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EYE STRUCTURE and VISUAL TESTS

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EYE STRUCTURE and VISUAL TESTS Powered By Docstoc
					                                     EYE STRUCTURE and VISUAL TESTS
                                                 GENERAL EYE ANATOMY
The fibrous tunic is the outermost covering of the eye and is composed primarily of collagen proteins that form a dense fibrous
connective tissue. This connective tissue is similar in structure to a tendon and makes the eye a very rigid or fixed shape
container. The anterior portion of the fibrous tunic is called the cornea and the posterior region is the sclera. The sclera or white
of the eye is vascularized. The cornea is transparent because the collagen fibers in this region are more regularly arranged and
do not reflect light. The cornea lacks blood vessels and receives its nutrients from the aqueous humor; the cornea does some
gas exchange with the atmosphere. The cornea functions to aid the bending of light and in part, contributes to the formation of a
clear image. If the cornea is not shaped properly or if its transparency is lost, the image will not reach a single focal point on the
retina and blurred vision results.
The middle layer is highly vascularized, hence its name the vascular tunic. It is composed of three structures: the choroid,
ciliary body, and the iris. The choroid is a thin, darkly pigmented membrane rich in blood vessels. The pigment is melanin,
a blackish or brown pigment that absorbs light so it does not reflect within the eyeball. Cow eyes and many nocturnal mammals
have a reflective, almost metallic looking layer in part of the choroid, called tapetum lucidum. Reflection inside the eyeball will
distort the image - but the animal has a higher probability of stimulating visual receptor cells in the dark. The ciliary body is
composed of ciliary processes and the ciliary muscle. The tiny ciliary processes secrete aqueous humor. Aqueous humor
supplies nutrients to the lens and the cornea of the eye and then drains back into the cardiovascular system via small canals.
The ciliary muscle is made of smooth muscle cells in a ring or sphincter around the lens. This muscle is controlled by the
autonomic nervous system. The cilary muscle is connected to the suspensory ligaments that are directly attached to the lens.
The ciliary muscle and these suspensory ligaments thus function to control the shape of the lens. When the ciliary muscle
contracts, the muscle moves closer to the lens and releases the tension on the suspensory ligaments. The lens is then free to
shrink to its thickest shape. When the ciliary muscle relaxes, they move away from the lens and tighten the suspensory
ligaments. The lens is effectively stretched into a thin, flat shape when the ciliary muscle is relaxed.
The lens is a biconvex, transparent, avascular structure made of epithelial cells. These cells are highly organized so that light
passes through it, and the cells contain highly elastic proteins so its shape can be altered. When the lens is flat or thin, it bends
light less than when it has a thicker or wider shape. The ability of the lens of the eye to produce a sharp image on the retina is,
partially, a function of its elasticity. When focusing on close objects, the lens must be more spherical (convex), than when
focusing on distant objects. As an individual gets older, the lens gradually loses this ability to accommodate or adjust to near
vision. The lens is progressively less able to bend light, probably due to protein denaturation within the cells of the lens. The
iris is thinner than the ciliary body and partly overlaps the front of the lens. It is composed of circular and radial smooth muscle
fibers. In the center of the iris is an opening called the pupil. Light passes through this opening to reach the lens and the interior
of the eye. Through contraction or relaxation of its smooth muscle cells, the diameter of the pupil can be carefully regulated.
Responses of the iris are also controlled by the autonomic nervous system. The space behind the lens is filled with a clear,
non-replaceable, jelly-like substance called vitreous humor. This substance helps maintain intraocular pressure, the shape of
the eyeball, and also contributes to the focusing of light on the retina.
The innermost area of the eye is the nervous tunic or retina. It contains a series of photoreceptor cells, sensory neurons &
blood vessels. Cones are photoreceptors specialized for color vision. There are three types: red, blue and green cones. Each
type of cone is most sensitive to that particular wavelength of light. There is a small "yellowish" region in the center of the retina
called the macula lutea (yellow spot), Within the macula lutea is a small depression that contains cones (and only cones), This
smaller region within the macula lutea is called the fovea centralis. The fovea centralis represents the exact center of the retina
on a perfect visual axis of the eye. This location and the high density of cones insure that this region has the highest visual
acuity. Rods are photoreceptors specialized for black and white (contrast) or night-vision. These receptors work well in very low
light conditions when cones may fail to be stimulated. The density of rods increases as you move away from the fovea centralis
(again, no rods are in the macula lutea). In contrast, at the very edges of the retina, all cones may be absent.
Where the optic nerve leaves the eye, is a region of the retina that has neither rods nor cones and as a result, no perception of
light. This area is called the optic disc or blind spot. The sensory neurons in the optic nerve are wrapped with connective tissue
for protection and support, so the nerve feels very hard and strong.




