Temporal Vision: Flicker Detection

Question 1

What is temporal vision?

Answer 1

Temporal vision is the ability to perceive changes in luminance over time.

Question 2

What is temporal acuity?

Answer 2

Temporal acuity is the minimum interstimulus interval detectable by the visual system i.e. the fastest speed of flashing lights we can detect.

Question 3

Which pathway is temporal vision mediated by?

Answer 3

The magnocellular pathway

Question 4

True or False- We are unable to perceive stabilised retinal images.

Answer 4

True- Spatial vision impossible unless the retinal image changes with time. If there is no change the images fade in a few seconds Thus even when focussing we make small involuntary eye movements continuously so image does not stabilise on retina.

Question 5

Why don’t we see the shadows of retinal vasculature on photoreceptors? Essentially these vessels are on the retina so would therefore obscure blood vessels

Answer 5

We don’t see them because vessels are stabilised relative to the retina (temporal frequency 0 Hz) i.e. the retinal image produced by the vessels i.e. the shadows do not move and any image on the retina that doesn’t move fades away.

Question 6

How may one see a purkinje tree image and what is this?

Answer 6

A purkinje tree image is an image formed by the shadows of retinal blood vessels. In order to see this you would keep having to move a light around the eyes so that the retinal vessel shadows would keep falling in different places giving rise to the purkinje tree.

Question 7

What is the Troxler Effect?

Answer 7

The disappearance of low temporal frequency stimuli –> temporary fading of image possible during steady fixation

Question 8

What is the difference between a spatial sine wave and a temporal sine wave?

Answer 8

Spatial sine wave varies sinusoidally over space.

Temporal sine wave varies sinusoidally over time.

Question 9

What is modulation amplitude/depth?

Answer 9

It is the temporal contrast of the stimulus i.e how bright and dark the flashing light gets. In the diagrams it is A.

Question 10

Is visibility higher or lower if the modulation depth stimuli is greater?

Answer 10

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

[Just as in spatial vision – the greater the spatial contrast the greater the visibility]

Question 11

What is the difference between modulation depth and modulation amplitude?

Answer 11

Modulation amplitude is from the peak or from the trough to the midway average line.

Modulation depth is an equation which takes into account the modulation amplitude divided by the mean luminance.

Question 12

How do you calculate the percentage modulation?

Answer 12

Modulation depth x 100% = Percentage Modulation

Question 13

What is temporal frequency?

Answer 13

Rate of flicker or rate of change of stimulus over time (in cycles per second i.e. Hz).

Question 14

What does a low temporal frequency mean?

Answer 14

Low temporal frequency – stimulus appears to flicker/move slowly

Question 15

What does a high temporal frequency mean?

Answer 15

High Temporal frequency – stimulus appears to flicker/move quickly

Question 16

What is relative sensitivity in regards to luminance and describe what a high relative sensitivity means and a low relative sensitivity means?

Answer 16

Relative Sensitvity is 1/percentage modulation.

If sensitivity is high you can detect dim contrasts.

If sensitity is low you can only detect high/obvious contrasts.

Question 17

What does the Temporal Contrast Sensitvity function look like and how do you interpret it?

Answer 17

Question 18

What is the peak temporal contrast sensitivity?

Answer 18

Peak sensitivity shifts from 5 Hz to 20 Hz with increasing retinal illuminance i.e. frequency increases as luminance contrast increases in that range of 5 to 20 cycles per second.

Question 19

What is Low-temporal frequency reduction in temporal Contrast sensitvity (termed ‘low-frequency roll-off’) ?

What are examples of this?

Answer 19

It’s when we don’t percieve really slow moving things despite whether they have a high luminance or not.

Examples of this is are:

The Troxler effect

Fact that we don’t notice the sky going from light to dark because it is so slow and gradual

Blood vessels on the retina

The slow movement of the minute hand on the clock

[The red circle in the graph represents the area of ‘low frequency roll-off’]

Question 20

What is lateral inhibition?

Answer 20

lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbors

Question 21

What is ‘low frequency roll-off’ caused by?

Answer 21

Lateral inhibition of the retina

Question 22

What is high temporal frequency cut off?

Answer 22

Max temporal frequency resolvable at 100% modulation depth (i.e. at 100% luminance contrast).

Question 23

What is the average high temporal frequency cut-off?

Answer 23

It shifts from 15 Hz to 60 Hz with increasing retinal illuminance.

Question 24

How does something flickering above our high temporal frequency cut off appear?

Answer 24

It appears stationary. e.g.

Light bulbs flicker at 60 Hz – appear stationary.

Question 25

What is the high temporal frequency cut off point limited by?

