2120: Brain Hemispheres

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Brain Hemispheres
Neurologically speaking, the LEFT hand is actually the one at the end of the RIGHT arm.
Title text: Neurologically speaking, the LEFT hand is actually the one at the end of the RIGHT arm.


Ambox notice.png This explanation may be incomplete or incorrect: Created by an AUTONOMOUS LEG and a CHICKEN. Needs some more links, and several of the explanations in parenthesis aren't needed. Do NOT delete this tag too soon.
If you can address this issue, please edit the page! Thanks.

As a general rule, each cerebral hemisphere innervates (feeds/supplies/controls) the contralateral (opposite side) portion of the body. So things on the left half of the body are controlled by the right side of the brain and vice-versa. As previously noted however, this is simply a rough approximation. Biology is complicated, and as with most biology "rules" there are exceptions.

A notable exception are the cranial nerves; some do not decussate (cross over) as would be predicted from the rule above and directly innervate the ipsilateral (same side) side. And of course, many cranial nerves innervate both ipsilateral and contralateral sides. This phenomenon is often seen, when everything is working properly, in things like the pupillary reflex.

While the motor and sensation aspects of nerve innervation have been relatively well-established from studies, experiments, and dissections, there is probably always going to be an exception, as you might imagine. Any number of factors may cause deviation from the normal physiology: trauma, disease, congenital birth defects, brain plasticity, etc.

With less concrete aspects of human brain function, such as logic, emotion, language processing, and creativity, establishing which brain hemisphere has control is obviously more complicated. Because a lot of these are higher order functions, establishing which hemisphere has control of which function are obviously more complicated. Due again to brain plasticity or other factors, different developing brains may grow to wire control of these functions differently. So while studies have established which hemisphere is more likely to be involved with which function, again mostly through knockout studies, these generalizations are not necessarily true for every individual.

Randall spoofs these by suggesting that the right brain instead controls the upper torso, while the left brain still controls the right side. The product of this partitioning in two dimensions gives four areas of the human body (upper left, upper right, lower left, lower right) and would eventually suggest that your left leg moves independently of your brain. To explain the areas of the body controlled by both halves of the brain, Randall declares those sections "disputed," echoing a note added on maps that must display a border which is part of a territorial dispute. This suggests that the halves of your brain fight for control of the region, and is also described similarly to two countries disputing territory. Alternatively, he states there would be cooperative shared control (Dual control) like in an airplane, where the pilot and the copilot both can control the plane with their respective yoke or stick at any time. His theory would explain, why most people are more skilled with their hands than their feet and with their right side than their left.

The title text further confuses the aspects of this decussation. The fact[dubious] that hands are an exception in neurology might be a confusion based to the common argument explaining the actual exceptions mentioned above, that a strictly divided control over body halfes would make coordinated tasks with both hands close to impossible.


Ambox notice.png This transcript is incomplete. Please help editing it! Thanks.
[Cueball is shown with the right half of his brain (on the viewer's left) colored in orange and the left half (on the viewer's right) in iris blue. An iris blue box is overlaid over the right half of the body (on the viewer's left), and an orange box is overlaid over the top half. The boxes are overlapping in a greenish color on the upper right quarter of the body (on the viewer's left).]
Neuroscience Fact:
[An arrow pointing to the iris blue rectangle on top with the text above:]
The left half of the brain actually controls the right half of the body...
[An arrow pointing to the orange rectangle at the right, the text reads:]
...while the right half of the brain actually controls the top half of the body.
[An arrow pointing to the overlapping area (the top left body from the viewers perspective) with the text below:]
Disputed/dual control
[An arrow pointing to Cueball's left leg area (on the viewer's right), not highlighted by any color, and the text is:]
This leg is fully autonomous

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https://en.wikipedia.org/wiki/Cerebral_hemisphere#Hemisphere_lateralization If the left side controls the top half of the body, wouldn't that mean it also controls the right half? 20:04, 6 March 2019 (UTC)

It is uncontroversial that many senses and motoric functions are swapped between the right and left side. The anatomy of the nerve swaps can also be shown. But it is still under discussion, why evolution led to this swap (source: Contralateral brain and the even better organized German version Kontralateralität des Vorderhirns) Sebastian -- 08:41, 7 March 2019 (UTC)
I'm referring exclusively to the hypothetical model posed by the comic; rather than the reality.