BIOLOGY 119                                                                                                         AUTUMN 2010
                                                     COW EYE DISSECTION

Locate the parts listed below in a beef eye & know their functions. Use illustrations in your textbook and the handout for
guidance. Wear gloves to protect your hands. Each group should get a dissecting tray with scissors, razor blade, forceps, a
probe and a preserved beef eye. If the smell bothers you, rinse the eye with more water & move closer to an opened window.

•     Look at the external features first. These eyes have been packed tightly together, so the normal shape is rather distorted.
•     Locate the cornea. It will be rather cloudy or gray from the preservative.
•     The optic nerve will also be visible, but may be partly hidden by fat and muscle on the posterior side of the eye because it
      is packed with lots of fat for cushioning.

•     Make an incision with the scissors/razor blade about a quarter of an inch away from the edge of the cornea in the sclera.
•     You should be able to remove the lens & ciliary body simultaneously with the cornea afterwards.
•     Note the positional relationship between the lens and ciliary body. The ciliary body surrounds the lens.
•     Gently pull the lens free of the ciliary body. As you do this, you can see the suspensory ligaments "tear" free of the lens.
•     The lens is hardened and cloudy because of the preservative. Peel the lens like an onion. It is made of layers of long, thin
      epithelial cells. Once you have a small piece of the lens, note the elasticity of the cells.
•     After the lens is removed from the ciliary body, you can see the thinner iris that was in front of the lens.
•     The pupil is the opening in the center of the iris.

•     The posterior section of the eye is filled with vitreous humor. Remove this gelatinous material carefully.
•     The retina will be the thin, easily torn beige or tan layer attached to the sclera only at the optic disc. The rest of the retina is
      held in place by the vitreous humor.
•     Beneath the retina is a very thin black layer called the choroid that has a reflective zone called the tapetum lucidum.
      This reflective layer is absent in human eyes but it helps a cow see in the dark by causing light to reflect repeatedly across
      the retina. This reflection increases the number of rods that are stimulated, but the resulting images are blurred.
•     The thickest and toughest part of the eye is the sclera. Note how the sclera retains the shape of the eye, even when the
      eye is empty.

                                            Structures to Locate in Your Dissection
            Fibrous Tunic                                                               Cornea: anterior, transparent
 Outer layer: tough connective tissues
                                                                                        Sclera: posterior, white


             Vascular Tunic                                                             Iris: anterior, controls pupil diameter
    Middle layer: pigmented (melanin)
               vascularized                                                             Ciliary Body: middle, controls lens shape

                                                                                        Choroid: posterior, absorbs light
             Nervous Tunic                                                              Retina: posterior, has rods & cones
        Inner layer: neural tissue
                                                                                                   Fovea: highest visual acuity
                                                                                                   Blind Spot: exit point for neurons
            Vitreous Humor                         Lens: middle, transparent            Optic Nerve: posterior, exits eyeball
         Gelatinous, transparent;                   focuses light on retina
       in posterior chamber of eye




BIOLOGY 119                                                                                                            AUTUMN 2010
                                               Eye Anatomy

Identify these labeled structures that you saw in the eye model:


    A                                                                    D




B                                                                                E



                                                                             F
        C
                                                                             G


            H (fluid)




                                               J (gelatinous material)




    I




BIOLOGY 119                                                                      AUTUMN 2010
                                                    Eye Anatomy
Identify these labeled structures that you saw in the cow eye dissection:


 K                                                                              M




                                                                            N (specific place)
L (transparent gel)


