, and easily proven. When people have a stroke on the left side of their head, detectable by MRI or CT scan, it is the right side of their body that is affected.

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YEah, MRI, and PET scanning have proved this too. But if there is an evolutionary advantage...who knows?

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I was always a fan of the "alien hand" experimentation. I have been intriqued about how the lack of communication between hemispheres via the corpus collosum causes the strange behaviors including not being able to identify certain unseen objects with one or another hand. Very strange, and very interesting indeed.

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I'm guessing that it goes back to early embryonic development, the blastula forms a sphere, then the gastrula is formed by one side of the sphere "caving in",

 

look here:

 

http://images.google.com/imgres?imgurl=http://www.nap.edu/readingroom/books/test_site/moore/images/p20007713g237002vpp.png&imgrefurl=http://www.nap.edu/readingroom/books/test_site/moore/chap10.html&h=263&w=381&sz=29&tbnid=SkSWPO9iehMJ:&tbnh=82&tbnw=118&start=25&prev=/images%3Fq%3Dneurula%26start%3D20%26hl%3Den%26lr%3D%26sa%3DN

 

The Neurula. The next prominent external change is the development of the nervous system. Approximately 40 hours after the beginning of development, the neural folds make their appearance on the top of the embryo (Fig. 10–17). These folds extend, as paired structures, from the blastopore region across the top of the embryo to a point where they join one another. These folds will eventually grow together, and in so doing they will form the neural tube. The neural tube will develop later into the brain and spinal cord. In the anterior region the neural folds are widely separated. This area will form the brain and the narrower posterior part will form the spinal cord. In Figure 10–17 the blastopore is not visible, being below the posterior edge of the embryo.

 

The growth of the neural folds is a rapid process and by 47 hours the folds are much better developed (Fig. 10–18). The section that will form the brain is clearly separated from the section that will form the spinal cord. The area between the folds is the neural groove. The embryo has begun to elongate.

 

 

I'm betting that gastrulation is the key. Does anyone know whether the crossover occurs in all vertebrates?

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Guest Viaggio

I haven't done any research into this "crossover" idea, but I figure it's mostly a matter of blood supply. Our circulatory system is not symetric.

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As everyone probably knows' date=' in the human body, the right hemisphere controls the left side, and vice versa. A little while ago I was struck by a question: Why is this the case?

 

Is there some sort of efficiency gain, in spite of what I'd think? Or is it a retained primitive trait? If so, what other groups of animals have this, and why did it occur when it did? Is it the product of some sort of developmental/evolutionary constraint (if so, which), or is it genuinely adaptive?

 

In short, why does this crossover exist?

 

Mokele[/quote']

 

So the second half of the question has not been answered definitively (but there are a ton of thories).

 

As for it existin in lower vertebrates...that is pretty much true. In vertebrates, most sensory modalities cross the midline to inervate the contralateral portion of the CNS. However, there are a few exceptions to this rule (the optic tectum is the one that most redily comes to mind).

 

This crossing gets even more complex when you consider the visual system. In the visual system right visual space is represented by the left hemisphere of cortex. This means that neurons from the retina that are on the left side of the retina do not cross the mid-line b/c they represent right visual space.

 

Even some invertebrates have crossed nervous systems. Some neurons in the legs of grasshoppers, for instance, cross the mid-line to inervate the contra-lateral portion of the body.

 

As to why this came up evolutinarily...I got nothin'

 

haven't done any research into this "crossover" idea, but I figure it's mostly a matter of blood supply. Our circulatory system is not symetric.

 

Why would you propose this?

 

Edit: I did not mean to say grasshopper, I should have said Fruit fly.

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I read somewhere (sorry can't cite it, it was quite a while ago) that studies have shown that lower order primates tend to be left-handed, and the ratio of right-handedness gradually increases as you get closer to the great apes and humans. Does anyone know if that's valid?

 

On the topic of the corpus callosum, a famous autistic savant (the model for Hoffman's character in "Rainman") named Kim Peek has been shown to lack that link between the hemispheres, and he can open a book and read each page with a separate eye, simultaneously and with 98% tested comprehension.

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