L07 & L10- Light and Dark Adaptation

Question 1

Mechanisms of retinal adaptation during:

1) Sunrise to sunset (global, slow, large)
2) Eye movements (local, fast, small)

Answer 1

1) Changes in pupil size, switch from cone to rod vision, changes in photopigment content of photoreceptors
2) Changes in the gain of phototransduction and postreceptoral circuitry

Question 2

In terms of vision: what is adaptation? Weak inputs are lost in ___. Strong inputs saturate the __ response

Answer 2

Adaptation acts to match the limited neuronal response range to the visual input range

Noise, GC

*GCs have a response range similar to a digital camera

Question 3

Long exposure makes things darker/brighter?

Answer 3

Brighter - more detail in shadowed region

Question 4

Does retina use local or global adaptation?

Answer 4

Local

Question 5

Laughlin’s Rule - says nothing about spatial extent or dynamics

Answer 5

Encoding efficiency is maximized by a stimulus-response relation that makes each response equally likely given the distribution of input signals

Question 6

Natural images contain info in the same or many different statistical moments of the intensity distribution? What two factors are most important?

Answer 6

Different
Mean intensity (intensity overall) and contrast (differences in intensity in a scene)

Question 7

Is fixation stable?

Answer 7

No, it is small, random movements.

Question 8

Behavioural threshold is determined by what? What is the problem with this?

Answer 8

Combination of the gain of the phys response to light and the noise that obscures this signal - PROBLEM: can’t tell whether an increase in threshold is due to a decrease in gain and/or an increase in noise

Question 9

“Dark light” region (rods and cones) - Is threshold dependent or independent of background? What is the dark light?

Answer 9

Independent

Dark light refers to spontaneous activation of rhodopsin that generates intrinsic noise

Question 10

Rose-DeVries region (rods)

a) Threshold increases in proportion to?
b) Classical explanation? Increase in background intensity = ?
c) Support for classical explanation?
d) Alternate explanation? Increase in background intensity = ?
e) Support for alternate explanation

*Explanations are not mutually exclusive

Answer 10

a) Threshold increases in proportion to the square root of the background

b) NO adaptation - photon noise obscures the test stimulus. Background lights produce noise (due to statistical fluctuations in the number of quanta delivered by a fixed intensity background)
We cannot control how much quanta we get - there is variation. (Poisson distribution is the probability distribution that governs how many quanta is delivered. The variant is the square root of a mean). Increase in background intensity = increase in noise = increase in flash to compensate

c) Parsimonious model - simple explanation based on physics
d) Adaptation - photon noise is regulated by adaptation. Increase in background intensity = decrease in gain = increase in flash to compensate
e) Suprathreshold brightness matching -situation where noise is irrelevant leads to a slope of a 1/2

Toad RGC show square root adaptation but mammalian RGCs do not

Question 11

What is gain?

Answer 11

It is a type of adaptation that controls level of response
High gain - amplifies response from neuron
Low gain - decreases response from neuron

e.g. High input going into system means you will use a low gain and vice versa

Question 12

Weber regions (cones)

a) Threshold increases in proportion to?
b) It has a slope of _ (what number)
c) Is contrast independent or dependent on the intensity of the illuminating light?
d) Explanation
e) The explanations are based from behavioural measures, what factors do they not take into consideration?

Answer 12

a) Threshold increases in direct proportion to background
b) 1 - as threshold and background are directly equal, if threshold = 1, background = 1, then slope is 1/1 = 1

c) Contrast in an image is independent of the intensity of the illuminating light
e. g. Car on the road illuminated by sunlight. Decrease in sunlight affects both the car AND road - hence ratio doesn’t change and contrast is the same. Even if you wear sunglasses during the day - it doesn’t affect what you can or can’t see as you cut down on both the amount of light coming from the stimulus (e.g.car) and light by the same proportion

d) Cone photoreceptors and GCs show Weberian adaptation

e) The fact that light adaptation causes changes in response kinetics
- e.g. Faster responses in cones, horizontal cells and RGCs
- These changes are not apparent in the TVI data
- Role of noise is not well understood

Question 13

Weber’s law

Answer 13

The law states that the change in a stimulus that will be just noticeable is a constant ratio of the original stimulus

Question 14

What happens to Weber’s law when you wear sunglasses at NIGHT?

