There are numerous amount of different changes that occur within the visual system both anteriorly and posteriorly, in the first 3 years of life as the visual function of acuity, contrast sensitivity and visual field would develop. These developments and changes occur through the growth of the infants because when they are new born they are physically not capable to see clearly which is due to the brain being not fully developed so it would not process the visual information, this would have an impact on the visual function. There are several different way to test VA, for example an teller acuity test could be carried out to test the child’s vision ability to identify an image as it gradually disappears.. In addition, an optotype test such as Kay pictures could be done to assess the visual acuity of each eye at a certain distance. Optics of the eyeAfter birth the infants eye is approximately 65 percent of its full size, this would result for the eye to go through a series of changes that could possibly take as long as a year. These changed consist of the growth of the orbit, developments to the crystalline lens and final maturation of the macula and the pigmentation of the iris. The corneal curvature also develops after birth. In infants their receptive field of the retinal ganglion cells are smaller. Part of the pathway between the retina and the visual cortex contain numerous cells in the lateral geniculate nucleus they are not able to yet respond to the visual input. This is because it is unable to develop fully to accommodate, visual acuity in infants 20/400, however when the child is at the age of 6 months, this improves to a 20/25. This procedure is known as emmetropization1. . At birth, the light detection is low which leads to emmetropia to occur which result in growing certain eye components, this would allow the light rays to focus on the retina. The growth of the cornea and the lens are in diameter which cause the two to flatten and decrease the refractive power. The reason of the cornea flattening is because of the increase in endothelium cells which lower the haziness in the vision so it could allow more light to go through the back of the eye, this would improve the visual acuity. Retinal development The retina is a light-sensitive layer of tissue which is the third and inner coat of the eye. Through the cornea and the lens, the optics of the eye would produce an image of the visual world on the retina, The retina does not easily send an image to the brain, this is because the retina spatially compresses the picture to go through the limited capacity of the optic nerve. Nerve cells that are located in the retina have the control of the vision, so it allows a child to see clearly, because it sends an image to the brain of what it sees exactly, however at birth the nerve cells are undeveloped. The improvement of the child’s visual system and ability to process information increases because of the amount of myelin sheath that is around the axon on the nerve also increases, this causes the impulse to travel at a quicker rate to the brain, and information to be sent faster which allows the child to identify images better. A great part of the axial length elongation is found in the first 3 to 6 months of life and a gradual reduction of growth over the next two years, then when they reach the age of 3 the adult size is attained. By one month, the child’s visual cortex cells would be undeveloped and it would not be specified into function types. Furthermore, the dendrites that are found in the neurons do not have the full length to reach the layers of the cortex as it short. This has an impact on the visual fields of the child eye, this due to the fact that they have a low colour vision and contrast sensitivity. On the other hand, when a child reaches the age 2 to 3 months the improvements of their contrast sensitivity get better, which is because the rods cells advanced which allows them to see more clearly in black and white pictures, although the cones cells are not completely developed or improved. Due to the reason of neurons and cones are not yet advance, it leads to the them not having the ability to perceive depth until the child is approximately six months. The development of the visual cortex would increase significantly as the child is at the age of 6 months and it would allow them focus better and perceive depth2. As the child is at the age of 6 months, in their visual cortex the amount of synaptic connections would raise between the neurons helps the child to be able to familiarise and translate pictures faster. Changes in the fovea There are numerous amount of photoreceptors in the retina such as rods and cones, these would change light rays into neural impulses that would be sent to the brain and processed as an image. Cones are in control of colour vision and allow the an individual to see light at a higher intensity and spatial frequencies, as a infant grows and reaches the age of 3 years their development of the cones cells would increase and migrate towards the fovea , however the rods would be pushed out the retina. The role of the rods is to processed vision at a low light intensity and low spatial frequency and they do not mediate colour vision as it is only used in dimmed light because it does not work in bright light,. There is a shallow foveal depression a week after an infant is born, however the thick cones would still lack outer segment and are only 1 cell deep in the fovea. 