Post natal development of the visual system Flashcards
what do infants show a visual preference to
boldly patterned surfaces over homogenous gray surfaces (Fantz)
infants like to look at patterns and faces, not boring gray stimuli
the child prefers the grating even though both have equal brightness/space average luminance to each other
what does an infant’s spatial vision consist of and which 2 ways is it measured
VA and CS
measured using:
Forced choice preferential looking
Visual evoked potentials
describe how forced choice preferential looking works
- looking at 2 displays of equal brightness and space average luminance but just a different spatial frequency
- between the display = hole and behind is the experimenter (so child cannot see)
- The experimenter makes the forced choice when observing the child’s gaze on the grating
- it can either be below or above the child’s threshold
- the experimenter’s result is analysed
- if for a given grating, if the experimenter scores above 50% then can infer that infant can equally see the grating
explain how the va of a human increases when measured by preferential looking tests
and how this compares to when tested on monkeys
- va increases in a monotonic fashion (with PL)
- va improved by 1 cpd per month between 1-12 months = 8 fold increase in object size
- if look between age of 1 - 12 months, the rule of thumb = visual acuity increases by 1 cpd
Monkey:
- similar type of monotonic increase as humans with visual acuity
- has values of adult like va, beyond 30 cpd
- age is expressed in weeks instead as they develop quicker by 4x
name an adaptation of the forced choice preferential looking test and explain how it works
what is the advantage of this test over the conventional forced choice preferential looking test
- The Acuity Card Procedure
- cards of different gratings/spatial frequency
- held up in a puppet stage
- we judge which side the child is looking
- increase the spatial frequency until you dont get a response
= a judgement of the child’s visual acuity and compare it to the age norms data - this test is less time consuming than the conventional forced choice preferential looking test
explain how Visual Evoked Potentials is used to measure va in an infant
- patterns that can be either gratings or checkerboard seen on a screen
- the test will find the change in spatial frequency of patterns and the patient has scalp electrodes attached
- this records the visual cortical activity and also at high enough spatial frequency beyond the child’s resolution where you will not find a response
how are the results of an infants va found to be with VEP and explain why
- va values are higher with VEP data
- because they elicit a cortical response but not a behavioural response in the early stages of their visual development
what is a more comprehensive measure of spatial vision than simple acuity
contrast sensitivity
define what contrast sensitivity is and the shape of the contrast function in adults and where their peak contrast sensitivity is
- Contrast sensitivity = 1/minimum contrast to detect (grating) target
i. e. the reciprocal of the minimum contrast of the target you can see - in adults the contrast function is a inverted U shape
left = lower, sf right = higher sf
the peak contrast sensitivity for adults is in the medium grating acuity (not low or high) so best vision is for medium sized objects
where in the contrast sensitivity function is the limit of someones va measured
at the higher spatial frequency seen at maximum contrast
at the botton right hand side of the graph
what happens to an infants contrast sensitivity with age
shift in sensitivity (upwards) and spatial range (rightwards) with age
= infant becomes more sensitive to contrast with age and also the range of grating acuity increases/expands
what also increases as spatial frequency increases
constrast sensitivity
what explains the low resolution of infant acuity
The wider cone spacing (lower density) in infants means that fine-grain spatial details could fall in-between adjoining cone outer segments rendering them ‘invisible’
describe the difference in foveal pit at birth vs adult
Birth:
Ill-defined pit, inner retinal cell layers present
Adult:
Obvious pit, inner retinal cell layers moved aside
describe the difference in cone outer segment at birth vs adult
Birth:
Cone outer segment length, 3 µm
Adult:
Cone outer segment length, 45 µm (x 15)
describe the difference in cone spacing at birth vs adult
Birth:
Cone spacing, every 2.1 mins arc
Adult:
Cone spacing, every 0.3 mins arc (x 7)
scribe the difference in cone density at birth vs adult
Birth:
Density, 36,000 per mm2
Adult:
Density, 108,000 per mm2 (x 4)
what is most part of the improvement for visual acuity in infants down to
the foveal development
what is visual acuity and contrast sensitivity (spatial vision) measure by
FPL and VEP
what is a significant limiting factor of a child’s spatial vision
Foveal immaturity (cone OSs and packing density) is significant limiting factor)
what is the definition of colour vision
The capacity to discriminate among lights of different wavelength composition on the basis of the difference in wavelength composition
(we have to exclude any factor of brightness)
what is used to measure colour vision in infants and what are the 2 components that its made up of
- preferential looking procedure
a tool that can be adapted to various measurements
made up of 2 components:
1) Measure Weber fraction (preference) with WHITE light stimulus
measuring the minimal amount/just noticeable of brightness change between target and background that the infant makes a preference for with the difference in brightness only = weber fraction
2) Measure preference for COLOURED light stimulus in intensity steps less than established Weber fraction
increase the coloured light intensity and attempt to find the preference again, but increase it in steps that are less than the weber fraction and by doing that, we know if the child responds and they’re responding because they see colour and not because they’re seeing a change in brightness
so to detect colour, is not confounded by their ability to detect their brightness or not
from what age can a child start to detect colour, and what type of colour vision do they have
2-month-old infants can make colour discriminations so at a minimum they are dichromatic
compared the photopic luminosity function between 2 and 4 month old infants with adults = not much difference in colour vision with adults
so…
By 2 months = we know the infant is trichromatic at this point (adult like)
found from VEP based tests
explain how an infant developes stereo acuity
and how this compares to the development of spatial acuity
the earliest stereo acuity starts at around 3 months of age and then can jump rapidly to near adult like within 24 hours, just like a switch
so there can be nothing measurable before 3 months of age. it is a neural change but we don’t know what is responsible for that
so there can be little change in the progression of grating acuity, but at the same time the same infant can have a large change in stereo acuity
