1. We can see objects even though the background luminance levels change over a range of more than 10 orders of magnitude (10 10 ). How do we do it?

2. Reminder about why we are doing all this:  As a clinician, you need to understand the scientific basis on which measurements of vision are made and how they can be made in the future as new tests of visual function are developed and put into clinical practice.  For instance, dark adaptation rate may turn out to be a way to diagnose Age-related Macular Degeneration (AMD) very early – trials underway

3. Three main purposes of course 1)Learn how vision is measured 2)Basic facts about monocular visual function (What is normal?) 3)Neural basis of visual function (Why does the visual system respond as it does?)

4. Three main purposes of course - Adaptation 1)Learn how vision is measured Will measure a group dark adaptation curve in lab 2)Basic facts about monocular visual function (What is normal?) Different curves from different test flash & adapting light conditions 3)Neural basis of visual function (Why does the visual system respond as it does?) mechanisms

7. “Typical” Dark Adaptation Curve Adapting light goes off at time = 0

8. The task: Measure the threshold intensity as the visual system dark adapts This is a “moving target” because the threshold decreases over time. Dark Adaptation

9. The task: measure a “group dark adaptation curve” Everyone in the group will light adapt. Then everyone will take a turn as a subject (have your threshold measured) and as an examiner (measure the threshold intensity of your classmate) as the visual system dark adapts This is a “moving target” because the threshold decreases over time. The winning group will be awarded two six-packs* The winning group gets to decide the content of each six-pack (water, beer, Coke, Pepsi, etc.) Dark Adaptation lab on Thursday

13. 1) Rods and cones both start dark adapting at time 0 2) the more sensitive system at that time determines the threshold 3) cones dark adapt faster than rods 4) the lowest thresholds obtained using cones are much higher than the lowest thresholds obtained with rods (rods, potentially, are more sensitive than cones) Note: If using the Method of Limits, must only use the ascending branch to avoid changing the time-course of the dark adaptation “sneak up” on threshold from below

14. The 2009 winning group

15. * * * * * ** = important parameters in dark adaptation studies

16. Variations in the dark adaptation curves help to illustrate the importance of knowing what you are doing when making psychophysical measurements. What you get depends on how you make the measures Different situations give very different results

18. Fig. 2.1 In order to see both the rod and cone branches during dark adaptation, the adapting light and test spot must stimulate both rods and cones

19. Retinal Location (2 deg spot) Broadband 300 millilambert adapting field, 2 min exposure 2° Spot flashed 1 s every 2 s (μmillilamberts)

20. Retinal Location (2 deg spot) Broadband 300 millilambert adapting field, 2 min exposure 2° Spot flashed 1 s every 2 s (μmillilamberts)

21. Test flash size (centered on fovea) (μmillilamberts)

22. Effects of Test Flash Wavelength on the Shape of the Dark Adaptation Curve 400 nm 500 nm600 nm700 nm | | Peak rod absorption

23. Effects of Test Flash Wavelength on the Shape of the Dark Adaptation Curve -Rods absorb poorly at long wavelengths 400 nm500 nm 600 nm700 nm | Peak rod absorption

24. Effect of Test flash Wavelength “decibels” (dB) is a log scale

25. Adapting Light Wavelength 400 nm 500 nm600 nm700 nm | | | Test flash

26. Effect of Adapting Light Wavelength 400 nm500 nm600 nm700 nm | | | Test flash |

27. Adapting light wavelength (blue test flash) (μμlamberts)

29. Adapting light intensity Illuminance (μTroland)

30. Adapting light duration 333 millilamberts Luminance (millilamberts)

31. Luminance needed to detect grating orientation If you need cones to do the task, then do not get a rod branch (millilamberts)

33. Illuminance (trolands)

35. Response to threshold test flash alone Response to adapting light offset alone Test flash long before adapting light offset Test flash just before adapting light offset Test flash same time as adapting light offset Test flash long after adapting light offset All of these action potentials are needed to see the test flash

36. Response to threshold test flash alone Response to adapting light offset alone Test flash long before adapting light offset Test flash just before adapting light offset Test flash same time as adapting light offset Test flash long after adapting light offset All of these action potentials are needed to see the test flash The response to the test flash is “cut off”; not enough APs to detect

37. Response to threshold test flash alone Response to adapting light offset alone Test flash long before adapting light offset Test flash just before adapting light offset Test flash same time as adapting light offset Test flash long after adapting light offset All of these action potentials are needed to see the test flash The response to the test flash is “cut off”; not enough APs to detect What happens when the test flash is presented at different times, relative to the adapting light offset? Remember, we are looking at the response of just ONE neuron, responding to BOTH the test flash and the adapting light offset.

