Week 3 - Postnatal development of visual function

Question 1

Whats the development of visual pathway and perceptual development?

Answer 1

• Development of the Visual Pathway
- optical
- retina
- lateral geniculate nucleus
- striate cortex

• Perceptual Development
- visual acuity
- contrast sensitivity
- stereopsis
- vernier acuity

Question 2

Visual pathway: Optical: What should we expect of a healthy child on first examination?

Answer 2

• ophthalmoscopic examination reveals clear media

• no optical aberrations in the cornea or lens

• full term human neonate is hyperopic, approximately +2.00 ± 2.00 DS

• preterm infant is slightly more variable, +2.00 ±2.50 DS

• infants also show a degree of astigmatism at birth (majority corneal)

Question 3

Visual pathway: Optical: Emmetropization

Answer 3

• process by which the refractive state of the eye changes is termed EMMETROPIZATION

• emmetropization appears governed by both active and passive factors

• passive emmetropization refers to normal eye growth as eye size increases the power of the optical components decrease proportionally reducing refractive error and maintaining emmetropia
• active emmetropization describes a visual feedback mechanism in the control of eye growth (visual experience)

Question 4

Visual pathway: Optical: How does emmetropization progress?

Answer 4

• hyperopic increases during first six months before reduction towards emmetropia
• significant reduction in astigmatism occurs in infants first year (until 18 to 48 months)
- result of increase in eye size + concurrent flattening of cornea changes in mean refractive error

• infants have some ability to accommodate at 2 weeks of age, increasing during the first 3 months
- cues to accommodation include blur, vergence, chromatic aberration and disparity
• need for infants to accommodate is much less (depth of focus as a result of a smaller pupil and visual acuity )
• the fact they do not make large accommodative efforts is related to lack of need, as much as lack of ability

Question 5

Visual pathway: Retina: Where does most retinal changes occur in infants?

Answer 5

• Anatomical immaturity of the retina at birth, around the fovea results in majority of postnatal changes occur in macular

• Diameter of fovea itself reduces from 5.4° at birth to around 2.3° at maturity (the peripheral retinal anatomy, in comparison, is relatively mature)

Question 6

Visual pathway: Retina: How do the segments of the retina in an infant change?

Answer 6

• foveal cones of a neonate are immature with abnormal shape, inner segments and outer segments are broader in comparison to adult
- adult the inner segments are highly effective in capturing light and funnelling this energy to the outer segment

• during development the outer segment increases in length and the cones become thinner as the effective receptor aperture increases in size and the inner segment begins to act as a funnel increasing light gathering properties
• the effective light collecting area covers 2% in the fovea of a newborn in comparison with an area covering 62% in adults

Question 7

Visual pathway: Retina: How does receptor density differ between adults and infants?

Answer 7

• fourfold increase in the number of cones in the central fovea between birth and adulthood
• receptor packing density increasing from 2.3 minutes to 0.58 minutes in adult fovea
- These modifications in both cone density and light gathering properties increase of acuity during visual maturation

• the human fovea remains immature until after 15 months of age appearing adult like at an age of 4 years

Question 8

Visual pathway: describe LGN

Answer 8

• The high resolution capabilities in the retina is of little use if neural connections are inadequate

• Therefore the major source of visual function improvement can be credited to changes in the neural organisation

• the LGN consists of six distinct capped-shaped layers of neurons
- layers 1, 4 and 6 receive input from the contralateral eye and layers 2, 3 and 5 from the ipsilateral eye
- ganglion cell bodies in layers 1 and 2 are larger (magnocellular) than the other layers (parvocellular)

Question 9

Visual pathway: How does the LGN change between adult and infant?

Answer 9

• LGN doubles in the first** six months ** of life with appearing adult-like by 9 months
- parvocellular reaching adult levels within the first year
- magnocellular layers reaching adult size at nearer two years

• spatial resolution of LGN cells with receptive fields in the fovea improves sevenfold from birth to adulthood
• resolution in the LGN continues to improve until at least five months of age with adult levels not being reached until seven months

• comparability of resolution improvement in LGN cells to development of behavioural resolution, implies that the behavior may be limited by the development of subcortical structures in the visual pathway

Question 10

Visual pathway: Summary of changes in retina

Answer 10

1. differentiation macular region
2. migration cells as foveal pit develops
3. foveal cones thinner and more elongated
4. foveal cone density increases

Question 11

Visual pathway: Summary development of changes in optical:

Answer 11

1. clear optical media
2. full term +2 dioptres
3. accommodation ‘on target’ 3/4 months
4. astigmatism correlates corneal curvature first year
   - small eye, more curved cornea, fatter lens=small eye phenomenon