                                                               S
  O (seen from the
  inside of the eye)                                                            T


P




Q
                                                                             U (iridescent
       R                                                                     portion of T)




       V

       posterior view,
       exterior of eye




BIOLOGY 119                                                                     AUTUMN 2010
                                                         VISUAL TESTS

A. Visual Acuity
•      We are using the Snellen eye charts placed around the room.
•      Colored tape on the floor marks 20 feet from each eye chart. You should stand at this line.
•      Cover left eye with your hand as you attempt to read the chart with the right eye.
•      Try to read the letters in the second from the bottom row that marks what an average person can read at 20 feet.
•      If you can read this row of letters correctly, you have what is called 20/20 vision.
•      If you can read the bottom row of letters, you have better than average vision (20/15).
•      If you cannot read all of these letters correctly, go to the next higher line with larger letters.
•      Continue until you can read all of the letters on a line. Record that line’s visual acuity value.
•      Repeat the process, covering your right eye & attempt to read the chart with your left eye.

                   Visual Acuity                                     Left Eye                               Right Eye
                  20/20 is normal


                        Snellen Eye Chart                                                 Astigmatism Chart




B. Astigmatism
•      Astigmatism is a small defect(s) in the refracting surface of the lens or cornea.
•      Even if you have normal "distance" vision, you may still be astigmatic. Each eye can have a different astigmatic pattern.
•      You can test your eyes using the astigmatism charts at any distance, but stand directly in front of the chart.
•      Cover your left eye & focus on the center of the chart using your right eye.
•      If all of the radiating lines appear distinct and equally dense, there is no deformation in your cornea or lens.
•      If some lines are blurred or if lines show differences in density (lighter or darker), then some astigmatism is present.
•      Repeat this process covering your right eye so that you may test the left eye for astigmatism.
•      Record below which lines appear to be blurred or out of focus, if any.

             Astigmatism                                  Left Eye                                       Right Eye

    Record the lines (axes) that are
            faint or blurred.




BIOLOGY 119                                                                                                       AUTUMN 2010
C. Near Point

•    Near point is a way to measure the ability of the lens to accommodate or refract light.
•    The lens loses this ability with age because of damage to the proteins within the lens cells.
•    The near point is the closest distance at which one can see an object in sharp focus.
•    Put a small lettered card in the slot on the near point ruler.
•    Move the slot so you start with the card 20 inches from your eye.
•    Hold 1 end of the ruler on your cheekbone below 1 eye, focus on the letters on 1 of these rows: sized "8, 9 or 10".
•    Close your other eye & gradually move the card closer to the open eye.
•    Stop at the closest point where the letters are still sharply focused.
•    The near-point ruler lets you measure this in several different units. - Use Inches.
•    Compare your value with the value expected for your age. If your values are not normal, do you know why?

                                               Near point ruler laid sideways




     Your Age (yrs)                                                          20 yrs     30 yrs      40 yrs      50 yrs     60 yrs

                                   R eye                   L eye
    Your Near Point
                                                                             3.5 in.    4.5 in.     8.6 in.    20.5 in.     33 in.
        Inches



D. Binocular vs Monocular Depth Perception
Monocular Perception: There are several visual cues that we try to use when we use just 1 eye to measure distances of
2 objects. We attempt to compare the relative larger size of 2 similar objects to decide which of 2 objects is closer. When our
heads move from side to side, objects at different distances move at a different relative velocity. Closer objects move "against"
the direction of head movement and farther objects move "with" the direction of head movement. In this trial you should keep
your head in line with the objects to see how well you can judge distances without this aid.
Binocular Perception: You should more accurately measure the distance between 2 objects when you use both eyes. Each
eye sees slightly different images because our eyes are approximately 2-3 inches apart. This gives us stereoscopic vision.


