Temporal Processing

Question 1

What is temporal vision and luminance?

Answer 1

-Temporal Vision is studied with stimuli of which luminance varies over time
-Study of temporal vision concerns how we perceive changes in luminance over time
-If pattern of luminance follows sine wave function, then called a temporal sinusoid
-Not the same sine-wave gratings as seen previously in spatial vision
-Even though these stimuli are temporal equivalent of spatial sine wave gratings, but they don’t necessarily look like a sine wave grating
-While a spatial grating manifests a sinusoidal change in luminance across space, a temporal sinusoid manifests a sinusoidal change in luminance over time  appears to be a flickering light
-Defined by flickering contrast or depth of modulation and temporal frequency

Question 2

Describe temporal contrast?

Answer 2

• Visibility of a temporally modulated stimulus
• Temporal equivalent of contrast in spatial vision
• % temporal contrast (modulation) = (A / luminanceaverage) x 100

Question 3

What is temporal frequency?

Answer 3

• Temporal equivalent for spatial frequency
  o Low temporal frequency –> slow flicker
  o High temporal frequency –> fast flicker
• Hertz (Hz): unit of temporal frequency
  o No. of (flicker) cycles per second (cycles/sec)
• More cycles in unit time w/ high temporal frequency

Question 4

What is the stimulus and what is the luminance profile of it in temporal processing?

Answer 4

Stimulus is 2D Gaussian and contrast, the luminance profile of that 2D Gaussian changes following sine wave function in time. Doesn’t look like a sine wave grating – it is only the luminance profile of the 3D Gaussian that follows the sine wave function, not stimulus itself

Question 5

Describe temporal summation & resolution?

Answer 5

• Temporal summation: ability to sum light over time
  o When light level is limited long integration time is preferred for detection whereas a short integration time is preferred for fine temporal discrimination when there is enough light
• Temporal resolution: ability to resolve 2 events of light occurring in temporal succession
• Critical duration (or period): time period over which light is summed
  o Photopic system: 10 ~ 50ms
  o Scotopic system: 100 ~ 200ms
• No matter how many flashes fall within the critical duration, they will be seen as one

Question 6

Describe scotopic temporal summation?

Answer 6

• For 2 flashes to be temporarily resolved as 2 then they must be separated in time by at least a critical duration
• E.g. critical duration for scotopic vision is 150ms (temporal summation window for scotopic system)  no. of quanta to reach threshold is constant
• One example: 2 lights, light intensity is ~2 thirds of threshold intensity & they are flashed in time separated by >100ms  these 2 flashes won’t be summated & as a consequence, no flash will be seen by the system.
• Next example: 2 flashes w/ same light intensity, 2 thirds of threshold intensity flashed one after other within critical duration of 100ms. Then one flash will be detected because their sub-threshold intensities are added up & summed intensity becomes greater than threshold light intensity
• Next example: 2 lights w/ a suprathreshold intensity one after other within the critical period. Even though these 2 flashes are suprathreshold flashes because they fall within this critical summation period the visual system will only be able to see one flash instead of 2 flashes
• Next example: 1 light w/ suprathreshold flash & other light w/ same intensity after 200ms –> 2 flashes are seen as 2nd flash is subject to next summation period

Question 7

Describe photopic temporal summation?

Answer 7

• Summation window much narrower (30ms)
• First example: no flash seen because one flash was sub-threshold (fall within critical period) & other one w/ same intensity (which is 2 thirds of one threshold) flashed outside the time window so they won’t get summated over – it never reaches threshold light intensity so no flash will be seen
• Next example: one flash seen as they fall within the critical period – the summation period. 2 thirds of threshold light intensity falls within critical period so they will be summated to signal one light flash event
• Next example: 2 flashes will be seen because they’re both suprathreshold light flashes. One falls within summation period & other falls outside critical period – 2 flashes signalled for this visual system

Question 8

Temporal summation & resolition: Photopic vs Scotopic Vision?

Answer 8

Time window for summation:
Photopic vision - 10-50ms, Scotopic vision - 100-200ms

Question 9

Describe Bloch’s Law (time-intesity reciprocity)?

Answer 9

• Relationship between time & light intensity
• State: within the critical duration, total number of quanta needed to reach threshold remains same
  o K = I x t
• Intensity & duration are inversely proportional – they have reciprocal relationship
• E.g. when luminance is halved, a doubling in stimulus duration is required to reach threshold
  o When luminance is doubled, threshold can be reached in half the duration
• Beyond the critical duration, threshold is only dependent upon luminance rather than product of luminance & duration

Question 10

Describe Temporal Contrast Sensitivity Function (CSF)?

Answer 10

• To study how visual system perceives changes in luminance over time, need to test eye with temporal sinusoids w/ various contrast & temporal frequencies
• Plots limits of our ability to perceive flicker as stimulus varies in terms of temporal contrast & frequency
• In many respects, spatial CSF & temporal CSF are v similar
• Modulation here means temporal contrast
• High contrast easier to see than low contrast
• Measures sensitivity to temporal changes in luminance
• Analogous to CSF in spatial vision
• Like in spatial CSF, peak sensitivity occurs at mid-range temporal frequencies but is worse at lower & higher temporal frequencies
• Our eyes appear to be most sensitive to temporal frequencies range from ~10-20Hz in photopic condition
• High temporal frequency a person can resolve is indicate by the extreme right value – Critical Flicker Fusion Frequency (CFF)
  o This is a temporal counterpart to high spatial cutoff frequency in spatial CSF

Question 11

Describe psychophysical determination of tCSF?