Answer 25

Limitations in the nervous system’s processing speed and temporal summation.

Question 26

What is Critical Flicker Frequency (CFF)?

Answer 26

Highest or lowest flicker frequency at which flicker can be resolved for a given percentage modulation. It is a measure of resolving power.

Question 27

When the percentage modulation hasn't been specified and on,y one value has been given what does the Critical Flicker Frequency (CFF) refer to?

Answer 27

When not specified CFF always refers to upper limit for temporal resolution, at maximum modulation depth.

Question 28

What is temporal summation in photoreceptors?

Answer 28

Temporal summation = Summation of all quanta landing on an area of the retina within a certain period of time to give single output. i.e: Any stimuli which fall on same retinal area within the temporal summation period will be summed i.e. will appear as SINGLE LIGHT.

Question 29

What is the critical duration?

Answer 29

The maximum period over which temporal summation can occur (10 – 100 ms in humans).

Question 30

How does a reduced critical duration affect critical frequency flicker (CFF) and light sensitivity?

Answer 30

Increased critical duration à lower CFF (poorer temporal acuity) but improved absolute sensitivity to dim lights (- this would mean a reduced absolute luminance threshold). [A luminance threshold is the lowest perceivable absolute values - i.e. you can detect to a smaller degree in this case]

Question 31

What factors is the critical duration dependant on?

Answer 31

Critical duration depends on properties of stimulus and background.

Question 32

Which type of photoreceptor has a longer critical duration?

Answer 32

Rods integrate over a long time period - i.e. have a longer critical duration Cones integrate over a short time period thus have a shorter critical duration

Question 33

Describe the Critical Flicker Frequency (CFF) and light sensitvity of Rods

Answer 33

Rods integrate over a long time period (critical duration approx 100 ms) → improved light sensitivity but low CFF (poor temporal acuity).

Question 34

Describe the Critical Flicker Frequency (CFF) and light sensitvity of Cones

Answer 34

Cones integrate over a short time period (approx 10-50ms) →lower luminance sensitvity but high CFF (better temporal acuity).

Question 35

Describe the Neural limitation of the Critical Flicker Frequency

Answer 35

Retinal Ganglion Cells produces action potentials in response to flashes of light. When there is a very high temporal frequency →no time for neuron to stop responding between flashes thus brain interpretes as one stationary light source. The more transient the response of the RGC (Retinal Ganglion Cell) → better temporal resolution.

Question 36

Which cells, magno or parvo, have better temporal acuity and why?

Answer 36

Magno cells are better for temporal acuity as they give out short bursts of action potentials allowing you to detect a quicker rate of flickering whereas parvo cells give out more prolonged responses.

Question 37

What factors affect CFF?

Answer 37

1) Retinal Illuminance 2) Eccentricity 3) Stimulus Size 4) Wavelength

Question 38

How does retinal illuminance affect CFF? What is Ferry-Porter's law?

Answer 38

As the luminance of the stimulus increases, CFF increases Ferry-Porter's law- CFF increases linearly with log stimulus luminance.

Question 39

How does Eccentricity affect CFF?

Answer 39

At high luminance CFF increases with eccentricity to a maximum in the mid-periphery. We are more sensitive to flicker in our mid periphery than central vision WITH CONES i.e. sometimes when you look at a light with your fovea it appears still but when you look at it from the corner of ur eye it will appear flickering. Magno cells in periphery respond faster than those in periphery – evolutionary POV - you would want to detect something from the corner of your eye about to eat u. WITH RODS – CFF is constant all over no eccentricty.

Question 40

How does Stimulus size affect CFF? What is Grannit-Harper Law?

Answer 40

Granit-Harper law →CFF increases with log stimulus size →Thus Easier to detect flicker for larger stimuli.

Question 41

How does Wavelength affect CFF?

Answer 41

In photopic conditions CFF equal for all wavelengths In scotopic conditions, CFF highest for short wavelengths. (because rods are more sensitive to short wavelengths thus it is easier to detect flashing blue lights than flashing red lights in scotopic conditions)

Question 42

Which photoreceptor has a higher temporal frequency?

Answer 42

Rods: low temp. resolution ~ 15 Hz (long critical duration) Cones: Higher temporal resolution (50-60 Hz)

Question 43

Which area, fovea or periphery, has a higher temporal frequency?

Answer 43

Periphery has higher temporal resolution than fovea even though more rods/less cones (as Magno- cells predominate in periphery & cones in periphery process faster)

Question 44

What is frequency doubling perimetry and what may it be used for?

Answer 44

The Frequency doubling test measures contrast threshold at which flicker just appears. It may be sensitive to early Glaucoma.