Vision and Vision Problems

3 O’s~ Optometrist, Opthalmologist, Optician  Corrections: Opthalmoscope: Instrument which illuminates eye interior—can see retina, optic disk, blood vessels—can detect diabetes, arteriosclerosis, optic nerve or retina damage  

Pathway of light through eye: Light Rays are refracted (Bent) as they pass thru things.

Structure of the Eye

Light Moving Through Eye

Pathway of Light Through the Eye Figure 8.9

Images Formed on the Retina Figure 8.10

Image seen at the back of eye

Refractive Power of Cornea and Humors is constant Refractive Power of Lens can change with shape of Lens. Ability of lens to change shape is called accommodation.   Lens is a biconvex structure.

Figure 8.4a

Lens Figure 8.7

It takes object viewed and reverses it left to right, turns it upside down and makes it smaller—this is called a real image.  

Image seen at the back of eye

Vision Problem Emmetropic Eye—lens accomdates properly—perfect vision. Myopic Eye (myopia)—near sighted/ trouble seeing far away. Lens too strong or eyeball too long. Distant objects fall in front of retina.

Correction of Myopia

Vision Problem Hyperopic Eye (hyperopia)—far sighted/trouble seeing up close Lens too weak or eyeball too short Objects fall “behind” the retina.

Emmetropic Eye—lens accomdates properly—perfect vision. Myopic Eye (myopia)—near sighted/ trouble seeing far away. Lens too strong or eyeball too long. Distant objects fall in front of retina.

Correction of Hyperopia

Vision Problem   Astigmatism—Unequal curvature of cornea or lens—

Results of Astigmatism Blurred vision—specially ground lens needed if severe enough. Example: football shaped lens Visual Acuity—sharpness of vision

Testing of Eyes Tested with snellen eye chart—series of letters of different sizes which can be seen by a normal vision eye at a specific distance

Snellen Eye Chart

20/20--Perfect Vision 40/20--What a normal person can see at 40 feet away, “X” can see it only at 20 feet away

20/20--Perfect Vision 40/20--What a normal person can see at 40 feet away, “X” can see it only at 20 feet away 20/10-- Sharper than normal

20/20--Perfect Vision 40/20--What a normal person can see at 40 feet away, “X” can see it only at 20 feet away 20/10-- Sharper than normal 200/20--Legally blind

Eye Prescription

Visual Fields and Visual Pathways Figure 8.11

Retina The retina is a light-sensitive layer at the back of the eye that covers about 65 percent of its interior surface. Photosensitive cells called rods and cones in the retina convert incident light energy into signals that are carried to the brain by the optic nerve.

Retina

Rods and Cones The retina contains two types of photoreceptors, rods and cones. The rods are more numerous, some 120 million, and are more sensitive than the cones. However, they are not sensitive to color.

Cones The 6 to 7 million cones provide the eye's color sensitivity The experimental evidence suggests that among the cones there are three different types of color reception.

The Fovea Centralis

Posterior Wall of Retina as Seen with Ophthalmoscope Figure 8.8

Optic Nerve The optic nerve is the cable of nerve fibers with carries the electrical signals from the retina to the brain for processing. The point of departure of that optic nerve through the retina does not have any rods or cones, and thus produces a "blind spot".

Blind Spot

Color Blindness Color blindness occurs when there is a problem with the color-sensing materials (pigments) in certain nerve cells of the eye. These cells are called cones. They are found in the retina, the light-sensitive layer of tissue at the back of the inner eye.

Most Common Color Blindness If you are missing just one pigment, you might have trouble telling the difference between red and green. The most severe form of color blindness is achromatopsia. A person with this rare condition cannot see any color.

Color Blindness Most color blindness is due to a genetic problem. (See: X-linked recessive) About 1 in 10 men have some form of color blindness. Very few women are color blind. The drug hydroxychloroquine (Plaquenil) can also cause color blindness. It is used to treat rheumatoid arthritis, among other conditions.

Color Blind Chart

Homeostatic Imbalances of the Eyes Night blindness—inhibited rod function that hinders the ability to see at night Color blindness—genetic conditions that result in the inability to see certain colors Due to the lack of one type of cone (partial color blindness) Cataracts—when lens becomes hard and opaque, our vision becomes hazy and distorted

Homeostatic Imbalances of the Eyes Glaucoma—can cause blindness due to increasing pressure within the eye Hemianopia—loss of the same side of the visual field of both eyes; results from damage to the visual cortex on one side only