temporal vision: Flicker detection Flashcards
what is temporal vision?
. is the ability to perceive changes in luminance over time
. this is related by motion perception
. mediated by magnocellular pathway
what is temporal acuity?
. 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?
what happens without temporal vision?
we wont be able to see anything
why are we not able to see anything without temporal vision?
. 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
how do make images not stable on retina?
. by making small involuntary eye movements continuously so image does not stabilise on retina
why don’t we see retinal vasculature ?
. because vessels are stabilised relative to retina
( temporal frequency 0HZ)
why do we see vessels ( also known as purkinje tree)
. when ophthalmoscopy is performed
. this is because there is a moving light which moves shadows of vessels on retina
what is Troxler effect?
. disappearance of low temporal frequency stimuli
. temporal fading of image possible during steady fixation
why do the blurred borders disappear when we steadily fixate the X?
. this is because involuntary eye movements across a blurred border result in very low temporal frequency retinal illuminance changes
why does the sharp border not disappear when fixating on the X?
. across a sharp border retinal illuminance changes at higher temporal frequency
what kind os stimuli do we use to investigate temporal vision?
. usually have luminance which varies sinusoidally over time ( temporal sinusoid)
. produced by light source turning on and off in sinusoidal manner
how do spatial sine waves vary?
. vary sinusoidally over space
how do temporal sine waves vary ?
. vary sinusoidally over time
what does modulation amplitude describe ?
. describes the contrast of temporal stimulus
what happens in high modulation amplitude?
. high modulation amplitude = we go from bright flash to really dark amplitude between flashes
what happens in low modulation amplitude?
. low modulation amplitude = less variation in light levels , so goes from little bit bright to little bit dark
what is modulation depth ?
. temporal contrast of the stimulus
. describes how great change in luminance over time
what does a greater modulation depth mean?
. the greater the modulation depth ( temporal contrast ) of a stimulus, the greater the visibility
what does greater spatial contrast mean?
. the greater the spatial contrast , the greater the visibility , the easier it is to see a grating
what is equation for modulation depth ?
. modulation depth = modulation amplitude / mean luminance
. 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)
what does low temporal frequency mean?
. stimulus appears to flicker/move slowly
what does high temporal frequency mean?
. stimulus appears to flicker/move quickly
how to investigate temporal contrast sensitivity function?
. this is done by recording the temporal modulation transfer ( temporal CSF ) characterises temporal vision
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
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
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
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
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
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
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
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
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
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
why are we not able to see very rapid rates of flicker?
- 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
what does increased critical duration mean?
. lower CFF ( poorer temporal acuity) but improved absolute sensitivity to dim lights ( reduced absolute luminance threshold)
what does critical duration depend on ?
. critical duration depends on properties of stimulus and background
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)
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 )
what is another reason to why we are not able to see very rapid rates of flicker ?
- 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)
what happens with high temporal frequency stimulus ?
. the action potential all merged together and we are not able to detect gap between stimuli
what factors affecting the CFF?
- 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
- 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) - stimulus size
. Granit-harper law: CFF increases with log stimulus
. easier to detect flicker for larger stimuli - wavelength
. in photopic conditions CFF equal for all wavelength
. in scotopic conditions, CFF highest for short wavelength
. rods are more sensitive to short wavelength
what is ferry-porter law ?
. CFF increases linearly with log stimulus luminance
. ferry-porter law is steepest in the periphery ( fastest processing)
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)
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
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)
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
conditions that affect flicker sensitivity?
. retinitis pigmentosa
. ARMD
