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.
Description
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.
Function
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.
Ocular dominance
Studies strongly indicate there is a critical period during which normal development of the visual system takes place and environmental information is permanently encoded within the brain. Although the exact time frame is not clear, it is believed that by age six or seven years, visual maturation is complete. Animal studies show that if one eye is covered during the critical period, neurons in the visual pathway and brain connected to the covered eye do not develop to optimal performance. When that eye is uncovered, only neurons relating to the unrestricted eye function in the visual process. This is an example of "ocular dominance," when cells activated by one eye dominate the cells of the other. It is not an abnormal development.
Memory
The same way in which vision plays an important role in memory, memory plays an important role in vision. The brain accurately stores visual data which it draws upon every time the eyes look at something.
Electrochemical messengers
The entire visual pathway—from the retina to the visual cortex—is paved with millions of neurons. From the time light enters the eye until the brain forms a visual image, vision relies upon the process of electrochemical communication between neurons. Each neuron has a cell body with branching fibers called dendrites and a single long, cylindrical fiber called an axon. When a neuron is stimulated it sends chemicals called neurotransmitters, which cause the release of electrical impulses along the axon. The point where information passes from one cell to the next is a gap called a synapse, and neurotransmitters affect the transmission of electrical impulses on to an adjacent cell. This synaptic transmission of impulses is repeated until the message reaches the appropriate location in the brain. In the retina, approximately 125 million rods and cones transmit information to approximately 1 million ganglion cells. As a result, that many rods and cones must converge onto one single ganglion cell. At the same time, however, information from each single rod and cone "diverges" on to more than one ganglion cell. This complicated phenomenon of convergence and divergence occurs along the entire optic pathway. The brain must transform all this stimulation into useful information and respond to it by sending messages back to the eye and other parts of the brain before we are able to see.
Although the pupil regulates to some degree the amount of light entering the eye, the rods and cones ennable vision to adapt to extremes. Vision ennabled by rods begins in dim light. Cones function in bright light and are responsible for color vision and visual activity.
When light hits the surface of an object, it is absorbed, reflected, or passes through it. The amount of light absorbed by an object is determined by the amount of pigment contained in that object. The more heavily pigmented the object, the darker it appears because it absorbs more light. A sparsely pigmented object, which absorbs little light and reflects a lot of back, appears lighter.
Color vision
Humans have three types of eye pigments: blue, green, and red. This combination, the primary colors, composes every impression of colors for humans. Human color vision extends 30 degrees from the macula, and after that distance, red and green are indistinguishable. That occurs due to the fact that in the periphery of the retina only a few cones are present that detect motion. Because rods are present, the periphery cannot determine colors. For example, a red object that is brought closer from the periphery will at first appear colorless. When the object is moved closer, the eyes will eventually pick up the red pigment.
Perception of color is dependent on three conditions. First, whether people have normal color vision; second, whether an object reflects or absorbs light; and third, whether the source of light transmits wavelengths within the visible spectrum. Rods contain only one pigment which is sensitive to very dim light, and which facilitates night vision but not color. Cones are activated by bright light and let us see colors and fine detail. There are three types of cones containing different pigments that absorb wavelengths in the short (S), middle (M), or long (L) ranges. The peak wavelength absorption of the S (blue) cone is approximately 430 nm; the M (green) cone 530 nm; and the L (red) cone 560 nm.
The range of detectable wavelengths for all three types of cones overlap, and two of them—the L and M cones—respond to all wavelengths in the visible spectrum. Most of the light we see consists of a mixture of all visible wavelengths which results in "white" light, like that of sunshine. Cone overlap and the amount of stimulation they receive from varying wavelengths produces the vivid colors and gentle hues present in normal color vision.
Optic pathway
Only about 10% of the light which enters the eye reaches the photoreceptors in the retina. This is because light must pass first through the cornea aqueous, pupil, lens, and vitreous humors (the liquid and gel-like fluids inside the eye), the blood vessels of the lining of the eye, and then through two layers of nerve cells (ganglion and bipolar cells in the retina).
