BIOMEDICAL SCIENCE 100 LAB - SENSES II: VISION
PUT REQUESTED INFORMATION (NOTED IN GREEN) ON RESPONSE SHEET
  1. MAPPING THE RELATIVE POSITIONS OF RODS AND CONES ON THE RETINA - "map" of retina.
    • Subject places Vision Disk against forehead and remains looking straight ahead throughout the experiment.
    • Tester puts the movable arm as far from the midline as possible. Randomly select either red, blue, or green paper and place a piece in the slot of the movable arm. The subject must not know the color of the paper.
    • Tester slowly brings the paper around toward the midline until the subject can see the object (but not necessarily the color of it). Record this angle as tne "Rod Field."
    • Continue bringing the paper around until the subject perceives its color. Record this angle as the "Cone Field."
    • Repeat the above steps until data has been obtained for red cones, blue cones, and green cones.
      colorangle at which object is perceived angle at which color is perceived
      red _______ degrees _______ degrees
      blue    _______ degrees _______ degrees    
      green _______ degrees _______ degrees

  2. DEMONSTRATION OF SUPERIMPOSITION AND THE “BLIND SPOT” - ophthalmoscope view | fundus interpreted
    • Subject holds the “blind spot test figure” about 18 inches from his or her face (“X” on the left).
    • Close your left eye, and look at the “X” with your right eye.
    • Move the test figure slowly toward your face until the dot disappears. If necessary, reverse the right/left orientation for all steps.
    • To demonstrate superimposition, roll a piece of paper into a tube. Look through the tube with your left eye with the tube angled slightly to the right. Put your right hand in front of your right eye, approx. one half of the way down the tube. If you seem to be able to see through your hand, then you have proven that your brain superimposes (combines together) the information received from both of your eyes.
    • If you were not able to experience the effect, then move your hand up or down along the tube, or try reversing the right/left orientation of the procedure.

  3. SNELLEN TEST (VISUAL ACUITY)
    • Face the Snellen eye chart from twenty (20) feet away.Cover your right eye.
    • Determine the lowest line (smallest font) that can be read with no errors. Tester records the number to the side of this line (the “visual acuity”).
      Snellen score: _____ / _____
    • A result of “20/15” means that at 20 feet, the subject can see the amount of detail that most people see at 15 feet.

  4. COLOR VISION
    • Types of Color Defiencies
    • Effect of various color combinations on vision.
      • Use face in upper right.
      • Upper button turns "room light" on or off.
      • Color buttons (at bottom) turn colored lights on or off.
      • Click upper buttom so that background is dark, then turn on blue light and shine on face. What can you see with the blue light only? Try again with all the lights.
    • Tester shows color test plate No. 5 to subject for three (3) seconds. Subject states the number seen and tester circles the response to the right of No. 5 on the Score Sheet. If two numbers are seen, the subject is to state the number seen more strongly (clearly). Repeat this procedure through Plate No. 14.
    • Add the number of circled responses in each column. A score of 8 or more “Normal” responses indicates normal color vision.
      Score - ___________ correct (without blue filter)
    • Repeat above procedure with the blue filter held before the face to simulate a red-green color vision defect.
      Score - ___________ correct (with blue filter)

  5. DETERMINATION OF THE VISUAL NEAR POINT - Amsler grid | Near points at various ages.
    • View a small object at arm’s length with one eye closed, then slowly move the object to the visual near point (the minimum distance at which the object can be kept in focus).
    • Measure the visual near point, in centimeters, of both eyes in both bright (outdoor) and dim (darkened room) conditions.
      in bright light in dim light
      right eye___________ cm. ___________ cm.
      left eye ___________ cm. ___________ cm.

  6. ACCOMMODATION (The “Model Eye”) - General Eye Anatomy.
    1. ORIENTATION TO THE MODEL EYE
      • Identify the parts of the model that represent the cornea and the retina with macula lutea / fovea centralis.
      • Slots 1 and 2 (anterior to the cornea) and 3 and 4 (posterior to the cornea) are provided for the placement of the iris and additional lenses.
      • Put the “iris” in Slot 3.
      • Pour water into the “eye" until it is nearly full.

