Vision Our most dominant sense

1. VisionOur most dominant sense

2. Our Essential Questions • What are the major parts of the eye? • How does the eye translate light into neural impulses?

3. Vision Purpose of the visual system • transform light energy into an electro-chemical neural response • represent characteristics of objects in our environment such as size, color, shape, and location

4. Light: The Visual Stimulus

5. Light: The Visual Stimulus • Wavelength of a light is the distance of one complete cycle of the wave. • Visible light : 400nm - 700nm. • Wavelength of light is related to its perceived color

6. The Structure of the Visual SystemSo how does this stimulus (light) transform into messages in our brain?

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

8. Parts of the Eye – Cornea

9. Iris • Colored portion of the eye • Does color affect vision? • A ring of muscle tissue that regulates the size of the pupil • Allows more or less light to enter the eye

10. Parts of the Eye - Iris

11. Pupil • Opening in the center of the eye • Controls the amount of light entering the eye • bright conditions - iris expands, pupil gets smaller • dark conditions - iris contracts, pupil gets larger

12. Parts of the Eye - Pupil

13. Lens • A transparent structure behind the pupil • Focuses the image on the back of the eye • Muscles change the thickness of the lens  change how light is bent  focuses the image • Glasses or contacts correct problems

14. Parts of the Eye - Lens

16. Retina • At the back of the eyeball • Light-sensitive surface with cells that convert light energy to neural impulses • This is where the magic happens!

17. Parts of the Eye - Retina

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

19. Parts of the Eye - Fovea

22. Receptor Cells • In sight they change light into neural impulses the brain can understand • Visual system has two types of receptor cells – rods and cones

23. Distribution of Rods and Cones • Cones—concentrated in center of eye (fovea) • approx. 6 million • Rods—concentrated in periphery • approx. 120 million • Blind spot—region with no rods or cones

24. Differences Between Rods and Cones • Cones • allow us to see in bright light • allow us to see fine spatial detail • allow us to see different colors • Rods • allow us to see in dim light • can not see fine spatial detail • can not see different colors

25. Receptive Fields and Rod vs. Cone Visual Acuity • Cones—in the fovea, one cone often synapse onto only a single ganglion cell • Rods—the axons of many rods synapse onto one ganglion cell • This allows rods to be more sensitive in dim light, but it also reduces visual acuity

26. Let’s Review • Cone Characteristics • Rod Characteristics

27. Rods • Located in the retina • Can only detect black and white • Respond to less light than do cones

28. Cones • Located in the retina • Can detect sharp images and color • Need more light than the rods • Many cones are clustered in the fovea

29. Let’s do an experiment now • What do you see in your peripheral vision (that’s the stuff on the side)?

30. Get into groups of 3

31. Pick an A, B, and C

32. The Experiment • A will look straight ahead • B will look A in the eyes – to make sure that A doesn’t cheat! • C will move various colored pieces of paper in A’s peripheral vision • A will guess the color • Note: if the person is consistently guessing correctly then they are cheating!

33. Write up the results… • Results – correct guess versus bad • Your conclusion • What do your results tell you about our vision? • How do the different kinds of receptor cells affect our vision?

34. Distribution of Rods and Cones • Cones—concentrated in center of eye (fovea) • approx. 6 million • Rods—concentrated in periphery • approx. 120 million • Blind spot—region with no rods or cones

35. Let’s Compare… Cones Rods allow us to see in dim light can not see fine spatial detail can not see different colors • allow us to see in bright light • allow us to see fine spatial detail • allow us to see different colors

36. Visual Processing in the Retina

37. Optic Nerve • The nerve that carries visual information from eye  occipital lobes

38. Parts of the Eye – Optic Nerve

39. Blind Spot 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

40. Parts of the Eye – Blind Spot

41. What do you see in your blind spot?

43. The Visual System: Color VisionHow do we see color?

44. Color Vision • Differences in wavelength of light = color • Rods are color blind, but cones can see different colors • We have only one type of rod but three types of cones

45. Color Vision • Two theories of color vision: • Trichromatic Theory • Opponent-Process Theory

46. Trichromatic (3-Color) Theory • Cones are “tuned” to be sensitive to red, green and blue light • All the colors we see are a combination of these 3 colors • Similar to the design of a color TV

47. 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

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

49. 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

50. Afterimage Effect