Introduction: not develop properly. General developments of the eye:

Introduction:

In this essay I will be explaining how the visual system of a
new-born develops over the first 12 months of life. I will be covering topics
such as general development of the eye, light sensitivity, colour sensitivity, depth
perception, changes in sensitivity to slow displacements and what can go wrong
if the visual system does not develop properly.  

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General developments of the eye:

Infant vision changes by gaining increased visual acuity. A
new-borns visual acuity is 12 to 25 times weaker than an adult’s visual acuity (Dobson
and Teller, 1978). At 2 months of age, infants begin to strengthen their eye movement,
so they can focus onto objects. At 2 months visual acuity is still weak even
with eye movement because other areas of the visual system are still immature.
These areas are the inputs from retina to the lateral geniculate nucleus, myelination
of nerve fibres, distance from cornea to retina is shorter at birth than at one
year of age, synaptic density and size of cells changes.

The visual acuity of a one-month infant ranges from 6/240 to
6/60 if you use a Snellen Acuity chart (Courage and Adams, 1990). By 2 months
of age, visual acuity is improved to 6/45 then at 4 months visual acuity
improves by a factor of 2 which is 6/18. At sixth months of age visual acuity
of an infant is comparable to an adult in that their visual acuity becomes 6/6.
(Sokol, 1978).

The diagram
on the right shows visual acuity of infants using VEPs as a method of measurement.

As a general trend, every month until the 8th
month of age, visual acuity increases then remains constant until 12 months. This
is because of post-retinal factors such as the increase in cell size, density,
waveguide property of photoreceptors, increase in myelination of neurons and
increase in synaptic density.

Between 2 and 4 months of age, synaptic density doubles within
the primary visual cortex in area V1.

At 2 months of age, Lateral inhibition starts around this time
in infants.

 

 

Infants are also exceptionally good at facial recognition after
birth because they can quickly identify who their mother are (Frank and Johnson,
2009). Infants prefer their mother’s face after 2 weeks of birth. For example,
if the infant is shown 2 pictures, one of their mother and one of other things they
will attend for longer at the picture of their mother as opposed to the picture
of other things. (Bushnell, 2001).

Lateral
inhibition begins at about 2 months of life. Lateral inhibition is when you can
detect edges and contrast of an object. The graph confirms that lateral inhibition
starts at 2 months.

At 1 month of age the graph shows that the contrast sensitivity
is constant and then drops after 0.5 (c/deg) until 2 (c/deg). 1 month has a
lower contrast sensitivity compared to 2 and 3 months.

At 2 months of age the graph shows that the contrast sensitivity
increases until 0.35 (c/deg) and then drops until 2 (c/deg). This shows a
slight increase in contrast sensitivity compared to one month of age. This is
due to lateral inhibition development.

At 3 months of age the graph shows that the contrast sensitivity
increases until 0.5 (c/deg) and then a gradual decrease until 2 (c/deg) however
the contrast sensitivity past 0.5 (c/deg) is much higher compared to 1 and 2 months
of age.

Light sensitivity:

An infant’s light threshold sensitivity is extremely high at
birth. In one month of age, there will need to be 50 times more light in order
for it to be detected in relation to an adult’s light sensitivity. At two
months of age, threshold is decreased dramatically to about 10 times more than
an adult. This increase is because of the shape change of the photoreceptors and
maturing of the retina (Brown, 1986).

The reason the threshold of light sensitivity decreases so
rapidly is due to the waveguide property of photoreceptors. It is estimated that
the new-born foveal cone absorbs 350 times less quanta than an adults foveal
cone. This is because the cone for new-born is not funnel shaped as it has not
fully matured at this early stage (Banks and Bennett, 1988).

Colour sensitivity:

Colour sensitivity increases at a fixed rate over the first
12 months of life due to the increase in strength of cones. Infants have the ability
to use chromatic discrimination using the 3 photoreceptors: long-wavelength,
medium-wavelength and short-wavelength cones. Cones recombine to form luminance
channels which help the infant see brightness and colour. The parvocellular
pathway is used in colour discrimination (Thomasson and Teller, 2000).  

In the early period of an infant first 12 months, there is a
general consensus that infants prefer high contrast colours as opposed to
saturated colours (Teller, Peeples and Sekel, 1978). Infants cannot discriminate
between different types of colours until about 3 months of age, this is because
after 3 months of age infants preferred longer wavelength colours such as red
and yellow as opposed to shorter wavelength colours such as green and blue. Before
3 months of age infants show no preference to different types of colours
(Adams, 1987).

Depth Perception:

In order to achieve depth perception, infants use distance of
objects to help. For example, closer objects appear larger as they fill more of
the visual field of view and distant objects fill less of the visual field of
view. New-born babies eyes do not work as well as an adults in that the pair of
eyes move in the same direction only half of the time (Kellman and Banks, 1998).
The strength of eye movement control is heavily correlated to depth perception.
Depth perception begins at around 4-5 months of age before an infant begins to crawl
based on the experiments performed by Gibson and Walk known as the “visual
cliff”.

Changes in sensitivity to slow displacements:

Infants slow displacement sensitivity increases dramatically
over the first 6 months of life (Sroessler
and
Ldannemiller, 1998). Slow displacement
is known as slow movement in simple terms. The need to assess this area of
visual development is important because it links with other areas of visual
development such as resolution, spatial and temporal contrast sensitivity function
as they all develop at the same time.