    2. ABOUT LENSES - Normally the lens system focuses the image on the retina.
      • The refractive index (light-bending ability) of lenses is measured in diopters, abbreviated "D".
      • A lens marked with a +D rating is a positive (converging) lens. It is "convex" - its center is thicker than its edges. View a printed page with a positive lens. What is the effect of a positive lens on the apparent size of the print? How does a positive lens affect the light rays passing through it?
      • A lens marked with a -D rating is a negative (diverging) lens. It is "concave" - its edges are thicker than its center. View a printed page with a negative lens. What is the effect of a negative lens on the apparent size of the print? How does a negative lens affect the light rays passing through it?
      • Also note the difference between spherical lenses and cylindrical lenses. View a printed page with a cylindrical lens and rotate the lens while holding it parallel to the object. What is the effect of the cylindrical lens on the appearance of the print?
      • Go through steps a. through c. with a pair of eyeglasses borrowed from a classmate, if available. Are the lenses converging or diverging lenses? Spherical or cylindrical?

    3. ACCOMMODATION - Far Vision | Near Vision
      • Put the retina in the normal (middle) position.
      • Put +7 D lens in slot 4. This represents the crystalline lens of the eye.
      • Put the lamp at arm’s length or more.
      • Is the image inverted in any way? If so, how?
      • Move the lamp to about one foot from the “eye”.
      • Try placing different lenses in slot 1 to “re-focus” the image. What lens must be used in slot 1 to get the image back in focus? Identify the necessary lens as either positive or negative and state the diopter rating.
      • Remove the lens presently in slot 1. Move the lamp to approx. two inches from the cornea.
      • What lens must be used in slot 1 to get the image in focus? Identify the necessary lens as either positive or negative and state the diopter rating.

    4. HYPEROPIA (“FAR-SIGHTEDNESS”) - The retina is "too close" to the lens system.
      • Put the retina in the normal position.
      • Put the lamp approx. 8 inches from the eye.
      • Remove all existing lenses then put the +20 D lens in slot 4.
      • Move the light back and forth to determine the range in which it appears in focus (and record this range on your Response Sheet).
      • Move the light to the point at which its image seems to be in best focus.
      • Put the retina in the “hyperopic” (anterior) position. Is the image in focus now? If not, what is the new range of focus?
      • Return the lamp to a location approx. 8 inches from the eye. What lens must be added to slot 1 to get the image in focus? Identify the necessary lens as either positive or negative and state the diopter rating.

    5. MYOPIA (“NEAR-SIGHTEDNESS”) - The retina is "too far from" to the lens system.
      • Put the retina in the normal position.
      • Put the lamp approx. 8 inches from the eye.
      • Remove all existing lenses then put the +20 D lens in slot 4.
      • Move the light back and forth to determine the range in which it appears in focus (and record this range on your Response Sheet).
      • Move the light to the point at which its image seems to be in best focus.
      • Put the retina in the “myopic” (posterior) position. Is the image in focus now? If not, what is the new range of focus?
      • Return the lamp to a location approx. 8 inches from the eye. What lens must be added to slot 1 to get the image in focus? Identify the necessary lens as either positive or negative and state the diopter rating.

    6. ASTIGMATISM - Astigmatism test chart | asymmetrical lens
      • Put the retina in the normal position and remove the iris.
      • Put the lamp approx. 8 inches from the eye.
      • Put the +20 D lens in slot 3.The image should appear as usual.
      • Now put the –5.5 D lens (a cylindrical lens) in slot 4. This configuration simulates an eye with astigmatism. Note the effect of the cylindrical lens on the image.
      • In order to "correct" the eye with astigmatism, find another cylindrical lens from the set and place it in slot 1. It may be necessary to rotate the corrective lens to make the image clear. Why is the orientation of the lenses important in correction of astigmatism?

Cleanup
Turn off and unplug lamp.
Remove all lenses, dry with a soft tissue and place upright in storage box.
Pour water into sink and place model eye upside down on a paper towel to drain.