Answer 11

1. Select a temporal freq of grating to be tested, for e.g., 10Hz (c/sec)
2. Determine contrast threshold using one of psychophysical methods (e.g. staircase)
3. Compute the inverse of contrast threshold to get the contrast sensitivity
4. Repeat for other temporal frequencies
5. Plot sensitivity (y-axis) as a function of temporal frequency (x-axis) in log-log scale – temporal contrast sensitivity function (tCSF)

If something flickers over 60Hz, we cannot really see that it is moving – some sensitive people can still see flicker at 60Hz – gaming monitors now go up to 75Hz or 144Hz (basically doubled CFF to provide more smooth visual experience)

Question 12

Describe Critical Flicker Fusion (CFF)?

Answer 12

• As frequency of a spatial grating is increased it appears to consist of increasingly thin bars
  o As frequency of temporally modulated stimulus is increased, the flicker appears more rapid
• In the case of spatial CSF, the bars are not resolvable at high spatial frequency cut off & stimulus appears as a uniform grey surface & the frequencies eventually reach to the point where they cannot be resolved & stimulus appears steady
• This temporal frequency & the CFF represents the high temporal resolution limit of visual system for a given contrast – can be thought of as a temporal acuity
• When refer to CFF, usually mean the high contrast critical flicker fusion like in high spatial frequency cut off in spatial CSF
  o However, if using a lower contrast flicker then the CFF could refer to either the lowest or highest flicker detectable
• Limit of temporal vision – statistical rather than absolute
• Temporal frequencies of a flicker beyond which human eyes cannot perceive as a flicker – its appears to be steady
• When not specified, the CFF almost always refer to the high-frequency CFF
• 15-20Hz for scotopic vision & 60-70Hz for photopic vision

Question 13

Describe tCSF/CFF Change with retinal illuminance?

Answer 13

• No. of factors affecting value of CFF e.g. modulation depth – temporal contrast, background illumination, or eccentricity because of the different distribution of rods & cones
• For e.g. temporal CSF changes w/ increasing retinal illumination
• In general, sensitivity increase w/ greater retinal illumination across a range of temporal frequencies

Question 14

Describe Ferry-Porter Law?

Answer 14

• If plot CFF as function of log retinal illumination then the CFF (100% modulation, high frequency cut-off) increases nearly linearly with the log of the retinal illumination over 4 log units w/ foveal observation
• CFF = klogL+b
  o k = slope of line
  o L = luminance
  o b = a constant

Question 15

Describe Critical Flicker Fusion & Eccentricity?

Answer 15

• As CFF is different for rods & cones, the CFF will change depending on proportion of rods & cones being stimulated
• As proportion of rods & cones change w/ changes w/ changes w/ eccentricity, a foveal test stimulus will follow the Ferry-Porter law & show no kink – just one branch
  o In foveal only cones are present so curve at 0 eccentricity is same as one shown above under “Ferry-Porter Law”
• Extrafoveal test stimuli will now show a kink – 2 branches in CFF as rods now determine the CFF at low retinal illuminances & the cones determining the CFF at higher retinal illuminance
• Ferry-Porter law applies over decreasing range as eccentricity increases & temporal resolution becomes poorer for eccentric locations
• As eccentricity increases, the Ferry-Porter law now breaks down

Question 16

Describe Granit-Harper Law?

Answer 16

• Because of different population of rods & cones in retina & different spatial summation properties, the CFF will be dependent upon the area of retina stimulated
• Instead of varying retinal eccentricity, now the size of the centrally fixated test field is varied here
  o As test field increases, 2 branches begin to appear w/ lower left branch representing the rod function
• Granit-Harper law is relationship between CFF & area of stimulus
  o CFF = klogA + b
   CFF is a function of slope of line (k) and the log of the area. b is the constant
• CFF increases linearly with log of stimulus area – easier to detect a large flicker
• Larger targets fall onto more peripheral retina that is more sensitive to flicker of low contrast

Question 17

Describe decreased sensitivity at low Temporal Frequency?

Answer 17

• Low TFs refer to slowly changing luminance levels
• Humans are very insensitive to slow moving objects or static images
• Stabilised Retinal Images – Purkinje Tree
  o Shape of retinal BVs  do not see these BVs in normal everyday visual activities & the reason has to do with way our visual system ignores the objects w/o any motion contrast
  o BVs are fixed forever w/ respect to retina  as move eye around, they move around together  fixed relationship  pattern of retinal BVs does not change even if move eye
  o Stabilised image: pattern that is fixed w/ respect to retina, even as position of eye changes
  o Our eyes are v insensitive to these stabilised images
• Troxler’s effect:
  o Shows how quickly our sensitivity wanes when an object ceases to move within our VF
  o Temporary & irregular fading or disappearance of a small object in the VF during a state of fixation of another object
  o When we are fixating on something static, we have a constant involuntary & tiny eye movements called microsaccades  this microsaccades in effect creates a tremble in our retinal image like a flicker
   However, changes in contrast of background are so gradual that it does not produce noticeable contrast difference w/ microsaccades.
   However, a sharp boundary against the fading background creates a big enough contrast difference that can be picked up by the microsaccades & work like a temporal flicker