Topic 7 Duplex Nature Of Retina Flashcards
what wavelength is rhodopsin most sensitive?
Rhodopsin is most sensitive to light at 507nm.
rhodopsin doesn’t need a 507nm-light to be very bright in order for it to be detected by an observer. In contrast, a 700nm-light will need to brighter in comparison for it to be detected by an observer.
what is the peak sensitivity of the 3 photopigments? what is their combined peak?
L cones contain erythrolabe, which is sensitive to long wavelength light (peak sensitivity at 565nm)
M cones contain chlorolabe, which is sensitive to medium wavelength light (peak 535nm)S cones contain cyanolabe, which is sensitive to short wavelength light (peak 430nm)
Combined, they create a single broad peak at 555nm
what are the characteristics of scotopic and photopic vision?
Scotopic vision
is in operation when light levels are below 0.1 cd/m2 (e.g. nighttime).
is mediated by rod photoreceptors. It is very sensitive to dim light but has poor visual acuity (6/60) and there is no colour discrimination.
Photopic vision
is in operation when light levels are between 10 – 107 cd/m2 (daytime) whereas
is mediated by cones. It has poor sensitivity under dim conditions, but has excellent acuity (6/6) and colour discrimination.
When light levels are in the mesopic range, both photopic and scotopic systems will be in use.
what is photochromatic interval?
The difference in sensitivity between the scotopic and photopic systems is referred to as the photochromatic interval.
describe the relationship between scotopic and photopic system from 350- just before 650nm and after 650 nm?
Around 350nm to just before 650nm, the scotopic system has higher sensitivity than photopic system
The 2 lines meet (scotopic and photopic systems are equally sensitive to that wavelength) around 650nm. After 650nm, the photopic system is slightly more sensitive to light than the scotopic system.•
what is purkinje shift?
The change in peak spectral sensitivity towards shorter wavelengths (blue) in scotopic conditions compared to in photopic conditions is known as the Purkinje shift
The reason for this shift is the different spectral sensitivities of the scotopic compared to photopic systems, which peak at 507nm and 555nm respectively
what are the 5 sections of a DA curve?
0-10 mins: cone sensitivity
quick reduction in threshold (quick improvement in sensitivity)
cones are not very sensitive in dim conditions, the threshold is quite high.
10-12 mins: cone plateau
sensitivity does not increase. This is the cone plateau. This plateau represents the absolute threshold of the photopic system.
During this period (time = 0 to < 12 min), rods are not in use as they are still recovering from the photobleaching.
approx 12 mins: rod-cone break
sudden drop in threshold again (sensitivity increases). This drop is known as the rod-cone break and represents the point in time when rods have recovered from bleaching enough to surpass cones in sensitivity.
12-35 mins: Rod sensitivity continues increasing
the rods gradually increase in sensitivity as more rhodopsin is regenerated.
end of graph: Maximum rod sensitivity
The tail end of the curve where rod curve plateaus represents the absolute threshold of the scotopic system (ie maximum sensitivity of rods)
what are the 4 factors affecting DA?
- Intensity and/or duration of pre-adapting light
- Size of the stimulus
- Retinal location that the stimulus falls on
- Wavelength of stimulus light used
how does Intensity and/or duration of pre-adapting light affect DA?
The brighter or longer the duration of pre-adapting light, the more photopigment is bleached. Rhodopsin will take a longer time to regenerate hence it will take longer for the rods to overtake cones in sensitivity.
This translates into a delayed rod-cone break. the absolute threshold for rods will also take longer time to be reached.
If the pre-adapting light is so dim or of such short duration as to bleach very little rhodopsin, we might not see the cone portion of the curve at all. This is because from the beginning of the experiment time=0, rods would already be sensitive enough to be used.
how does size of stimulus affect DA?
if the test spot is very small and central, only cone sensitivity will be captured.
If the test spot is large enough to include rods, then both rod and cone portions of the curve can be captured.
•As the size of the test spot increases and more rods are included in the measurement, the absolute sensitivity of the rod portion of the curve increases.
Retinal location that the stimulus falls on
Fovea: only cones present so only cone branch captured
Periphery: both rods and cones present so both rod and cone branches will be captured.
how does WL of stimulus used affect DA?
Long wavelength used, there will be no rod-cone break.
At long wavelengths, rods and cones have similar sensitivity.
\Using a short wavelength light will give the most prominent rod-cone break. Again, because the photochromatic interval is greatest at the short – medium wavelengths
what are the 4 sections of the rod branch of the LA curve?
- dark light
- Square root law
- weber’s law of rods
- Rod saturation
describe what happens during dark light
as background luminance increases, the gradient remains constant.
This shows that the retinal sensitivity is limited by neural noise / dark light.
describe what happens during square root law? What is retinal sensitivity limited by here?
Gradient= 0.5
↑in threshold is proportional to the square root of background luminance
Retinal sensitivity is limited by quantal fluctuation in the background.
describe what happens during weber’s law? what is webers constant for rods?
Gradient = 1
Relationship between background luminance and increment threshold is constant
As the background brightness is increased, the increment intensity must be increased such that the ratio of the increment intensity to the background intensity remains constant.
Weber constant for rods: 0.14
