Lecture 7: Duplex nature of Retina Flashcards
Compare/contrast scotopic & photopic systems
S. vs P.
mediated by: rods vs cones
light sensitivity: higher vs lower
used at light levels: 0.1cd vs 10-107cd
photopigment: rhodopsin (507nm) vs cyanolabe (430), chlorolabe (535), erythrolabe (565)
explain photochromatic interval
it is the difference in peak sensitivity between scotopic and photopic systems.
from 350-650nm, scotopic system is more sensitive than photopic
@650nm, scotopic and photopic systems are equally sensitive
explain purkinje shift
the change in peak spectral sensitivity towards shorter wavelengths at low illumination levels (scotopic conditions) vs at high illumination levels (photopic conditions)
this change is due to the different spectral sensitivities between scotopic and photopic systems (507 & 555)
purkinje shift occurs when daytime changes to nighttime since scotopic system will replace photopic system.
state the 5 portions of a normal dark adaption curve
cone sensitivity, cone plateau, rod-cone break, rod branch, rod plateau
explain cone sensitivity portion of DA curve
at t(min)=0, there is a quick reduction in cone threshold until about t=10. since cones are not very sensitive in dim conditions, threshold at t=10 is quite high.
explain cone plateau portion of DA curve
from t=10 to t=12, cone threshold stops decreasing. this is the maximum sensitivity of cones
explain rod-cone break portion of DA curve
at t=12, there is a sudden drop in threshold. This is the point in time when the rods have finished unbleaching/recovered enough to surpass cones in sensitivity.
explain rod branch portion of DA curve
t=12 to about t=35, the rods continue to gradually become more sensitive/threshold gradually decreases as more rhodopsin is generated
explain rod plateau of DA curve
eventually, the rods will stop decreasing in threshold. this is the maximum sensitivity of rods. it will not increase after this period of time
discuss factors to DA curve
(1) pre adapting light: brighter/longer duration leads to delayed R-C break since it will take longer for rhodopsin to regenerate VS dimmer/shorter duration leads to cone portion not seen as very little rhodopsin is bleached and rods would be sensitive enough to be used right at t=0
(2) stimulus wavelength: long (650nm & onwards) result in no R-C break because scotopic and photopic systems are equally sensitive VS short(-med) results in very prominent R-C break. photochromatic interval is greatest at short-med wavelengths
(3) size of stimulus: small/central results in no rods captured VS big/inclusive of rods results in both rods and cones captured (the more rods the stimulus includes, the higher the absolute rod sensitivity)
(4) retinal location ^^
state portions of light adaptation curve
dark light, square root law, weber’s law (scotopic), saturation, weber’s law (photopic)
explain dark light portion of LA curve
it is the first segment of LA curve (m=0). as the BG luminance increases, there is no increment of stimulus due to presence of neural noise. neural noise/dark light limits retinal sensitivity
NOTE: neural noise is the ongoing activity in retina even when not stimulated
explain square root law portion of LA curve
NOTE: light source/stimulus is constantly fluctuating. brightness of light source is not constant
m=1/2. increment threshold will increase directly proportionate to the square root of background luminance. this is due to the fluctuations of the stimulus intensity
light must sufficiently exceed BG fluctuation to be detected
explain weber’s law (rods) portion of LA curve
m=1. increment threshold will increase directly proportionate to BG luminance. this is to keep the contrast constant for stimulus to be detected
WBL CONSTANT (rods) = 0.14
eg. BG=1cd/m^2, stimulus must be 0.14cd/m^2 BRIGHTER to be detected
explain saturation portion of LA curve
m=infinity. at high BG luminance, rods independent of stimulus increment. rods do not perceive the stimulus as getting brighter anymore .
this is due to a significant amount of rhodopsin being bleached
explain weber’s law (cones) portion of LA curve
m=1. increment threshold will increase directly proportionate to BG luminance. this is to keep the contrast constant for stimulus to be detected
WBL CONSTANT (rods) = 0.015
eg. BG=50cd/m^2, stimulus must be 0.75cd/m^2 BRIGHTER to be detected
define stiles-crawford effect of the first
this effect only applies to cones where if the light strikes cones at a perpendicular angle, it will be perceived as the brightest.
this is because of the waveguide properties of cones. The funnel shape of cones it funnels the light in and bleaches more ppigments in this angle of light striking
define spacial summation
how well a system sums up light over an area
explain spacial summation in scotopic vs photopic system
scotopic system has bigger area to sum up light because there are many rods connected to a single ganglion cell layer (10,000:1) - hence signals from many rods get summed up
since scotopic has bigger spacial summation, even in dim light it still collects a substantial amount and hence able to still detect it
TRADEOFF: when summative field is large, resolution is compromised
VS photopic system with small summative field (fewer cones connected to single ganglion cell layer - 100:1). hence it does not have the ability to sum up as much light especially so in dim conditions
explain temporal summation in scotopic vs photopic system
scotopic = longer temporal summation period (100ms)
photopic = shorter temporal summation period (10-50ms)
2 factors to NOTE:
as long as light falls within one summation period/window, it will be perceived as one light PROVIDED that the subthreshold pulse(s added together) is bright enough to be detected
If light is of sufficient brightness, it will be perceived as flashing ONLY if it exceeds one summation window (more than 100ms for scotopic & more than 50ms for photopic)
summarise tradeoffs for scotopic/photopic spacial and temporal summation
S vs P
spacial: greater summative field hence greater sensitivity, poorer resolution VS smaller summative field hence poorer sensitivity, greater resolution
temporal: longer summation period allows more summation to detect dimmer light but poorer resolution VS shorter period hence greater resolution (detect faster flicker of light) but poorer sensitivity
explain ricco’s law for spacial summation
States that when a light source of given size and intensity is just capable of producing visual sensation, reduction of either size or intensity will make it invisible
I * A = constant
Brighter light can be smaller in size to be detected
Dimmer light needs to be larger in size to be detected
Explain bloch’s law for temporal summation
States that the detectability of otherwise identical visual stimuli depends only on its energy - product of illuminance and duration
I * T = constant
Eg. Light flashed for 30ms at 80cd/m^2 is equally detectable as light flashed for 60ms at 40cd/m^2
