how to assess human visual function Flashcards
what is visual function ?
- seeing is a complex task and it can be helpful to break it down conceptually into a series of individual modules or function
what are some the most obvious visual function?
. perceiving fine spatial detail . perceiving differences in luminance . perceiving differences in wave length of light . perceiving information in the periphery . adjusting to bright light . adjusting to low light . depth perception . motion perception
how is perceiving spatial detail assessed and what is associated visual impairment ?
- assessed in terms of visual acuity
- associated visual impairment is blurred vision
how is perceiving differences in luminance assessed and what is associated visual impairment ?
- assessed in terms of contrast sensitivity
- associated visual impairment is hazy/dim vision
how is perceiving differences in wave length assessed and what is associated visual impairment ?
- assessed in terms of colour vision
- associated visual impairment is colour blindness
how is perceiving information in the periphery assessed and what is associated visual impairment ?
- assessed in terms of visual fields
- associated visual impairment is loss of peripheral vision
how is adjusting to bright light assessed and what is associated visual impairment ?
- assessed in terms of light adaptation
- associated visual impairment is glare or photophobia
how is adjusting to low light assessed and what is associated visual impairment ?
- assessed in terms of dark adaptation
- associated visual impairment is night blindness
how is depth perception assessed and what is associated visual impairment?
- assessed in terms of stereopsis
- associated visual impairment is amblyopia
how is motion perception assessed and what is associated visual impairment ?
- flicker fusion rate
- associated visual impairment is motion blindness
how are different eye diseases associated with particular set of visual functions ?
- different eye diseases are often associated with particular set of visual functions
- e.g. AMD - acuity and contrast
Glaucoma - field - though no function is unique to a single disease and often multiple different functions are disrupted
what can each function be divided into ?
- each function can be subdivided into many sub functions
- e.g. acuity is sub divided into resolution acuity and recognition acuity
how to measure visual acuity ?
1- letter charts
. Snellen chart
. LogMAR chart ( logarithm of the minimum angle of resolution )
. various electronic chart
2- symbol chart
. tumbling E’s or Landolt C’s
. HOTV - optional plate for child to hold and point to
. various optotype charts (e.g. Lea symbols; auckland optotypes; kay pictures; cardiff acuity charts )
3-acuity cards
. preferential looking cards; grating cards
. teller cards, keeler cards, lea paddles ( clinician judges if infant saw the grating )
4-EEG ( checker boards)
. electrodes record cortical activity in response to a dynamic ( phase-reversing) stimulus
how to measure contrast sensitivity ?
- letter charts
e.g. Pelli-Robson ( letters get dimmer ) - grating detection charts
e.g. Vistech contrast test - pen-and- paper tests
e.g. spot checks
mark each circle with an X - modified ‘acuity’ cards
e.g. ohio contrast cards
-clinician judges if infant saw the grating
how to measure colour vision ?
- Ishihara plates - chromatic difference between colour and background
- Farnsworth-munsell 100 hue test
- computerised tests
. Cambridge colour test
. universal colour discrimination test
. these test are not independent of acuity , so you don’t know if px really doesn’t have colour vision
how to measure visual fields ?
- confrontation testing
‘say when you see my hand ‘ - kinetic perimetry
.e.g. arc perimetry
.e.g. Goldmann perimetry
. look into screen and dots are presented and say when you see them - standard automated perimetry (SAP)
e.g. HFA, octopus, Topcon , compass
. measuring contrast threshold - SAP variants
e. g. blue-on-yellow perimetry , microperimetry , eye-movement perimetry
5.fMRI retinotopic mapping
what is the outcome variable of test of visual function ?
- the key outcome measure of most functional test is an estimate of the patient’s threshold
what is detection threshold ?
- the weakest stimulus that can be detected X% of the time (e.g. 50% , 75%, 95%)
- exact value varies between different tests, and not always clearly defined
what do different clinical tests measure ?
- not every clinical test actually measures a threshold.
- some (suprathreshold) tests are just pass/fail at a single stimulus level. These may be suitable for crude applications (e.g. screening) , but not for staging disease severity or monitoring progression
how do we measure a threshold ?
- Fechner (1860) defined 3 basic methods:
1.method of constant stimuli
( the gold standard, but slow; mainly for research )
2.method of limits (adaptive )
( primary clinical method)
3. method of adjustment
( if speed is critical )
what is method of constant stimuli ?
- the crudest possible test you can do
- gold standard in research
- easy
- long process
- present every light possible ( bright , dim)
- you see whether patient saw them
- if you present enough you will be able to find the point which you go from not being able to see it to seeing it and that is your threshold
what is method of limits ( adaptive )?
