The Eye

First, a brief introduction to how vertebrate vision works:

The part of the eye that is responsible for seeing things is the retina. The retina is a thin, light-sensitive membrane that lines the inner-surface of the eye. It is analogous to the film in a camera in that images are projected onto it. The numerous photoreceptors on the retina (more than 125 million!) receives these images and in turn transmit them through the optic nerve to the brain in the form of electrochemical signals.

Anatomy of an eye

The two types of photoreceptors in the retina are:

• Rods: The rods are more numerous (120 million) and are more sensitive than the cones. However, they are not sensitive to color. Rods are able to function in low light and can create black-and-white images without much light.

• Cones : When enough light is present, the 6 to 7 million cones give us the ability to see color and detail of objects. Cones provide the eye's color sensitivity and they are much more concentrated in the central yellow spot known as the macula. In the center of that region is the fovea centralis, a 0.3 mm diameter rod-free area with very thin, densely packed cones.

The rods and cones transmit electrochemical signals to the ganglion cells in the retina, which in turn interpret these signals and send the information to the brain. It is important to note that blindness often occurs when photoreceptors are rendered inoperative due to common diseases such as age-related macular degeneration, which affect 10 million Americans, but ganglion cells or the optic nerve are almost never affected. In fact, in 1988, Dr. Mark Humayun at the University of North Carolina demonstrated that a blind person could be made to see points of light by stimulating the nerve ganglia behind the retina with electrical current. This served as the basis for the research that followed: scientists focused on creating a device that could translate images to electrical impulses- basically an artificial retina.


In the next post we will be focusing on the artificial silicon retina- one of the first artificial vision devices that was implanted into humans and have rescued vision in patients suffering from retinitis pigmentosis.

Some important terms...

- Retinitis Pigmentosis

" Retinitis pigmentosa is an eye disease in which there is damage to the retina...The condition may eventually lead to blindness, but usually not complete blindness. Signs and symptoms often first appear in childhood, but severe vision problems do not usually develop until early adulthood. The main risk factor is a family history of retinitis pigmentosa. It is an uncommon condition affecting about 1 in 4,000 people in the United States." (NIH)

- Age-related Macular Degeneration
"Age-related macular degeneration (AMD) is a disease associated with aging that gradually destroys sharp, central vision. Central vision is needed for seeing objects clearly and for common daily tasks such as reading and driving.AMD affects the macula, the part of the eye that allows you to see fine detail. AMD causes no pain.n some cases, AMD advances so slowly that people notice little change in their vision. In others, the disease progresses faster and may lead to a loss of vision in both eyes. AMD is a leading cause of vision loss in Americans 60 years of age and older." (Medicinenet.com)


- Legal blindness

 "In the US, the criteria for legal blindness are:  1) Visual acuity of 20/200 or worse in the better eye with corrective lenses (20/200 means that a person at 20 feet from an eye chart can see what a person with normal vision can see at 200 feet); 2) Visual field restriction to 20 degrees diameter or less (tunnel vision) in the better eye. "      

 (Medicinenet.com)