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OV2023 visual perception > dark adaptation > Flashcards

dark adaptation Flashcards

1
Q

what is the light levels the human visual system operates ?

A
  • the human visual system operates over about 14 log units ( threshold is about 10^13 x diameter than the brightest light we can operate under )
  • we can detect a few quanta of light against complete blackness
  • we can see a white polar bear against dazzling white snow
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2
Q

what are the 4 mechanisms which allow the visual system to operate over this huge range ?

A
  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)
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3
Q

what is the duplex retina mechanism ?

A
  • two sub-systems of photoreceptors- rods and cones
  • when fully dark adapted, rods are highly sensitive to dim light 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 )
  • nearly doubles the range of light levels over which the visual system can operate
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4
Q

explain the mechanism of changes in pupil size ?

A
  • pupil light reflex helps us deal with range of luminance experienced by visual system
  • average min pupil = 2mm and average max pupil = 8mm
  • area of biggest pupil is 16x bigger than area of smallest this means that retinal illuminance can be increased by 16x by dilating pupil
  • this only accounts for 1.2 log units of 13 log unit range in intensity encountered by visual system
  • therefore we cannot keep retinal illuminance constant
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5
Q

what do we need to function over different light levels ?

A
  • we require photochemical and neural adaptation mechanism to allow us to function over different light levels
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6
Q

what is adaptation ?

A
  • adaptation is an increase or decrease in retinal sensitivity with changing light levels
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7
Q

what is dark adaptation ?

A
  • retinal adaptation to increase sensitivity in response to reduction in illuminance
  • takes up to 50 minutes to be complete when moving from very bright to very dim light levels
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8
Q

what is light adaptation ?

A
  • is reduction in sensitivity to light when we move to higher retinal illuminance level rapidly
  • very fast ( seconds )
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9
Q

where does adaptation occur ?

A
  • the fact that we can remain dark adapted in the covered eye, but light adapted in the uncovered eye
  • this tells us that the mechanism for light and dark adaptation must take place in the retina this is before the optic chiasm
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10
Q

what does it mean that one part of the retina can be dark adapted and one part can be light adapted ?

A
  • the fact that one part of the retina can be dark adapted and one part can be light adapted tells us that this is something that occurs on a very localised level
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11
Q

what is the importance of adaptation ?

A

. increasing light intensity increases retinal ganglion cell response ( action potentials/second)

. the response of retinal ganglion cells is limited ( maximum around 500 spikes/second )

. without adaption there would be saturation of the RGC response at 500 spikes/second

. adaptation allows the RGC to respond to a large range of intensities

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12
Q

how to record dark adapation function ?

A
  • subject in bright light
  • switch light off
  • measure how their visual threshold changes over time in the dark
  • we do this by increasing the intensity of target until subject sees it
  • repeat at regular intervals , record time and ‘ threshold ‘ intensity
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13
Q

explain the assessment of dark adaptation graph ?

A
  • after exposure to the brightest lights , the dark adaption function has two distinct curves
  • first curve - describes cone adaption ( cone branch ) after 10 minutes we have kink in the curve called the rod cone break
  • the rods take over as being the most sensitive photoreceptor
  • we get a rod branch of the dark adaptation function
  • where the graph levels off is known as absolute threshold when completely dark adapted
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14
Q

why does the dark adaption curve look this way ?

A
  • cones dark-adapt faster than rods - cones determine threshold initially but cone absolute sensitivity is worse
  • at rod-cone break rod sensitivity exceeds cone sensitivity
  • after rod-cone break rod determine threshold
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15
Q

what are the two components of photopigment ?

A
  • photopigments is protein ( opsin) + chromophore ( retinal )
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16
Q

what is the structure of photopigment in dark ?

A
  • unbleached ( purple ) photopigment ( opsin + 11-cis retinal ) absorbs light to initiate phototransduction cascade
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17
Q

what happens in the phototransduction cascade?

A
  • photopigment breaks down into components ( opsin + all trans retinal ) and loses colour ( is bleached ), cannot absorb light
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18
Q

how does photopigment regenerate ?

A
  • all-trans retinal diffuses to RPE where its isomerised to 11-cis
  • then diffuses back to photoreceptors , recombines with opsin , ready to absorb new photon
  • regeneration of photopigment follows exponential function ( like radioactive decay )
  • half time = time take for amount of bleached photopigment to be halved
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19
Q

what is the halftime of cones?

A
  • 1.7 minutes
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20
Q

what is the halftime of rods ?

