Why can (most) eyeballs be controlled to go in the same direction as well as cross-eyed, but not in separate directions?
In: Biology
Going cross-eyed has practical application, mainly focusing on things very close to your face. At straight ahead, you’ve got that infinite focal distance some cameras have for landscapes. But going “wall-eyed” doesn’t help you focus on anything, it only makes it harder to see. There’s no reason for your eyes to have ever evolved to be able to do that.
muscle wise, of course moving your eyes in opposite directions is possible. But the control of our eye muscles is only partially conscious, and the brain just doesn’t allow us to do it. I presume with heavy training early in life it might be possible, but very uncomfortable.
As a side note, if you damage one of the nerves controlling you eye muscles, your eyes will both be looking in different directions and you will see 2 separate images, that you brain will try to reconcile into a single image. I imagine this would give you a serious headache!
what i’ve always wondered is why some people can do other stuff, like i can cross one eye without moving the other, what’s the difference that makes it so i can do that when others can’t? am i missing something that’s supposed to prevent that or what?
Because looking right or left or at something close at the tip of your nose is something we do but we don’t look in both opposite directions at once. Animals that can look in opposite directions are usually herbivore/prey animals where that’s beneficial to see danger coming whereas stereoscopic vision, like we have where both eye face forwards, is generally how hunting animals have evolved as it helps with judging depth and makes us better hunters.
To facilitate depth perception. We achieve depth perception by having slightly different views in each eye and our brain “calculates” the difference between those 2 images to give us a sense of “depth” (how far away something is in relation to something else).
In terms of evolution, this was likely more useful for us than being able to move our eyes independently (and thus losing depth perception whenever they aren’t “synchronized”), or it was a good enough compromise that there wasn’t enough pressure to evolve something better.
Going cross-eyed still counts as going in the same direction (at least as far as maintaining depth perception goes), but optics being what they are, the more versatile an optical system has to be, the more complex it has to be. At some point you hit diminishing returns where increasing complexity doesn’t increase versatility that much; at that point it isn’t worth “investing” the resources needed for a more complex system.
Generally, evolution is a cost/benefit ecuation.
Other animals (usually prey animals) have far less depth perception since for them it was more valuable to have a wider field of view (by having eyes that don’t overlap, or not as much as our eyes).
Other animals do have independently moving eyes (like Chameleons) and benefit from both a wider field of view and depth perception when needed. Likely, for these animals, there was enough pressure to evolve this system, and therefore the benefit likely outweighed the cost.
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