Post-Natal Development of the Visual System Flashcards
What do infants show preference to:
Infants show a visual preference for boldly patterned surfaces over homogenous gray surfaces
Describe test of Forced-Choice Preferential Looking (FPL):
- Baby sits facing screen with two monitors left and right
- Behind display is experimenter who sits and can view the infant through an aperture between the two monitors
- Present stimuli to left or right of infant at random intervals
- The spatial frequency of stimulus can be changed
- Aim of experimenter is to make a choice every time stimulus is presented
- Analysis of experimenters response is done = threshold generated
VA of humans vs monkeys:
- Monotonic increase of VA over time for humans and monkeys
- Infants VA i.e grating acuity increase at approx one cycle per degree per month of age
- Monkeys develop 4x as quickly as humans = vision increases faster
What is the Acuity Card Procedure:
- Cards of different patterns of various spatial frequencies, fineness and coarseness of gratings arranged on cards
- Infants eye and head movement is observed
- Average luminance on both sides of card stimulus and blank are identical
Visual Evoked Potentials:
- Trying to find the finest pattern the highest spatial frequency that produces an electrical response measured at scalp
- Better more accurate electrophysiology methods = better real world result
What is contrast sensitivity:
- More comprehensive measure of spatial vision than simple acuity
- Contrast sensitivity = 1/minimum contrast to detect (grating) target
- Inverted ‘U-shaped’ function in adults.
What is visual acuity:
Highest spatial frequency which can be seen at maximum contrast
Fovea development at birth vs adult:
Birth:
* Ill-defined pit, inner retinal cell layers present
* Cone outer segment length, 3 µm
* Cone spacing, every 2.1 mins arc
* Density, 36,000 per mm2
Adult:
* Obvious pit, inner retinal cell layers moved aside
* Cone outer segment length, 45 µm (x 15)
* Cone spacing, every 0.3 mins arc (x 7)
* Density, 108,000 per mm2 (x 4) = Cone photoreceptors more packed in fovea
What does visual pathway start with:
Starts with tear film and ends with extra striate cortex
Why is VA poor in infants:
Because tear film lacks clarity - FOVEA AT BIRTH IS ANATOMICALLY AND MORPHOLOGICALLY IMMATURE
Photoreceptors in infant vs adult:
- For 10 triggered photoreceptors that span the object, it will trigger and fire for much smaller object for adult than infant
- Cone fovea packing density underpins limit of infant va
What is infant spatial vision (VA & CS ) measured using:
FPL or VEP techniques
What does CS function show:
Maturation of sensitivity and spatial scale
What is limiting factor for infant spatial vision i.e VA and CS:
Foveal immaturity (cone OSs & packing density) is significant limiting factor)
What is colour vision:
“The capacity to discriminate among lights of different wavelength composition on the basis of the difference in wavelength composition”
Measuring cv in infants:
FPL method
1. Measure weber fraction with white light stimulus – try to find preference depending on brightness of target against background i.e find minimal amount of brightness difference infant can detect
2. Measure preference for coloured light stimulus in intensity steps less than established weber fraction but only show brightness differences between the stimuli which already infant cant see
- This is to make sure the colour preference is not effected by brightness differences
What age do infants have colour vision:
2 months
Colour Vision in Infants:
- Established by Peeples & Teller (1975) that 2-month-old infants can make colour discriminations so at a minimum they are dichromatic
- Subsequent (VEP based) techniques show infant spectral sensitivity (V lambda) is adult-like therefore trichromatic vision is present
What is vernier acuity:
- A positional acuity
- A measure of the smallest offset between two objects
- A hyperacuity in adults
- Processed in striate and extra - striate cortices
- Minimum offset of two targets
What is hyperacuity:
Acuity that has lower thresholds than standard resolution acuities like grating acuities
Difference between vernier acuity and standard grating acuity:
- Vernier acuity and grating acuity proceed in same manner as standard grating acuity
- But vernier acuity start off poorer than gratings acuity
- Vernier acuity matures from a lower baseline than grating acuity
Stereovision development in infants:
- Sub-cortical switch occurring
- Early infant vision is subcortical mediated = cortex switches on and that is what mediates rapid turning on of maturation of stereo acuity
Vision at Birth:
- VA & CS just measurable
- OKN eye movements present
- Probably no colour perception
Vision at 1-month:
- 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
Vision at 2-months:
- Rudimentary colour vision (not yellows)
- VA & CS have improved slightly
- Response to motion (other than OKN)
CFF
Vision at 3-months:
- VA now 3 cycles per degree
- CFF adult-like (50Hz)
- Still no stereovision
Vision at 6-months:
- Stereovision now ‘online’ and 1 min arc minimum
- Peak of CSF near that of adult
- Steady rise of functions to adult level asymptotes
Anatomical Development of the Human Visual System:
- Rapid changes 1st year of Infancy, but most continue into early childhood:
- Eyes: grow & change shape (~6 years)
- Retina: fovea differentiates (~5 years)
- Optic & Visual Pathways: myelination (~2 years)
- Visual Cortex (& other Brain Centres): growth & refinement of synaptic connections (~8 years)
- Ocular muscles: grow & differentiate (~6 months)
What happens to astigmatism as older:
Incidence of astigmatism reduces, from 20-30% at birth to 8% at 1 year
