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TMTV > Chapter 2-Light sensitivity > Flashcards

Chapter 2-Light sensitivity Flashcards

1
Q

what is the duplex retina

what must the 2 thigns operate

A
  1. one operates under dim conditions and is more sensitive to short wavelengths
  2. a second that operates under bright conditions and is more sensitive to long wavelenths
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2
Q

what is scotopic vision

A

rod mediated mech
operates under dim lighting conditions
doesnt perceive color
poor VA

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3
Q

what is photopic visoin

A

cone mediated mech
operates under bright lighting conditoins
perceives colors
best VA

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4
Q

what do rods contain in their outer segment?

how is that activated?

A

rhodopsin
it is very sensitve to light and it can be activated by only 1 photon
-can trigger rod to transmit signal to rod-bipolar cells
-DOES NOT mean we perceive this photon as light

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5
Q

what happens when rhodopsin is activated by a photon

A

becomes inactive and it is not sensitive to light anymore
BLEACHING is the inactive site

  • cont. regenerates and reverts to the active state after some time
  • after regnerated it is able to absorb another photon and etc.
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6
Q

how many rhodopsin molec does a single rod contain, what about the whole retina
how long does it take for all of these molec to become active (NOT BLEACHED)

A

10^7
10^15
40 min (in ocmplete darkness this is when we can measure scotopic threshold)

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7
Q

how can we obtain a similar function to scotopic spectral sensitivie for photopic conditions?

A

no dark adaptation, bright background and stimulus to stimulate cones

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8
Q

how long does it take for the regneration of the 3 cone opsins

A

5-10 min

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9
Q

what is the photochromatic interval

A

the difference btwn the scotopic and photopic spectral sensitivity function

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10
Q

in a radiometry, where will the output be

A

the radiant power (or radiant flux) of the source in Watt (W)

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11
Q

what does photometry deal w/

A

the effect a certain wavelength has on the visual system and how it compares w/ other wavelengths
or
how efficient a wavelength is at stimulating the visual system

photopic and scotopic luminosity functions

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12
Q

what is luminous power

A

the perceived power of light, not the physical power

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13
Q

what is abney’s law of additivity

A

add the total luminous power by adding all of the indiv wavelenghts

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14
Q

what is dark adaptation?

waht is light adaptation

A

dark: adaptation to dark conditions (or scotopic conditions(
light: adapatation to light conditions (or photopic)

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15
Q

what are the 2 def of dark adapattation

A
  1. how fast our visual system adapts to darkness after exposure to bright light
  2. how fast our sensitivity recovers after exposure to bright light
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16
Q

after bleaching w/ a very intense light the subject sets his/her threshold for up to _______ in complete darkness

A

40 min

17
Q

what are the factors affecting dark adaptation curve

A 
bleaching
stimulus size
stimulus location
stimulus wavelength 
rhodopsin regneration
18
Q

what is bleaching

A

the process when the photopigments become inactive and the regeneration process starts

19
Q

what does the amt of bleached photopigment depend on

A

the intensity (I) and the duration (t) of the light used for bleaching

bleached photopigment = f(I,t)

20
Q

what is the photochromatic interval

A

the difference in spectral sensitivity btwn photopic and scotopic system

21
Q

what is the half life of rhodopsin regneration

cone?

A

Rods: T=5 min
in other words, in 5 min, 50% of the bleached rhodopsin is regnerated

Cones: 1.5 min

22
Q

when causes dark adaptations to start sucking for pts?

A
  1. age-photoreceptor loss and thickening of bruch’s
  2. retinal/neural sources-RP, AMD, congenital stationary night blindness
  3. vascular changes
    - choroideremia,
    - gyrate atrophy
  4. vit A deficiency
    - chronic liver disease (cirrhosis)
    - chronic GI tract dieases (malabsorption)
    - alcoholism
23
Q

what is the visual cycle also known as

A

retinoid cycle

24
Q

when photopigments molec are active (ready to react to light) they are opsins bound with a …..

a photon is absorbed by that and it chnages its form to what?

A

chromophore called 11-cis-retinal (rhod etc)

a photon is absorbed by 11-cis-retinal which then changes its form to all trans retinal

25
Q

what starts the phototransduction cascade

A

when rhodopsin is transformed to metarhodopsin

26
Q

the photopigment is inactive until its all trans retinal is transformed back to

A

11 cis retinal

27
Q

the all trans retinal undergoes a series of rxns to convert back to 11 cis retinal and combine w/ what again

A

opsins

known as the retinoid cycle (when all trans goes to a series of rxns)

28
Q

what does RPE do for the visual cycle

A
  • part of the bbb
  • recycling of retinoid
  • active transport of nutrients from choriocap to photorecep (vit A)
  • phagocytosis of the aged OS tips
  • contains melanin granules that absorb excess light and therefore reduce glare
29
Q

what happens to the rod system as vit a decreases

what happens when it sig deceases

A
  • rod system shows a slower rate of DA and photopigment regeneration
  • abs threshold elevates (sensitity drops) and all rod function might be lost
30
Q

what happens to the cone system as vit a decreases

what happens when it sig decreases

A

the cone system isnt too severely affected w/ normal DA threshold
-as it significantly decreases the cones degenerate

31
Q

if vit a deficiecny is not too severe, how can it be reversed?
what does vit A restore w/in the RPE

A

effects can be reversed w/in days w/ oral vit a supplementation
-vit a restores 11 cis retinal w/in the RPE

32
Q

what is weber’s law

A

the relationshiop btwn the initial intensity or quantity and the smallest detectable increment

-the increment threshold (just noticable difference) in a psychophyscial task will be a fixed fraction (weber fraction) of the background

33
Q

our threshold for detecting an increment in the quantity or intensity of something changes depending on what

A

how much there is before we add the increment

34
Q

weber’s law says that the just noticeable difference for a quantity is a …..

A

fixed percentage of this quantity

35
Q

what is a visual field test
under what conditions
what is it useful for detecting/monitoring

A

measures the subject’s threshold on detecting a small light against a background
-done under photopic conditoins and proved to be useful for detecthing/monitor glaucoma

TMTV (5 decks)

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