A
  • 5.2 minutes
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21
Q

how do we know about photopigment regeneration ?

A
  • it is investigated using retinal densitometry
  • light is shone through in the eye through a half silvered mirror
  • some of the light is absorbed by photopigment as it hits the retina
  • some of the light is absorbed by other structures such as the RPE
  • some of the light reflects off the sclera at the back of the eye and bounces back through pupil
  • when the retina is dark adapted there is more unbleached pigment available to absorb light
  • when retina has more bleached photopigment in light adapted state , there is less light absorbed by the photoreceptor
  • as the eye gradually dark adapts , the proportion of light reflected back out pupil gradually decreases
  • by monitoring this using a photocell and galvanometer we are able to monitor the rate of dark adaptation
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22
Q

what is photochemical adaptation ?

A
  • change in ratio of bleached: unbleached photopigment in retina
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23
Q

does the process of photopigment regeneration control rate of adaptation ?

A
  • time course of rhodopsin regeneration ( measured by retinal densitometry ) mirrors time course of rod branch of dark adaptation function
  • photopigment regeneration is at the bottom of the process of dark adaptation
24
Q

why isn’t the concentration of unbleached pigment what limits threshold during dark adaptation ?

A

. bleached photopigment can’t absorb light, so require unbleached photopigment for vision

. if it’s a limitation in concentration of unbleached photopigment which limits sensitivity in dark - we would expect 50% regeneration of rhodopsin to cause 50% recovery of threshold

. yet when 90% of rhodopsin has regenerated, threshold is still elevated by 1000x

25
Q

does the concentration of photoproducts of bleaching limit threshold during dark adaptation ?

A

. rhodopsin breaks down into other photoproducts e.g.opsin and metarhodopsin II , when it absorbs light and is bleached

. good correlation found between threshold and concentration of bleached photoproducts e.g. opsin

26
Q

what actually limits threshold during dark adaptation?

A
  • the presence of bleached photoproducts like opsin rather than the lack of unbleached photopigment
27
Q

what is dowling-rushton relationship ?

A
  • log sensitivity during dark adaptation is linearly related to concentration bleached photopigment
28
Q

how does the concentration of photoproducts limit threshold ?

A

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

. has same effect on retina as adapting to real light, but stable on retina , son not seen as light ( troxler effect )

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

. retina therefore responds to photoproducts by having low sensitivity
. as photoproducts decay sensitivity recovers

29
Q

what is the equivalent background theory ?

A

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

. e.g. at 17 mins, 10% of rhodopsin is still bleached
. detecting light at this time, would be like detecting light against a background which bleached 10% of rhodopsin

30
Q

why is it in the first 0.3 seconds during dark adaption threshold drops very steeply ?

A

. this is due to neural not photopigment changes

31
Q

what is the Arden and weale theory ?

A

. post-receptoral factors
e.g. changes in receptive field size ( increase in size, loss of centre surround organisation ), also involved in dark adaptation

32
Q

how does retinal location of target affect the dark adaption function ?

A

. sensitivity improves with eccentricity up to 15 degrees due to greater rod density
. at fovea - no rod cone break
. if we move target to 2.5 deg - rod-cone break and better absolute sensitivity
. if we move target to 5 deg- prominent rod-cone break, lower threshold

33
Q

how does target size affect dark adaptation ?

A

. small test spot at fovea - cone branch only

. larger stimulus - rod branch present ( stimulus incorporates rods also)

. increase in stimulus size - lower absolute threshold ( lots of spatial summation makes us more sensitive )

34
Q

how does target wavelength affect dark adaptation ?

A

. cone peak sensitivity is at longer wavelength than rod

. cones more sensitive than rods to long wavelength stimuli (>700nm) even in the dark adapted eye - no rod branch

. test flash - 507nm ( peak rod sensitivity ) gives prominent rod branch and low final threshold

35
Q

how does adapting light wavelength affect the dark adaptation function ?

A

. red adapting light will not bleach much rhodopsin
. rods remain sensitive when light turned off
. red lights used in military ready rooms
. allows cones to function
. doesn’t bleach rods, so night vision remains good

36
Q

how does adapting light intensity and duration affect the dark adaptation function ?

A

. higher initial threshold
. more prominent cone branch
. later rod-cone break
. longer time to reach final threshold

37
Q

explain the process of light adaptation ?

