Temporal Vision Temporal Contrast Sensitivity Function (TCSF) also called the

1. Temporal Vision Temporal Contrast Sensitivity Function (TCSF) also called the Temporal Modulation Transfer Function (TMTF) Temporal Phenomena: Troxler Effect Ferry-Porter Law Granit-Harper Law Broca-Sultzer Effect Brucke-Bartley Effect Talbot-Plateau Law Schwartz Chapter 8 Ref-4 EW8250F07

2. Troxler EffectSmall involuntary eye movements – microsaccades, slow drifts, and high-frequency tremors – cause small temporal transients to the spatial luminance profile of retinal images. These temporal transients displace images by seconds to tens of minutes of arc and are necessary to maintain spatial vision.The Troxler Effect occurs when the retinal image is stabilized so that it does not change over time and vision fades.“The temporary and irregular fading or disappearance of a small object in the visual field during steady fixation of another object…” (Dictionary of Visual Science) EW8250F07

3. Troxler EffectThe Troxler Effect is the reason why we do not perceive the shadow cast by our retinal blood vessels (the Purkinje tree),which is stabilized on the retina. These shadows don’t move and the eye is very insensitive to non-changing retinal images, i.e. luminance patterns.Often during direct ophthalmoscopy light enters the eye at an unusual angle, e.g. on peripheral retina observation.This light causes the shadow of the retinal vasculature to shift and the they become visible to the patient. EW8250F07

4. Spatial VisionHow we perceive changes in luminance across space.Temporal VisionHow we perceive changes in luminance over time.Temporal vision is best studied with a flickering light,i.e. a light having a luminance profile that changes sinusoidally over time. A = Amplitude Use Lmax – Lavg to avoid negative values since both sinewave and squarewave contrast is defined as being positive 0 – 100%. Ref-4 EW8250F07

5. Temporal Contrast is calculated the same ways as spatial contrast.Just realize that contrast is varying over time.Michaelson Contrast: Contrast = (Lmax – Lmin) / (Lmax + Lmin)orContrast = L / Lavg = Amplitude / Lavg = (Lmax – Lavg) / Lavg(50 nits – 10 nits) / (50 nits + 10 nits) = 0.67 = 67% contrastor(50 nits – 30 nits) / 30 nits = 0.67 = 67% contrast Ref-4 EW8250F07

6. Temporal Contrast Just like spatial contrast, A contrast < B contrast for temporal contrast. A = Amplitude Use Lmax – Lavg to avoid negative values since both sinewave and squarewave contrast is defined as being positive 0 – 100%. Ref-4 EW8250F07

7. In addition to temporal contrast, a temporal stimulus varies in terms of temporal frequency or flicker rate.This is analogous to spatial frequency (c/deg) but instead describes the number of on-off flicker cycles per second, i.e. c/sec or Hertz (Hz). The cycle is the length of time taken for one complete flicker of light and dark, and the flicker frequency is the number of cycles/second.Low Hz = slow flicker, and High Hz = rapid flicker. A Hz (squarewave) and B Hz (sinewave) is 1 c / 500 msec = 2 c/sec = 2 Hz Where 1 cycle is one peak and one trough. A Hz (slow) less than B Hz (fast). Ref-4 Ref-1 EW8250F07

8. Low Temporal FrequenciesThese refer to very slowly changing luminance levels. An extreme example is the change from daylight to darkness (1 cycle / 24 hours).A very slow moving object is difficult to detect due to lateral inhibitory processes in the retina.This insensitivity is the basis for the Troxler Effect. The shadow of the retinal vasculature has temporal frequency of 0 unless light is directed in the eye at an unusual angle. Then the shadows shift, i.e. obtain a high temporal frequency, and the patient sees the Purkinje tree.The difference is sensitivity to low and high temporal frequencies is summarized by the temporal contrast sensitivity function (TCSF). EW8250F07

9. Temporal Resolution AcuityTemporal resolution acuity refers to the smallest time interval that can be resolved as flickering light.It is measured psychophysically and is called the critical flicker frequency (CFF), which is the frequency at which a flickering light is perceived as a steady light 50% of the time (a psychophysical threshold). The temporal contrast of the flickering light is fixed at 100%.This represents the smallest time interval between flashes that the eye can detect as two flashes instead of one—it is generally measured using a train of flashes. The flash train is the temporal analog of a spatial sinewave grating.The CFF is the temporal resolution limit and is analogous to the spatial frequency resolution limit or the high spatial frequency cut-off. EW8250F07

