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psychopysics > Dark Adaptation > Flashcards

Dark Adaptation Flashcards

1
Q

what do psychophysical Procedures?

A

procedures developed top measure threshold. aim to give accurate and reliable results

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2
Q

How do they get reliable results?

A
  • With the fewest number of stimulus presentation
  • Without requiring sophisticated judgments from observer
  • with minimal opportunities for psychophysical bias
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3
Q

what do psychophysical procedures involve?

A

presenting visual stimuli that vary along one of a number of possible dimensions but all parameters are kept the same

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4
Q

What example is a psychophysical procedure?

A

Visual Acuity

Amplitude of accommodation

Near point of convergence

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5
Q

What does visual acuity do?

A

Measured the threshold for resolution

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6
Q

How is VA a psychophysical procedure?

A

We only change the size of the letter everything else such as contrast, size of separation, stays the same

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7
Q

What does Amplitude of accommodation do?

A

Measures threshold for near resolution

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8
Q

What does near point of convergence do?

A

Measures threshold for BSV

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9
Q

what are the classics psychophysical methods?

A

Method of adjustment

method of limits

method of constant stimuli

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10
Q

What is method of adjustment?

A

Gradual smooth adjustment of intensity (usually by patient) until threshold is reached

can be ascending or descending

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11
Q

Give example of method of adjustment?

A

Bring reading chart until can see it

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12
Q

What is the advantage of method of adjustment?

A

Fastest technique
good for obtaining estimate
px more likely to pay attention

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13
Q

What is the disadvantage of method of adjustment?

A

psychological bias (differences in confidence)

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14
Q

What is method of Limits?

A

Adjustment of intensity in regular steps until threshold reached

can be ascending or descending

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15
Q

What is ascending method of limits?

A

Dark adaptometry- measurement of threshold during dark adaptation minimise light adaptation

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16
Q

What is descending method of limits?

A

Used in letter charts start with contrast and decrease until they cant detect it

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17
Q

What is descending method of limits?

A

Used in letter charts start with contrast and decrease until they cant detect it

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18
Q

Give example of method of limits?.

A

Peli robsopn

bailey lovie

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19
Q

What is the advantage of method of limits?

A

Quick and simple

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20
Q

What is the disadvantage of method of limits?

A

Errors of habituation
errors of anticipation
difference in confidence between subjects

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21
Q

How is method of limits modified?

A

Staircase

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22
Q

What do multiple simultaneous staircases do?

A

Increases accuracy

23
Q

Give example of staircase method?

A

Visual Field screeners

24
Q

What is method of constant stimuli?

A

Most accurate way of measuring threshold and most time consuming

Fixed intensity presented in random order

25
Q

What is the advantage of method of constant stimuli ?

A

Most accurate technique as

  • px cannot anticipate the next stimuli
  • lots of presentation of each stimulus intensity
  • can include blank frames and frames well above threshold to check reliability false +ve and -ve
26
Q

What is the disadvantage of method of constant stimuli?

A

Still problems with differences in confidence

time consuming

27
Q

How do inaccuracies occur?.

A

Fatigue
psychophysical bias
Attention and motivation
Habituation

28
Q

What do we look for in dark adaptation?

A 
How threshold drops over time in dark, bright light and then dark
Cone recovery 
rod cone break 
rod recovery 
absolute threshold
29
Q

How to we measure dark adaptation? procedure

A

1) Bleach substantial proportion of photopigments
2) Measuring thresholds at regular intervals in the dark
3) Shape of curve affected by how much photopigment bleached to begin with

30
Q

What limits threshold during dark adaptation ?

A

Rhodopsin breaks down into other photoproducts when it absorbs light and is bleached

31
Q

How does rhodopsin break?

A

Rhodopsoin-> transduction cascade initiating visual signal =opsin + retinal by metarhodopsin 11 intermediate step

32
Q

What is Dowling Rushton Relationship?

A

Log sensitivity during dark adaptation is linearly related to concentration BLEACHED photopigment

33
Q

What is Arden and weale (1954)

A

Post receptoral factors- changes in receptive field size, also involved in dark adaptation

34
Q

What is possible causes for delayed dark adaptation/ Poor night vision ?

A

Choroidal Circulation abnormality

Bruchs membrane abnormality

RPE abnormality

Photoreceptors abnormality

Post receptoral abnormality

35
Q

What effects rate of dark adaptation?

A

Age
Retinal disease - retinitis pigmentosa
systemic condition- vitamin A deficiency

36
Q

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

A

psychophysical techniques

Fundus and anterior eye examination

Electrophysiology etc

37
Q

How do you measure dark adaptation?

A

Goldmann adaptometer

38
Q

How does Goldmann Adaptometer work ?

A
  • –Adapting light in Ganzfeld Bowl
  • –All lights extinguished
  • –Threshold measured at intervals post bleach
  • –Take more than 30 mins for full dark adaption curve
39
Q

What is photostress test?

A

Much quicker
Bleach with ophthalmoscope
time taken to return to within 1 line best VA
>60 sec pathological

40
Q

What are age related problems?

A

Miosis and lens opacities -> retinal illuminance reduced!

41
Q

What are night myopia?

A

Eye focuses at about 1m in dark
usually only when no visual stimulus, nut possible when stimulus is degraded
may benefit from -0.50 over correction

42
Q

How does age effect the retina?

A

Reduced rate of photopigment regeneration causes slowed dark adaptation

43
Q

How does AMD effect the retina?

A

Worsening of normal ageing changes to retina
Rod cell death raises absolute threshold
Thickening of Bruchs membrane, RPE and photoreceptor damage and reduced choroidal circulation slow photopigment regeneration

44
Q

How does glaucoma effect the retina?

A

Chronic open angle glaucoma causes ganglion cell loss which coincides with area of max rod density 15 degrees and reduced scotopic sensitivity

45
Q

How does Vitamin A deficiency effect the retina?

A
  • Vitamin A important part of rhodopsin
  • Deficiency reduces rhodopsin concentration and slows photopigment regeneration
  • Results in raised absolute threshold and prolonged dark adaptation
46
Q

What is CSNB?

A

Congenital stationary night blindness

47
Q

What is type 1 CSNB?

A

Normal Fundus

  • Most common type
  • Normal fundal appearance, VA normal
  • Rod branch of dark adaptation function absent
  • Cones less effected
48
Q

What is type 11 CSNB?

A

Abnormal fundus

  • fundus albipunctatus
  • autosomal recessive
  • affects rod and cone photopigment kinetics
  • grossly extended adaptation times
  • eventually normal threshold
49
Q

What is Oguchi’s Disease?

A 
Rare autosomal disease
Fundus looks grey/yellow metalic
regenration of pigment normal 
post receptoral abnormality 
Cone adaptation normal, rod adaptation grossly delayed( 2-24 hours)
50
Q

What is type 111 CSNB?

A
  • X linked recessive
  • Associated with myopia, nystagmus, and low vision
  • Cone dark adaptation is normal, rod phase of recovery is absent
51
Q

What is retinitis Pigmentosa?

A
  • -Autosomal recessive, autosomal dominant or X linked recessive
  • -Progressive photoreceptor degeneration
  • Caused by progressive shortening of rod and cone outer segment due to normal disc shedding
  • -First symptoms is night blindness(20 year old)
  • -Central vision initially good later effected
52
Q

What does retinitis pigmentosa look like on fundus?

A

Pigment clumping in mid periphery (bone spicule appearance) and attenuated blood vessels

53
Q

What is the threshold in retinitis pigmentosa?

A

Grossly elevated rod and cone threshold

psychopysics (10 decks)

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