VISFUN: SEMINARS - amblyopia, SDT, psychophysics Flashcards
Define amblyopia. What does it result in [Amblyopia #1]
neuro-developmental disorder of the visual cortex arising from abnormal visual experience in early life
results in decreased VA in one or both eyes
List and describe the 3 types of Amblyopia [Amblyopia #1]
- Strabismic Amblyopia - caused by active inhibition of a deviated/turned eye
- Anisometropic Amblyopia - caused by inhibition of the eye with the stronger refractive power (poorer vision eye)
- Deprivation Amblyopia - poor vision in one or both eyes due to under-stimulation of the retina via other causes
List 6 possible causes of deprivation amblyopia [Amblyopia #1]
- congenital cataracts
- corneal opacities
- blepharospasm (involuntary tight closure of eyelids)
- surgical lid closure
- unilateral complete ptosis
- prolonged patching or use of atropine drops
How does amblyopia affect contrast sensitivity? [Amblyopia #2]
Generally decreases at all spatial frequencies
Which type of amblyopia has a larger impact on stereopsis? [Amblyopia #2]
‘Strabismic amblyopia’ has a much larger effect on stereopsis than anisometropic
In fact, many anisometropic amblyopes retain some stereopsis
Where do the predominant neurophysiological changes occur in amblyopia development? What other areas are affected? [Amblyopia #3]
Predominant changes occur in V1
There are changes to areas downstream also:
- V2, LGN, MT, Macula and Optic disc
What is Signal Detection Theory? [SDT #1]
The idea that the detection of a stimulus depends on its intensity and the physical/psychological state of the individual
I.e relates to our ability as researchers/optom’s to discriminate signal from noise of stimulus and noise of observer
I.e true vs false positives/negatives
What is the clinical application of signal detection theory? [SDT #1]
Can be used to detect ocular disease (e.g glaucoma) by setting an expected response criterion
I.e probability of a ‘hit’ or ‘miss’
Define ‘sensitivity’ and ‘specificity’ [SDT #2]
Sensitivity = ability to correctly identify those with the target condition (true positives)
Specificity = ability to correctly identify those without the target condition (true negatives)
Describe the receiver-operator characteristic (ROC) curve. What are it’s axis? and how does it relate to sensitivtiy and specificity? [SDT #2]
Plots the Hit rate (sensitivity) against the rate of False alarms (False positives; 1 - specificity)
Each point on ROC curve represents a sensitivity/specificity pair corresponding to a particular decision threshold
How do you calculate sensitivity and specificity [SDT #3]
Sensitivity = True positive/True positive + False negative
Specificity = True negative/True negative + False positive
List the features of a good clinically chosen test [SDT #4]
- appropriate criterion level
- allow for early detection
- constancy in test conditions
- sensitivity + specificity
- high reproducibility + reliability
- test multiple prediction parameters for stronger prediction
- patient comfort
- within time and cost constraints
- low risk
- easy to follow + interpret
- objective + quantitative analysis
List issues for using clinical tests as a screening tool [SDT #5]
- cost + time + funding
- availability of large population
- sensitivity: which stage to screen and with which combination
- screening not absolute confirmation of disease
- screening often not sensitive to detect early stages
- bias: selection, observer/expectation and verification bias
- psychology
- variable time factors
Describe the methods to determine psycho-physical paramaters [Psychophysics #1]
- Method of Constant Stimuli - randomly presenting stimuli. threshold at 50%
- Method of Adjustment - subject increase/decrease until seen/unseen
- Method of limits - ascending or descending steps until seen/unseen
Adaptive psychophysical methods:
4. Staircase Method, 5. Maximum likelihood estimation
What is Bayesian Data Analysis? [Psychophysics #2]
Is a method of statistical inference where you use probability to represent uncertainty in a statistical model
As you get more info, the probability is updated and becomes more accurate for the chosen parameter
Also, the gathering of data gives you a distribution. With enough data, you can obtain a generative model of what to expect
What are the challenges of the Bayesian Adaptive Method? [Psychophysics #3]
- The influence of prior knowledge: researcher bias + scientific validity
- requires planning + good experimental design
- is computationally intensive
How do we overcome the challenges of the Bayesian Adaptive Method? [Psychophysics #3]
Overcome by:
- justify why the prior information included was chosen
- specify the algorithm for analysis in advance
What are the 2 different bayesian adaptive methods in perimetry? [Psychophysics #4]
- SITA - Swedish Interactive Thresholding Algorithm
2. ZEST - Zippy Estimation by Sequential Testing
Describe the difference between SITA and ZEST [Psychophysics #4]
SITA: Model of visual field is constructed before actual test; threshold is estimated based on age and neighbouring threshold
ZEST: uses a maximum likelihood threshold strategy
What is Maximum Likelihood Estimation? [Psychophysics #4}
Responses from start of session by the subject are used to determine stimulus intensity at each trial through statistic estimation
List Advantages of Full Threshold method of perimetry visual field testing {Psychophysics #5]
- Gold standard
- accurate
- sensitive
- reliable
List Disadvantages of Full Threshold method of perimetry visual field testing [Pscyhophysics #5]
- long test duration: result in px fatigue and malcompliance
- high retest variability
Advantages of SITA [Psychophysics #5]
- High sensitivity (95.12% for mild defects, higher for severe)
- over 50% faster than full threshold method
- similar re-test variability to full threshold (so not worse)
- low intra- and inter-test variability
Disadvantages of SITA [Pscyhophysics #5]
- Higher prevalence of VF loss
- sensitivity measurements higher and lower
- SITA can pick up more abnormal points: i.e. false positives
- intolerant to: refractive error, pupil size, blur
- size of defects larger compared to full threshold
List Advantages of Adaptive Perimetry [Psychophysics #5]
- no data wastage
- decreased likelihood for fatigue artifacts
- increased efficiency
- increased sensitivity
- low test-retest variability
List Disadvantages of Adaptive Perimetry [Psychophysics #5]
- technique lacks age-matched data for children
- difficulty in learning tasks required of px
- can only be used for yes-no/forced-choice designs
