using psychophysics in clinic: assessment of vision in infants and children Flashcards
what is the wikipedia definition of psychophysics
“psychophysics quantitatively investigates the relationship between physical stimuli and the sensations and perceptions they affect”
you are getting a number in the end that describes the extent to which they’re able to see this stimulus or the way they can see this stimulus
how is visual acuity a psychophysical test
the set of letters on a letter chart is the visual stimulus, when asking what is the lowest line of letters they can read, when the patient tells us, they are telling us about their perception, this gives a quantitative output i.e. va is a number
when is using a psychophysical test such as visual acuity from a letter chart not possible and why
for people who cannot respond for some reason e.g. children, you need to find out their perception to a stimulus without asking them what they can see as they can’t tell us, so need a different method
name 2 main types of psychophysical tests used on children who cannot respond to visual acuity charts, and name 2 other methods which are not psychophysical tests
- preferential looking (PL)
- forced-choice preferential looking (FPL) = slight adaptation of preferential looking
other methods:
- OKN
- VEP
how is a preferential looking test carried out
- pattern on one part of a display which is held up infront of an infant
- remainder or display has the same average luminance
- observe, whether the infant reliably looks towards the pattern
- or point toward it, or say where it is (which in this case is not PF)
what will the card look like to the baby, in preferential looking test if a baby has not acuity and why
the whole card will appear to look grey as they can’t discriminate the pattern of one side
how is bias avoided when carrying out a preferential looking test on a child
the practitioner only holds the cards and does not know which side the pattern is on, they just observe the patient’s eyes/behaviour
what type of stimulus is displayed on cardiff cards and which type of children are they used on in a preferential looking test
familiar objects such as a house or a duck drawn on a grey background
so used on a child who is old enough to recognise these objects
how are the objects drawn on cardiff cards when assessing acuity in a preferential looking test and how will it appear if a child is unable to resolve the object
objects are drawn in white with a black border,
thickness of the white line denotes acuity level, if unable to resolve the black and white lines then the child will see no picture and the whole card will appear uniformly grey.
the black and white contrast will always be the same, as you are not assessing contrast sensitivity here
how are the objects drawn on cardiff cards when assessing contrast sensitivity in a preferential looking test
objects are drawn in light grey with a darker grey border
how do cardiff cards avoid discrimination during a preferential looking test
the whole card is grey with the same luminance/average brightness over the whole card = same mean luminance as the picture
swell as assessing a child’s resolution acuity and contrast sensitivity, what else can the pictures on a cardiff card assess
higher level acuity, such as asking the child to identify the picture = recognition acuity
how does the target appear on a keeler/teller card
sine wave grating on one half and uniform grey on the other half
how do keeler cards avoid the effect of an ‘edge artifact’ caused by gratings abruptly stopping at their border with the grey background
theres a white circle around the same region on both halves
(this doesn’t say anything about acuity, but the baby prefers the pattern when the artifact is there, if the baby can’t perceive the pattern but can perceive the artifact, then the artifact will be on both sides)
list the method of how you will test a child with cardiff and keeler cards
- cards presented at about 40cm
- higher spatial frequency or lower contrast lines until no reliably correct response/baby shows no preference
- response could be verbal, pointing or preferential looking
- if involves identification (cardiff cards), measures recognition acuity (a different type of acuity)
- otherwise involves resolution acuity
- is the response correct?
