Lecture 7: Duplex nature of Retina

Question 1

Compare/contrast scotopic & photopic systems

Answer 1

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)

Question 2

explain photochromatic interval

Answer 2

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

Question 3

explain purkinje shift

Answer 3

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.

Question 4

state the 5 portions of a normal dark adaption curve

Answer 4

cone sensitivity, cone plateau, rod-cone break, rod branch, rod plateau

Question 5

explain cone sensitivity portion of DA curve

Answer 5

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.

Question 6

explain cone plateau portion of DA curve

Answer 6

from t=10 to t=12, cone threshold stops decreasing. this is the maximum sensitivity of cones

Question 7

explain rod-cone break portion of DA curve

Answer 7

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.

Question 8

explain rod branch portion of DA curve

Answer 8

t=12 to about t=35, the rods continue to gradually become more sensitive/threshold gradually decreases as more rhodopsin is generated

Question 9

explain rod plateau of DA curve

Answer 9

eventually, the rods will stop decreasing in threshold. this is the maximum sensitivity of rods. it will not increase after this period of time

Question 10

discuss factors to DA curve

Answer 10

(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 ^^

Question 11

state portions of light adaptation curve

Answer 11

dark light, square root law, weber’s law (scotopic), saturation, weber’s law (photopic)

Question 12

explain dark light portion of LA curve

Answer 12

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

Question 13

explain square root law portion of LA curve

Answer 13

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

Question 14

explain weber’s law (rods) portion of LA curve

Answer 14

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

Question 15

explain saturation portion of LA curve

Answer 15

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

Question 16

explain weber’s law (cones) portion of LA curve

Answer 16

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

Question 17

define stiles-crawford effect of the first

Answer 17

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

Question 18

define spacial summation

Answer 18

how well a system sums up light over an area

Question 19

explain spacial summation in scotopic vs photopic system

Answer 19

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

Question 20

explain temporal summation in scotopic vs photopic system

Answer 20

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)

Question 21

summarise tradeoffs for scotopic/photopic spacial and temporal summation

Answer 21

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

Question 22

explain ricco’s law for spacial summation

Answer 22

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

Question 23

Explain bloch’s law for temporal summation

Answer 23

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