39. Response to threshold test flash alone Response to adapting light offset alone Test flash long before adapting light offset Test flash just before adapting light offset Test flash same time as adapting light offset Test flash long after adapting light offset All of these action potentials are needed to see the test flash The response to the test flash is “cut off”; not enough APs to detect The response to the test flash is supporessed; not enough APs to detect How do you make the test flash visible again? Raise the intensity to restore the needed number of action potentials

41. Response to threshold test flash alone Response to adapting light offset alone Test flash long before adapting light offset Test flash just before adapting light offset Test flash same time as adapting light offset Test flash long after adapting light offset All of these action potentials are needed to see the test flash The response to the test flash is “cut off”; not enough APs to detect The response to the test flash is supporessed; not enough APs to detect How do you make the test flash visible again? Raise the intensity to restore the needed number of action potentials

43. Response to threshold test flash alone Response to adapting light offset alone Test flash long before adapting light offset Test flash just before adapting light offset Test flash same time as adapting light offset Test flash long after adapting light offset All of these action potentials are needed to see the test flash The response to the test flash is “cut off”; not enough APs to detect The response to the test flash is suppressed; not enough APs to detect How do you make the test flash visible again? Raise the intensity to restore the needed number of action potentials

45. Response to threshold test flash alone Response to adapting light offset alone Test flash long before adapting light offset Test flash just before adapting light offset Test flash same time as adapting light offset Test flash long after adapting light offset All of these action potentials are needed to see the test flash The response to the test flash is “cut off”; not enough APs to detect The response to the test flash is suppressed; not enough APs to detect

46. Illuminance (trolands)

49. New research (Greg Jackson, just moved from CEFH, UAB) suggests that dark adaptation is slower in people who are developing age-related macular degeneration Clinical trial ongoing on HPB 4 th floor

50. Dark Adaptation

52. Both for light adaptation and dark adaptation

53. Half-time for cones = 1.7 min rods, 5.2 min Regeneration of rhodopsin follows a exponential decay function

54. At a practical level, the amount of bleached photopigment is cut in half every 1.7 min for cones and every 5.2 min for rods

55. Both for light adaptation and dark adaptation If you bleach half of the photopigment, how much does the threshold rise? If you bleach ¼ of the photopigment, is the threshold elevated half as much (linear increase)?

56. The log of the threshold elevation (above absolute threshold) is related to the fraction of bleached rhodopsin This gives how much the threshold is raised above absolute threshold

59. Time constant for cones = 1.7 min rods, 5.2 min

60. Symbols = threshold Lines = bleached pigment

62. Another way the amount of bleached pigment sets the threshold: This ties together thresholds during light adaptation (real background light) and during dark adaptation (“equivalent background” set by the fraction of bleached pigment)

63. deVries-Rose But plots threshold L not ΔL Dark Adaptation

64. DA-threshold drops as bleached rhodopsin level drops As background L rises, more rhodopsin is bleached When the thresholds are the same, the amount of bleached rhodopsin is the same

65. This is the x- axis from the right side of the previous figure This is the x-axis from the left side of the previous figure “equivalent background” works for all target sizes

66. Light adaptation alters the responses of the photoreceptors (looking at the neural changes that occur during light adaptation)

67. What happens to the response of rods as the background L is raised?

68. We know that the threshold ΔL rises as the background L is increased (Ch. 3) We also know that the amount of bleached photopigment increases as L is increased. Look now at what effect increasing L has on photoreceptor responses. This should explain the increase in threshold ΔL.

69. Low intensity, brief flash of light produces a small hyperpolarization with longer latency As the flash intensity rises, the amount of hyperpolarization rises, an overshoot develops, and the latency is shorter. The membrane is slow to return to baseline These are the responses (hyperpolarization) of a rod to different flash intensities

70. Low intensity, brief flash of light produces a small hyperpolarization with longer latency If you slow down time on the x-axis, this just looks like a line of differing lengths For simplicity, represent the responses just with vertical lines

71. Top: no adapting light; bottom: with increasing adapting light

73. ΔV is the Key! Change in membrane potential codes brightness

75. Neural (“network”) (non-photopigment) Early dark adaptation “early” Light adaptation – non-photopigment based photoreceptor changes; Ganglion cell sensitivity changes even though photoreceptors are dark adapted “Loss” (disconnection) of receptive-field surround in full dark adaptation Circadian changes – dark adaptation is more complete at night

77. Ganglion cells can show dark adaptation when photoreceptors do not

78. This figure is misleading. The network changes really are here