Five Parts to this Chapter Temporal Acuity (critical flicker frequency [CFF])

3. The Troxler effect: the fading of a large stimulus with blurred edges presented in the peripheral visual field X

4. Five Parts to this Chapter Temporal Acuity (critical flicker frequency [CFF]) The Temporal Contrast Sensitivity Function Temporal Summation Masking Motion Detection (Real and Apparent)

5. Part One Temporal Acuity: The critical flicker frequency (CFF)

6. Measure CFF using an episcotister (a rotating sectored disk used to produce square-wave flickering stimuli)

8. How bright does a fused flickering light appear? To convert duty cycle to f, divide the first number by the sum of the two numbers: 1:1 means f=0.5

9. If a square-wave flickering light has a duty cycle of 4:1, what is f? • 0.1 • 0.2 • 0.4 • 0.8 • 1.0

10. To convert duty cycle to f, divide the first number by the sum of the two numbers: 4:1 would be 4/(4+1) so f=0.8

11. How bright does a flickering light appear? At flicker rates slightly below the CFF, brightness is enhanced beyond the mean luminance of the flicker (the Brücke-Bartley phenomenon) This is related to the Broca-Sulzer effect described later in the chapter

12. The neural basis of the CFF is the modulation of firing rates of retinal neurons (ganglion cells)

13. Cone flicker response (pig). Contrast 0.49; mean light level 48,300 photon/ square micron Courtesy of Dr. Tim Kraft

14. Rat ganglion cell responses showing CFF

15. In order to see a light as flickering • The flicker rate must be above the CFF • The Troxler effect must occur • Retinal neurons must be able to respond with gaps in their firing pattern • All of the other answers are correct

16. How does this measure of temporal acuity (CFF) change under different conditions (changes in the stimulus dimensions listed in Chapter 1)? First: stimulus luminance (intensity)

18. Note: if the luminance of the stimulus increases by one log unit, so does the retinal illuminance

19. 3 1 2 demo • Find CFF • Raise intensity (luminance) by 1 log unit. The more intense stimulus is below CFF (flicker is seen). • Have to increase the flicker rate to again find CFF.

20. Critical Flicker Frequency (Hz) 100 80 60 40 20 0 -4 -3 -2 -1 0 1 2 3 4 o o Log Retinal Illuminance for 10 -85 (td) -1 0 1 2 3 4 5 6 7 o o Log Retinal Illuminance for 0 and 3 (td) o 0 o 3 o 10 o 35 o 65 o 85

21. The CFF is highest in the midperipheral retina at high luminance, but nearly constant across the retina at low luminance. Critical Flicker Frequency (Hz) 100 80 60 40 20 0 0 20 40 60 80 100 Retinal Eccentricity (deg) Retinal Illuminance (td) 2500 250 25 2.5 0.25 This is why you can see flicker on some PC monitors if you look slightly to the side

22. CFF increases • In direct proportion to the log of the stimulus luminance • In the periphery at all luminance levels • In response to the Brücke-Bartley phenomenon • None of the above

23. Second: area (size) CFF = k logA + b Where k and b are constants and A is the area of the flickering stimulus Demo, since I haven’t found a good figure showing this relationship

24. Demo – Granit-Harper • Find CFF • Increase stimulus area by 1 log unit. The more intense stimulus is below CFF (flicker is seen). • Have to increase the flicker rate to again find CFF.

25. Chapter 7 – Temporal Factors in Vision • Main points so far: • CFF is a measure of temporal acuity – analogous to VA (how small a temporal interval can you detect – in time)? • CFF increases linearly with log stimulus luminance (Ferry-Porter Law) • CFF increases linearly with log stimulus area (Granit-Harper)

26. You will not be responsible for the material starting on page 188, “flicker sensitivity increases….” and including all of page 189 and 190 (Figs. 7.7 and 7.8). You will be responsible for material starting again on page 191, “Temporal Contrast Sensitivity”

27. Five Parts to this Chapter Temporal Acuity (critical flicker frequency [CFF]) The Temporal Contrast Sensitivity Function Temporal Summation Masking Motion Detection (Real and Apparent)

