What is vision?
Vision is one of five special senses. Vision provides information about the environment and is important in balance. Loss of vision limits activities but is not life-threatening. “Low vision” is a term to describe partial vision loss that can be treated with visual aids such as magnifiers. The word “blind” is often used incorrectly. Criteria for legal blindness varies from country to country and is less than 20/200 in the United States. No light perception means that there is no vision at all.
The most common method to measure vision is the standard eye chart. A vision of 20/20 is considered normal vision. This means that at a standard distance of twenty feet, a person can read a specific line on a standardized eye chart. Vision of 20/40 means that the individual has to be at a distance of twenty feet to see what normally can be seen at forty feet. When the second number is less than twenty—for example, 20/15—then the vision is better than 20/20.
Other vision functions can also be measured. Color vision measures the ability to distinguish colors. Red-green color blindness is the most common color deficit. Contrast sensitivity measures the ability to distinguish shades of gray. It is possible to read 20/20 but to have reduced visual function, such as reduced contrast sensitivity, which would make it difficult to do activities in low light. Stereo vision is made possible by the two eyes working together. Stereo vision is important for tasks such as threading a needle. If vision is impaired in one eye, then stereo vision is reduced. If vision is similar in the two eyes but they are not aligned properly, then double vision (diplopia) results. Peripheral vision is the ability to see to the sides and is measured by visual field testing. Finally, the lens in the eye can focus but becomes weak with age. Focus ability allows the vision to transition between near and far. By forty years or older, the focus ability is usually weak enough that bifocal or reading glasses are needed.
The process of vision begins when light enters the eye. Light travels first through the clear central cornea, which is analogous to a car windshield. The cornea is approximately 550 microns thick at its thinnest central point. Corneal structure includes collagen connective tissue molecules that are aligned to allow for the cornea to be clear. Light rays entering the eye are parallel and must be focused to allow for vision to be possible. The corneal curvature begins the process of bending the light rays to bring them into focus on the retina at the back of the eye. The cornea provides the majority of focusing power for the eye.
After the cornea, light traverses the anterior chamber, which is filled with a watery substance called aqueous. The anterior chamber is the space between the iris (the colored part of the eye) and the cornea. Light will then travel through the pupil, the space in the center of the iris. The iris is similar to the shutter in a camera and will expand and constrict to let more or less light into the eye. The pupil usually looks dark because the light enters this space but does not exit. The exception is when light bounces off the retina and exits through the pupil, such as happens with red-eye effect from a camera flash.
Behind the iris is the lens. The lens focuses light onto the retina and allows for the transition of vision between far and near. The change in focus, called accommodation, decreases with age. After passing through the lens of the eye, the light enters the posterior chamber, which is filled with a clear gel called vitreous. The posterior chamber is the space between the lens and the retina.
The retina lines the back of the eye and contains approximately 120 million specialized cells called photoreceptors. Photoreceptors around the peripheral retina are called rods and are important for peripheral vision and night vision. The central part of the retina is called the macula, and at the center of the macula is the fovea, which is the most sensitive to color and provides sharp central acuity. The fovea contains approximately 35,000 photoreceptors called cones and no rods. There are about twenty rods for each cone.
The layers of the retina include the pigment epithelium, outer segments of the photoreceptor cells, outer nuclear layer, outer plexiform layer, inner nuclear layer, inner plexiform layer, ganglion cell layer, and axon layer. Underneath the retina is the choroid vascular, which provides blood flow to the retina. Retinal veins and arteries also nourish the retina. There are no blood vessels in the fovea.
When light strikes the retina, a molecule within the photoreceptors called rhodopsin undergoes a chemical structural change from a cis to a trans configuration. This hyperpolarizes the cell, causing an increase in negative charge. The cells within the retina have a hierarchy of on-center and off-center receptive fields that interact to distinguish gradations of light intensity and color. These electrical messages are passed from the photoreceptor cells to bipolar cells and then to ganglion cells. The ganglion cells then communicate with the visual areas of the brain via the optic nerve.
After light is translated into neural information by retinal cells, information from each eye travels through the optic nerve to the optic chiasm. At the chiasm, the two nerves meet and the central half of each set of nerve fibers crosses and joins with fibers from the fellow eye. The neural pathways continue in parallel to the right and left brain, along the optic tract to the lateral geniculate nucleus and to the primary visual cortex. The visual cortex provides an interpretation of the visual images from each eye and fuses them into a single image. A stroke to these areas of the brain will cause an identical defect to the opposite side of the vision in each eye. For example, a right stroke will cause a left field defect that will affect both eyes.