Visual discrimination
The retina has the ability to distinguish between visual stimuli, and the greater this ability, the greater the sensitivity in making such distinctions. The retina distinguishes visual stimuli in three ways: light discrimination (brightness sensitivity), spatial discrimination (ability to recognize shapes and patterns) and temporal (sensations) discrimination. Human temporal discrimination is limited. For example, this allows people to watch television without noticing the wavy lines that would distort the picture.
Optic chiasma
Vision functions in the brain are divided into two areas: the afferent (sensory) system and the efferent (motor) system. Synaptic transmission of impulses from retinal cells follows the optic nerve (an extension of the brain) to the optic chiasma, also referred to as the optic chiasm, an x-shaped junction in the brain where half the fibers from each eye cross to the other side of the brain. Consequently, visual information from the right half of each retina travels to the right visual cortex, and visual information from the left half of each retina travels to the left visual cortex. Information from the right half of our environment is processed in the left hemisphere of the brain, and vice versa. Damage to the optic pathway or visual cortex in the left brain—perhaps from a stroke—can cause loss of the right visual field. As a result, only
information entering the eye from the left side of our environment is processed, even though information still enters the eye from both visual fields.
Visual cortex
Each visual cortex is about 5 cm (2 in) square and contains about 200 million nerve cells which respond to elaborate stimuli. In primates, there are about 20 different visual areas in the visual cortex, the largest being the primary, or striate, cortex. The striate cortex sends information to an adjacent area which in turn transmits to at least three other areas about the size of postage stamps. Each of these areas then relays the information to several other remote areas called accessory optic nuclei.
Visual acuity
Visual acuity, keenness of sight and the ability to distinguish small objects, develops rapidly in infants between the age of three and six months, and decreases rapidly as people approach middle age. Optometrists and ophthalmologists test visual acuity during a routine examination, and poor acuity is often correctable with glasses, contact lenses, or refractive laser surgery. Visual acuity is highly complex and is influenced by many factors.
Retinal eccentricity
The area of the retina on which light is focused influences visual acuity, which is sharpest when the object is
projected directly onto the central fovea—a tiny indentation at the back of the retina comprised entirely of cones. Acuity decreases rapidly toward the retina's periphery, as well as the number of cones. Studies have indicated recently that this may result from the decreasing density of ganglion cells toward the retina's periphery.
Luminance
Luminance is the intensity of light reflecting off an object, and influences visual acuity. Dim light activates only rods, and visual acuity is poor. As luminance increases, more cones become active and acuity levels rise. Pupil size also affects acuity. When the pupil expands, it allows more light into the eye. However, because light is then projected onto a wider area of the retina, optical irregularities can occur. Two issues are key regarding pupil size: light to the retina—more is better, up to a point; and, whether or not the light is hitting the rods or the the cones—for example, with bright illumination, the pupil naturally constricts because only cones are stimulated and thus increase visual acuity. A very narrow pupil can reduce acuity because it greatly reduces retinal luminance; but a small pupil (for example, a "pinhole") will increase acuity in people with refractive errors. Optimal acuity seems to occur with an intermediate pupil size, but the optimum size varies depending on the degree of external luminance.
Accommodation
Accommodation is the eye's ability to adjust its focus in order to bring about sharp images of both far and near objects. Accommodation begins to decline around age 20 and is so diminished by the mid-50s that sharp close-up vision is seldom possible without corrective lenses. This condition, called presbyopia, is the most common vision problem in the world.
Role in human health
Human memory and mental processes rely heavily on sight. There are more neurons in the nervous system dedicated to vision than to any other of the five senses, indicating vision's importance. The almost immediate interaction between the eye and the brain in producing vision makes even the most intricate computer program pale in comparison. Although sighted individuals might seldom pause to imagine life without sight, vision is considered to be the most desirable of all human senses. Without it, a person's relationship to the surrounding world and the ability to interact with the environment, is considered seriously diminished.