    Monocular                                    Monocular                                   Binocular
distance between                             distance between                            distance between
    bars (cm)                                    bars (cm)                                   bars (cm)
     Left Eye                                    Right Eye                                   Both Eyes




BIOLOGY 119                                                                                                     AUTUMN 2010
E. Ciliary Muscle Reflex

•       Place a square of wire screen about 5 inches in front of your eyes.
•       Look out the window at the trees through the screen, but keep your eyes focused on the screen. (The trees are blurry).
•       Then, close your eyes for a 15 sec. count.
•       When you open them again, immediately note whether the screen or the trees are in focus.
•       Do NOT give your eyes "time" to adjust - record the instantaneous response.
•       Were your ciliary muscles relaxed or contracted when your eyes were closed (they will be in the same state when you first
        open your eyes). Record your answer.

Your eyes when first reopened are at near or far focus?

Were your ciliary eye muscles relaxed or contracted when your eyes were closed?



F. Pupillary Reflexes

    1. Adjustment to Light

    Sudden exposure of the retina to a bright light causes immediate reflex reduction in pupil diameter in direct proportion to the
    degree of light intensity. The pupil can reduce to 1.5 mm in intense light and enlarges to 10 mm in complete darkness. This
    approximates a total difference in pupillary area of 40X.

    •     Select a subject with light colored eyes so that changes in pupil diameter are easily observed.
    •     Have the subject hold a piece of paper or their hand close to the forehead pointing forward, so the paper/hand separates
          the right & left eyes without covering either eye.
    •     Shine a light into the subject’s right eye, while watching the pupil in the right eye.
    •     Does the right pupil constrict (get smaller), dilate (enlarge) or stay the same?
    •     Next watch what happens to the pupil of the left eye when the RIGHT eye is exposed to that light.
    •     Did both pupils react in the same way? Why or why not?

                          Right eye received light
             Pupil diameter (constrict, dilate or no change?)
                          Left eye didn’t get light
             Pupil diameter (constrict, dilate or no change?)

    2. Adjustment to Distance

    •     Select a subject with light colored eyes.
    •     Have the subject look at something across the room so they are focused for far vision.
    •     While watching the subject's pupils, place a pencil or pen (held vertically) to within 5-6 inches of the subjects face and ask
          them to focus on the object as it gets closer & closer.
    •     The light has not changed so would you expect the pupil size to change?
    •     Record any change in pupil diameter when looking at a near object & then try to explain your result.



               Pupil diameter as object gets close to eyes
                     (constrict, dilate or no change?)




BIOLOGY 119                                                                                                          AUTUMN 2010
G. Reading & Peripheral Vision Tests
Peripheral regions of the retina are at a distance from the macula lutea. Most of the retina has more rods than cones, but at the
extreme edges of the retina there may be no cones at all. The distribution of cones (red, green & blue) varies across the retina,
although that is not visible in this graph.




•    Your TA will show you how to use a large visual field detector to measure the following 3 aspects of your
     peripheral vision.
•    Work in teams: You need at least 1 subject, 1 watcher & 1 person to move the ruler on the vision disk.
•    These tests work well if the subject sits with their right side along an empty space of the white board
•    Test just the right eye of your subject since these test take so much time.

                                                Vision Disk & Detail of Disk




•    Maximum Visual Range: Maximum peripheral vision. Record your results for the right eye, in degrees.
•    At what angle can you detect a black or darkly marked card?
•    These images will be fuzzy & indistinct because this region has only rods present.
•    Color Vision Range: At what angle can you correctly identify a particular color? Record your results, in degrees.
•    Don't let the subject know the paper color in advance. Try 2-3 different colors.
•    Does the angle vary with the color of the paper? Can you explain why?
•    Reading Range: At what maximum range would you prefer to read? Record your results, in degrees.
•    At what angle can you get a SHARP, CLEAR image and focus & read letters on the card?
•    This will be almost right in front of your face - in line with the fovea of the eye.

Maximum Range             Red Color             Blue Color           Yellow Color           Green Color          Reading Range
    “Black”                Range                  Range                 Range                 Range




BIOLOGY 119                                                                                                      AUTUMN 2010
H. Macular Degeneration Test

The Amsler grid is used to test individuals for macular degeneration. Eye exams should include this test if you are over 50 years
of age. The macula lutea is the region that contains & surrounds the fovea centralis. In macular degeneration, neurons within this
area begin to die off. The mechanism is not yet clear, but smoking & diabetes are two major risk factors besides aging.
     •    The table below mimics an Amsler grid.
     •    Hold this paper at a comfortable reading distance (1 foot) and follow these steps:
          — Cover your left eye.
          — Look at the black center of the table with your right eye.
          — Note any irregularities: Are all lines straight? Do some appear wavy?
          — Are there any blank spots?
          — Repeat the test with the other eye.




