1. Vision Our 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
Hockenbury Powerpoint, slides (Schulman)

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 System So how does this stimulus (light) transform into messages in our brain?

7. 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

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

20. Disc psy p 87 Figure 3.3

21. Discovering PSY p 90 Fig 3.5

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
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. A will look straight ahead
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… allow us to see in bright light
Cones
Rods
allow us to see in bright light
allow us to see fine spatial detail
allow us to see different colors
allow us to see in dim light
can not see fine spatial detail
can not 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 Vision How 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. 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”
red green
green red
blue yellow
yellow blue
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

52. Can you see what is in the middle?

53. Red-Green Color Blindness

54. 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
Inherited and found more in males

55. Discovering Psy p 92 Photo at top of page

56. Discovering Psy p 92 Photo at top of page

57. Overview of Visual System
The eye is like a camera; instead of using film to catch the light, we have rods and cones.
Cones allow us to see fine spatial detail and color but cannot function well in dim light.

58. Overview of Visual System
Rods enable us to see in dim light but at the loss of color and fine spatial detail.
Our color vision is based on the presence of 3 types of cones, each maximally sensitive to a different range of wavelengths.