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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
how to record temporal CSF?
. measure modulation depth at which flicker first appears for a range of temporal frequencies . CSF is a band -pass function
26
what does it mean that CSF is band-pass function?
. 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
what is the peak of temporal contrast sensitivity function ?
. 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
why is there a roll-off in sensitivity when we go to lower temporal frequencies ?
. 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
what is high temporal frequency cut-off?
. this is the max temporal frequency resolvable at 100% modulation depth . fastest flicker we can see at maximum stimulus contrast
30
what is high temporal frequency cut-off caused by?
. 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
what does high temporal frequency cut-off shift from?
. 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
what are examples of how our visual system is not able to detect very fast flicker?
. e.g. light bulbs flicker at 60Hz so appear stable
33
what is critical flicker (fusion) frequency (CFF) ?
. 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
what does CFF refer to when not specified ?
. when not specified CFF always refers to upper limit for temporal resolution, at maximum modulation depth . equivalent to grating acuity in spatial domain
35
why are we not able to see very rapid rates of flicker?
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
what does increased critical duration mean?
. lower CFF ( poorer temporal acuity) but improved absolute sensitivity to dim lights ( reduced absolute luminance threshold)
37
what does critical duration depend on ?
. critical duration depends on properties of stimulus and background
38
what is critical duration of rods?
. 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
what is critical duration of cones ?
. cones integrate over a short time period ( approx 10 - 50 ms ) - less sensitive but high CFF ( better temporal acuity )
40
what is another reason to why we are not able to see very rapid rates of flicker ?
2. 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
what happens with high temporal frequency stimulus ?
. the action potential all merged together and we are not able to detect gap between stimuli
42
what factors affecting the CFF?
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 2. 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) 3. stimulus size . Granit-harper law: CFF increases with log stimulus . easier to detect flicker for larger stimuli 4. 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
what is ferry-porter law ?
. CFF increases linearly with log stimulus luminance . ferry-porter law is steepest in the periphery ( fastest processing)
44
rods vs cones?
rods: low temporal resolution - maximum flicker we can detect is 15 Hz ( due to long critical duration) cones: higher temporal resolution ( 50-60Hz)
45
magno vs parvo?
. 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
fovea vs periphery ?
. 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
how is flicker used in clinical test ?
. 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
conditions that affect flicker sensitivity?
. retinitis pigmentosa | . ARMD

OV2023 visual perception (20 decks)

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