Dark Adaptation

Question 1

True or False- Human visual system operates over about 13 log units

Answer 1

True - that means that the brightest light level we can function under is 10 trillion times brighter than the dimmest!

Question 2

What are the four mechanisms that allow the visual system to operate over this huge range of light levels?

Answer 2

1) Duplex retina (2 photoreceptor sub-systems)
2) Changes in pupil size
3) Photochemical adaptation (changes in concentration of bleached and unbleached photopigment)
4) Neural adaptation (neural responsiveness e.g. changes in receptive field size, temporal summation, negative feedback loops).

Question 3

What do we mean by the term duplex retina?

Answer 3

The fact that we have two photoreceptor types which function over different ranges of light levels. We call this having a duplex retina

Question 4

How does changing pupil size allow for operation under a large range of light levels?

Answer 4

Changes in pupil size help us to adjust for changes in ambient light levels

Question 5

Which of the four mechanisms for dealing with light levels over a large range, are kinetic processes?

Answer 5

Photochemical adaptation and Neural adaptation – are kinetic processes which allow us to adjust the sensitivity of our retina to adjust for the brightness of the ambient light.

Question 6

Describe the mechanism of the Duplex Retina and its effect on the range of light levels that can be dealt with.

Answer 6

When fully dark adapted, rods are highly sensitive to dim lights in low light levels (scotopic conditions) but saturate under higher light levels.

Cones are used in high light levels (photopic conditions), but are less sensitive than rods in low light levels.

In intermediate light levels both are active (mesopic conditions)

This mechansim nearly doubles the range of light levels over which the visual system can operate.

Question 7

Describe the mechanism of changing pupil size in order to deal with a range of light levels and how much of the range this mechanism accounts for.

Answer 7

Question 8

What does dark adaptation refer to?

How long does dark adaptation take?

Answer 8

The retinal adaptation to increase sensitivity in response to a reduction in illuminance.

50 mins

Question 9

How long does light adaptation take?

Answer 9

It is very fast (can take barely seconds to adapt to light).

Question 10

What is light adaptation?

Answer 10

A process whereby, sensitivity is reduced as background illumination increases

Question 11

How do we know adaptations for light and dark occur in the retina?

Answer 11

It is possible for one eye to be light adapted while the other is dark adapted.

It must therefore be a process taking place before the optic chiasm as, after this point, information from the two eyes is no longer kept separate.

(Well, if you are in complete darkness for a while – say if you wake up in the middle of the night, try covering one of your eyes and switching the light on, then switching the light off again. The eye that had been covered will remain completely dark adapted, whilst the other eye will have lost all sensitivity to dim lights. It takes a number of minutes for the eye exposed to the light to dark adapt again.

This tells us that it is possible for one eye to be light adapted while the other is dark adapted. It must therefore be a process taking place before the optic chiasm as, after this point, information from the two eyes is no longer kept separate.)

Question 12

Describe this dark adaptation function graph whereby a px has been seated in a dark room and tested for the dimmest light they can see as time goes on.

(I.e explain the shape and why it is like that)

Answer 12

Cones dark adapt faster than rods thus they determine the initial threshold. (By initial threshold we mean the dimmest light that can be seen initially - remember dark adaptation is a process that takes around 5o mins to complete).

There is then a ‘rod-cone’ break where rods take over as the dominant photoreceptor in the mesopic condition.

At the rod cone break the rod sensitivity to light exceeds that of cones.

After the rod cone break, the rods determine the absolute threshold (i.e. the absolute dimmest light the px can see).

Question 13

True or False- rods dark adapt quicker than cones.

Answer 13

False - Cones dark adapt quicker than cones - which leads to the appearance of a rod cone break in dark adaptation function graphs

Question 14

True or False- Absolute threshold is determined by cones

Answer 14

False - Absolute threshold is determined by Rods

Question 15

True or False - Initial threshold for light sensitivity in mesopic conditions follwoing dark adaptation is determined by cones.

Answer 15

True

Question 16

Describe the mechanism of Photochemical Dark Adaptation

Answer 16

A Photopigment is made up of protein (opsin) + chromophore (retinal)

Unbleached (purple) photopigment (opsin + 11-cis retinal) absorbs light to initiate phototransduction cascade.

Durring this process the photopigment breaks down into itscomponents (opsin + all trans retinal) and loses colour (it is bleached). [When bleached it cannot absorb light].

(The photopigment is then regenerated).

Question 17

How do photopigments regerate follwoing Photochemical adaptation?

