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
2
Q
what are the 4 mechanisms which allow the visual system to operate over this huge range ?
A
- duplex retina ( 2 photoreceptor sub-systems)
- changes in pupil size
- photochemical adaptation ( changes in concentration of bleached and unbleached photopigment )
- neural adaptation ( neural responsiveness e.g. changes in receptive field size, temporal summation, negative feedback loops)
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
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
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
6
Q
what is adaptation ?
A
- adaptation is an increase or decrease in retinal sensitivity with changing light levels
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
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 )
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
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
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
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
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
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
15
Q
what are the two components of photopigment ?
A
- photopigments is protein ( opsin) + chromophore ( retinal )
16
Q
what is the structure of photopigment in dark ?
A
- unbleached ( purple ) photopigment ( opsin + 11-cis retinal ) absorbs light to initiate phototransduction cascade
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
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
19
Q
what is the halftime of cones?
A
- 1.7 minutes
20
Q
what is the halftime of rods ?
A
- 5.2 minutes
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
22
Q
what is photochemical adaptation ?
A
- change in ratio of bleached: unbleached photopigment in retina
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
does the concentration of photoproducts of bleaching limit threshold during dark adaptation ?
. 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
what actually limits threshold during dark adaptation?
- the presence of bleached photoproducts like opsin rather than the lack of unbleached photopigment
27
what is dowling-rushton relationship ?
- log sensitivity during dark adaptation is linearly related to concentration bleached photopigment
28
how does the concentration of photoproducts limit threshold ?
. 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
what is the equivalent background theory ?
. 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
why is it in the first 0.3 seconds during dark adaption threshold drops very steeply ?
. this is due to neural not photopigment changes
31
what is the Arden and weale theory ?
. post-receptoral factors
e.g. changes in receptive field size ( increase in size, loss of centre surround organisation ), also involved in dark adaptation
32
how does retinal location of target affect the dark adaption function ?
. 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
how does target size affect dark adaptation ?
. 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
how does target wavelength affect dark adaptation ?
. 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
how does adapting light wavelength affect the dark adaptation function ?
. 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
how does adapting light intensity and duration affect the dark adaptation function ?
. higher initial threshold
. more prominent cone branch
. later rod-cone break
. longer time to reach final threshold
37
explain the process of light adaptation ?
. 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
what are the factors that affect the rate of dark adaptation ?
. age
. retinal disease e.g. AMD, retinitis pigmentosa, CSNB
. systemic conditions e.g. vitamin A deficiency
39
how do we investigate and diagnose different causes of poor night vision ?
. psychophysical techniques
. fundus and anterior eye examination
. electrophysiology
40
how is the goldmann adaptometer used in the clinical measurement of dark adaptation ?
. 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
how does the goldmann adaptometer show retinal disease ?
. thresholds are either very elevated or dark dark adaptation is very delayed ( fundus albipunctatus )
42
what happens in fundus albipunctatus ?
- this is where the process of dark adaptation takes up to four hours to complete
43
what are computer based systems used in measurement of dark adaptation ?
. 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
how does the photostress test used in the clinical measurement of dark adaptation ?
. 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
what are some non-retinal causes of not seeing well in the dark ?
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
what are the retinal causes of not seeing well in the dark ?
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
how does chronic open angle glaucoma reduce the ability to see in the dark ?
. ganglion cell loss coincides with area of max rod density ( 15 degrees) which is reduced scotopic sensitivity
48
how does vitamin A deficiency reduce the ability to see in the dark ?
. vitamin A important part of rhodopsin
. deficiency reduces rhodopsin concentration slows photopigment regeneration
. results in raised absolute threshold and prolonged dark adaptation
49
what inherited night blindness ?
. inherited conditions giving symptoms of night-blindness
50
what are progressive conditions that cause gradual degeneration ?
. retinitis pigmentosa - condition where vision get worse over time
51
what are the stationary conditions of inherited night blindness ?
. theses conditions do not change over time
. congenital stationary night blindness (CSNB)
. ( types I, II and III)
52
how do differentially diagnose inherited night blindness ?
. dark adaptometry
53
what is Type II CSNB fundus albipunctatus ?
. autosomal recessive
. affects rod and cone photopigment kinetics
. grossly extended adaptation times
. eventually normal thresholds
54
what is retinitis pigmentosa ?
. 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
how is the visual system able to function ?
. 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
what is the limiting factor in photochemical adaptation ?
. the limiting factor in photochemical adaptation is the rate of removal of photoproducts of bleaching
57
what is the benefit of the assessment of dark adaptation ?
. assessment of dark adaptation can help with the diagnosis of congenital and acquired retinal disease