A

. this is when we are going from lower to higher light levels
. increment threshold , that is the amount by which the target needs to be brighter than its background to be seen increases
. our visual system becomes less sensitive to light as background light levels increase
. at low levels of background light this is determined by neural factors such as changes in receptive field size
. at higher light levels its determined by the amount of bleach pigment present in the retina
. reduction in sensitivity as we move to conditions of high illuminance is very important in stopping the response of our retinal ganglion cells from becoming saturated at high light levels
. this process happens rapidly

38
Q

what are the factors that affect the rate of dark adaptation ?

A

. age

. retinal disease e.g. AMD, retinitis pigmentosa, CSNB

. systemic conditions e.g. vitamin A deficiency

39
Q

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

A

. psychophysical techniques

. fundus and anterior eye examination

. electrophysiology

40
Q

how is the goldmann adaptometer used in the clinical measurement of dark adaptation ?

A

. adapting light in hemispherical bowl

. all light extinguishes

. threshold measured at intervals post bleach

. takes more than 30 mins for full dark adaptation curve

. uses method of ascending limits ( yes/no paradigm )

41
Q

how does the goldmann adaptometer show retinal disease ?

A

. thresholds are either very elevated or dark dark adaptation is very delayed ( fundus albipunctatus )

42
Q

what happens in fundus albipunctatus ?

A
  • this is where the process of dark adaptation takes up to four hours to complete
43
Q

what are computer based systems used in measurement of dark adaptation ?

A

. AdaptDx- presents a brief flash of light which bleaches the central part of the retina and then measures the time taken for retinal sensitivity to recover to a criterion value
. its measuring how long it takes for threshold to drop to a certain point
. the test takes 20 minutes

44
Q

how does the photostress test used in the clinical measurement of dark adaptation ?

A

. measure best VA
. bleach with ophthalmoscope
. time taken to return to within 1 line of best VA
. >60 sec pathological
. can be completed readily in practice by Optom

45
Q

what are some non-retinal causes of not seeing well in the dark ?

A

1- age related problems
. Miosis
. lens opacities
. both these things mean that retinal illuminance is reduced and absolute threshold elevated

2- eye myopia
. eye focuses at about 1m in dark
. usually only when no visual stimulus, but possible when stimulus is degraded e.g. driving at night
. may benefit from -0.50 over-correction

46
Q

what are the retinal causes of not seeing well in the dark ?

A
  1. age
    - reduced rate of photopigment regeneration causes slowed dark adaptation. photoreceptor death ( particularly of rods ) elevates absolute threshold
  2. 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
47
Q

how does chronic open angle glaucoma reduce the ability to see in the dark ?

A

. ganglion cell loss coincides with area of max rod density ( 15 degrees) which is reduced scotopic sensitivity

48
Q

how does vitamin A deficiency reduce the ability to see in the dark ?

A

. vitamin A important part of rhodopsin

. deficiency reduces rhodopsin concentration slows photopigment regeneration

. results in raised absolute threshold and prolonged dark adaptation

49
Q

what inherited night blindness ?

A

. inherited conditions giving symptoms of night-blindness

50
Q

what are progressive conditions that cause gradual degeneration ?

A

. retinitis pigmentosa - condition where vision get worse over time

51
Q

what are the stationary conditions of inherited night blindness ?

A

. theses conditions do not change over time

. congenital stationary night blindness (CSNB)

. ( types I, II and III)

52
Q

how do differentially diagnose inherited night blindness ?

A

. dark adaptometry

53
Q

what is Type II CSNB fundus albipunctatus ?

A

. autosomal recessive
. affects rod and cone photopigment kinetics
. grossly extended adaptation times
. eventually normal thresholds

54
Q

what is retinitis pigmentosa ?

A

. autosomal recessive, autosomal dominat or X-linked recessive
. progressive photoreceptor degeneration
. caused by progressive shortening of rod and cone outer segment due to abnormal disc shedding
. first symptoms is ‘night blindness’ ( aged 20 )
. central vision initially good, later affected
. fundus shows pigment clumping in mid-periphery ( bone-spicule appearance ) and attenuated blood vessels
. grossly elevated rod and cone threshold

55
Q

how is the visual system able to function ?

A

. the visual system is able to function over a wide range of retinal illuminance levels as a result of the duplex retina , pupil diameter changes, and retinal adaptation ( photochemical and neural )

56
Q

what is the limiting factor in photochemical adaptation ?

A

. the limiting factor in photochemical adaptation is the rate of removal of photoproducts of bleaching

57
Q

what is the benefit of the assessment of dark adaptation ?

A

. assessment of dark adaptation can help with the diagnosis of congenital and acquired retinal disease

OV2023 visual perception (20 decks)

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