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 )