Light perception Flashcards
what was the objective of the experiment on absolute threshold performed by, Hecht, Schlaer and Pirenne
to determine the minimum amount of light/photons needed to elicit a visual response i.e. for the eye to detect it
list the 5 experimental factors that were investigated in the experiment on absolute threshold performed by, Hecht, Schlaer and Pirenne
- stimulus location
- the state of the subject’s eye: i.e. dark adaptation (sitting in the dark)
- wavelength of the test flash: spectral sensitivity curve (λ)
- the size of the stimulus: spatial summation
- duration of the test flash/stimulus: temporal summation (which is also linked to the size of the stimulus)
where in the fundus is the peak density of cones and only the cones located
the centre of the fovea
which type of cones are only present at the centre of the fovea and what does that cause as a result
only L and M cones = small field tritanopia
what is not present at the centre of the fovea
rods
at which eccentricity from the fovea does the rod density peak
20 degrees from the fovea is where rod density peaks
what are the rods that are most dense at 20 degrees from the fovea good at doing here
can detect the dimmest stimulus, as it has the strongest response at 20 degrees away from the fovea
when do transient cells only respond and why
when there are changes in light, as we respond to change and don’t like steady fixation
what do our eyes do as a result of not liking/wanting a steady image
our eyes make continual small movements i.e. drifts and tremors avoid a steady image, even if we try to sustain a steady image, we will have tremors
why do our eyes make tremors, even when we try to sustain a steady image
because our sustained cells get bored if we keep our fixation on them for too long
what do sustained cells do
give a steady response with light falling on them, but get bored i.e. switch off so they’re not sustained permanently
what is the first stage of the state of a subject’s eye during dark adaptation
very bright light causes rhodopsin and cone pigments to be bleached. since the system is saturated, there is no response to changes in luminance from both rods and cones
what happens 5 minutes into dark adaptation, after the rhodopsin and cone pigments are bleached
rods start to recover, but as cones are more sensitive they dominate i.e. the cones recover more quickly than the rods, so the cones can respond to changes and the threshold needed to get a response from the visual system starts to fall
what happens 10 minutes into dark adaptation, after the cones start to recover more quickly than the rods and the threshold needed to get a response from the visual system starts to fall
the cones are now as sensitive as they can get giving the lowest threshold
what happens 10 >min into dark adaptation, once the cones are as sensitive as they can get and give their lowest threshold
the rod-cone break occurs, where cones are as sensitive as they can get so can’t do better here (don’t help), and as rods are now more sensitive they take over as the more sensitive system
when into dark adaptation do rods start to recover i.e. to become more sensitive after the lights go out
5-10 min
how long does it take in dark adaptation to reach maximum sensitivity when in the dark
30-40 min
list the 4 main points that occur in dark adaptation
- rods and cones start dark adapting, rods start bit after the cones
- the more sensitive system determines the threshold at any one time, i.e. the cones are more sensitive in the beginning so they determine the first curve at the start of dark adaptation
- cones adapt faster than rods at 8-10 minutes vs 30 minutes for rods
- the lowest threshold obtained with rods is much less than that with cones, because they’re more sensitive
list the 4 factors that affect us with adapting to different light levels
- rods and cones
- pupil size
- concentration of photopigment: rods and cones don’t have equal/same concentrations of photopigment
- neural responses
what causes the normal retina to look orange
photopigments absorb blue-green and a bit of yellow light, leaving red-yellow light to be reflected
when does light reflected back from the eye appear white
in a bleached retina, when photopigments are saturated and can’t absorb anymore, so all the light is reflected back
what demonstrates how photopigment density/concentration can contribute to adaptation
there are changes in threshold with photopigment concentration, different amounts of initial bleaching of photopigments shows that they recover at different times.
