ABOUT PERIPHERAL VISION: Peripheral vision refers to what we can see out of the corners of our eyes. The retina contains light-sensitive cells called rods and cones. The cones sense color and are found mostly in the central region of the retina. When you see something out of the corner of your eye, the image focuses on the periphery of the retina, where there are very few cones, so it's difficult to distinguish the colors of objects. Rods also become less densely packed toward the outer edges of the retina, reducing your ability to resolve the shapes of objects at the periphery. But our peripheral vision is highly sensitive to motion, probably because it was a useful adaptation to spot potential predators in the earlier stages of human evolution.
HOW WE SEE "DEPTH": The human visual system is designed to allow us to detect fine detail, track a moving object, see colors, and perceive depth. All these components of a visual scene are processed and merged by the brain so that we observe them as one visual experience. How we recognize that different objects are at different distances from us depends on visual cues. For objects beyond 100 feet, the image that's projected on to the back of the eye is basically the same size to both eyes, so cues of depth perception would include knowing the relative range of sizes of objects in general. If one object partly hides another, we know that the object in front is closer. And as we move our heads and bodies, nearby objects will seem to move more quickly than distant objects, an effect called motion parallax.
For objects closer than 100 feet, we need three-dimensional vision. Because the eyes are separated by about six centimeters, each eye gets a slightly different view of the same object. When we fixate on one object, we can tell if another object is in front of or behind it, because the object is located in two different places on the images that reach the retinas, or backs of the eyes. This is called disparity. Experiments have found that depth perception likely occurs in the primary visual cortex, where individual neurons receiving input from the retinas of the two eyes fire specifically when retinal disparity exists.
