Postnata Development Of Visual Function

Question 1

What is emmetropization

Answer 1

• process by which the refractive state of the eye changes

Question 2

Which 2 factors control emmetropizion

Answer 2

• passive factors
• active factors

Question 3

What are passive factors

Answer 3

• normal eye growth
• as eye size increases the power of the optical components decrease proportionally reducing refractive error

Question 4

What are active factors

Answer 4

• visual feedback mechanism that controls eye growth

Question 5

How does hyperopia progress during emmetropisation

Answer 5

• hyperopic element increases during the first 6 months before any reduction to emmetropia

Question 6

Cues for accommodation

Answer 6

• blur
• vergence
• chromatic aberration
• disparity

Question 7

Why do infants not need to accommodate as much

Answer 7

• increased depth of focus
• due to reduced pupil

Question 8

Summary optical factors relating to development

Answer 8

• clear optical media
• full term +2.00D
• accommodation on target after 3/4 months
• reduced astigmatism

Question 9

Macular changes in development

Answer 9

• vast majority of postnatal changes occur at the macula
• foveal diameter reduces from 5.4% to 2.3%
• neonate cones are immature, and have an abnormal shape and broad inner segments
• increase in receptor density
• 4 fold increase in cones at central fovea
• immature until 15 months

Question 10

How are cones affected during development

Answer 10

• outer segments increase in length and become thinner as the receptor aperture increases in size and the inner segment begins to act as a funnel

Question 11

Summary retina

Answer 11

• differentiation macular region
• migration cells as foveal pit develop
• foveal cones thinner and more elongated
• foveal cone density increases

Question 12

Development of the LGN

Answer 12

• LGN consists of 6 distinct capped shaped layers of neurons
• layers 1,4,6 receive input from the contralateral eye
• layers 2,3,5 from the ipsilateral eye
• the LGN approximately doubles in first 6 months of life, adult like at 9 months
• parvocellular adult like in 1st year
• magnocellular adult like at 2 years

Question 13

LGN Summary

Answer 13

• parvocellular 1 year
• magnoelluar 2 years
• body LGN max 4 months,adult like at 9 months
• closely parallels development of behavioral acuity

Question 14

Development of striate cortex

Answer 14

• spatial pattern of representation of the retina in the visual cortex
• 6 principal layers and 7 sub-layers
• axons from LGN terminate on cortical neurons in layer 4
• cells from on layer of LGN send signals to target cells in layer 4
• most cells are binocular

Question 15

What are simple cells

Answer 15

• respond to lines darker or bighter than the background

Question 16

What are complex cells

Answer 16

• in upper layers of the striae cortex show a strong selectivity for the direction
• movement in one direction provides a strong response

Question 17

What are end stopped complex cells

Answer 17

• respond to short bars because of inhibitory influences
• best stimuli is a line that stop

Question 18

What age does the striae cortex develop

Answer 18

• 8 months
• synapses are selectivity eliminated until adult density is reached at 11 years

Question 19

Development of ocular dominance columns

Answer 19

• not complete at birth
• segregation of geniculate afferants evident several weeks later
• when the LGN axons retract to establish separate alternating areas in layer 4
• completed by 4-6 weeks
• dependent on input from both eyes

Question 20

Summary striae cortex

Answer 20

• 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 21

Development of visual acuity

Answer 21

• rapid improvement in first 6 months
• 1 c/degree at birth
• 8-12 c/degree at 1 year
• 30 c/degree at 3 years
• 40-50 c/degree at 5/6 years

Question 22

Maturation of VA on test charts

Answer 22

• maturation of linear visual acuity is thought to occur later at around 10 years of age

Question 23

Summary visual acuity

Answer 23

• rapid 6 months
• grating acuity 1 c/degree at birth
• 8-12 c/degree 1 year
• 30 c/degree 3 years
• 40-50 c/degree at 5/6 years
• recognition acuity
• VEP initial levels poor
  -adult values 6-12 months

Question 24

Development of contrast sensitivity

Answer 24

• between birth and 10 weeks contrast sensitivity improves at all spatial frequencies
• rapid in first few months of postnatal life and then gradual until 5 to 8 years
• cones become longer and thinner
• sensitivity to high spatial frequencies continue to grow beyond 33 weeks
• the presence of inhibitory responses improves he sensitivity of the visual system

Question 25

Development of depth perception and stereopsis

Answer 25

• depends on quality of binocular vision
• by 3 months infants can use disparity information to receive depth
• at 3 months infants are able to detect disparities of about 60 mins/arc
• increases rapidly to less than 1 min of arc within 3 to 4 weeks
• development of stereopsis is linked with the segregation of ocular dominance columns

Question 26

Development of vernier acuity

Answer 26

• hyperacuity
• seller acuity limited
• visual system has the capability to make much finer discriminations
• remarkable performance
  -smallest foveal cones are separated by 30 degrees
• point spread function