I. Blind Spot Test
     •    You can try this test at home if you do not have time in lab to try it.
     •    Hold the image below at approximately 18-20 inches from your eyes.
     •    Close your right eye and position the square directly in front of your left eye, so that the circle is even further off
          to your left.
     •    Focus on the square with your left eye. Do not let your eye wander back & forth between the circle & the square.
     •    Slowly move the figure toward your eye, keeping the square in line with your left eye.
     •    At some point the circle will disappear.
     •    You have found the blind spot for your left eye.
     •    Now close your left eye. Focus on the circle with your right eye.
     •    Position the circle so that it is directly in front of the right eye & the square is further off to your right side.
     •    Repeat as above to find the blind spot for your right eye.




BIOLOGY 119                                                                                                          AUTUMN 2010
                              These sites contain useful animations & illustrations.

Eye Anatomy
Bascom Palmer Eye Institute. 1997-2009. Eye Care: Anatomy of the Eye.
          http://www.bpei.med.miami.edu/site/disease/disease_anatomy.asp
Chudler, E.H. 2008. The Eye. Neuroscience for Kids. Univ. of Washington. http://faculty.washington.edu/chudler/bigeye.html
The Exploratorium. 2009. Cow’s Eye Dissection. Museum of Science, Art & Human Perception.
          http://www.exploratorium.edu/learning_studio/cow_eye/step01.html
Kolb, H., E. Fernandez, R. Nelson. 2009. Gross Anatomy of the Eye, Webvision, The Organization of the Retina & Visual
System. John Moran Eye Center, Univ. of Utah. http://webvision.med.utah.edu/anatomy.html
Blind Spot
Serendip. 2010. (updated 2007). Seeing more than your eye does. Bryn Mawr. http://serendip.brynmawr.edu/bb/blindspot1.html
Chudler. E.H.. 2008. The Blind Spot. Neuroscience for Kids. Univ. of Washington.
          http://faculty.washington.edu/chudler/chvision.html
Kaiser, P. 2010. The Joy of Visual Perception: Blind Spot (Optic Disc). York Univ. & National Eye Instit.
          http://www.yorku.ca/eye/blndspo1.htm

Color Vision & Peripheral Vision
Chandler, A. 2010. Color Vision Tests. Richmond Eye Associates. http://www.richmondeye.com/colortest.asp
Cooper, K. & C.J. Kazilek. 2010. Ask a Biologist: Seeing Color. Arizona State Univ.
          http://askabiologist.asu.edu/research/seecolor/
Exploratorium. 2010. Snacks. Peripheral Vision: We are not usually aware of our eyes' limitations. Mus. Science, Art & Human
          Perception. http://www.exploratorium.edu/snacks/peripheral_vision/index.html
Olympus Microscopy Resource Center. Physics of Light & Color. 2010.
          Human Vision and Color Perception. http://www.olympusmicro.com/primer/lightandcolor/humanvisionintro.html
          Ishihara Color Blindness test. http://www.olympusmicro.com/primer/java/humanvision/colorblindness/index.html

Visual Disorders:
Astigmatism
Kaiser, P. 2010. The Joy of Visual Perception: Astigmatism. York Univ. & Natl. Eye Instit. http://www.yorku.ca/eye/astigmat.htm
Chandler, A. 2010. Simulations of Eye Disorders. Richmond Eye Associates. http://www.richmondeye.com/simulation.asp
Cataracts
Action for Vision Eye Foundation. 2005. Cataract. http://www.afv.org.hk/ecataract.htm
National Eye Institute. 2010. Cataract: What you should know. http://www.nei.nih.gov/health/cataract/cataract_facts.asp
Glaucoma
Chandler, A. 2010. Glaucoma. Richmond Eye Associates. http://www.richmondeye.com/eyehealth_glaucoma.asp
National Eye Institute 2010. Glaucoma. NEI Health Information. http://www.nei.nih.gov/health/glaucoma/glaucoma_facts.asp
Macular Degeneration
Action for Vision Foundation 2005. Macular Degeneration. http://www.afv.org.hk/emacular.htm
Macular Degeneration Foundation. 2008. Macular Degeneration.
           Pictorials. http://www.eyesight.org/Macular_Degeneration/Pictorials/pictorials.html
           Adult Macular Degeneration. http://www.eyesight.org/Macular_Degeneration/Adult_MD/adult_md.html

