Dark Adaptation Flashcards

1
Q

In the dark, our visual system must maximize the ability to capture photons AKA ____ sensitivity

A

Increase

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2
Q

Three things that occur to increase dark adaptation (which is the recovery of visual sensitivity that occurs in the dark)

A
  1. Pupils dilate
  2. Increased sensitivity of photoreceptors
  3. Increased dominance of rod activity with decreasing ambient light.
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3
Q

We are way more efficient at detecting light under (scotopic or photopic) conditions

A

Scotopic.

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4
Q

Photochromatic interval

A

The change in sensitivity to light of a specific wavelength as one adapts to light or darkness.

can be seen on a luminous efficiency graph

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5
Q

Spatial summation

A

The eyes ability to sum, or add, quanta over a given area. The consequences of summation is that we get an increase in light sensitivity as stimulus size increases (up to a limit).

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6
Q

Spatial summation is based upon the ____

A

Neural convergence of signals generated by the retinal photoreceptor, bipolar, and ganglion cell.

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7
Q

Critical angle and diameter of complete spatial summation

A

The critical area of complete spatial summation is the retinal area over which all light energy is summed to contribute to perception.

Limit of critical angle is dependent on retinal location.

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8
Q

Riccos law

A

Applies only to small images on the fovea at near threshold luminance. Tells us that the same total amount of light energy will have the same effect, no matter how it is distributed as long as its within the critical area of complete spatial summation.

Ex: two stimuli, one large and one small in diameter, will provide the same total energy. They just distribute it differently. As long as each is within the critical area, they will produce the same response change in the neuron.

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9
Q

Riccos law formula

A

LA= C

L- Stimulus luminance
A- Stimulus area
C- constant

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10
Q

Larger areas of spatial summation produces (2)

A
  1. Greater light sensitivity and brightness due to increased space over which photons can be captured.
  2. Poorer spatial resolution (VAs). If you use a bigger net, you can’t be asa discriminating about what you catch.
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11
Q

___ convergence determines the critical area for complete spatial summation.

A

Neural.
The critical area (diameter) is larger under scotopic than under photopic conditions and larger for peripheral than for foveal retinal stimuli.

Bc cones to bipolar cells are one to one whereas rods to bipolar cells are many to one. Convergence occurs.

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12
Q

When viewing a VF screening, what is the trend you should expect of the numbers in the center vs numbers in the periphery

A

Numbers in the center should be higher than numbers in the periphery. This is normal because we expect the periphery to not be as sensitive to light as the center.

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13
Q

Spatial antagonism

A

Under photopic conditions, if the stimuli is larger than the critical area. The energy falling just outside of the critical area actively decreases its perceived brightness.

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14
Q

Under scotopic conditions, what happens if energy falls outside of the critical area?

A

The energy falling outside of the area does not contribute to its brightness.
Given two otherwise identical stimuli with the same total energy, one smaller and the other one larger than the critical area , the smaller one would be brighter bc more photons are in the area that can be summed for the smaller target.

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15
Q

Temporal summation

A

The eyes ability to sum the effects of individual quanta of light over time.
Temporal summation operates across time like spatial summation operates across space.

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16
Q

For stimulus durations less than the critical duration, a decrease in the luminance requires that

A

the stimulus be shown for a longer time in order to be detected.

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17
Q

For stimulus durations longer than the critical duration, the threshold for detection depends only on

A

The luminance of the stimulus

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18
Q

Bloch’s Law

A

Stimulus luminance x stimulus duration= constant

The constant must reach a certain level in order to be seen.

Larger C value? That light will look brighter.

Same C value but different sizes? The same total amount of light will have the same effect, no matter how it is distributed across time as long as all is received within the critical duration for complete temporal summation.

19
Q

Temporal summation is longer at low or high light levels?

A

Low. Scotopic vision, rods

20
Q

Decreasing stimulus size results in a longer or shorter temporal summation?

A

Longer. Result is poorer temporal discrimination. Poor ability to discriminate flicker.

21
Q

The goldmann-Weekers Dark Adaptometer Testing Procedure

A
  1. Light adapt for 5 minutes. Head in bowl. Light on.
  2. Turn off the background light in bowl, leaving the patient in total darkness.
  3. Over the next 40-45 mins, repeatedly measure the patient’s light detection threshold using the ascending method of limits. Use this in order to record the absolute lowest amount of illumination every time.
  4. Plot the measured threshold. Y axis- log of threshold, X axis- time in darkness in mins,
22
Q

What should the graph look like for the goldmann-weekers dark adaptometer testing

A

Y axis- log of threshold. X axis- Time in darkness in minutes.

On the left side of the graph, the cones will capture light quicker. They were hardly bleached during the light adaption step, so they recovered quickly and were able to capture light. In the middle and right side of the graph, the rods will start to recover slower. They were heavily bleached during the light adaption step, so it took them longer before they were able to capture light again.

Results in a rod/cone break.

23
Q

Photochromatic interval

A

The difference in sensitivity between the rod and cone systems for a given visual stimulus.

24
Q

For then Goldman seekers dark adaption test, what happens when you dim the pre-adaption light?

A

Fewer rods and cones will be bleached. Therefore, both systems will recover faster when in darkness.

Results in a graph shifting down and to the left. May even eventually lose the rod/cone break.