Answer 17

All trans retinal diffuses into the RPE, where it is isomerised to form 11-cis retinal . (The original component of the photopigment).

This then diffuses back into photoreceptors, recombines with opsin and is ready to absorb new photons.

Question 18

What do we mean when we talk about half time following photopigment regeneration?

Answer 18

Regeneration of photopigment follows exponential function (like radioactive decay)

Half time (t) = time taken for amount of bleached photopigment to be halved.

Question 19

What is the half time for Rhodopsin and iodopsin?

Answer 19

Rhodopsin = 5.2 minutes (rod pigment)

Iodopsin (cone pigment) = 1.7 minutes

(Thus you can see why cones dark adapt faster)

Question 20

How is photopigment regeneration investigated?

Answer 20

Photopigment regeneration is investigated using retinal densitometry.

(In simple terms, a light is shone into the eye through a half silvered mirror. Some of this light will be absorbed by photopigment, some will be absorbed by other structures, such as the RPE, and some light will reflect off the sclera back out through the pupil. When the retina is dark adapted, there is more unbleached pigment available to absorb light, so less light is reflected back out of the eye. When the retina has more bleached photopigment in a light adapted state, there is less light absorbed by the photoreceptor outer segments, and more light is reflected out of the eye. As the eye gradually dark adapts, the proportion of light reflected back out of the pupil gradually decreases. By monitoring this using a photocell and galvanometer, we are able to monitor the rate of dark adaptation).

Question 21

Does the process of photopigment regeneration control rate of adaptation?

Answer 21

Whether it controls we cannot be sure but there is definitely a direct link as they mirror each other.

Question 22

What factors are suspected to limit the threshold of dark adaptation?

Answer 22

Perhaps the concentration of unbleached photopigment (remember that only unbleached photopigment can absorb light). (This theory is not accepted as the main reason).

The concentrtaion of photoproducts. Threshold during dark adaptation is limited by the presence of products like opsin. This is described by the Dowling-Rushton relationship.

Question 23

Why is the idea that the concentration of unbleached photorceptor limits the threshold of dark adaptation not widely accepted?

Answer 23

If this was the case you would expect when 50% regeneration occurs, there would be a 50% recovery of threshold.

However in real life when 90% of rhodopsin is recovered, threshold is still elevated by 1000x.

So it can’t be that there isn’t enough photopigment around to absorb photons, that is our main limitation on dark adaptation threshold.

Question 24

What is the Dowling-Rushton relationship?

Answer 24

Log sensitivity during dark adaptation is linearly related to concentration BLEACHED photopigment.

(There is a good correlation)

Question 25

By what mechnanism/process does the concentration of photoreceptors limit the threshold of dark adaptation function?

Answer 25

Bleached photoproducts (e.g. opsin) weakly activate further visual transduction.

This has the same effect on the retina as ‘adapting to real light’ however this light is deemed as stable (i.e. that it is not chnaging in brightness or luminance) and so is not ‘seen’ as light by the retina (this is called the troxlear effect).

Retina responds to background light by reducing retinal gain (amplification) resulting in low sensitivity.

Retina therefore responds to these photoproducts by having low sensitivity.

As photoproducts decay light sensitivity recovers.

Question 26

What is the Equivalent Background theory?

Answer 26

Equivalent background Theory (Crawford, 1947):

At any time during dark adaptation, retinal sensitivity is equivalent to sensitivity caused by adaptation to a background light.

(i.e if your background was 20 light measures and your stimulus was thirty light measures , then your sensitivity would be ten light measures - basically that your sensitivity is relative to background ‘noise’).

Question 27

Describe the mechanism of Neural Adaptation in dark adaptation and when it occurs.

Answer 27

It occurs in the first 0.3 seconds when threshold drops very quickly.

[Photopigment changes occur very slowly - changes in the first 0.3 seconds are purely neural adaptation).

These neural adaptations are post receptal factors -e.g. changes in receptive field size (increase in size, loss of centre surround organisation).

For example in term of receptive field size, when we go from high to low levels of light, the lateral lead of information in the retina, leads to receptive field sizes getting larger to allow for more spatial summation and increase light sensitivity rapidly. - This is an example of neural adaptation.

Question 28

The dark adaptation function doesn’t always look the same.

What affects the shape of the dark adaptation function?

Answer 28

• The retinal location of the target
• Target size
• The target Wavelength
• The adapting light’s wavelength
• The adapting light’s light intensity and duration

Question 29

How does the retinal location of the target affect the dark adaptation function?

