As a physics student with very little understanding of biology, in a course about physics foundations diagnostic techniques I have come up with this question. I don't even know if it does make sense or not; in the latter case, please excuse me.

As far as I know, in the human brain the right hemisphere controls the motion of the left side of the body, while the left hemisphere controls the right side. Moreover, the part related to sight is on the back of the brain.

To me, it seems that there is a sort of opposition between the part of the body considered and the area of the brain that have to control it. Is this statement right? Only for human beings or also for animals? Are there any exception to this sort of pattern (if there is anyone)? Is there a logical reason to the fact that we show this characteristic and it couldn't be different from how it is?

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    $\begingroup$ See the linked answer, but a tl;dr is that this organization is very very old, as old as bilateral symmetry. In my opinion the best explanations are developmental ones, and the lack of any particular evolutionary pressure to untangle the connectivity. For most movements an organism makes, both sides of the body are involved. Even a reaching motion with one arm requires activation of postural muscles in the other flank to prevent you from tipping over, for example. $\endgroup$
    – Bryan Krause
    Commented Apr 20, 2017 at 21:37
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    $\begingroup$ As far as exceptions, on the sensory side olfaction doesn't cross, probably for anatomical reasons (the olfactory bulbs are part of the brain and reach out almost to the nostrils), for vision the crossing is not by eye but by visual field: your left visual field from both eyes goes to the right side of the brain; also, since the image is flipped onto the retina, that means the left side of the retina stays on the left, and vice versa. Lastly, the cerebellum does same-side control: information to the cerebellum from the rest of the brain has to cross over before entering the cerebellum. $\endgroup$
    – Bryan Krause
    Commented Apr 20, 2017 at 21:41
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    $\begingroup$ Another exception is hearing, the cochlear nerve enters the brainstem on the ipsilateral side and only downstream processing neurons cross the midline (and some descending projections from the cortex I believe). $\endgroup$ Commented Apr 21, 2017 at 9:25

1 Answer 1


"How" is easy to answer (signals from the right side of the body cross the midline and end up in the left cerebral hemisphere; answered well in a previous question). This pattern of wiring appears to have existed since bilateral symmetry first evolved.

"Why" is not a well-defined question; the possible evolutionary advantage gained by having cross-wiring is unclear.

To answer your explicit question: there is no "opposition" in any meaningful sense between the location of a sensory nerve on the body and the location at which the information is processed in the brain. There are a raft of visual areas in the neocortex, spanning from the back but extending forward. The areas that control eye movement are much more frontal ("frontal eye fields"). Before arriving in the neocortex, signals from the eyes pass through the dLGN, a nucleus in the thalamus which is a subcortical structure closer to the middle of the brain than the rear. This flow of visual information is similar across many (most? all?) mammals.


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