Each eye, also called the globe, is about twenty-four millimeters long and can distinguish light from three hundred nanometers (near ultraviolet) to two thousand nanometers (near infrared). The globes are positioned within the bony space within the skull called the orbit. Orbital bones are particularly strong around the orbital rim but very thin and weaker toward the back of the orbital roof and orbital floor. This feature protects the eye from trauma, as the orbital bones will fracture before the eye is ruptured. This is called a blowout fracture.
The orbit contains fatty tissue, connective tissue, the lacrimal gland (important for tearing), and the extraocular muscles. Six extraocular muscles are attached to each eye to provide the ability of the eyes to move synchronously. The inferior oblique, inferior rectus, medial rectus, superior rectus, iris, and lid are supplied by the third nerve. Injury to the third nerve can lead to diplopia, ptosis (droopy lid), and pupil abnormalities. The lateral rectus is supplied by the sixth cranial nerve, and the superior oblique is supplied by the fourth cranial nerve. The oblique muscles provide rotational movement. The extraocular muscles coordinate the eyes to provide a single image. An imbalance of muscles can cause diplopia.
The orbit is covered by the eyelids. To maintain clear vision, the cornea must be covered with a film of tears. If the cornea becomes too dry or swollen, then it will lose clarity. Disorders of the lacrimal gland and lid disorders can lead to dry eyes, which will impair vision. The lids, lacrimal gland, extraocular muscles, structures within the eye, optic nerve, and brain must all be working properly to provide normal vision.
Numerous diseases and disorders can affect vision. The most common disorders are nearsightedness (myopia), farsightedness (hyperopia), and astigmatism, which can be corrected by glasses and contact lenses.
A nearsighted eye is too long or the cornea is too steep, which results in the light from a distance coming to a focal point in front of the retina. The farsighted eye is too short or the cornea is too flat, and light from a distance will come to a focal point past the retina. Astigmatism usually means the cornea is not spherical but is shaped more like a football, with two different radii of curvature. In the astigmatic eye, light from a distance will have two focal points, which results in a tilted image. Refractive errors can be treated with glasses, contact lenses, or laser vision correction.
Uncorrected refractive error or other childhood conditions can lead to amblyopia, also known as a lazy eye. If these problems are not corrected before age seven, then the eye will not develop the proper neural connections and vision is permanently reduced. Treatment with surgery, glasses, or patching can help many patients with amblyopia.
Strabismus is also called a wandering eye or squint. Strabismus commonly develops in childhood and causes an eye turn. Adult causes of strabismus include stroke, thyroid orbitopathy, or trauma. Strabismus can lead to amblyopia in a child or diplopia. Prism glasses, patching, or surgery may be recommended.
The focus ability of the lens, accommodation, decreases with age, resulting in the need for reading glasses or a bifocal eyeglass for reading sometime after the age of forty. Other common eye diseases include cataract, glaucoma, and macular degeneration. Cataract is clouding of the lens and is treated by cataract removal (removal of the lens of the eye) and replacement with a clear lens implant. Glaucoma can be hereditary and involves the loss of peripheral vision due to damage to the optic nerve as a result of high eye pressure. Glaucoma can also occur with a normal pressure and, if not treated, will result in tunnel vision. Macular degeneration may be hereditary and can lead to the loss of central vision. Vitamins, injections, and laser treatment may sometimes be helpful in the treatment of macular degeneration.
Numerous other eye disorders and diseases can lead to a partial or complete loss of vision. Healthy lifestyle choices such as not smoking, eating a healthy diet, and exercising may help prevent many eye disorders. Eye examination by a qualified optometrist or ophthalmologist in childhood and routinely throughout life can aid in the early detection of eye disorders. Eye disorders can often be prevented or treated if detected early.
Millions of people are affected by eye disorders and injury, and billions of dollars are spent each year on such things as contact lenses and cataract surgery to correct these issues. Many eye injuries can be prevented with the proper protective eyewear for sports and other activities, such as using power tools.
Vision has fascinated scientists and writers since classical Greek writers theorized that vision was made possible by light rays emanating from the eyes. Other theories included a process by which images send copies of themselves to the eye. It was the work of Johannes Kepler, Galileo Galilei, Christoph Scheiner, and Willibrord Snellius (Snell) in the seventeenth century that began the true understanding of the vision process.
Some research suggests that many people are more afraid of going blind than of dying, and those with eye disorders such as an eye turn have lower socioeconomic success. However, there are many examples of success in spite of vision loss, including musician Stevie Wonder, artist Claude Monet, and activist Helen Keller. Fortunately, vision loss can often be avoided by proper eye protection and regular eye examinations.
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