Answer 29

Remember in the central fovea there are no rods.

Question 30

How does target size affect dark adaptation function?

Answer 30

Question 31

How does target wavelength affect dark adaptation function?

Answer 31

Question 32

What is an adapting light?

Answer 32

The light you use to bleach the photopigment before you measure the dark adaptation.

Question 33

How does the adapting light’s intensity and duration affect the dark adaptation function?

Answer 33

. A longer or more intense bleaching light results in

• Higher initial threshold,
• More prominent cone branch,
• Later rod-cone break
• Longer time to reach final threshold.

Question 34

True or False- A Red adapting light will not bleach much rhodopsin.

Answer 34

True - A Red adapting light will not bleach much rhodopsin as it falls outside rod sensitivity.

Question 35

True or False - After a red adapting light has been shone, rods remain sensitive.

Answer 35

True

Question 36

Why are red lights used in military ready rooms?

[A ready room is the room a pilot is in before they go on night flights]

Answer 36

Allows cones to function (can see to read etc.)

Yet Doesn’t bleach rods, so night vision remains good.

Question 37

What are the mechanisms behind light adaptation?

Answer 37

Neural changes (at very low light levels) and photopigment bleaching (at higher light levels).

Question 38

What is the increment threshold?

Answer 38

The amount by which a target needs to be brighter than its background to be seen.

Question 39

True or False- Our visual system becomes less sensitive to light at low levels of background light

Answer 39

True

Question 40

What is the benefit of our visual system becoming less sensitive to light when there are low levels of background light?

Answer 40

Stops the retinal ganglion cells from becoming oversaturated at high light levels

Question 41

What factors affect the rate of dark adaptation?

Answer 41

Age

Retinal Diseases e.g. AMD, retinitis pigmentosa,

Systemic conditions e.g. Vitamin A deficiency

Question 42

How do we investigate and diagnose different causes of poor night vision?

Answer 42

Psychophysical techniques

Fundus and anterior eye examination

Electrophysiology etc

Question 43

What are non-retinal causes for why people struggle with their night vision?

Answer 43

Question 44

What are retinal causes for why px may struggle to see in the dark?

Answer 44

• Age:
• Reduced rate of photopigment regeneration causes slowed dark adaptation. Photoreceptor death elevates absolute threshold.
• Age-related Macular Degeneration (AMD)
• Pathological worsening of normal ageing changes
• Photoreceptor death raises absolute threshold.
• Thickening of Bruch’s membrane, RPE and photoreceptor damage and reduced choroidal circulation slow photopigment regeneration.
• Chronic open angle glaucoma
• Ganglion cell loss coincides with area of max rod density (15º) thus leads to reduced scotopic sensitivity
• Vitamin A deficiency
• Vitamin A important part of rhodopsin
• Deficiency reduces rhodopsin concentration and slows photopigment regeneration.
• Results in raised absolute threshold and prolonged dark adaptation.

Question 45

How do we group inherited night blindness?

Answer 45

By whether they are a progressive or stationary condition.

If the disease is progressive (i.e. gets worse- gradual degradation) we refer to it as retinitis pigmentosa ( this is a broad term).

If the disease does not change over time we refer to it as Congenital Stationary Night Blindness (CSNB). We may further categorise this into type I, II and III.

Question 46

Is Congenital Stationary Night blindness (CSNB) autosomal recessive or dominant?

Answer 46

Autosomal recessive

Question 47

What does type II CSNB Fundus Albipunctuas (Congenital Stationary Night Blindness) affect and what effect does this have on night vision?

Answer 47

Rods and cone photopigment kinetics are affected.

This condition causes longer adaptation periods however eventually normal thresholds are reached.

Question 48

Is Retinitis Pigmentosa autosomal recessive or dominant?

Answer 48

Can be either (and/or even X linked recessive)

Question 49

Describe retinitis pigmentosa?

What is it?

What is it caused by?

What are its symptoms?

Answer 49

Question 50

What will be seen in the fundus of someone with retinitis pigmentosa?

Answer 50

Question 51

Will rod and cone thresholds in someone with retinitis pigmentosa eventually reach normal levels but just take longer to?

Answer 51

No ( that’s the case for someone with CSNB). Px with retinitis pigmentosa have grossly elevated rod and cone thresholds.

Question 52

What are some tests to clinically measure a Px’s dark adaptation?

Answer 52

Historically a Goldmann adaptometer.

More recently Computer based systems e.g. AdaptDx

Or for a quick screening : A photostress test