- start with easy stimulus
- if you get it right you make it harder
- stop when you no longer get it right and that is your threshold
what is method of adjustment ?
- you hand control to px
- press button when you no longer can’t see light any more
- rarely used
- very quick
what to note about the three basic methods of measuring threshold?
- not every test fits into these 3 categories
- e.g. the F-M 100Hue test, or new statistical methods in which the stimulus is adapted in complex ways ( e.g. used in standard automated perimetry )
why are there many letter in letter charts ?
- responses must generally be repeated to ensure accuracy. any single response cannot be 100% trusted due to a variety of factors, including:
. intrinsic noise in the visual system ( false positives and false negatives)
. lapses in concentration ( false negatives )
. lucky guesses ( false positive )
why is measuring threshold hard ?
- due to a complex balance between many, often competing practical considerations . speed . accuracy . reliability . bias . ease of use . validity . cost
what does step size mean in a practical ?
- how quickly to vary the stimulus magnitude represents a trade off between accuracy and speed of test
- many steps risk the patient becoming bored. A lapse in concentration can cause their threshold to be wildly underestimated
what is the significance of test duration in a practical ?
- short test are important to ensure compliance and patient flow
- some functional measures are intrinsically more time consuming ( data hungry ) than others
- some test have more measurements
- e.g. SAP test ( 54 threshold)
- contrast ( 1-5 threshold )
- colour ( 2-3 threshold )
what is the significance of criterion effects ( aka response bias )
- letting people choose not to answer ( I don’t know ) is problematic , as you may end up measuring a person’s confidence , not their vision
- similarly with a yes/no task ( e.g. SAP ), some people may be shy about saying yes if not certain
- response bias can be minimised by using forced choice tasks ( e.g. correctly administered letter chart ) , though bias can never be eliminated 100%
what is the significance of response method ?
- often trade off between accuracy and task complexity
- more response options mean more information ( less likely to respond correctly by chance, so less measurement error )
- but can make for a more complicated test ( requires greater patient ability/comprehension )
what is the significance of stimulus >
- specific stimulus can crucially dictate not only other aspects of the test (e.g. the response method) but what is actually being measured
- grating acuity ( resolution acuity ) and letter acuity ( recognition acuity ) are measuring two different things
- standard letter charts aren’t even measuring acuity , but combination of acuity and contrast sensitivity
- pure letter stimuli do exist ( e.g. Moorfields acuity chart ) , but are not yet widely used/known about
- similarly colour test may actually be picking up acuity deficits in some patients
what is the significance of practice inertia, clinical resources , and sustainability )?
- many of today’s tests are old
- this is because they are tried and tested with established normative data
- but there are often other considerations too, like cost, legal hurdles or general inertia ( this is how we’ve always done it )
- it’s also important to ensure the test is sustainable ( will it work in 20 years )
what makes a good test ?
- accurate ( un-biased )
- precise ( reliable/repeatable )
- fast
- easy to use/administer
- inexpensive
- standardised
- sustainable
what is the summary of psychological theory ?
- we are trying to measure a threshold
- we do this by observing responses to a stimulus variable magnitude
- often we use 1 of 3 principle methods ( most often method of limits)
- any single observation is intrinsically noisy, so generally responses are repeated to minimise measurement error
- the final test is a complex balance of many practical considerations , including speed, accuracy , reliability, bias , easy of use , validity and cost
what are other ways is functional measures used ?
used in behavioural measures; psychological measures
a. stimulus image is presented
b. the patient is asked to make a response (e.g. press a button, point , say what they see)
c. based on this response we infer what they can and cannot see
d. key outcome measure: threshold ( or sometimes pass/fail)
what is another way structural measures is used ?
used in objective measures; passive measures
a. special equipment is used. sometimes to measure a threshold (e.g. VEPs). sometimes to quantify anatomy of physiology
b. possible outcome measures include; threshold, retinal thickness (OCT), strength of electrical response ( ERG’EEG), number of photoreceptors ( AOSLO) , amount of blood flow (fMRI ) , volume of white/grey matter in brain
c. sometimes involves presenting a stimulus, but not always ( sometimes don’t even need to be awake )
d. at present, it always possible to relate structure to function
what is functional vision ?
- the person’s ability to perform everyday vision- related activities
- what patients really care about
- often measured by questionnaire
- tends to correlate poorly with more basic measures of visual function
- perhaps not surprising, given that even these more basic measures often correlate poorly with themselves
- no hard distinction between visual function (VF) and functional vision (FV)