10. Temporal Resolution AcuityWhen the flicker rate is low an observer will see a series of flashes.As the flicker rate increases a steady light with no flicker will be seen.The flicker frequency at which the light is seen as flickering 50% of the time and as steady or fused 50% of the time is called the critical flicker frequency or the CFF.The CFF is a determination of the temporal resolving capability of the visual system and is limited by the processing speed of the retina.The minimum interval of resolution (temporal acuity) is analogous to the minimum angle of spatial resolution. The CFF is like grating acuity in the spatial domain (detecting high contrast, alternating light and dark bars).The CFF changes with retinal illumination, stimulus size and location, etc. Generally the expected CFF for foveal viewing is about 50 Hz.This is why standard light bulbs flicker at 60 Hz and CRTs and monitorsrefresh at at minimum 60 Hz. EW8250F07

11. Temporal Resolution AcuityThe mechanism for the CFF is likely early in visual processing. Recordings from photoreceptors and ganglion cells show similar relationships between the CFF and light intensity as measured in human psychophysics.At low flicker rates, the firing of a cell increases during Lmax and decreases during Lmin.Response latency means that the burst of action potentials caused by Lmax extends a certain amount of time into Lmin before ending.Increasing flicker rate eventually surpass the neural resolving ability. Ref-4 EW8250F07

12. Temporal Resolution AcuityIt is likely the lack of gaps between AP bursts that produces the CFF. Ref-1 EW8250F07

13. TCSF (Temporal Contrast Sensitivity Function)Temporal contrast sensitivity measurements indicate how the visual system responds to combinations of temporal frequencies and contrasts.The previous Temporal Resolution Acuity only provides information about the highest detectable temporal frequency at a fixed high contrast.The TCSF provides a complete picture of temporal vision; analogous to what the spatial CSF does in the realm of spatial vision. Low temporal contrast Sensitivity High temporal contrast Sees fused light (no flicker seen) Sees flicker Slow to fast flicker rate (Hz) Ref-4 EW8250F07

14. TCSFs measured in usual lighting have band pass characteristics:Peak contrast sensitivity occurs at an intermediate frequency (5–8 Hz).High temporal frequency cutoff (~ 50 Hz) occurs at the upper limit of flicker rate above which flicker can’t be seen even with contrast of 100%.Low temporal frequency rolloff from poor sensitivity at low frequencies. Like the spatial CSF, high contrast, or modulation, is always easier to see than low contrast. Sensitivity CFF is measured for the TCSF but is typically extrapolated for the SCSF. Ref-4 EW8250F07

15. Retinal Illuminance Affect on the Temporal CSF No affect on low frequencies until lighting is scotopic. Remaining TCSF shifts down and left for decreased retinal illuminance. Affect is much greater for high Hz. Peak shifts from 5 – 8 Hz to 20 Hz. CFF shifts from 20 Hz to ~80 Hz. Usual photopic luminance (3 – 300 cd/sqm) Ref-4 EW8250F07

16. Background Luminance and the CFF: The Ferry-Porter LawThe CFF (100% contrast stimulus; high frequency cutoff) increases nearly linearly with the log of the background luminance. That is, temporal resolution acuity improves with increasing retinal illuminance. Ref-4 EW8250F07

17. Background Luminance and the CFF: The Ferry-Porter LawMathematical definition: CFF = klogL + bWhere k is the slope, b is a constant, and L is the luminance. If a patient complains of computer flicker or fluorescent light flicker it is worth trying to decrease retinal illumination through changes in ambient lighting, monitor contrast, and tinted lenses. And for the monitor, purchase one with capacity for much higher refresh rates. EW8250F07

18. Background Luminance and the CFF: The Ferry-Porter LawMathematical definition: CFF = klogL + bWhere k is the slope, b is a constant, and L is the luminance. Ferry-Porter Law approaches linearity for both scotopic and photopic vision. Scotopic CFF is less than photopic CFF. The peak of 70 Hz is not for usual photopic levels of luminance. Ref-4 EW8250F07

19. The Ferry-Porter law also holds outside the fovea and the slope of the CFF function becomes steeper with eccentricity.This indicates that theretinal periphery hasgreater temporal acuity than the central retina. Ref-1 EW8250F07

20. Stimulus Size and the CFF: The Granit-Harper LawThe larger the flickering stimulus, the higher the CFF – temporal resolution acuity improves. Improvement is linear with the log of the stimulus area. Mathematical definition: CFF = klogA + bWhere k and b are constants, and A is the area of the flickering stimulus.Holds for 3 log unit range of luminance and for stimuli ≤ 10° eccentricity. EW8250F07

21. Stimulus Size and the CFF: The Granit-Harper LawMathematical definition: CFF = klogA + bLarger stimuli fall on peripheral retina which is more sensitive to flicker and motion due to predominantly magnocellular neurons.Note this by viewing a ceiling fan directly then with your peripheral vision. EW8250F07

