Why can’t we control each of our eyes to see different things simultaneously?
Answer by Paul King:
The reason for this is that the human visual system is wired throughout for stereo vision.
Some animals, such as chickens, have an eye on each side of their head. For these animals, the eyes function fairly independently, just as our left and right hand function independently.
However animals with front facing eyes such as humans, monkeys, and owls, are wired at the lowest level for stereo vision. To make 3D stereo vision work, both eye movement control and the visual processing pathway is shared and unified across both eyes.
While the human eye muscles do function independently, there is a central brain circuit that controls eye movements. As a result, there is only one visual focal point for the whole brain, and both eyes track to it. That is why you can’t move the eyes independently.
But even if you could use mirrors to look in two different directions at the same time, your brain cannot process it. The very first step after the retina is a crossover point (the “optic chiasm”) which sorts all the nerve fibers so that the left side of both eyes goes to one side of the brain and the right side of both eyes goes to the other. At the first full processing stage, called the “primary visual cortex” or area “V1”, the image from both eyes are merged into a single image. From that point on throughout the rest of low-, mid-, and high-level visual processing in the brain, there is only one unified image.
If the eyes are forced to look at different things, what happens is that the V1 processing stage selects one or the other eye’s image to pass to the next stage (called V2). As a result, you will see only see the image from one eye at a time, and often it alternates. This is called “binocular rivalry,” and there are some visual illusions based on it.
The reason “lazy eye” is a concern in children is that when the eyes don’t overlap properly, the brain will favor one eye, which will keep getting stronger while the other eye gets weaker. There is an urgency to correct lazy eye before the “critical period” of vision ends, or one may grow up with limited or no ability to form 3D stereo images, even after correction. At the same time, the notion of a fixed critical period is controversial, see the story of Sue Barry: http://www.stereosue.com/