Dimension being tested

2. Dimension being tested Remained constant during testing Remained constant during testing Remained constant during testing Remained constant during testing Remained constant during testing Remained constant during testing Remained constant during testing Remained constant during testing Remained constant during testing

3. Making threshold measurements requires choices about the values to select for the stimulus dimensions that are not being measured Five needed special care in their selection: Wavelength, size, and duration are three characteristics of the stimulus. Two characteristics related to the observer also had to be selected with care: The retinal location that was stimulated and The adaptive state of the observer’s eye Only the stimulus intensity was altered from trial to trial

4. Retinal Location

5. Retinal Location They aimed the spot of light 20 degrees temporal on the retina

6. Fig. 2.1

7. Wavelength 400 nm 500 nm 600 nm 700 nm

9. They used “monochromatic” light – but what wavelength to use to get lowest threshold? Left eye, temporal retina Fig. 2.2

10. Wavelength | | 400 nm 500 nm 600 nm 700 nm

11. Stimulus Size 10’ (minutes of arc) = 1/6th of a degree Any bigger would have “wasted” light

12. Fig. 2.3

13. Photons that are absorbed anywhere within a small region (<15 min. of arc) will add together

14. Photons that are absorbed outside a certain size area do not add together This is because of the wiring of the retina – which produces the “receptive field” of cells

15. The receptive field of a neuron is the region on the retina where light can produce a response from the cell

16. Receptive field (center) of on bipolar is the diameter of the single cone that connects to it (Not concerned here with the R-F “surround” because: dark adapted & very low stimulus levels)

17. Receptive field of this bipolar is the diameter of the all the cones that connect to it

18. Receptive field of this ganglion cell is the diameter of all the cones that connect to it through the bipolar cells

20. Fig. 2.3 Rods show even more convergence (larger receptive fields)

21. Fig. 2.3 Rods show even more convergence (larger receptive fields)

22. Fig. 2.3 Rods show even more convergence (larger receptive fields)

23. For maximum effect (lowest threshold) want all of your photons to “count” – to add together

24. Hecht, Shlaer & Pirenne didn’t know about receptive-field sizes – the wiring of the retina had not been as thoroughly studied as it now has been. What they did know was that the number of photons needed to reach threshold was a constant until a test spot became too large. This is summarized as Ricco’s Law

26. Fig. 2.4

30. Ricco’s Law Fig. 2.4 Piper’s Law

32. Back to the parameters used by Hecht et al. Location: 20 degrees temporal Wavelength: 510 nm Size: 10’ diameter Duration: Adaptation:

33. Stimulus duration

34. Fig. 2.2

35. Fig. 2.3 Time (msec)

36. Fig. 2.3 Time (msec)

37. Fig. 2.3 Time (msec)

38. Fig. 2.3 Time (msec)

39. Fig. 2.3 Time (msec)

41. Fig. 2.5

44. The Adaptive State of the Subject’s Eye

46. Location: 20 degrees temporal Wavelength: 510 nm Size: 10’ diameter Duration: 1 ms Adaptation: 30 min in dark before starting The Method of Constant Stimuli was used to measure the threshold

47. Fig. 2.6

48. Fig. 2.2

49. Fig. 2.3

50. Because the number of rods in the area stimulated by the flash was large in comparison to the number of quanta, the odds of two quanta being absorbed by one rod were small. Therefore, one quanta must be sufficient to stimulate one rod.