Question 12

Visual pathway: Summary of changes in LGN:

Answer 12

1.parvo cellular 1 year
2. magno cellular 2 years
3. body LGN max 4 months, adult like 9 months
4. closely parallels development of behavioural acuity

Question 13

Visual pathway: Striate cortex:

Answer 13

• the axons whose cell bodies are in the LGN reach the primary visual cortex (also known as the striate cortex or area V1) by passing through the optic radiations
• as in the LGN, there is a spatial pattern of representation of the retinae in the visual cortex
- macula is represented in the posterior third of the visual cortex (25% = 2.5° )

Question 14

Visual pathway: Striate cortex: layers

Answer 14

• Six layers (and several sub-layers) arranged in bands
• LGN terminate in layer 4 with parvocellular layer neurons sending their axons to neurons in the deeper part of this layer
• Layer 4 has separation of inputs from the two eyes, if cells in one layer of the LGN layers receive their input from one eye, the next layer will receive input from the contralateral eye
• These groups of cells form alternating stripes or bands in layer 4, above and below this layer, most cells are driven binocularly, although one eye is usually dominant; Ocular dominance columns

Question 15

Visual pathway: Striate cortex: types of neurons

Answer 15

• simple cells respond to lines darker or lighter than the background
• Complex cells in the upper layers of the striate cortex show a strong selectivity for the direction in which a stimulus is moving. Movement in one direction provides strong response from the cell but it is unresponsive to movement in other directions

• end stopped complex cells (previously called hypercomplex cells) respond optimally to short bars because of inhibitory influences. The best stimuli for these cells requires not only a certain orientation but also a discontinuity, such as a line that stops, an angle or a corner

Question 16

Visual pathway: Striate cortex: development (synapses)

Answer 16

• volume of the striate cortex reaches adult levels by eight months of age

• Early stages of visual development cortical cells refined during a period of intense synaptogenesis
- The number of synapses per neuron at the time of eye opening is only about 1% of the adult figure; thereafter the number increases dramatically to a maximum value at eight months of age
- after this, synapses are selectivity eliminated until adult density is reached at about eleven years of age

• These abundant synaptic connections present in childhood support the plasticity of the cerebral cortex, which is lost or reduced in adult life

Question 17

Visual pathway: Striate cortex : development (cells)

Answer 17

• progressive increase in proportion of selective cells at expense of nonselective cells first six weeks
• orientation selective tuning reaches an adult value between five and six weeks of age
- similar developmental sequence occurs for directional selectivity

Question 18

Visual pathway: Striate cortex: development (organisation)

Answer 18

• Organisation of the cortex into ocular dominance columns not complete at birth
• segregation happening several weeks later, when the LGN axons retract to establish separate, alternating areas in layer 4 that are supplied by one or other eye
• this process of segregation is completed by four to six weeks.
• The segregation of the LGN axons to form ocular dominance columns is dependent on equal input from either eye

Question 19

Visual pathway: Summary development striate cortex

Answer 19

• synapses double first 8 months
- decline to adult levels
• orientation selectivity 5/6 weeks
• directional selectivity 5/6 weeks
• ocular dominance columns complete 4/6 weeks

Question 20

Perceptual development: visual acuity improvement evidence

Answer 20

• Rapid improvement in visual acuity during the first six months of life reported with both behavioural methods
- such as forced choice preferential looking and acuity card procedures and pattern evoked visual potentials

Question 21

Perceptual development: visual acuity (cycles per degree)

Answer 21

• Studies Show binocular acuity of 1 cycle per degree at birth, increasing to 8-12 cycles per degree at one year of age and reaches around 30 cycles per degree at three years
• adult levels of 40-50 cycles per degree are reached at five to six years or later
• initial acuity levels are poor at first with a steep rise approaching close to adult values over the first six months of life, adult levels being finally reached at around the end of the first year

Question 22

Perceptual development: visual acuity (development)

Answer 22

• recognition acuity (letters, numbers or similar optotypes) in children, as in adults tends to be lower than grating acuity
• six years recognition acuity lies between 1-1.5 minutes of arc and continues to improve until puberty
• surrounding contours, targets or shapes have a deleterious effect on recognition acuity - crowding phenomenon
• majority of children are not able to achieve levels of 6/6 with a linear format test, this supports the suggestion that maturation of linear visual acuity is thought to occur later at around 10 years of age

Question 23

Perceptual development: why do infants have poor visual acuity?

