Senses Lab: Sight How do our eyes work? Why do they work this way? What if they don’t work correctly?

Some Background How does vision work? Rods and Cones?

1 - Demonstrating the Blind Spot Hold the Blind Spot Card about 18 inches from your eyes. Close your left eye, and focus your right eye on the X, which should be positioned so that it is directly in line with your right eye. Move the figure slowly toward your face, keeping your right eye focused on the X. When the dot focuses on the blind spot, which lacks photoreceptors, it will disappear. Have your laboratory partner record in metric units the distance at which this occurs. The dot will reappear as the figure is moved closer. Repeat the test for the left eye. This time close the right eye and focus the left eye on the dot. 1- Demonstrating the Blind Spot P1 - Right eye: ________; Left eye: ___________ P2 - Right eye: ________; Left eye: ___________ What causes you to have a blind spot? (What is it?)

2 - Determining Near Point of Accommodation To determine your near point of accommodation, hold a sharpened pencil at arm's length in front of one eye. Slowly move the pencil toward that eye until the pencil image becomes distorted. Have your lab partner measure the distance from your eye to the pencil at this point, and record the distance below. Repeat the procedure for the other eye. 2 - Determining Near Point of Accommodation P1 Near point for right eye: ___________________ Near point for left eye: ____________________ P2 Near point for right eye: ___________________ Why might two people have different near points? What happens to the light rays hitting the retina that causes the blurring?

3 - Testing Visual Acuity Visual acuity, or sharpness of vision, is generally tested with a Snellen eye chart, which consists of letters of various sizes printed on a white card. The distance at which the normal eye can read a line of letters is printed at the end of that line. Have your partner stand the appropriate distance and as your partner reads each consecutive line aloud, check for accuracy. If this individual wears glasses, give the test twice – first with glasses off and then with glasses on. Record the number of the line with the smallest-sized letters read. If it is 20/20, the person's vision for that eye is normal. Give your partner the number of the line corresponding to the smallest letters. Repeat the process for the other eye. 3 - Testing Visual Acuity (without glasses / with glasses) P1 - Visual acuity, right eye: _______/ _______ Visual acuity, left eye: ________/_______ P2 - Visual acuity, right eye: _______/_______ Why do you think that people lose their visual acuity as they age?

This is the same chart (Snellen) that your doctor uses to determine your visual acuity.

4 - Testing for Astigmatism In patients who have astigmatism, a normally spherical shaped eye is instead slightly oval (“squished”). The astigmatism chart on the next page tests for detects in the refracting surface of the lens and/or cornea. View the chart first with one eye and then with the other, focusing on the center of the chart. If all the radiating lines appear equally dark and distinct, your refracting surfaces are not distorted. If some of the lines are blurred or appear less dark than others, you have at least some degree of astigmatism. 4 - Testing for Astigmatism – Is astigmatism present: P1 - in your left eye? _________ Right eye? _____________ P2 - in your left eye? _________ Right eye? _____________ What problems might a person with astigmatism have when trying to converge light rays onto the retina? What would happen to the images at which they are looking?

Uncorrected Astigmatism Astigmatism Test Example of Uncorrected Astigmatism

Background – Generic Eye Diagram Intrinsic Muscles Control the lens to allow it to focus Control the iris to allow more/less light in Extrinsic Muscles Attached to the outside of the eye moving it up/down and left/right

Eye Reflexes The convergence reflex mediated by the extrinsic eye muscles and the accommodation reflex mediated by the intrinsic eye muscles are described here. The photo-pupillary reflex protects the delicate photoreceptor cells from damage due to excessive light and which also involves the intrinsic muscles.

5- Accommodation Pupillary Reflex Have your partner gaze for approximately 1 minute at a distant object in the lab – not toward the windows or another light source. Observe your partner's pupils. Then hold some printed material 6 to 10 inches from his or her face, and direct him or her to focus on it. How does pupil size change as your partner focuses on the printed material? Repeat the previous experiment, this time noting the position of your partner' s eyeballs both while he or she is gazing at the distant and at the close object (a pen or pencil). Do they change position as the object of focus is changed? In what way? 5- Accommodation Pupillary Reflex Describe what happens to the diameter of the pupils. Explain the value of this reflex. Without it, what ability of the eye would be lost?

6 - Photopupillary Reflex Have your partner close their eyes. After about 1 min. have your partner remove his/her hands and watch what happens to his/her pupils. Describe what happens to the diameter of the pupils. 6 - Photopupillary Reflex How does pupil size change as your partner focuses on the printed material? Do they change position as the object of focus is changed? In what way? Explain the importance of this reflex. What damage may come to the eye if we didn’t have this reflex? Give a real-life scenario where you might observe the photopupillary reflex.

7 – Test for Color Blindness Were there any numbers you or your partner could not see? Which ones?

Colorblind? is the decreased ability to perceive differences between some of the colors that others can distinguish.

What do things look like to those who are colorblind What do things look like to those who are colorblind? (most common types of colorblindness) Normal   Protanope  Deuteranope  Tritanope

What do things look like to those who are colorblind? Regular Colorblind

Nothing to do with colorblind – but cool anyway!