Postnatal Development of Visual Fixation (in normal neonate) Flashcards
What is included in the visual pathway?
What is included in perceptual development?
Optical, retina, LGN, striate cortex
VA, Contrast Sensitivity, stereopsis, vernier acuity
Before dealing with development of responses to visual stimuli what do you need to determine?
If image on infants retina is clear.
Is the limitation in image analysis due to image clarity or the processing within the nervous system?
If blurred image sent to retina, nervous system will not process it – if not clear brain will not process it – just switches off
What should you expect to see in ophthalmoscopic exam of neonate? What isusual Rx (fullterm and preterm)?
- Ophthalmoscopic examination reveals clear media - look for corneal opacities, lens opacities, is lens obscuring visual axis?
- 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)
o Have v steep cornea at birth
Describe emmetropisation - passive and active?
- The process by which the refractive state of the eye changes is termed EMMETROPISATION
- Emmetropisation appears governed by both active and passive factors
o Passive emmetropisation refers to normal eye growth as eye size increases the power of the optical components decrease proportionally reducing refractive error and maintaining emmetropia
Eye grows anyway, little baby has little eeyes, as you grow eye will grow too
Small eye – focus falls behind retina – as eye grows this becomes easier
o Active emmetropisation describes a visual feedback mechanism in the control of eye growth (visual experience)
If image not developing clearly & falling on retina, there is blur. Blur gets sent along visual pathway, blur stimulates more growth. This is normal in process of emmetropisation
Myopia management – inducing blur at certain points to change shape of eye
What happens to the hyperopic element in 1st 6months?
The hyperopic element increases during the 1st 6mths before any reduction towards emmetropia becomes apparent
The amount of hyperopia the child has will depend on the baby, family hx etc on how hyperopic you start life.
First 4 years, astig & hyperopia reduces.
What happens to degree of astigmatism in infants and why?
A significant reduction in the degree of astigmatism occurs in the infants 1st year (until 18months to 48months) result of increase in eye size, concurrent flattening of cornea
Describe accommodation and infants? What are the cues to accommodation?
- Infants have some ability to accommodate to objects at different distance at 2 weeks of age, this ability increases during the first 3 months
- Cues to accommodation include blur, vergence, chromatic aberration and disparity - which are used by infants
- Need for infants to accommodate is much less (increases 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
- Accommodative strabismus comes in about 18months to 2 – they are practicing using accommodation & getting gain from it. They get clear vision from this so they over accommodate to keep that clear if they are a big hyperope
o Minute squint starts, emmetropisation will stop
What are the 5 aspects to summarise optical development?
- Clear optical media (no ptosis, no cong cataract, full term)
- Full term +2 dioptres
- Accommodation ‘on target’ - 3/4 months
- Astigmatism correlates corneal curvature - first year
- Small eye, more curved cornea, fatter lens=small eye phenomenon -> becomes hyperopic as small eye – emmetropisation starts to occur – normal
Describe development of fovea? Foveal hypoplasia?
One main change to structures in development is in fovea. Density of cones starts to change, starts thick then gets smaller – so can get more cones in at fovea – need these to get clear image at fovea.
Foveal hypoplasia – underdevelopment of fovea, can happen anatomically in pxs, may not get vision past 0.4 (6/18). Look on OCT for nice foveal pit w/ healthy amount of cones, they cannot increase this – need to know this as 0.4 vision wont allow them to drive. Under development of fovea could run in families, genetic component.
Corneal power reduces, power of lens reduces, axial length increases – foveal development is v key
What are the 4 aspects to summarise development of retina?
- Differentiation macular region – fovea not like rest of retina
- Migration cells as foveal pit develops
- Foveal cones thinner & more elongated
o Need them elongated so more light can be collected by them
- Foveal cones thinner & more elongated
- Foveal cone density increases
Summarie the development of LGN?
- Parvo cellular developed by 1 year – 4 out of 6 layers of LGN
- Magno cellular developed by 2 years – 2 out of 6 layers of LGN
- Body LGN grows rapidly in 1st 4mths, adult like 9mth (in normal stimulation – no cong cataract)
- Closely parallels developmental of behavioural acuity
- Layers 1, 4 and 6 receive input from the contralateral eye and layers 2, 3 and 5 from the ipsilateral eye
- The ganglion cell bodies in layers 1 and 2 are larger (magnocellular) than the other layers (parvocellular)
o Cells shrink if not being used or stimulated – LGN layers will then be in different format
o Only way to get them to grow and work again is by patching – stimulating growth of those cells
o When giving glasses/patch not just fixing eye now, it is brain that we are working with – if don’t develop these cells in critical or sensory period – we can do no more after age of 8/9 years – if you don’t improve it now, & don’t get glasses or patch worn then wont be able to work with child’s brain plasticity
Summarise development of Striate Cortex? DEscribe layer 4 and ocular dominance columns?
- Synapses double first 8 months – rapid development – critical period
o Decline to adult levels - Orientation selectivity 5/6 weeks – normal environment, seeing different pattern forms, different light forms
- Directional selectivity 5/6 weeks – normal environment, seeing different pattern forms, different light forms
- Ocular dominance columns complete 4/6 weeks
o Layer 4 - The axons from the LGN terminate on cortical neurons in layer 4 with the parvocellular layer neurons sending their axons to neurons in the deeper part of this layer
- Also preserved in layer 4 is the separation of inputs from the two eyes, if the cells in one layer of the LGN receive their input from one eye, the next layer will receive input from the contralateral eye
o Each layer takes RE, LE, RE, LE
o Then can see binocularly as stimulate each side of striate cortex from each eye – doesn’t happen in amblyopia. Need this process to then develop binocular single vision - The cells from one LGN layer will project to groups of target cells in layer 4
- Theses groups of cells from alternating stripes or bands in layer 4, above & below this layer, most cells are driven binocularly, although one eye is usually dominant
o Ocular dominance columns
Describe categories of Neurons in Visual Cortex (Simple & Complex)?
- 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 therefore in one particular direction provides a 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
- Recently a new class of neuron has been reported in the visual cortex which is unresponsive to lines, bars or edges but which responds to texture
What happens if px not presented to complex world?
What happens if get problem in 1st year of life?
If px is not presented to complex world then vision will only be process in one plane. Need all these different systems in order to develop normal binocular system
Visual system going through dramatic changes through 1st year of life – problem in this first year has detrimental impact