Answer 14

Uses your rods instead where the law is non-existent and you won’t be able to see, low contrast things = invisible, contrast is no longer constant

Question 15

There is always uncertainty. Low light levels = noisy from stimulus noise or spontaneous firing, hence adaption will be noisy. How do you deal with this?

Answer 15

Spatial and/or temporal pooling reduce stimulus and photoreceptor noise

1) Pool photoR then average = reliable response
2) Responses increase at subsequent stages of retinal processing
3) Adaptation at late stages - reduces risk of fluctuations in gain, but increases risk of saturation

Question 16

Adaptation pools for rod vision
At LOW light levels:
-Adaptation at rod bipolar/All amarine cell synapse?
-Adaptation at level of GCs?

At HIGH light levels:
-Adaptation in?

Answer 16

At LOW light levels:

• Adaptation not feasible for rods as stimulus and intrinsic noise makes signal unreliable (photons absorbed rarely)
• Adaptation at rod bipolar/All amarine cell synapse? YES
• Adaptation at level of GCs? NO

At HIGH light levels:
-Adaptation in rods

Question 17

Adaptation pools for cone vision
At LOW light levels:
-Adaptation at level of GCs?

At MOD/HIGH light levels (~room levels):
-Adaptation in?

Answer 17

At LOW light levels:

• Adaptation not feasible for cones: stimulus noise and intrinsic noise (mainly intrinsic noise being a factor)
• Adaptation at level of GCs? YES

At MOD/HIGH light levels (~room levels):
-Adaptation predominantly in cones but also in BP and GC

Question 18

Local and rapid adaptation is potentially unreliable

Answer 18

• Pooling is used at low light levels (where stimulus and neural noises are problematic)
• As light levels increase, mechanisms with less pooling and better spatial resolution contribute, and eventually dominate

Question 19

What is dark adaptation?

Answer 19

The recovery of visual sensitivity in the dark after exposure of eye to intense light that bleaches photo pigment.

Question 20

How is dark adaptation measured?

Answer 20

It is a slow process that is measured by plotting the intensity of a just-detectable light (e.g. threshold) as a function of time after extinction of the bleaching exposure

Question 21

What does the recovery of sensitivity represent?

Answer 21

The fading (hence increase in sensitivity) of the equivalent background in the visual system. It is the rate that the background fades away over time that affects the threshold overtime. It is linked to the “retinoid cycle” of photo pigment regeneration

Question 22

Is DA beneficial?

Answer 22

No - it is too slow, it may even take up to 40 mins (rarely encountered in daily life) to recover sensitivity. 10-15 mins is average recovery time

Question 23

Component S2 is visible for __ bleaches (red). What is it? Rate of recovery?

Answer 23

ALL bleaches

S2 represents the exponential decay in concentration of a photoproduct that decreases sensitivity

Rate of recovery = 0.24 log10 units min

Question 24

Component S2, for bleaches that exceed ~ _%, the time taken to recover increases linearly with the size of the bleach. For each additional 1% of bleached pigment, an extra _ seconds is required for the removal of the photoproduct underlying component S2

Answer 24

10, 7

Question 25

Component S2 - what is the photoproduct? How does it work?

Answer 25

Opsin (unreg rhodopsin) causes suppression of rod circulating current and elevation of threshold by activating Gprotein cascade of phototransduction

Question 26

Component S3 - visible for bleaches > _% (green)

Rate of recovery?

Answer 26

20 - it occurs in the last stages of DA recovery

Rate of recovery = 0.06 log10 units min