22. Stimulus Duration and Brightness: The Broca-Sultzer EffectA light of fixed luminance that is flashed for a duration of 50 – 100 msecwill appear brighter than if it were flashed for a shorter or longer time.Thus the perceived brightness of a suprathreshold stimulus cannot be predicted based only on its luminance, it also depends on its duration. Ref-4 EW8250F07

23. Stimulus Duration and Brightness: The Broca-Sultzer EffectA light of fixed luminance that is flashed for a duration of 50 – 100 msecwill appear brighter than if it were flashed for a shorter or longer time.The Broca-Sultzer Effect holds for scotopic and photopic vision, all areas of the retina and all colors.Schwartz correctly points out that the Broca-Sultzer Effect is not due to the time course of ganglion cell action potentials. The underlying neural mechanism is unknown. This effect may in part be due to individual cone’s maximum response occurring for flash durations of 50 msec. Ref-4 EW8250F07

24. Flicker and Brightness Enhancement: The Brucke-Bartley EffectWhen the flicker rate is varied without changing the time-averaged luminance, the brightness is enhanced at certain frequencies (5-20 Hz); this is called the Brücke-Bartley phenomenon or effect.That is, a light flickering at 10 Hz will look brighter than a steady light of the same luminance. Steady state light (same luminance as flicker light) 10 Hz peak enhancement Brightness perception EW8250F07

25. The Brucke-Bartley Effect. A light flickering at 10 Hz will look brighter than a steady light of the same luminance.The underlying mechanism is actually the Broca-Sultzer Effect:A 10 Hz stimulus = 10 cycles / sec = 100 msec / cycle = 50 msec / flash.The Broca-Sultzer Effect states that a light flashed for 50 msec appearsbrighter than if it is flashed for shorter or longer durations.Thus the 10 Hz stimulus produces maximum brightness enhancement while still being within the CFF, i.e. flicker is still visible.An application is the production of flashing lights that are most visible (brightest) but uses the least energy (10 Hz flicker rate). Ref-4 EW8250F07

26. Flicker Exceeds CFF, Affect on Brightness: Talbot-Plateau LawWhen flicker rate exceeds the CFF, the brightness of the flickering light (now seen as fused) is dimmer than a steady light of the same luminance. The apparent brightness is equal to the time-averaged brightness of the steady light. This known as the Talbot-Plateau law and the time-average brightness is called Talbot brightness.Talbot brightness = Lmin + (Lmax – Lmin)*fWhere f is the fraction of time Lmax is on.Examples:A light is on and off 50% of the time and it flickers greater than the CFF.It will appear 50% as bright as a steady light with the same “on” luminance.Talbot brightness = 0 + (100 – 0)*0.50 = 50Talbot brightness = 10 + (50 – 10)*0.50 = 25 Ref-4 EW8250F07

27. Talbot-Plateau LawWhen flicker rate exceeds the CFF, the brightness of the flickering light (now seen as fused) is dimmer than a steady light of the same luminance. EW8250F07

28. Pathology and the TCSFAlthough the TCSFis not a clinical measure, it does have application to clinical conditions. These include aiding the early detection of glaucoma due to disproportionate loss of large axon ganglion cells. Namely, magnocellular neurons which predominantly encode motion information. TCSF is likely more sensitive to early ganglion cell loss from primary open angle glaucoma than conventional automated perimetry using white light. Ref-4 EW8250F07

29. References Norton, T., Corliss, D., & Bailey, J. (2002). The psychophysical measurement of visual function. Butterworth–Heinmann. Palmer, S. (1999). Vision science: photons to phenomenology. MIT Press. Goldstein, B. (1996). Sensation and perception. 4th Ed. Brooks/Cole Publishing. Schwartz, S. (2004). Visual perception: a clinical orientation. 3rd Ed. McGraw-Hill. Atchison, D., & Smith, G. (2000). Optics of the human eye. Butterworth–Heinmann, Oxford. Wandell, B. (1995). Foundations of vision. Sinauer Associates. Regan, D. (2000). Human perception of objects: early visual processing of spatial form, defined by luminance, color, texture, motion, and binocular disparity. Sinauer Associates. Snowden, R.,Thompson, P., & Troscianko, T. (2006). Basic vision, an introduction to visual perception. Oxford University Press. Kaufman, P., & Alm, A. (Eds.). 2003. Adler’s Physiology of the Eye, 10th Ed. Mosby. Chalupa, L, & Werner, J. (Eds.), 2004. The Visual Neurosciences. Bradford. Practice questions for the final exam upcoming. EW8250F07