The Visual System: The Structure of the Visual System

1. The Visual System: The Structure of the Visual System

2. Cornea • The clear bulge on the front of the eyeball • Begins to focus the light by bending it toward a central focal point • Protects the eye

3. Parts of the Eye – Cornea

4. Iris • A ring of muscle tissue that forms the colored portion of the eye; creates a hole in the center of the iris (pupil) • Regulates the size of the pupil by changing its size--allowing more or less light to enter the eye

5. Parts of the Eye - Iris

6. Pupil • The adjustable opening in the center of the eye that controls the amount of light entering the eye (surrounded by the iris) • In bright conditions the iris expands, making the pupil smaller. • In dark conditions the iris contracts, making the pupil larger.

7. Parts of the Eye - Pupil

8. Lens • A transparent structure behind the pupil; focuses the image on the back of the eye (retina) • Muscles that change the thickness of the lens change how the light is bent thereby focusing the image • Glasses or contacts correct problems in the lens’ ability to focus.

9. Parts of the Eye - Lens

10. Nearsighted - Myopia

11. Farsighted - Hyperopia

15. Retina • Light-sensitive surface with cells that convert light energy to nerve impulses • At the back of the eyeball

16. Parts of the Eye - Retina

17. Receptor Cells • These cells are present in every sensory system to change (transduce) some other form of energy into neural impulses. • In sight they change light into neural impulses the brain can understand. • Visual system has two types of receptor cells – rods and cones

18. Rods • Visual receptor cells located in the retina • Can only detect black and white • Respond to less light than do cones

19. Cones • Visual receptor cells located in the retina • Can detect sharp images and color • Need more light than the rods • Many cones are clustered in the fovea.

21. Fovea • The central focal point of the retina • The spot where vision is best (most detailed)

22. Parts of the Eye - Fovea

23. Visual Processing in the Retina

24. Optic Nerve • The nerve that carries visual information from the eye to the occipital lobes of the brain

25. Parts of the Eye – Optic Nerve

26. Blind Spot • The point at which the optic nerve travels through the retina to exit the eye • There are no rods and cones at this point, so there is a small blind spot in vision.

27. Parts of the Eye – Blind Spot

29. The Visual System: Color Vision Module 9: Sensation

30. Color Vision • There are two theories of color vision: • Trichromatic Theory • Opponent-Process Theory

31. Trichromatic (three-color) Theory • Theory of color vision that says cones are “tuned” to be sensitive to red, green and blue light • All the colors we see are a combination of these three colors. • Similar to the design of a color TV

32. How do we see color? • Trichromatic (three color) Theory • three different retinal color receptors • Red green blue

33. Can you see what is in the middle?

34. Color Deficient Vision • People who lack one of the three types of cones • Usually the red or green receptors are missing • Usually referred to as color blindness • In inherited and found more in males

35. Red-Green Color Blindness

36. Opponent-Process Theory- Vision from opposing pairs of color receptors- only one “side” ON at a time

37. Opponent-Process Theory • Sensory receptors in the retina come in pairs: • Red/Green • Yellow/Blue • Black/White • Only one side is “on” at a time

38. Opponent Process Theory ON” “OFF” redgreen greenred blueyellow yellowblue black white white black

39. Afterimage Effect

45. Opponent-Process Theory • If one sensor is stimulated, the other is inhibited • If one sensor is over-stimulated, and fatigues, the paired sensor will be activated, causing an afterimage