Vision Health Article

Definition

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.