lecture 4

• Electrophysiological responses


  Gross measurements that represent the average of the firing of all the neurons involved, which may underestimate the infant's real visual capabilities

  
• Eye movement responses


  • Fixation on an object of interest
  • Optokinetic nystagmus (OKN)

  
• Optokinetic nystagmus


  An oscillation of the eyes produced when looking at a stimulus slowly moving in one direction. The eyes follow the stimulus, and then flick back in the reverse direction with a saccade like movement

  
• Forced-choice preferential looking procedure (FPL)


  1. Infant is seated looking at two displays with a peep hole between them

  2. An observer, who does not know which display is which, decides whether the infant looks more at the left display or the right display

  3. The position of the interesting display is changed to left or right in a random order

  4. A positive result is when the records from the observer show that the infant is looking at the interesting display more often than the other more than 75% of the time
• A negative result in the forced-choice preferential looking procedure could be due to inattentiveness on the part of the infant or inexperience on the part of the observer rather than failure of the infant to discriminate the displays
• Spatial frequency


  A measure of the number of lines in a grating that fit into 1 degree(°) of visual angle as seen by the observer

  
• 6/6 vision corresponds to 30 cycles per degree
• Correlation between Snellen VA, stripe width and spatial frequency


  • Snellen (meter): 6/6, Spatial frequency (cycle/degree): 30, Strip width (minutes of arc): 1
  • Snellen (meter): 6/12, Spatial frequency (cycle/degree): 15, Strip width (minutes of arc): 2
  • Snellen (meter): 6/30, Spatial frequency (cycle/degree): 6, Strip width (minutes of arc): 5
  • Snellen (meter): 6/60, Spatial frequency (cycle/degree): 3, Strip width (minutes of arc): 10
• Snellen acuity


  • Acuity measured with letters in a line, which is less distinguishable than letters seen by themselves because the neighboring letters interfere with each other's visibility (crowding)
  • Continues to develop after 5 years of age and does not reach adult levels until 10 years of age
• Acuity changes


  Due to the development of the cones in the fovea, where the cones become longer and narrower, increasing the efficiency of their light catching ability and consequently increasing the contrast sensitivity
• Expected development of Visual Acuity from birth to 3 years of age

  • Newborn: 6/240
  • 1 month: 6/180- 6/90
  • 4-6 months: 6/18-6/6
  • 24 months: 6/12- 6/6
  • 36 months: 6/6

  
• Sheridan Gardner test


  • A matching test designed for use with children but also suitable for those with learning difficulties or who do not share a common language with the examiner
• Kay Picture Test


  • Only pictures proven to be both recognizable and accurate are used, with a matching card to help shy or under-confident children
• Methods to measure visual acuity in young children


  • Sheridan Gardner test
  • Kay Picture Test
  • Allen Cards
  • Allen preschool cards
  • Maclure Test Type

  
• Allen preschool cards


  Acuity is recorded as test distance over which the pictures are detected over 30ft, i.e. if pictures are seen at 30ft, acuity is 30/30, equivalent to 20/20 acuity
• Maclure Test Type


  Designed to differentiate between the ability to see and the ability to read, based on the Ladybird Keywords Reading Scheme using a non-serif font to help the child appreciate the link between handwriting and print
• Developing contrast sensitivity in children


  1. Between birth and 10 weeks of age, contrast sensitivity improves at all spatial frequencies

  2. Improvement at high spatial frequencies (low temporal frequency) is rapid, continuing until 4 years of age (Parvocellular system)

  3. Improvement at low spatial frequencies (high temporal frequency) is slower, but goes on for longer, until 9 years of age (Magnocellular system)
• Overall contrast sensitivity


  • Due largely to the increase in the length of the photoreceptors and the increase in funneling capacity
  • Increased sensitivity at high spatial frequencies is due to packing of the foveal cones closer together