the cone part and the rod part of dark adaptation contributes to the recovery of concentration of photopigments
e.g. the rhodopsin recovery time matches that of the dark adaptation curve i.e. rod only recovery
what does neural responsiveness show as a factor that affects us with adapting to different light levels
the changes in responsiveness in retinal cells, other than our photoreceptors
what is neural responsiveness faster than in relation to adapting to different light levels
faster than photopigment changes i.e. recovery
which other retinal cell shows adaptation to light, other than photoreceptors and how does it compare to photoreceptors
light adaptation is also present in RGCs and shows faster recovery in light adaptation than photoreceptors
between which wavelengths does the human eye respond
between approx. 380-780nm
at which wavelength does the eye response maximally to
555nm
what is the v λ curve also called
photopic response curve
when does the v λ curve change, and how
changes when its dark
it shifts our scotopic curve to the left as rods is more responsive i.e. sensitive to blue short wavelength light = perpendry shift
what is a perpendry shift in our v λ curve
when changes in the dark shifts our v λ curve to the left as rods is more responsive i.e. sensitive to blue short wavelength light
which two ways do you get a maximum sensitivity for luminance and most accurate response from rods
- have to get the wavelength right i.e. one that responds to the peak sensitivity for rods which is 498nm
and - at the right place i.e. 20 degrees away from the fovea
= maximum sensitivity for luminance detection in rods which give the most accurate response
what does the rod threshold curve show the peak rod sensitivity to be at as stated by Hecht, Schlaer and pirenne, and what has it been changed to
was chosen to be at 510nm by Hecht, Schlaer and pirenne, but now the rod threshold curve is said to peak at 498nm
what does how we respond to light depend on? in relation to the stimulus
depends on the wavelength of the stimulus
which types of wavelengths do we respond to
we respond to differently to different wavelengths
what is a receptive field
an area of space which we respond to equally
e.g. if we flash a stimulus in a receptive field, it doesn’t matter where, you can’t tell the difference
how are photoreceptors wired as we go away from the fovea and what is this called
groups of photoreceptors get wired together = spatial summation
(instead of single wired photoreceptors at the fovea)
how do rods lead to spatial summation
a large number of rods can synapse to a single RGC, giving a larger receptive field which leads to spatial summation
what is spatial summation
summing the number of different photoreceptors
i.e. groups of photoreceptors get wired together
how are rods more sensitive
as groups of them are wired together and synapse to a single RGC = single receptive field
what will happen with parafoveal cones synapsing to 3 RGCs when the same light is falling on the peripheral cones
it produces independent (lower) response from the RGCs
from which area of the retina do you get less summation
the closer to the fovea you go
why do you get less summation the closer to the fovea you go
there’s less synapses of photoreceptors to a single RGC therefore it is possible to resolve/discriminate between two patches of light
what is the outcome when a number of rods are synapsing to a single RGC, and a light happened to be incident on the peripheral rod
light on the peripheral rod sums to produce a response from the RGC, e.g. light coming from the left and another coming from the right, gives of a single response and can’t tell that they are both/two lights falling on a single receptive field, so you get a single percept from it i.e. you don’t see two lights = more summation
what does Ricco’s law and critical area state about when threshold is reached
threshold is reached if, luminance x area ( that falls on the retina) reaches a threshold constant (constant value)
what does Ricco’s law state about how lower luminances can reach the same threshold as brighter luminances
lower luminances with larger areas can reach the same threshold as a small area and bright stimulus. as long as the area/luminance is the same
what does Ricco’s law state about the stimulus area in relation to where summation occurs
stimulus area must be less than the critical area, over which summation occurs
what does Ricco’s law state about what detection depends on for large stimuli
for large stimuli, detection only depends on the luminance i.e. how bright it is
to whether we detect it and doesn’t depend on the critical area and how big it is and is relative to the critical area
according to Ricco’s law, what is the critical area of the parafovea (4-7 degrees) where we get spatial summation in
30’
according to Ricco’s law, what is the critical area of the periphery (35 degrees) where we get spatial summation in
2 degrees
what is the temporal summation window
10-100 msec = which our eyes sum information arriving within this time
why do we need a short temporal summation of between 10-100 msec
so that images don’t build up on top of one another
what will happen if two flashes arrive within the temporal summation window
the subject reports a single flash
what is Bloch’s law in relation to Ricco’s law
it is analogous (comparable) to Ricco’s law - temporal summation gives rise to Bloch’s law
what does Bloch’s law state
luminance x time = a constant
what does Bloch’s law state about a lower luminance and time in reaching threshold and give an example of this
a lower luminance for longer can have all the photons summed up to reach the threshold
e.g. within 100 msec (temporal summation) you can have a very short bright stimulus, and you can have one which is within 20 msec, it is the same effect if you have a 80 msec temporal summation which is 4x less bright because the duration is 4x longer and the brightness is 4x less, so when you multiply them together, you will still reach the constant thats needed for threshold. all you need is a stimulus thats less than 100 msec to keep in temporal summation window in order to avoid problems exceeding temporal summation
what does Hecht et al state about light reaching threshold detection
eyes that can detect between 5 and 14 photons of light could reach the threshold for detection
what is the equation for the amount go photons of light reaching threshold for detection, as stated by Hecht et al
E = h x v
what is the minimum luminance that can be detected under practical situations, as stated by Hecht et al
the minimum luminance that can be detected is 0.75 x 10-6 cdm-2 = 5-30% of the darkest sky measured
what 2 things does light perception depend on
- stimulus attributes e.g. brightness, location, size, wavelength, duration
- neural factors e.g. adaptation, temporal and spatial summation
list the 5 stimulus attributes that light perception depends on
- brightness
- location
- size
- wavelength
- duration
list 3 neural attributes that light perception depends on
- adaptation
- temporal summation
- spatial summation
what does troller fading demonstrate
that a stable image is not desirable and it is not achieved under normal conditions i.e. you don’t need a stable image
