Vision is sight, the act of seeing with the eyes. Sight conveys more information to the brain than either hearing, touch, taste, or smell, and contributes enormously to memory and other requirements for normal, everyday human functioning.
Because humans see objects with two eyes simultaneously, vision is binocular, and therefore stereoscopic. Vision begins when light enters the eye, stimulating photoreceptor cells in the retina called rods and cones. The retina forms the inner lining of each eye and functions in many ways like film in a camera. The photoreceptor cells produce electrical impulses which they transmit to adjoining nerve cells (neurons), which converge at the optic nerve at the back of the retina. The visual information coded as electrical impulses travels along nerve tracts to reach each visual cortex in the posterior of the brain's left and right hemispheres. Each eye conveys a slightly different, two-dimensional image to the brain,
which decodes and interprets these images into a colorful, three-dimensional view of the world. The speed of the completion of this task is sensitive enough that it can be registered only on scientific equipment, rather than by human observation.
Because human eyes are separated by about 6.5 cm (2.6 in), each eye has a slightly different horizontal view. This phenomenon is called binocular displacement. The visual images reaching each eye's retina are two-dimensional and flat. In normal binocular vision, the blending of these images into one single image is called stereopsis.
Monocular stereopsis, or depth perception, is also available. For example, even with one eye closed, a nearby car will appear much larger than the same sized car a mile away. The ability to unconsciously and instantaneously assess depth and distance allows humans to move without continually bumping into objects, also providing eye/hand coordination.
Mary Bekker, The Gale Group Inc., Gale, Detroit,