Postnatal Development of Visual Function: Perceptual Development

Question 1

Describe visual acuity in development?

Answer 1

• There is a 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
  Seeing baby will always choose to look at something with a pattern
  Will make quick fixation movement to B&W lines – depending on how close the lines are predicts how much they can see
  All these VA measurement in baby is overestimating the VA
  As soon as start testing them on Cardiff Cards/logMAR cards – VA will never be as good as what you thought
  Er on side of caution with babies – cannot tell which line of VA they can see – GROSS ESTIMATE – can never determine if they can see 6/9
  Want to know what angle each letter px is looking at is subtending on to px’s nodal point.
  Sheridan Gardner – single letter test will not pick up amblyopia
  Snellen VA can overestimate vision by up to 3 lines than logMAR
  Test to threshold – if only present v simple features then everyone can see well
  Need to use logMAR cards to test vision – to not miss amblyopia

Question 2

What to bear in mind when testing child?

Answer 2

Kay Pictures overestimates vision by 1 line
10 minutes of arc – large separation between B&W lines
VA always tested on logMAR and tested to threshold
Always test crowding
Compare RE & LE
Test near vision & distance vision
Near vision often improves first in amblyopia – before distance vision

Question 3

What age are adult levels of cyl/degree reached?

Answer 3

• No. of behavioural studies employing grating stimuli show a binoc acuity of 1 cycl/degree at birth, which increases to 8-12 cycl/degree at one year of age & reaches ~30 cycl/degree at 3 years
• Adult levels of 40-50 cycl/degree are reached at 5 to 6 yrs or later
• Although, VA values measured using visual evoked potentials (VEP) are slightly higher there is general agreement, at the rate of change in which acuity improves over 1st few months

Question 4

Describe Forced Choice Preferential Looking vs Visual Evoked Potentials?

Answer 4

• 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
• VEP:
  o Checking for stimuli – are cells firing?
  o Child may have developmental delay & that’s why cant get response
  o Find out whats happening on visual pathway
  o If they have amblyopia, firing will be less but may be other problem e.g. developmental
  o Use these tests to see if things are getting to cortex
  o This is to determine if there is potential for vision  will not tell you what the vision is
  o GROSS TEST

Question 5

Why do behavioural and electrophysiological testing give different results?

Answer 5

• Recognition acuity (letters, numbers or similar optotypes) in children, as in adults tends to be lower than grating acuity
• By 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. This is an important consideration in the clinical assessment of visual acuity and is termed the crowding phenomenon
• Ocular media are reasonably clear at birth and provide no hindrance to the passage of incident light. Because the presence of hypermetropia and astigmatism are common in infants it has been suggested that this is the reason for poor spatial resolution capabilities
• Optical blur, however, cannot account for poor acuity as the degree of refractive error is usually small to moderate and visual acuity is still found to be dramatically reduced at 3-4 months of age when accommodation responses are in fact reaching maturity
• Adult foveal acuity can be blurred by as much as +14.00 dioptres and levels of acuity are still found to be better than that of a six week old infant

Question 6

Why are there low levels of VA at birth?

Answer 6

The low density of cone photoreceptors & their cumbersome shape, which hinders light absorption combined with an underdeveloped cortex can explain the low levels of VA at birth. It is these changes, which have been discussed previously, in both cone density & functional properties & cortical architecture that are instrumental in rate of visual acuity development.
Cones that are short & fat are then not long enough to gather the light.
Need normal processes etc to have potential to get 6/5 vision for rest of life

Question 7

Summarise the development of VA?

Answer 7

• rapid increase in 1st 6 months of life
• grating acuity
  o 1c/deg birth
  o 8-12 c/deg 1 year
  o 30 c/deg 3 years
  o 40-50 c/deg at 5/6 years
• recognition acuity  grating acuity
• VEP initial levels poor (immaturity of structures they are used to assess) – improves
  o adult values 6 - 12 months
• c/deg -> bigger thee number – better the VA – max by 5/6yrs but could be by 4yrs
• VEP – what potential does child have to see? – if normal, child may just have delayed visual maturation

Question 8

Describe perceptual devlopment: contrast sensitivity?

Answer 8

• Pelli-Robson charts
• What is difference in luminosity/contrast to what px sees
• 24 yrs – optic neuritis – MS – may have INO – once had this optic neuritis they may have normal vision return – but they are still miserable. Px v aware of different in eyes – contrast sensitivity dramatically reduced
• When develop probably in optic nerve e.g. optic neuritis – VA may return but contrast sensitivity may not come back – if spatial frequency huge then px may be very aware of it
• 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
• Amblyopic – test contrast sensitivity – 1st point where it will not be normal
  o Then later in life next problem with contrast sensitivity may come with optic nerve problem

Question 9

Describe cones and constrast sensitivity function (CSF)?

Answer 9

• The cones become longer & thinner allowing the more effective funnelling & absorption of light, rendering an overall increase in sensitivity
• The peak of the CSF shifts towards the high spatial frequencies with age, a process which can be attributed to the potential increase in central receptor density
  o If amblyopic, not able to see high spatial frequencies
• Finally the roll off at the lower spatial frequencies is assumed to occur through the cortical development of lateral inhibitory interactions, as well as the development of bandpass spatial frequency & 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.
  If have not developed small cortical cells then cannot see finer spatial frequency.
  The presence of inhibitory responses improve the sensitivity of the visual system & hence has an effect on visual performance.
  Also ability to suppress too much visual info – otherwise would be completely overwhelmed by what looking at

Question 10

Describe development of depth perception and stereopsis?

Answer 10

• Quality of an individual’s ability to perceive depth can vary greatly depending on the quality of their binocular vision (the ability to use your eyes together)
• 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
• if going to appreciate depth then need stereo acuity
• Stereoacuity is a hyper acuity – doesn’t matter how straight the eyes are – if px doesn’t have adequate VA in both eyes – will never get normal levels of stereoacuity
  o Need good VA in both eyes to get good 3D
  o 3D vision will never be good until VA similar between 2 eyes
• Depth processing develops – before babies start moving
  o E.g. stereo develops before baby starts crawling – to protect them from table corners etc
  If have squint, never appreciated depth – don’t develop enough binocular cells to see stereo
  Other ways to determine how far away things are – use different cures – if haven’t ever had it, don’t know what you’re missing.
  You only miss 3D vision if you had it then lost it.

Question 11

Describe stereo and depth in infant? What do you do if testing VA but can’t get accurate test?

Answer 11

• At three months infants are able to detect disparities of about 60 minutes of arc
• Almost as soon as the onset of this ability to make stereoscopic discriminations occurs the 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
  Depth perception is v important
  If testing VA but cant get accurate test – try stereo test – if baby reaches for ball then get good idea that baby has similar level of vision in two eyes
  Need good vision to get good stereo vision

Question 12

Describe development of stereopsis and what it is related to in brain?

Answer 12

• 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
• If no ocular dominance columns (no level 4) – then will never have good stereopsis
• Pxs who are even slightly amblyopic – will never have good stereopsis

Question 13

Describe development of vernier acuity?

Answer 13

• 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 14

What are the suggestions relating to devleopment of vernier acuity?

Answer 14

• 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