VF - Dark Adaptation - Week 8

Question 1

Define light adaptation.

Answer 1

Increased ambient illumination descreases sesnsitivity and increases threshold.

Question 2

Define dark adaptation.

Answer 2

Decreasing ambient illumination increases sensitivity, and decreases threshold.

Question 3

When is dark adaptation most noticeable?

Answer 3

Large reductions in ambient light.

Question 4

Name 4 mechanisms by which the visual system adapts to a wide range on light levels in order of importance.

Answer 4

Duplex retina
Changes in pupil size
Changes in photopigment concentration
Changes in neural responsiveness

Question 5

Describe what is meant by a duplex retina.

Answer 5

Having a cone and rod system, each deals with about half the range of light levels.

Question 6

Describe the procedure to generate a dark adaptation map.

Answer 6

Exposure to bright adapting light.
Turned off at t=0.
Test flashes are used to measure threshold during recovery.

Question 7

On a dark adaptation map, what do the two branches represent?

Answer 7

The top left one is the cone system, and the bottom right one is the rod system.
The junction at which they meet is the rod-cone break.

Question 8

During early recovery, what is more sensitive, cones or rods? How do test flashes appear? What about late recovery times?

Answer 8

Cones are more sensitive during early recovery, flashes appear coloured and sharp.
Rods are more sensitive during late recovery times, test flash appears fuzzy and grey.

Question 9

What adapts quicker, the cones or the rods, and how is this represented on a dark adaptation map?

Answer 9

Cones adapt faster, this can be seen by a steeper curve on a dark adaptation map.

Question 10

Name 4 parameters of the test flash.

Answer 10

Retinal location
Size
Duration
Wavelength

Question 11

Name 3 parameters of the adapting light.

Answer 11

Intensity
Duration
Wavelength

Question 12

What happens to the dark adaptation map with increased test flash eccentricity?

Answer 12

The rod curve is shifted downwards.

Question 13

What happens to the dark adaptation map with increased test flash diameter?

Answer 13

The rod curve is shifted downwards.

Question 14

What happens to the dark adaptation map if the test flash uses peak sensitivity wavelengths?

Answer 14

The rod curve is shifted downwards.

Question 15

True or false

The duplicity model cannot be used to model the effect of wavelength on dark adaptation.

Answer 15

False, you can use the duplicity model.

Question 16

Name 4 assumptions made in the duplicity model.

Answer 16

• There are two detection systems (rods and cones)
• The systems have unique and invariant spectral sensitivities
• The systems are independent
• Shape of the dark adaptation curves are invariant with changes in test stimulus parameters

Question 17

Describe the limitations of the duplicity model in terms of changing wavelengths.

Answer 17

Shape of the cone branch can change with changes in test and adapting wavlength.
Multiple cone branches appear with chromatic adaptation.

Question 18

True or false
Rods can remain dark adapted with red light.
Explain why.

Answer 18

True
This is because at high enough wavelengths, the cone system is actually more sensitive to light than the rod system, and the eye can then remain dark adapted under red light.

Question 19

What effect does brighter adapting light have on the cone branch, and dark adaptation?

Answer 19

A more prominent cone branch, and prolonged dark adaptation.

Question 20

What effect does longer adapting light have on the cone branch, and dark adaptation?

Answer 20

A more prominent cone branch, and prolonged dark adaptation.

Question 21

What happens to threshhold in the early dark adaptation, and what can this be attributed to?

Answer 21

Very rapid decrease in threshold due to neural changes.

Question 22

Early during dark adaptation, in the moments before the adapting light is turned off, what happens to threshold?
How can this happen?

Answer 22

Moments before turning off the adapting light, threshold actually increases slightly, before decreasing.
This is due to the interaction between the neural response to the test flash, and the adapting light.

Question 23

What colour is a dark adapted retina, and what colour does it change to?

Answer 23

Pink, bleaches to white.

Question 24

What kind of curve is recovery oh photopigment like?

Answer 24

Exponential

Question 25

Define the Dowling-Rushton equation, and its units.

Answer 25

log (It / Io) = 10HB

• It is the threshold for detecting the test flash
• Io is the absolute threshold
• H is a constant (~2)
• B is the proportion of pigment bleached

Question 26

What is the limitation of the Dowling-Rushton equation?

Answer 26

Only holds when >10% of pigment is bleached

Question 27

Define component S2, the assumption made, and what it represents.

Answer 27

Assume the intensity of equivalent light during dark adaptation is directly proportional to the concentration of a presumed photoproduct of light exposure.
S2 represents the exponential decay in the concentration of the presumed photoproduct.

Question 28

At what bleach levels does the S2 increase in direct proportion to the magnitude of the bleach?

Answer 28

Small bleaches <10%.

Question 29

Describe the rate limited behaviour of S2.

Answer 29

For bleach levels >10%, time taken to recover to a criterion level increases linearly with the size of the bleach.

Question 30

During S2 recovery, what does opsin cause (2)?

Answer 30

• Suppression of rod circulating current.

- Elevation of psychophysical threshold.

Question 31

What are the kinetics of recovery during component S2 recovery determined by?

Answer 31

The time-course of the removal of opsin by its recombination with 11-cis retinal.

Question 32

Describe the component S3.

Answer 32

Region of the straight line recovery seen for bleaches >20%.

Question 33

Is the dependency on bleach magnitude the same or different compared to component S2?

Answer 33

Different.

Question 34

What does the shape of the dark adaptation map depend on?

Answer 34

Parameters of the test stimulus

Question 35

Describe the equivalent background theory.

Answer 35

The threshold for detecting a test stimulus is equivalent to the threshold for detecting the same stimulus against a background that bleaches the same proportion of photopigment that remains bleached at that time point in dark adaptation.

Question 36

Describe the changes that occur to light and dark adaptations according to the equivalent background theory

Answer 36

For a given test stimulus size, and after a set amount of time, the threshold increases by the same amount in both light and dark adaptations.

Question 37

What effect does the equivalent background theory eliminate?

Answer 37

The effect of stimulus size.

Question 38

For what 3 parameters does the equivalent background theory hold for?

Answer 38

• Rod system and changes in test stimulus duration
• Cone system and changes in test stimulus size
• Cone system and changes in test stimulus wavelength

Question 39

Neural mechanisms involved in light and dark adaptation are involved in chnages to what two cells?

Answer 39

Photoreceptors and post-receptoral cells.

Question 40

In which system do neural mechanisms play a large role in? Small role?

Answer 40

Rod system light adaptation

Small role for the cone system

Question 41

Does photopigment depletion play a role in rod light adaptation?

Answer 41

No.

Question 42

Does photopigment depletion play a role in rod light adaptation?

Answer 42

Yes.