3 When babies are born, they are unable to see clearly, this is because as infants grow, more complex skills such as visual perception would develop. As a new born, babies are unable to easily tell difference between two targets or movie their eyes between two images as they are unable to accommodate. They also do not have the ability to see in colour fully because the nerve cells in their retina is not fully developed, however a week after birth they are able to see red, orange, yellow and green. The reason why they are unable to see colour is because they have a smaller wavelength such as blue or violet as they have a few photoreceptors for blue light. As they reach the ages of two to three months, their vision has adapted on how to move the eyes better and quicker because they developed a sharper visual acuity. The retina is fairly developed around the first three to six months, as babies are able to visualize small objects, also occurs development of the depth perceptions. At six months of age, the child’s eye would of got to the point of reaching around two-thirds as their eyes are most likely to be working together.Test for visual acuity and procedures of 0-3 yearsVisual acuity enables the eye to differentiate objects and identify in detail what is seen at a specific distance, as each test carries out different distances. As infants are unable to communicate, so a different test is carried out compared to the adults to measure the visual acuity, for example the teller acuity is suitable at the ages of 6 to 12 months because infants have a prefer to look at a patterned stimuli as the this test is a type of preferential looking test.. At this stage, the parent of the child would need to know what the procedure is and how it will be carried out. This is important because they would need to be informed that they are not allowed to guide the child at any point, as they could indicate the child on where to look either with speech or with gesture. From the age of 6 to 12 months, a child is able to sit on their back however, they might not stay in the right position and distance, and therefore their parent would need to hold them to prevent any head lag or movement as it would make the procedure hard to follow. The distance depends on the age ranges, an infant between birth and six months tested from the distance of 38cm, then for seven to three years of age is at 55cm and for ages older than three it would be at 84cm. A monocular test would require a patch to be placed on one of the child’s eye at a time, however it is important for the guardian that’s holding the child to ensure the child does not take off the patch at any point during the test, the test would be carried out at from an eye level so that the results are accurate. The teller acuity test cards consists of 17 cards that has about 4mm diameter peephole at the center, each card contains a square wave grating opposite a blank field of equal average luminance, the stripes on the cards would fade away gradually. When the child can no longer follow the pattern shown on the cards, the results are recorded from the number written on the back of the card. Another method to test the visual acuity would be carried out for children at the ages of 2 to 3 years old such as kay pictures and lea symbols as they could possibly describe the images seen from the booklet at the distance of 3 meters. If the child is unable to describe what they see by speaking then their actions would be recorded through the cards given to them, they would receive matching cards and be informed to point at cards seen from the booklet. The optotype which are the shapes shown on the booklet, they would get smaller in size after each page. However, as they are identifying the images seen one of their eyes would be occluded, then at a certain page there are two rows of the optotype, one at the top and on underneath at the bottom which then the child would be required to cover one eye and read top rows whilst the bottom row is covered ,then cover the other eye and read the bottom row. This method would continue until the child is are no longer able to recognize the shape shown. The result would be recorded for both eyes as the child’s vision could be differentiated from each eye, the result is written at the back of the page depending on where they could no longer see the shapes and it would be written in log MAR. Gratings and optotype test At the age of 2.5 and 3, there are other optotype and grating test that could be carried such as contrast sensitivity VISTECH is a method of testing contrast sensitivity for the gratings. The VISTECH board is tested at a distance of 3 meters, and each row of the 5 consists of sine wave gratings that would decrease from the left side of the board to the right as the patient would read each line until they can are unable to see the lines appearing. The orthoptist would need to make sure that there is bright illumination in the room and to inform the patient that they are not allowed to do anything to help them see better such as, leaning forward or squinting. The test involves the patient having to explain at what orientation the lines are, if the child is unable to communicate verbally they would have an understanding on how to show the direction of the lines through gestures. In conclusion, the development of the visual system develops dramatically through the first 3 years of life and has a great effect on the child. At every stage of the growth of the child, every aspect of their development of the eye has a massive impact on their VA and contrast sensitivity. For example if the visual cortex does not develop accurately at the age of 6 months, then child could have difficulties to focus, translate images and perceive depth. Another example, if their cones do not develop fully the child would not be able to see colour clearly, at a high intensity and spatial frequency, as their development of the cones does not increase to migrate towards the fovea.