I don't think the sentence "all 3 claims are false" is accurate. I think the claim that the right side of your brain controls the left side of your body is accurate. It says so on the Wikipedia article mentioned and in several other sources. What the Wikipedia article disputes is whether or not "higher-level" functions are partitioned to one side of the brain. 20:29, 6 March 2019 (UTC) Harrison

With regard to the retina, the right half of the brain processes what the right half of each retina receives, and the left half processes what the left half of each retina receives (see e.g. optic nerve), but because our retina is behind the focal point of our lens so all the lightbeams cross and images hit the back of the eyeball upside-down and backwards, that means the halves of our brain process the opposite halves of what we see. But it's the same side of our body! I stopped learning neuroscience after we got to the optic nerve ;p 21:48, 6 March 2019 (UTC)
You must've stopped in the middle of the lesson, because the optic nerves split so that both brain halves get a copy of each eye. Your own link points it out in the figure as the "Optic chiasm".--Henke37 (talk) 13:09, 7 March 2019 (UTC)
If you look closer, you can see how the left half of each eye (retina) goes only to the left half of the brain, and same for the right, even though both eyes do go to both sides. The split is by what is seen, not which eye sees it, which specially maps to the left side or right side of the eyeball. 19:03, 7 March 2019 (UTC)
I think the "right side of your brain controls the left side of your body" is NOT accurate, it's just closer to truth than the reverse. Some parts of perception and motor control are divided that way, but unless you have corpus callosotomy the high-level control is centralized and/or distributed regardless the side. Would be hard to synchronize both hands if not. -- Hkmaly (talk) 02:05, 7 March 2019 (UTC)

Having survived an ischemic stroke on the left side of my brain, which temporarily paralyzed the right side of my body, this comic speaks to me like none other.

Is it possible that the title text is a reference to the whole "magnetic north is actually south" thing? (talk) (please sign your comments with ~~~~)

Completely rewrote the explanation. There are two key phrases that were removed with the following justifications:

"left arm and left leg, and vice versa for the left half of the brain, and competing theories such as these two attempt to explain why this is the case." Those are not competing theories. It is the abandonment of one old theory in favor of a one that is better supported with evidence! It's like saying "There are competing theories of what an atom is actually made out of. One says that they are the smallest indivisible part of all matter. Another says that they are like plum-puddings with with a positive charge particle studded with negative charge corpsucles. And the last theory of what an atom is is a core particle with positive charge, made of many many smaller parts, surrounded by an probability/statistical cloud of negative particles which can act as both particles and waves and cannot have their speed and location determined at the same time.

... And if you think these are all "competing" and equally valid theories of what an atom is... I can not help you. Please... I don't know. Take a class or read a textbook or something. Good luck. May God have mercy on you.

Similarly, many people incorrectly argue that different parts of the brain control logic and emotion, due to the importance of the left brain for language processing. Removed and replaced it with a better explanation. Not technically "incorrectly argue". There is basis for which the arguments are founded.

--Evilbob0 (talk) 03:12, 8 March 2019 (UTC)

The concern about the left leg being independent of the brain can be confirmed. Just scratch your left side, and watch your left leg kick. Keybounce (talk) 00:18, 9 March 2019 (UTC)

https://www.explainxkcd.com/wiki/index.php/212:_Brain coincidence? I prefer qwerty (talk) 11:38, 13 March 2019 (UTC)

I think maybe. Netherin5 (talk) 12:29, 13 March 2019 (UTC)

Conjoined twins analogous to "Disputed/Dual control"

I don't have time to properly research now, but could some conjoined twins be a real world example of disputed/dual control on some parts of their bodies? I think I read that in some cases they have to cooperate to control a shared (3rd) leg or similar things? PotatoGod (talk) 22:14, 11 March 2019 (UTC)