Pupil Reflex
Montgomery, T. 2005. Pupil Reflex Animation. http://www.tedmontgomery.com/the_eye/parasymresp.swf

Visual Acuity & Accommodation of Lens
Kaiser, P. 2010. The Joy of Visual Perception: Refraction. York Univ. http://www.yorku.ca/eye/lensfn1.htm
Olympus Microscopy Resource Ctr. 2010. Interactive Java Tutorials: Human Eye Accommodation. Physics of Light & Color.
          http://www.olympusmicro.com/primer/java/humanvision/accommodation/index.html
Faculty of Medicine. 2003. Ocular Anatomy: Video of Accommodation. Ophthalmology Teaching Website. Univ. of Toronto
          http://icarus.med.utoronto.ca/meaghan2/JuHo/BandB/download/accommodation_sm.mpg

Depth Perception
Kolb, H., E. Fernandez, R. Nelson. 2007. The Perception of Depth. Webvision, The Organization of the Retina & Visual System.
John Moran Eye Center, Univ. of Utah. http://webvision.med.utah.edu/KallDepth.html



BIOLOGY 119                                                                                                    AUTUMN 2010
                                     Review of Experiments - Group Discussion
Visual Acuity
1. How do you interpret 20/20 vision?


2. What does 20/40 or 20/200 mean?



Astigmatism
3. What do astigmatic students see when they look at the astigmatism charts?



4. What caused the change in their visual perception compared to a normal student - i.e. what is different about their eye(s)?



Near Point
5. Graph the data in your lab notes for near point distances along a horizontal axis (X-axis) for age.
    Describe the shape of this relationship for the class.




6. Why does your near point increase with age?




Accommodation
7. Describe & compare the angles of light sent to your eye from a near object vs a far object. Why does this matter?



8. What do the ciliary muscles do when you are reading or looking at a close object? How does this affect the shape of lens?



9. What do the ciliary muscles do when you close your eyes or look at a distant object? How does this affect the shape of lens?



10. Why can you suffer eyestrain or "get tired" from prolonged periods of reading? What can you do to reduce eyestrain?



Pupillary Reflexes
11. What happens to your pupil size when you increase the light intensity? Why does it do this?



12. Explain how & why the pupil diameter changes with distance to the object.



Reading & Peripheral Vision
13. What is a normal maximum peripheral visual range?


BIOLOGY 119                                                                                                    AUTUMN 2010
14. Do we have accurate color detection in the peripheral parts of the retina? If not, what is missing from that region of
    the retina?



15. Name the part of the retina we use when we must read or see detailed images clearly & explain why we use this region.




Depth Perception
16. Why does your depth perception improve when you use both eyes (binocular) compared to monocular vision.



17. Describe 1 other visual cue that helps you detect the distance between 2 objects. You may not have used this cue in our
    tests.


Blind Spot
18. How does the brain "cope" with this absence of information?




Visual Disorders
19. Describe 2 of the common, preventable, risk factors for both cataracts & macular degeneration. Describe how these factors
    can affect your eyes. (You may have to look this up online).

     a

     b.

20. What part of your visual field is damaged by macular degeneration? Explain how your vision would differ from normal.



21. What part of your visual field is damaged by glaucoma? Explain how your vision would differ from normal.



22. Explain what it means when we say that our vision is “trichromatic”. What are the color pigments of the cones in our eyes?



23. What is different in the retina of a person who is color-blind? Explain how this can alter your perception of color(s).



24. What are the common causes of color-blindness? Who is most likely to be affected?




BIOLOGY 119                                                                                                       AUTUMN 2010

				
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