Color-blindness
Approximately 8% of all human males experience abnormal color vision, or color "blindness" or deficiency. Women who experience color-deficiency will pass the X-linked recessive gene to any son, and each will be color-blind. Color-blindness is caused when one of the pigments in a person's photoreceptors is abnormal. Red deficient individuals are easier to categorize because that wavelength has minimal overlap with the other primary colors.
Various diseases and conditions can also cause color-blindness. These defects usually occur in one eye and can be intermittent, while congenital defects are present in both eyes and remain constant.
Strabismus
Strabismus is the condition whereby visualization of two images occurs when viewing a single object. This results from a lack of parallelism of the visual axes of the eyes. In one form (known colloquially as cross-eyes) one or both eyes turn inward toward the nose. In another form, (known colloquially as 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 occurs due to the brain's suppressesion of the image from the weaker eye, causing neurons associated with the dominant eye (ocular dominance) take over.
Amblyopia (known colloquially as 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 4 million people in the United States and is a common cause of blindness in younger people.
Strabismus appears to be hereditary, and is often obvious soon after birth. In many cases, strabismus is correctable. The critical period extends until a child reaches the ages of six or seven. It is involved in normal neuronal development of vision thus rendering it crucial that the problem be detected and treated as early as possible.
Other common visual problems
Slight irregularities in the shape or structure of the eyeball, lens, or cornea cause imperfectly focused images on the retina. Resulting visual distortions include hyperopia (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) and presbyopia. These distortions can usually be rectified with corrective lenses or refractive surgery.
KEY TERMS
Accommodation—The eye's ability to focus clearly on both near and far objects.
Cones—Photoreceptors for daylight and color vision are found in three types, each type detecting visible wavelengths in either the short, medium, or long, (blue, green, or red) spectrum.
Ganglion cells—Neurons in the retina whose axons form the optic nerves.
Ocular dominance—When cells in the striate cortex respond more to input from one eye than from the other.
Optic pathway—The neuronal pathway leading from the eye to the visual cortex. It includes the eye, optic nerve, optic chiasm, optic tract, geniculate nucleus, optic radiations, and striate cortex.
Rods—Photoreceptors which allow vision in dim light but do not facilitate color.
Stereopsis—The blending of two different images into one single image, resulting in a three-dimensional image.
Suppression—A "blocking out" by the brain of unwanted images from one or both eyes. Prolonged, abnormal suppression will result in underdevelopment of neurons in the visual pathway.
Synapse—Junction between cells where the exchange of electrical or chemical information takes place.
Visual acuity—Keenness of sight and the ability to focus sharply on small objects.
Visual field—The entire image seen with both eyes, divided into the left and right visual fields.
BOOKS
Billig, Michael D.; Gary H. Cassel; Harry G. Randall. The Eye Book: A Complete Guide to Eye Disorders and Health. Baltimore and London: The Johns Hopkins University Press, 1998.
Hart, William M., Jr., ed. Adler's Physiology of the Eye. St. Louis, Baltimore, Boston, Chicago, London, Philadelphia, Sydney, Toronto: Mosby Year Book, 1992.
Lent, Roberto, ed. The Visual System-From Genesis to Maturity. Boston, Basel, Berlin: Birkhauser, 1992.
Zinn, Walter J., and Herbert Solomon. Complete Guide to Eyecare, Eyeglasses & Contact Lenses, 4th ed. Hollywood, FL: Lifetime Books, 1996.
ORGANIZATIONS
The Lighthouse National Center for Education. 111 E. 59th Street. New York, NY 10022. (800) 334-5497. <http://www.lighthouse.org>.
National Eye Institute of the National Institute of Health. 9000 Rockville Pike, Bethesda, MD 20892. (301) 496-5248. <http://www.nei.nih.gov>.