- its the practitioner’s judgement
- practitioner should be unaware of the true position of the pattern
- the position is shuffled to avoid bias
why are the cards presented at 40cm from the child when using the cardiff and keeler cards
as babies can accommodate more accurately at 40cm, so they can see better
what do cardiff cards not have, in order to avoid distractions
they dont have pinholes in the middle
what is a problem with showing a card with higher spatial frequency or lower contrast lines until no reliably correct response, with cardiff and keeler cards
babies lose motivation quickly
what is being measured, by cardiff and keeler cards preferential looking test
a true measure of acuity or contrast sensitivity tells us the ability of the visual system (the best that the visual test can do for the px to resolve the finest detail)
what outcome does a preferential looking test have when a baby is not well fed or alert
they will not respond to the best of their ability (won’t get a true measure of acuity or contrast sensitivity etc)
PL tests are behavioural, so they require the infant’s interest and motivation
how reliable are the responses to a preferential looking test when a baby is alert
90% of responses are correct in infants, which is very reliable
how can you encourage an infant to stay alert as the pattern becomes harder to see in a preferential looking test, and what is this method called
the practitioner could order the cards so that an ‘easy’ pattern (e.g. low spatial frequency) appears occasionally between the harder patterns
called recovery method
what can a baby at 2 months of age respond to and not respond to
can respond to black and white patterns and when increase the brightness they show more interest, but do not respond to patterns that have colour
this is done in a 2AFC choice (the baby has 50% chance to guess the correct choice)
what can a baby at 4 months of age respond to
most can respond to the pattern, regardless of if its black and white or colour
what can older children e.g. 5 year olds be asked to recognise, in psychophysical tests used in children and what types of psychophysical tests are these
symbols or letters (can name the symbol or match it with a smaller card)
may also be asked to report the orientation of a pattern such as the letter ‘E’
they are forced choice and also a method of limits
when do you consider the acuity of e.g. a 5 year old child, when carrying out a psychophysical test using symbols and letters, and what method is used to do this
consider acuity to be the point at which they can no longer do this correctly
this is method of descending limits
how is methods of descending limits carried out on a child using psychophysical tests
the patient is shown targets that are above threshold, stepping towards threshold until they begin to make errors
(ascending method of limits makes it easier, as your going from hard to easy)
what are you measuring when using the ‘E’ target
resolution acuity - ability to resolve the lines
what are you measuring for symbols and letters
recognition acuity (higher level and more complex)
what type of psychophysical method does the ‘E’ target use
a four alternative forced choice method (up, down, left or right)
what type of psychophysical method do symbols and letters use
letters = a 26 alternative forced choice method symbols = a four or five alternative forced choice method (on a card)
what type of forced choice test do cardiff cards have
6 alternative forced choice (as 6 different pictures)
what type of acuity is it if a child looks or points towards the target on a cardiff card
resolution acuity (px just needs to resolve the target to see that its there)
what type of acuity is it if a child names the target on a cardiff card
recognition acuity (higher level)
how is a visually evoked potential VEP set up and used on a px
- electrical potentials are recorded from key locations on the scalp, including one or more over the visual cortex
(where we want to pick up a response from, at the back of the head is where v1 is, so getting a response from the cortex underlying that electrode) - this electrical activity is arising from transmission along the visual pathway
what does a VEP measure on a baby
how long it takes the visual system/v1 to respond (the latency) and how big the response is (the peak)
how can a VEP be used to track visual development on a baby
recordings from VEP can show a response to black and white stimuli from v1 but no response to colour stimuli from v1
what is a VEP response usually dominated by and therefore what does it not reflect
the response is usually fovea-dominated and therefore does not reflect cortical function beyond the recording point (so can only pick up early response i.e. from v1)
what 2 types of responses come from a psychophysical test
- visual
- behavioural
what is there no evidence of, if a child shows no preference i.e. gives no response to a target in a psychophysical test
there is no evidence that the stimulus can’t be seen
which tests dont require a behavioural response
objective tests such as VEP and OKN
what does a VEP passive (not asking baby to do anything) electrophysiological response not indicate
perception (that the baby can see the pattern in a normal way)
what type of response is OKN
involuntary eye movement response
what does a OKN response not indicate and what does it indicate
it just tells us the the visual system is working up to a point, and does not indicate perception
what are the two types of responses that partially come from OKN
partially cortical and partially sub-cortical
e.g. spinning drum: temporal to nasal uses cortical function and nasal to temporal uses sub-cortical function
babies can’t do nasal to temporal movement
with which two methods is contrast sensitivity in adults similar i.e. shows the same results/outcome
when measured using the sweep VEP or a psychophysical method (2-AFC)
both find similar acuities and contrast sensitivity, so both the methods seem to agree
what type of results does VEP and OKN give on acuity compared to preferential looking psychophysical test, and why
VEP and OKN methods tend to give higher estimates of acuity then preferential looking
because perhaps babies can perceive these types at this age but can’t make the behavioural response we need in order to know that they can see these things well
