Introduction:In this essay I will be explaining how the visual system of anew-born develops over the first 12 months of life. I will be covering topicssuch as general development of the eye, light sensitivity, colour sensitivity, depthperception, changes in sensitivity to slow displacements and what can go wrongif the visual system does not develop properly.
General developments of the eye:Infant vision changes by gaining increased visual acuity. Anew-borns visual acuity is 12 to 25 times weaker than an adult’s visual acuity (Dobsonand 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 evenwith eye movement because other areas of the visual system are still immature.These areas are the inputs from retina to the lateral geniculate nucleus, myelinationof nerve fibres, distance from cornea to retina is shorter at birth than at oneyear of age, synaptic density and size of cells changes. The visual acuity of a one-month infant ranges from 6/240 to6/60 if you use a Snellen Acuity chart (Courage and Adams, 1990). By 2 monthsof age, visual acuity is improved to 6/45 then at 4 months visual acuityimproves by a factor of 2 which is 6/18. At sixth months of age visual acuityof an infant is comparable to an adult in that their visual acuity becomes 6/6.(Sokol, 1978). The diagramon the right shows visual acuity of infants using VEPs as a method of measurement.As a general trend, every month until the 8thmonth of age, visual acuity increases then remains constant until 12 months.
Thisis because of post-retinal factors such as the increase in cell size, density,waveguide property of photoreceptors, increase in myelination of neurons andincrease in synaptic density.Between 2 and 4 months of age, synaptic density doubles withinthe primary visual cortex in area V1.At 2 months of age, Lateral inhibition starts around this timein infants. Infants are also exceptionally good at facial recognition afterbirth 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 theywill attend for longer at the picture of their mother as opposed to the pictureof other things.
(Bushnell, 2001). Lateralinhibition begins at about 2 months of life. Lateral inhibition is when you candetect edges and contrast of an object. The graph confirms that lateral inhibitionstarts at 2 months. At 1 month of age the graph shows that the contrast sensitivityis constant and then drops after 0.
5 (c/deg) until 2 (c/deg). 1 month has alower contrast sensitivity compared to 2 and 3 months.At 2 months of age the graph shows that the contrast sensitivityincreases until 0.35 (c/deg) and then drops until 2 (c/deg). This shows aslight increase in contrast sensitivity compared to one month of age.
This isdue to lateral inhibition development.At 3 months of age the graph shows that the contrast sensitivityincreases until 0.5 (c/deg) and then a gradual decrease until 2 (c/deg) howeverthe contrast sensitivity past 0.5 (c/deg) is much higher compared to 1 and 2 monthsof age.Light sensitivity:An infant’s light threshold sensitivity is extremely high atbirth. In one month of age, there will need to be 50 times more light in orderfor it to be detected in relation to an adult’s light sensitivity. At twomonths of age, threshold is decreased dramatically to about 10 times more thanan adult.
This increase is because of the shape change of the photoreceptors andmaturing of the retina (Brown, 1986). The reason the threshold of light sensitivity decreases sorapidly is due to the waveguide property of photoreceptors. It is estimated thatthe new-born foveal cone absorbs 350 times less quanta than an adults fovealcone.
This is because the cone for new-born is not funnel shaped as it has notfully matured at this early stage (Banks and Bennett, 1988).Colour sensitivity:Colour sensitivity increases at a fixed rate over the first12 months of life due to the increase in strength of cones. Infants have the abilityto use chromatic discrimination using the 3 photoreceptors: long-wavelength,medium-wavelength and short-wavelength cones. Cones recombine to form luminancechannels which help the infant see brightness and colour.
The parvocellularpathway is used in colour discrimination (Thomasson and Teller, 2000). In the early period of an infant first 12 months, there is ageneral consensus that infants prefer high contrast colours as opposed tosaturated colours (Teller, Peeples and Sekel, 1978). Infants cannot discriminatebetween different types of colours until about 3 months of age, this is becauseafter 3 months of age infants preferred longer wavelength colours such as redand yellow as opposed to shorter wavelength colours such as green and blue.
Before3 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 ofobjects to help. For example, closer objects appear larger as they fill more ofthe visual field of view and distant objects fill less of the visual field ofview.
New-born babies eyes do not work as well as an adults in that the pair ofeyes 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 crawlbased on the experiments performed by Gibson and Walk known as the “visualcliff”. Changes in sensitivity to slow displacements:Infants slow displacement sensitivity increases dramaticallyover the first 6 months of life (SroesslerandLdannemiller, 1998). Slow displacementis known as slow movement in simple terms.
The need to assess this area ofvisual development is important because it links with other areas of visualdevelopment such as resolution, spatial and temporal contrast sensitivity functionas they all develop at the same time.