  
• Dark adaptation


  It takes over half an hour for adults to get used to very dim light, due to the long time required for the rod pigment to regenerate
• Contrast sensitivity development in children


  • Overall sensitivity is due largely to the increase in the length of the photoreceptors and the increase in funneling capacity
  • Increased sensitivity at high spatial frequencies is due to packing of the foveal cones closer together

  
• The psychophysical results suggest that the increase in the length of the cones should be over around 10 weeks of age, and the increase in their packing density should continue after that
• Dark adaptation


  It takes over half an hour for adults to get used to very dim light, and this is due to the long time required for the rod pigment to regenerate
• The time course in infants is the same as in the adults, so one concludes that the kinetics of the infant rod pigment are the same as in the adult
• Orientation, direction, and movement sensitivity


  • Newborn infants can discriminate the orientation of a stimulus if they are familiarized to one orientation
  • Discrimination of orientations developed between 3 weeks and 6 weeks of age
  • Direction specification functions at age from 6 to 20 weeks
  • Development of the movement sensitivity complete at 7 to 8 years of age
• Contour integration


  A task in which the observer is asked to detect a circle of Gabor patches in a display of patches of all orientations, with varying amounts of noise

  
• There is significant improvement in contour integration task between 5 and 14 years of age
• Vision in the periphery of the visual field


  • At 3 months of age, infants do not notice the sphere until it is 40 degrees from the fovea in the temporal field
  • Their fields widen until they reach close to the adult value of around 80 degrees at 10 months of age
  • Nasal fields lag behind temporal fields in development

  
• Developing of binocular vision


  • Good acuity in both eyes, good control of eye movements, and the ability to tell whether an object is closer or further away than the point of fixation are required
  • Vergence movements initially depend on cues other than stereopsis
  • Binocular function matures as the acuity in the retina matures and the vergence movements become more accurate
  • The development of stereopsis, which is required for the fine tuning of depth perception, anticipates the development of some binocular coordination

  
• If the infant is unfortunate to have equal blurring in both eyes which is not remedied by 2 months, (e.g. bilateral congenital cataracts) then this can lead to severe amblyopia and sensory nystagmus
• Orthotropia


  The ability of the eyes to look together at an object, to give a single visual image

  
• Stereopsis


  The most precise cue to depth perception, which depends on disparity
• The display with apparent depth becomes more interesting to the infant very suddenly around 16 weeks of age
• Crossed disparity


  For objects closer than the fixation point, the disparity is called crossed because the lines of sight cross each other between the eyes and the surface on which the eyes are focused

  
• Uncrossed disparity


  For objects further away than the fixation point, the disparity is called uncrossed
• Binocular vision reflexes


  • The first reflexes to develop are the blink and uniocular fixation reflexes at 1 and 3 months respectively
  • By 4 months most of the reflexes are in place, including fixation and pursuit

  
• Age of onset for binocular reflexes and aspects of visual function


  • Birth: Blinking (to light stimulus)
  • 1 week: Vestibulo-ocular
  • 2 weeks: Small saccades
  • 2 months: Large saccades, Pursuit, Bifoveal fixation, Convergence
  • 3 Months: Uniocular fixation
  • 4 months: Fusional vergence, sensory fusion, stereopsis
  • 6 months: accommodation
• Visual development in infants


  • At birth, the fovea is immature and grating acuity is poor, but the eyes look in approximately the same direction
  • Soon after birth, there is some binocular coordination and there is consequently some ability to make vergence movements
  • Over the first 3 months the fovea matures, grating acuity improves, and the eyes become more able to fixate together on an object
  • Between 3 and 6 months of age, dramatic alterations occur in the visual cortex and in the ability of the eyes to work together. Stereopsis becomes detectable, followed by a rapid increase in stereoscopic acuity. The eyes develop the ability to make full convergence movements. Orthotropia is mature. Binocular summation occurs
  • After 6 months of age there is further development in some visual properties. Grating acuity, for example, continues to improve