Vision

Vision is sight, the act of seeing with the eyes. In humans, sight conveys more information to the brain than either hearing, touch, taste, or smell, and contributes enormously to memory and other requirements for our normal, everyday functioning. Because we see objects with two eyes at the same time, human 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 (flat) image to the brain, which has the amazing ability to decode and interpret these images into a clear, colorful, three-dimensional view of the world.

Only about 10% of the light which enters the eye actually reaches the photoreceptors in the retina. This is because light must pass first through the cornea, pupil, lens, aqueous and vitreous humors (the liquid and gel-like fluids inside the eye) then through the blood vessels of the lining of the eye and then through two layers of nerve cells (ganglion and bipolar cells in the retina).

The entire scene projected onto the retinas of both eyes is called the "visual field."

Accommodation is the eye's ability to adjust its focus to bring about clear, sharp images of both far and near objects. Accommodation begins to decline around age 20 and is so diminished by the mid-fifties that sharp close-up vision is seldom possible without corrective lenses. This condition, called presbyopia, is the most common vision problem in the world.

Strabismus

Strabismus is seeing two images of a single object. Strabismus results from a lack of parallelism of the visual axes of the eyes. In one form (cross-eyes) one or both eyes turn inward toward the nose. In another form, wall-eyes, one or both eyes turn outward. A person with strabismus does not usually see a double image—particularly if onset was at a young age and remained untreated. This is because the brain suppresses the image from the weaker eye, and neurons associated with the dominant eye (ocular dominance) take over.

While the causes of strabismus are not fully understood, it appears to be hereditary, often obvious soon after birth. In many cases, strabismus is correctable. However, the critical period (probably to age six or seven years) involved in normal neuronal development of vision makes it necessary that the problem be detected and treated as early as possible.

Amblyopia, or lazy eye, is the most common visual problem associated with strabismus. Amblyopia involves severely impaired visual acuity, and is the result of suppression and ocular dominance; it affects an estimated four million people in the United States alone. One study suggests it causes blindness in more people under 45 years of age than any other ocular disease and injury combined.

Slight irregularities in the shape or structure of the eyeball, lens, or cornea cause imperfectly focused images on the retina. Resulting visual distortions include presbyopia (far-sightedness, or the inability to focus on close objects), myopia (near-sightedness, in which distant objects appear out of focus), and astigmatism (which causes distorted visual images). All of these distortions can usually be rectified with corrective lenses.