Why is it so hard for humans to move their eyes in different directions? (Like chameleons)
In: Biology
ELI5: the human brain wants to focus very clearly on what it sees. In order to best do that, it uses both eyes to get a clear, sharp image. It has the ability to move the eyes independently a little bit, because the muscles closest to the nose are stronger than the muscles closest to the ears, that’s how people are able to do the “ cross eyed” look.
But If you’ve ever tried doing the “cross eyed” look, you probably notice a blurry double image.
Not focusing both eyes on the target doesn’t give your brain the information it needs to produce the clear image it likes.
Chameleons have their eyes on either side of their head. There is little to no overlap in the area each eye is able to see. They are independent in that regard.
Us humans on the other hand, have both our eyes on the front of our face, and the area we are able to see with one eye is almost the same as the area we are able to see with the other one. So our brain is hardwired to move the two eyes together so the two images line up correctly and combine the two images into a single one. There is little to gain in having the eyes be able to move independently since they’re mostly seeing the same thing anyways, but being able to have both eyes effortlessly move together to combine images and see in 3D and past obstacles like branches is pretty cool.
We have a system, in our brain (actually our brainstem), called [medial longitudinal fasciculus](https://en.wikipedia.org/wiki/Medial_longitudinal_fasciculus). It’s a nervous tract connecting the nuclei of the nerves that allow your eyes to move. By action of that nervous tract, those nuclei are wired in a way that when you move one eye to the left, the other eye moves automatically to the left too. It’s an automatic system, it’s just wired that way. That tract has other functions too, it allows us not just to coordinate eye movements with each other, but also to coordinate the movement of our neck with our eyes.
Chameleons don’t have the same system in their brains.
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