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OV2023 visual perception > temporal vision: Flicker detection > Flashcards

temporal vision: Flicker detection Flashcards

1
Q

what is temporal vision?

A

. is the ability to perceive changes in luminance over time
. this is related by motion perception
. mediated by magnocellular pathway

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2
Q

what is temporal acuity?

A

. is the minimum inter stimulus interval detectable by the visual system

. how close together can 2 flashes of light be presented where we can still see they are 2 flashes of light one after the other, rather than perceiving them as just one steady light?

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3
Q

what happens without temporal vision?

A

we wont be able to see anything

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4
Q

why are we not able to see anything without temporal vision?

A

. this is because we are unable to perceive stabilised retinal images
. spatial vision impossible unless the retinal image changes with time
. if there is no change 0 image will fade in few seconds

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5
Q

how do make images not stable on retina?

A

. by making small involuntary eye movements continuously so image does not stabilise on retina

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6
Q

why don’t we see retinal vasculature ?

A

. because vessels are stabilised relative to retina

( temporal frequency 0HZ)

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7
Q

why do we see vessels ( also known as purkinje tree)

A

. when ophthalmoscopy is performed

. this is because there is a moving light which moves shadows of vessels on retina

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8
Q

what is Troxler effect?

A

. disappearance of low temporal frequency stimuli

. temporal fading of image possible during steady fixation

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9
Q

why do the blurred borders disappear when we steadily fixate the X?

A

. this is because involuntary eye movements across a blurred border result in very low temporal frequency retinal illuminance changes

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10
Q

why does the sharp border not disappear when fixating on the X?

A

. across a sharp border retinal illuminance changes at higher temporal frequency

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11
Q

what kind os stimuli do we use to investigate temporal vision?

A

. usually have luminance which varies sinusoidally over time ( temporal sinusoid)
. produced by light source turning on and off in sinusoidal manner

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12
Q

how do spatial sine waves vary?

A

. vary sinusoidally over space

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13
Q

how do temporal sine waves vary ?

A

. vary sinusoidally over time

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14
Q

what does modulation amplitude describe ?

A

. describes the contrast of temporal stimulus

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15
Q

what happens in high modulation amplitude?

A

. high modulation amplitude = we go from bright flash to really dark amplitude between flashes

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16
Q

what happens in low modulation amplitude?

A

. low modulation amplitude = less variation in light levels , so goes from little bit bright to little bit dark

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17
Q

what is modulation depth ?

A

. temporal contrast of the stimulus

. describes how great change in luminance over time

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18
Q

what does a greater modulation depth mean?

A

. the greater the modulation depth ( temporal contrast ) of a stimulus, the greater the visibility

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19
Q

what does greater spatial contrast mean?

A

. the greater the spatial contrast , the greater the visibility , the easier it is to see a grating

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20
Q

what is equation for modulation depth ?

A

. modulation depth = modulation amplitude / mean luminance

. modulation depth x 100% = percentage modulation

21
Q

what is temporal frequency?

A

. rate of flicker or rate of change of stimulus over time ( in cycles per second)

22
Q

what does low temporal frequency mean?

A

. stimulus appears to flicker/move slowly

23
Q

what does high temporal frequency mean?

A

. stimulus appears to flicker/move quickly

24
Q

how to investigate temporal contrast sensitivity function?

A

. this is done by recording the temporal modulation transfer ( temporal CSF ) characterises temporal vision

25
Q

how to record temporal CSF?

A

. measure modulation depth at which flicker first appears for a range of temporal frequencies
. CSF is a band -pass function

26
Q

what does it mean that CSF is band-pass function?

A

. low temporal frequencies and high temporal frequencies = sensitivity is low and need lot of contrast to detect flicker
. medium spatial frequency = higher sensitivity = so need less contrast

27
Q

what is the peak of temporal contrast sensitivity function ?

A

. peak sensitivity shifts from 5 Hz to 20 Hz with increasing retinal illuminance
. high retinal illuminance = more sensitive to higher rates of flicker
. lower light levels = more sensitive to lower rates of flicker

28
Q

why is there a roll-off in sensitivity when we go to lower temporal frequencies ?

A

. this is caused by lateral inhibition within the retina
. this is why we get
- troxler effect
-blood vessels on retina
- slow changes from day to night not perceived
- movement of minute hand on watch

29
Q

what is high temporal frequency cut-off?

A

. this is the max temporal frequency resolvable at 100% modulation depth
. fastest flicker we can see at maximum stimulus contrast

30
Q

what is high temporal frequency cut-off caused by?

