To start off: I'm not a biology student, but a computer science major

It has always been my understanding that humans have 2 eyes so that we can have 3D vision: the left eye see more of the left side of an object than the right eye and vice versa, this helps us to estimate depth among other things.

Now when I close one eye, I still am able to perceive depth: I assume this is because my brain fills in the blanks? For how long does this work? Do people eventually lose the depth perception (or at least is diminishes significantly) when they lose a single eye?

If so, how low does it take? If not, clearly we are capable of perceiving everything with one eye: why do we then have two (besides redundancy and a larger field of view? What is considering the evolution of man better in having 2 eyes as opposed to 1 or 3,4,..?

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    $\begingroup$ Here's an open-access article on monocular stereopsis that might be interesting: researchgate.net/publication/… .. Being said this may very well be more suited on cognitive science SE. $\endgroup$ – CKM Jan 14 '16 at 23:03
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    $\begingroup$ @Kendall +1 for neat article. But I disagree about migrating the question. $\endgroup$ – theforestecologist Jan 14 '16 at 23:11
  • $\begingroup$ I guess distance can be perceived with information from lens focal length, but that is not sufficient for 3D vision. $\endgroup$ – busukxuan Jan 16 '16 at 2:35
  • $\begingroup$ Try getting a friend to throw a ball back and forth with you. Then attempt to close one eye while continuously tossing the ball. I think you will be surprised by the results if they are the same as mine.. It does in fact, from my experience becomes extremely difficult to make the judgements needed for the task with only one eye. $\endgroup$ – user21343 Jan 22 '16 at 0:51
  • $\begingroup$ I can accurately understand 3-d structure of an object in hand, from various perspectives when I see from 1 eye. Isn't it normal? $\endgroup$ – Always Confused Oct 14 '16 at 14:41

It seems like you suffer from a misconception. "The left eye sees more of the left side of an object..." is not how distance perception works. Otherwise we wouldn't be able to estimate the distance from flat objects, such as traffic signs and shooting targets.

The actual mechanism is parallax estimation, or Binocular disparity. In a nutshell, the closer an object is to your eyes, the bigger the difference of its position on the left and right eye's retina will be.

You may perform a simple experiment: find a place where several parallel wires hang in the air: a train line or aerial phone/power line. Look at the wires normally, and they will appear just as black lines in the sky, with no distance perception. Now tilt your head to the side, and you will instantly gain a feeling which one is closer, and which one is further away. Why the difference? Because any shift of a horizontal wire gives the same picture, meanwhile for vertical wires the difference is obvious.

When you close one eye, your brain loses the ability to estimate parallax. However, it is left with several options:

  1. Visual cues. If one object overlaps another, it's obviously closer. If two objects look the same but one is smaller, it's probably further away (or a child). If you know the expected size of an object (a mountain/a horse/a fly), you may get the feeling of distance as well.
  2. Focusing distance. When you focus your eye on a near and far object, the feelings are different.
  3. Memories. You remember what you've seen with two eyes.

Of these, only (3) depends on previous binocular vision. (1) and (2) are available even to those who were born with one eye. However, parallax estimation is much more fast and precise. With a single eye you will be able to hit a fly on the wall, but catching it in the air will be extremely difficult.

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