Post natal development of the visual system

Question 1

what do infants show a visual preference to

Answer 1

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

Question 2

what does an infant’s spatial vision consist of and which 2 ways is it measured

Answer 2

VA and CS

measured using:
Forced choice preferential looking
Visual evoked potentials

Question 3

describe how forced choice preferential looking works

Answer 3

• 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

Question 4

explain how the va of a human increases when measured by preferential looking tests
and how this compares to when tested on monkeys

Answer 4

• 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

Question 5

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

Answer 5

• 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

Question 6

explain how Visual Evoked Potentials is used to measure va in an infant

Answer 6

• 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

Question 7

how are the results of an infants va found to be with VEP and explain why

Answer 7

• 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

Question 8

what is a more comprehensive measure of spatial vision than simple acuity

Answer 8

contrast sensitivity

Question 9

define what contrast sensitivity is and the shape of the contrast function in adults and where their peak contrast sensitivity is

Answer 9

• 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

Question 10

where in the contrast sensitivity function is the limit of someones va measured

Answer 10

at the higher spatial frequency seen at maximum contrast

at the botton right hand side of the graph

Question 11

what happens to an infants contrast sensitivity with age

Answer 11

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

Question 12

what also increases as spatial frequency increases

Answer 12

constrast sensitivity

Question 13

what explains the low resolution of infant acuity

Answer 13

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’

Question 14

describe the difference in foveal pit at birth vs adult

Answer 14

Birth:
Ill-defined pit, inner retinal cell layers present

Adult:
Obvious pit, inner retinal cell layers moved aside

Question 15

describe the difference in cone outer segment at birth vs adult

Answer 15

Birth:
Cone outer segment length, 3 µm

Adult:
Cone outer segment length, 45 µm (x 15)

Question 16

describe the difference in cone spacing at birth vs adult

Answer 16

Birth:
Cone spacing, every 2.1 mins arc

Adult:
Cone spacing, every 0.3 mins arc (x 7)

Question 17

scribe the difference in cone density at birth vs adult

Answer 17

Birth:
Density, 36,000 per mm2

Adult:
Density, 108,000 per mm2 (x 4)

Question 18

what is most part of the improvement for visual acuity in infants down to

Answer 18

the foveal development

Question 19

what is visual acuity and contrast sensitivity (spatial vision) measure by

Answer 19

FPL and VEP

Question 20

what is a significant limiting factor of a child’s spatial vision

Answer 20

Foveal immaturity (cone OSs and packing density) is significant limiting factor)

Question 21

what is the definition of colour vision

Answer 21

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)

Question 22

what is used to measure colour vision in infants and what are the 2 components that its made up of

Answer 22

• 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

Question 23

from what age can a child start to detect colour, and what type of colour vision do they have

Answer 23

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

Question 24

explain how an infant developes stereo acuity

and how this compares to the development of spatial acuity

Answer 24

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

Question 25

describe 3 things about the state of vision at birth

Answer 25

• VA and CS just measurable (can elicit using acuity cards, by looking at responses to a pattern)
• OKN eye movements present
• Probably no colour perception (earliest that it can be measured is at 2 months old)

Question 26

describe 4 things about the state of vision at 1 month old

Answer 26

• VA is 1 cycle per degree
• CS is 10 times less than adult and peak shift 4 times to low SFs
• Flicker fusion near adult-like (40 Hz)
• Little response to colour or disparity

Question 27

describe 3 things about the state of vision at 2 months old

Answer 27

• Rudimentary colour vision (not yellows)
  referred to with behavioural measures using forced choice preferential looking
• VA & CS have improved slightly
• Response to motion (other than OKN)

Question 28

describe 3 things about the state of vision at 3 months old

Answer 28

• VA now 3 cycles per degree
• C Flicker Fusion adult-like (50Hz)
• Still no stereovision

Question 29

describe 3 things about the state of vision at 6 months old

Answer 29

• Stereovision now ‘online’ and 1 min arc minimum
• Peak of CSF near that of adult
• Steady rise of functions to adult level asymptotes

Question 30

there are rapid changes to the anatomical development of the human visual system, in the first year of infancy, list 5 other changes that occur into early childhood and by what age this happens until with the:
Eyes
Retina
Optic and visual pathways 
Visual cortex
ocular muscles

Answer 30

• Eyes: grow and change shape (~6 years)
• Retina: fovea differentiates (~5 years)
• Optic and Visual Pathways: myelination (~2 years)
• Visual Cortex (& other Brain Centres): growth & refinement of synaptic connections (~8 years)
• Ocular muscles: grow & differentiate (~6 months)

Question 31

list the values of the:
cornea and lens power (D)
axial length (mm)
refractive error (mean +/- SD)

at birth

Answer 31

cornea and lens power (D) = 55D, 34D

axial length (mm) = 17mm

refractive error (mean +/- SD) = +2.5D  (2.75)
(range –6 to +8)

Question 32

list the values of the:
cornea and lens power (D)
axial length (mm)
refractive error (mean +/- SD)

at 1 years old

Answer 32

cornea and lens power (D): 44D, 28D

axial length (mm): 20mm

refractive error (mean +/- SD): +1.5D (2.0)

Question 33

list the values of the:
cornea and lens power (D)
axial length (mm)
refractive error (mean +/- SD)

6 years old

Answer 33

cornea and lens power (D): 43D, 25D

axial length (mm): 23mm (is increasing = losing power)

refractive error (mean +/- SD): +1.0D (1.6)

Question 34

as well as cornea and lens power, axial length and refractive error, what else reduces and by how much from birth compared to 1 year

Answer 34

incidence of astigmatism also reduces, from 20-30% at Birth to 8% at 1 year