Temporal Vision: Flicker Detection Flashcards
What is temporal vision?
Temporal vision is the ability to perceive changes in luminance over time.
What is temporal acuity?
Temporal acuity is the minimum interstimulus interval detectable by the visual system i.e. the fastest speed of flashing lights we can detect.
Which pathway is temporal vision mediated by?
The magnocellular pathway
True or False- We are unable to perceive stabilised retinal images.
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.
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
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.
How may one see a purkinje tree image and what is this?
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.
What is the Troxler Effect?
The disappearance of low temporal frequency stimuli –> temporary fading of image possible during steady fixation
What is the difference between a spatial sine wave and a temporal sine wave?
Spatial sine wave varies sinusoidally over space.
Temporal sine wave varies sinusoidally over time.
What is modulation amplitude/depth?
It is the temporal contrast of the stimulus i.e how bright and dark the flashing light gets. In the diagrams it is A.
Is visibility higher or lower if the modulation depth stimuli is greater?
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]
What is the difference between modulation depth and modulation amplitude?
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.
How do you calculate the percentage modulation?
Modulation depth x 100% = Percentage Modulation
What is temporal frequency?
Rate of flicker or rate of change of stimulus over time (in cycles per second i.e. Hz).
What does a low temporal frequency mean?
Low temporal frequency – stimulus appears to flicker/move slowly
What does a high temporal frequency mean?
High Temporal frequency – stimulus appears to flicker/move quickly
What is relative sensitivity in regards to luminance and describe what a high relative sensitivity means and a low relative sensitivity means?
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.
What does the Temporal Contrast Sensitvity function look like and how do you interpret it?
What is Low-temporal frequency reduction in temporal Contrast sensitvity (termed ‘low-frequency roll-off’) ?
What are examples of this?
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’]
What is ‘low frequency roll-off’ caused by?
Lateral inhibition of the retina
What is high temporal frequency cut off?
Max temporal frequency resolvable at 100% modulation depth (i.e. at 100% luminance contrast).
What is Critical Flicker Frequency (CFF)?
Highest or lowest flicker frequency at which flicker can be resolved for a given percentage modulation.
It is a measure of resolving power.
Describe the Neural limitation of the Critical Flicker Frequency
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.
How does retinal illuminance affect CFF?
What is Ferry-Porter’s law?
As the luminance of the stimulus increases, CFF increases
Ferry-Porter’s law- CFF increases linearly with log stimulus luminance.
How does Eccentricity affect CFF?
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.