28. Contrast, modulation and amplitude The contrast of a temporal sine wave is defined the same way as the contrast of a spatial sine wave grating: - Contrast = (L L )/( L + L ) max min max min In Figure 7-1, L is 300, L is 100, so contrast = (200)/(400) = 0.5 Another term, modulation max min (abbreviated as m ),is sometimes used for sine-wave flicker, and may be used interchangeably with contrast. As illustrated in Figure 7-1, L is the maximum luminance of the flicker, and L is the max min minimum luminance. L and L are symmetrically arranged around the mean or average luminance, max min defined as: Mean Luminance = L = ( L + L )/2 max min m Hence, contrast or modulation can also be expressed as: - Contrast = m = (L L )/ L max m m In addition, L - L is also called the of the wave, and, therefore, amplitude max m Contrast = modulation = amplitude /L m Referring again to the sine wave at the bottom of Figure 7-1 , the mean luminance is 200 units, the amplitude is 100, and the contrast (modulation) therefore is 0.5. As was the case for spatial sine-wave gratings, contrast sensitivity is defined as the inverse of the threshold contrast.

29. This is like Figure 6.9 in the spatial domain

30. Temporal CSF Demo http://psy.ucsd.edu/~sanstis/TMTF.html

31. Change in the temporal CSF with luminance: As luminance decreases, • the peak contrast sensitivity becomes lower • the cutoff high temporal frequency decreases (Ferry-Porter law) • peak contrast sensitivity occurs at lower temporal frequency • the low temporal frequency rolloff disappears

32. The temporal contrast sensitivity function • Is the boundary between contrasts you can see and ones you cannot see • Has a peak contrast at around 1 Hz at high mean luminance • Is a measure of temporal acuity • Becomes more bandpass as the mean luminance is decreased

34. The delayed arrival of the surround signal, relative to the center signal can cause the surround to add with the center at some temporal frequencies

37. The delayed arrival of the surround signal, relative to the center signal can cause the surround to add with the center at mid-range temporal frequencies

38. This is like Figure 6.9 in the spatial domain

39. The temporal CSF is a useful measure for diagnosing retinal disorders • Artificially increased IOP produces reduced temporal CSF (but no effect on CFF) • Temporal CSF is reduced with glaucoma and ocular hypertension • Glaucoma - Frequency-doubling perimeter measures contrast threshold for 0.25 c/deg grating flickering at 25 Hz (mediated by MY [nonlinear magno] cells?) • Eyes at risk for exudative (wet) AMD show reduced sensitivity at 5 - 40 Hz (5 Hz & 10 Hz alone discriminate from healthy eyes) • Importance? Early diagnosis can lead to earlier treatments

40. The low temporal frequency rolloff of the temporal CSF • Is really a “mid-temporal frequency enhancement produced by the longer latency of the receptive field surround • Becomes more prominent at low mean luminance levels • Helps create Mach bands • Is related to the cutoff high temporal frequency

41. Five Parts to this Chapter Temporal Acuity (critical flicker frequency [CFF]) The Temporal Contrast Sensitivity Function Temporal Summation (Bloch’s Law & Broca-Sulzer) Masking Motion Detection (Real and Apparent)

42. Fig. 2.5

43. Bloch’s Law holds for durations shorter than the critical duration There is a constant # of quanta in a threshold flash as L decreases

44. Part A – threshold measures Temporal Summation and Bloch's Law When a brief flash is used to determine the threshold intensity, the visual system does not distinguish the “temporal shape” of the flash if the flash duration is less than the “critical duration”

45. Two ways to show Bloch’s Law: L x t = C Bloch’s Law holds Bloch’s Law holds “Holds” means that Bloch’s Law accounts for the threshold values

46. But I will not hold you responsible for this section Bottom of 198 & top of 199

47. Flashes of various durations shorter than the critical duration all have the same temporal frequency spectrum. Flashes longer than the critical duration contain less contrast at intermediate temporal frequencies, after filtering through the temporal CSF and are therefore less visible. Thus, more quanta are need to be added to bring them up to threshold.

50. For flash durations less than the critical duration, Bloch’s Law holds and • The flash cannot be seen when it is above threshold • The number of quanta in a threshold flash is the same for different flash durations • L x C = t • None of the above