A

. this is caused by limitations in the nervous system’s processing speed and temporal summation

. in order to increase sensitivity of our eye to dim light , our visual system adds together light that falls on retina within certain period of time , this is temporal summation

31
Q

what does high temporal frequency cut-off shift from?

A

. shifts from 15Hz to 60Hz with increasing retinal illuminance
. higher retinal illuminance = faster high temporal frequency cut-off
. high light levels we are able to detect fast flicker

32
Q

what are examples of how our visual system is not able to detect very fast flicker?

A

. e.g. light bulbs flicker at 60Hz so appear stable

33
Q

what is critical flicker (fusion) frequency (CFF) ?

A

. highest or lowest flicker frequency at which flicker can be resolved for a given percentage modulation

. for any temporal contrast , what is the lowest rate of flicker and fastest that is detectable

. measure of temporal resolving power

34
Q

what does CFF refer to when not specified ?

A

. when not specified CFF always refers to upper limit for temporal resolution, at maximum modulation depth
. equivalent to grating acuity in spatial domain

35
Q

why are we not able to see very rapid rates of flicker?

A
  1. temporal summation in photoreceptors
    temporal summation = summation of all light landing on an area of the retina within a certain period of time to give single output

. critical duration = maximum period over which temporal summation can occur ( 10-100 ms in humans )

. any stimuli which fall on same retinal area within temporal summation period will be summed i.e. will appear as single light

36
Q

what does increased critical duration mean?

A

. lower CFF ( poorer temporal acuity) but improved absolute sensitivity to dim lights ( reduced absolute luminance threshold)

37
Q

what does critical duration depend on ?

A

. critical duration depends on properties of stimulus and background

38
Q

what is critical duration of rods?

A

. rods integrate over long time period ( critical duration is approx 100 ms )
. this means improved sensitivity to dim light but poor CFF ( poor temporal acuity)

39
Q

what is critical duration of cones ?

A

. cones integrate over a short time period ( approx 10 - 50 ms ) - less sensitive but high CFF
( better temporal acuity )

40
Q

what is another reason to why we are not able to see very rapid rates of flicker ?

A
  1. modulation of firing rate of retinal ganglion cells
    . RGC produces action potentials in response to flash of light

. v high temporal frequency - no time for neuron to stop responding between flashes

. the more transient the response of the RGC - better temporal resolution ( mango cells are faster than parvo)

41
Q

what happens with high temporal frequency stimulus ?

A

. the action potential all merged together and we are not able to detect gap between stimuli

42
Q

what factors affecting the CFF?

A
  1. retinal illuminance
    - as the luminance of the stimulus increases, CFF increases
  • appearance of flicker on a computer screen can be reduced by reducing monitor luminance
  1. eccentricity ( retinal location )
    - at high luminance CFF increases with eccentricity to a maximum in the mid-periphery
    - max CFF - 90 Hz at 35 degrees
    - foveal - 50 - 60 Hz
    - more sensitive to flicker in the corner
    . at low luminance CFF constant with eccentricity ( rods only)
  2. stimulus size
    . Granit-harper law: CFF increases with log stimulus
    . easier to detect flicker for larger stimuli
  3. wavelength
    . in photopic conditions CFF equal for all wavelength
    . in scotopic conditions, CFF highest for short wavelength
    . rods are more sensitive to short wavelength
43
Q

what is ferry-porter law ?

A

. CFF increases linearly with log stimulus luminance

. ferry-porter law is steepest in the periphery ( fastest processing)

44
Q

rods vs cones?

A

rods: low temporal resolution - maximum flicker we can detect is 15 Hz ( due to long critical duration)
cones: higher temporal resolution ( 50-60Hz)

45
Q

magno vs parvo?

A

. magno - ( parasol) ganglion cells have transient response and good temporal resolution

. M-cells - layers 1 and 2 LGN and enter visual cortex in layer 4c alpha and information travels to thick stripes of V2 and then area MT

. parvo - ( midget ) ganglion cells have sustained response and poor temporal resolution

46
Q

fovea vs periphery ?

A

. periphery has higher temporal resolution than fovea even though more rods/less cones ( M-cells predominate in periphery and cones in periphery process faster)

47
Q

how is flicker used in clinical test ?

A

. an example is frequency doubling perimetry
frequency doubling:
- high temporal frequency , very low spatial frequency grating at high contrast - you will twice as many bars as there actually is

  • frequency doubling test measures contrast threshold at which flicker just appears
  • Humphrey frequency doubling perimeter test at 0.25 cycles per degree at 25 Hz
  • may be sensitive to early Glaucoma
48
Q

conditions that affect flicker sensitivity?

A

. retinitis pigmentosa

. ARMD

OV2023 visual perception (20 decks)

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