Answer 23

• ocular media clear at birth, however hypermetropia and astigmatism are common so this is the reason for poor spatial resolution capabilities

• optical blur, however, cannot account for poor acuity as degree of refractive error is usually small to moderate and visual acuity still found dramatically reduced at 3-4 months of age when accommodation responses reach maturity

• adult foveal acuity can be blurred by +14.00 dioptres and levels of acuity are still found to be better than that of a six week old infant

Question 24

Compare degree of arc vs minute of arc

Answer 24

• Consider a circle which contains 360 degrees… 1 degree of visual angle contains 60 minutes and 1 minute of arc contains 60 seconds of arc… Therefore, a visual angle of 1 minute of arc is 1/60 of a degree.
• When measuring acuity using gratings the grating size is specified as the number of whole light/dark cycles per degree (c/deg) of visual angle. This is termed spatial frequency (in cycles per degree).
• In a grating of 30 (c/deg), each light/dark cycle subtends 2 minutes of arc and the individual bars subtend 1 minute of arc ( this is equivalent to 6/6), while the individual bars of a 3 c/deg grating subtend 10 minutes (this is equivalent to 6/60)

……the low density of cone photoreceptors and their cumbersome shape, which hinders light absorption combined with an under developed cortex can explain the low levels of visual acuity at birth. It is these changes, which have been discussed previously, in both cone density and functional properties and cortical architecture that are instrumental in the rate of visual acuity development……

Question 25

Perceptual development: Summary of visual acuity

Answer 25

• rapid 6 months
• grating acuity 1c/deg birth
8-12 c/deg 1 year
30 c/deg 3 years
40-50 c/deg at 5/6 years
• recognition acuity < grating acuity
• VEP initial levels poor (immaturity of structures they are used to assess)
adult values 6 - 12 months

Question 26

Perceptual development: Contrast sensitivity: development

Answer 26

• between birth and ten weeks contrast sensitivity improves at all spatial frequencies indicating the overall improvement in the ability of the infant visual system to process spatial information
• this improvement is rapid in the first few months of postnatal life and then gradual until five to eight years of age when adult like values are attained

Question 27

Perceptual development: Contrast sensitivity: low and high spatial frequency development

Answer 27

• the cones become longer and thinner allowing the more effective funnelling and absorption of light, rendering an overall increase in sensitivity
• peak of the CSF shifts towards the high spatial frequencies with age, due to increase in central receptor density
• roll off at the lower spatial frequencies occurs through the cortical development of lateral inhibitory interactions as well as the development of bandpass spatial frequency and orientation tuning

• maturation of spatial frequency channels appear to occur at different rates in the neonate, mechanisms tuned to low spatial frequencies maturing earliest, while sensitivity to high spatial frequencies continue to grow beyond 33 weeks

Question 28

Perceptual development: Depth perception:

Answer 28

• quality of ability to perceive depth vary depending on quality of their binocular vision
• by about three months infants can use disparity information to perceive depth, in both behavioural responses to line stereograms using a preferential looking method and to random dot stereograms

• three months infants are able to detect disparities of about 60 minutes of arc
• as soon as the onset stereoscopic discriminations occurs neonates stereoacuity increases rapidly to less than 1 minute of arc within three to four weeks of discrimination onset
• in contrast to data on the development of visual acuity and contrast sensitivity, behavioral measurements of the development of stereopsis show remarkable agreement with electrophysiological data

Question 29

Perceptual development: Depth perception: ocular dominance columns

Answer 29

• the development of stereopsis has been found to be closely correlated with the segregation of ocular dominance columns

• the anatomical segregation of the input from either eye to the visual cortex ensures that the derivation of the information is conserved

• crossed (near objects) and uncrossed disparities (far objects) appear to develop differentially this
may reflect separate developmental rates of the near and far cortical cells responsible for disparity-sensitivity

Question 30

Perceptual Development:Vernier Acuity

Answer 30

• (Hyperacuity)
• Snellen acuity limited
• visual system has the capability to make much finer discriminations, 3-6”
• remarkable performance
• smallest foveal cones are separated by 30”
• point spread function

Question 31

Perceptual Development:Vernier Acuity: development

Answer 31

• Vernier acuity improves rapidly in the first few months of life, but is delayed in comparison to resolution, before eleven to twelve weeks of age. Vernier acuity then develops at a faster rate until thresholds are half that of grating acuity at eight months
- grating and Vernier acuity develop in parallel, with Vernier acuity being twice as good as grating acuity at all ages
- grating acuity reaches adult levels at around five years of age where as Vernier acuity may not reach adult levels until nearer ten years of age

• vernier acuity requires a high degree of spatial processing
- the immaturity of a neonates retina and cortex and under sampling by cortical neurones, would predispose poor vernier thresholds
- it is plausible then that improvements would become apparent, only as the subsequent alterations in the cortical architecture and connectivity occur during the process of visual maturation