Chapter 8 - Vision

1. Chapter 8 - Vision

2. The electromagnetic spectrum • The range of electromagnetic radiation, from gamma rays to radio. • The portion we can sense with out eyes is a very small portion, called the “visible light spectrum.

3. Diagram of the Electromagnetic and visible spectrum

4. Characteristics of visible electromagnetic radiation (light) our eyes can detect • Wavelength – interpreted by the brain as color • Wave amplitude – interpreted by the brain as brightness

5. The eye

6. Parts of the eye and their function • Pupil - The black portion of the eye. The size of the pupil changes to adjust the amount of light admitted; becoming smaller under bright conditions and enlarging in darkness. The size of the pupil is also affected by a variety of drugs and conditions affecting the brain. • Iris – The colored portion of the eye (hazel, green, blue, even violet). The iris is what actually changes the size of the pupils, covering part of the pupil in bright light. • Cornea – The transparent portion of the eye directly in front of the lens; actually functions to adjust focus in conjunction with the lens. • Lens – Portion of the eye that acts to focus incoming light. The lens changes shape so as to focus on near or distant objects.

7. More parts of the eye • Sclera – The white portion of the eye. • Retina – The back, inside surface of the eye. Light is focused on this surface, which contains light-sensitive cells. • Rods – Sensitive to light at very low levels of intensity. These provide black and white vision. The density of rods is greater near the periphery of the retina than at the center. • Cones – Sensitive to specific ranges of wavelengths of light. There are three types, each sensitive to a different range of wavelength. More dense near center of retina than at periphery. The cones provide color vision. Much less sensitive than rods. • Vitreous humor – Jelly-like, transparent liquid

8. More parts of the eye • Macula – The area around the fovea where visual acuity is best. • Ciliary muscles – Very tiny muscles that contract or relax to adjust the shape of the eye. These are how the eye can focus. • Fovea – The center of focus. When an individual looks directly at an object, the light from the object is centered on the fovea. • Aqueous humor – Clear fluid located between the cornea and the lens. Contains nutrients.

9. Yet more parts to the eye • Optic nerve – The nerve that carries information from the eyes to the visual cortex of the brain, where that information is interpreted. • Blind spot – The portion of the retina directly in front of where the optic nerve attaches. There are no rods or cones here; therefore no vision. Demonstration.

10. Color Vision • Three types of cones, each sensitive to a different, but overlapping, range of wavelengths. • 400 to 500nM is interpreted as blue or violet; 500 to 600nM is green to yellow, then orange; 600 to 700nM is orange to red. • Stimulation of two or more types of cones, in various ratios, is interpreted as the full variety of colors we can perceive. • Note: Different species can see different wavelengths of light, even outside what we call the visible spectrum.

11. Demonstration • The brain’s interpretation of different colors, based on the relative stimulation of different types of cones, will be demonstrated.

12. Color deficiency • The inability or a deficit in the ability, to perceive one or more colors. • Usually due to the absence of one, or more, type of cone. Usually genetic in origin. More common in males. • Color blindness – refers to the total inability to see any color.

13. Demonstration • Devices for detecting various types of color deficiency will be demonstrated. • Discuss: What significance would color deficiency have for human performance in sport.

14. Binocular Vision and Eye Dominance • Binocular vision – We have (usually) two eyes, each of which sees a slightly different image. The brain uses the differences in these images to interpret distances to things. This, however, also depends on knowledge and experience that provides a context within which such interpretation is made. • Dominance – In right or left “eyed” people, input from the dominant eye is “perceived” while that from the other is suppressed. • What percentage of people are “right-eyed” or “left-eyed.” • Note that eyedness and handedness are independent of each other.

15. Demonstration • How to test for eye dominance will be demonstrated and its possible significance to human performance will be discussed. • Significance of eye dominance? • Many believe eye dominance is important, especially in combination with handedness, in certain sports. However, this has yet to be conclusively demonstrated.

16. Demonstration • The measurement of Stereopsis (depth perception) will be demonstrated and discussed. • What would be the consequences to sports performance if depth perception did not exist, or was impaired in some way?

17. Visual Acuity • The ability see details sharply. • Static visual acuity refers to the ability to see details under conditions of high contrast and without motion, as when using an Snellen eye chart. • Dynamic visual acuity refers to the ability to see details when there is relative motion. [Relative motions means when the object, the person, or both, are moving, and not in tandem.]

18. More on acuity • Both types of acuity are increased by greater illumination, to a point. • Dynamic acuity generally improves from age 6 to about 20, but then begins to decline. • Children under 12 may lack the dynamic acuity they will have later. This can make certain activities hazardous.

19. Demonstration • The methods used for measuring visual acuity will be demonstrated and discussed.

20. Selected defects of vision • Astigmatism • Myopia (nearsightedness) • Presbyopia (farsightedness)

21. Accomodation • The ability to adjust eye focus. • Tends to decline with age. • Unclear (and doubtful) that accomodation can be improved in athletes, although it would be desirable.

22. Vergence • The ability to follow an object as it moves closer or further away. • Convergence = when the object is coming closer • Divergence = When the object is moving away • Unclear if this can be improved, though it is clearly a factor in performance.

23. Glare recovery • When rods and cones are exposed to very bright light they become temporarily “bleached.” While bleached, they can no longer sense light. They then recover and become able to function again. The time required for the rods and cones to return to full function is their glare recovery time. • This works in reverse also. After a period in very low light the rods and cones become more sensitive. Occasionally this can be painful when bright illumination returns.