25
Q

What happens if you do the goldman-seekers dark adaption test using a colored hue in place of your pre-adaption light?

A

It depends on the hue used. For example, if you use a red light, only the cones will become bleached since rods are not sensitive to red light.

Other colors that equally stimulate both photoreceptors will result in the same type of graph.

26
Q

goldman seekers dark adaption test- you have completed the light adaption portion of the test. You are now sitting in darkness and light flashes directly on the fovea. What will the graph look like.

What will the graph look like if the stimulus is 2 degree eccentric to the fovea?

A

Since the flash is only stimulating the cones, the cones will only show a response on the graph as they reset from being bleached. There will be no rod reaction since rods are not found in the fovea.

In the second example, the flash will stimulate both rods and cones. This will result in a rod cone break and response from each photoreceptor. The break will occur sooner. (Maybe cones further from fovea are quicker to reset?)

27
Q

Describe the concept of “equivalent background” by stiles and crawford

A

In bright room. Turn lights off. Retina continues to behave as if its looking at a uniform field of light.

after 10 minutes in total darkness, retinal function (sensitivity) is the same as though exposed to a light grey colored level of constant, full field illumination.

After 12 minutes, the retina will respond as though exposed to a full field of dark grey color.

As time continues in a dark room, fewer photoreceptors are bleached and dim objects can be easier detected. Threshold decreases over time as the retina becomes more sensitive (functioning in dark efficiently).

28
Q

It takes approx __ minutes for 50% of beached rhodopsin to regenerate

A

5 mins. That is their half life.

29
Q

Only __% of the total rhodopsin is bleached at the point of rod response saturation

A

10%. After 10% is bleached, no greater response can be made. Maxed out.

30
Q

Effective lifetime of activated Rhodopsin

A

100 ms.

31
Q

Steps for rhodopsin regeneration

A
  1. Rhodopsin kinase is attached to disc membrane and binds to R*
  2. Rhodopsin kinase phosphorylates R* using ATP.
  3. Now, phosphorylated R* can be recognized by arrestin, which binds to R* in place of the kinase.
  4. Chromophore (All trans retinal) is removed from R* and converted into all trans retinol by retinol dehydrogenase. It is then exported to the IPM.
  5. The opsin is still phosphorylated, so that is removed by a phosphatase before it can recombine with 11-cis retinal to form inactivated rhodopsin in the outer seg. Now, it can be activated by light again and 11-cis retinal will be converted to active all trans retinal.
32
Q

Where does disc renewal occur in rods?

A

New disc formation occurs only at the base of the outer segment (cilium). The plasma membrane of the photoreceptor infolds, then pinches off.

33
Q

Where does disc removal occur in rods?

A

Discs at outer aspect of rods are broken off and ingested by RPE cells daily, usually in the morning with light onset. Entire outer seg is replaced in 1-2 weeks.

34
Q

Where does disc renewal and removal occur in rods?

A

Renewal- similar to rods. Occurs at base of outer seg by cilium. Tapered shape is maintained via remodeling.

Removal- Phagocytosis occurs daily, with light OFFSET.

35
Q

Difference between rod and cone removal

A

Rod discs are removed/phagocytosed by RPE with light ONSET.

Cone discs are removed/phagocytosed by RPE with light OFFSET.

36
Q

What is the purpose of disc renewal and removal?

A

To prevent cell damage. Therefore, they remove oxidized (old and damaged) proteins.

37
Q

Retinitis Pigmentosa

A

a GROUP of inherited retinal disorders in which the dark adaptation function is impaired.

Rhodopsin is a large molecule with many amino acids. Many mutations may occur throughout the molecule, resulting in different behaviors. Ex: Some are folded but not transported to the outer seg. Some cannot easily bind with 11-cis retinal, some are unstable.

38
Q

When looking at a chart graphing dark adaption with Time in dark on the X and log of threshold on the Y, what is the slope of the scotopic recovery?

A

The slope of the rod’s recovery is the same no matter the level of bleach. This suggests that the rate of recovery of rods is tied to the REMOVAL of some substance.

The substance that is removed is opsin. After opsin and chromophore (all trans retinal) are separated, opsin is still slightly active until it binds with another 11-cis retinal molecule. At this point, it is now called rhodopsin and will be activated with light.

39
Q

The “removal” of opsin is dependent on

A

How quickly 11-cis retinal can be moved from RPE to the photoreceptor so they can bind and form rhodopsin.

40
Q

In full dark adaptation, all opsin in the rod outer seg is bound to

A

Chromophore. 11-cis retinal.

41
Q

Fundus Albipunctatus

A

A genetic eye disorder causing a flecked retina (unknown why flecks appear).
Nyctalopia (inability to see in dim light or at night) is the primary symptom.

Stationary night blindness, not progressive.

Mutation occurs in the RDH 5 gene.

42
Q

Difference between fundus Albipunctatus and retinitis punctata?

A

Fundus Albipunctatus is stationary night blindness.

Retinitis punctata is progressive night blindness.

43
Q

The slowness of rod recovery results from which rate limiting step?

A

Rate limiting delivery of 11 cis retinal to opsin to form rhodopsin.

This can be explained by a physical resistance from the 11 cis retinal reaching the outer seg or enzyme kinetics.

44
Q

The raised thresholds during the S2 phase result from

A

Opsin activation of the cascade.