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Discuss and describe infant vision development over 12 months

In this essay we will be discussing how vision changes in an infant over the first 12 months of life and a number of factors that can affect this development. Learning about infant vision is important because it can allow increase medical knowledge to allow prevention of conditions such as amblyopia which arise from strabismus. This can lead to the infant losing clarity of vision in one eye which happens by the nerve pathway to the brain doesn’t develop properly and if not treated promptly may cause permanent vision loss. Therefore, knowledge of vision development is important. Contrast sensitivity is the ability to distinguish an object from its background where contrast is affected by the brightness and the colour of the object. Contrast sensitivity is an important measure of infant vision because it measures a number of different aspects of vision and can also provide an estimate for visual acuity. Infants have a low contrast sensitivity when they are born and it increases over the first 12 months of life and Martin S Banks and Philip Salapatek have investigated further by setting up a number of experiments using the fixation and following card procedure on 1 to 3 month old infants. This procedure is where an infant is shown two stimuli and an observer tracks what the infant first locks onto and see how long they take to change focus to the other stimuli. Figure .. shows that for 1 and 2 month old infants the sensitivity is very low but starts to increase for 3 month olds with a peak of around 5 cy/deg. Norcia et al used a different method the ‘sweep VEP and sinusoidal grating method’ to find the contrast sensitivity for 48 infants and 10 adults. He found that neural development is rapid for the first 3 months which meant for low spatial frequencies the contrast sensitivity was at adult level. He also found that between 4 and 9 weeks there was an increase in all spatial frequencies but after 9 weeks the contrast sensitivity at low frequencies were stagnant but other frequencies were still increasing. This shows that infants are not very sensitive to contrast initially but as they develop the sensitivity increases and this leads to visual acuity increasing as well. Another factor that affects contrast sensitivity is lateral inhibition, which refers to inhibition of neighbouring neurons in brain pathways on each other and in the visual system neighbouring pathways from receptors of the optic nerve show lateral inhibition. This helps the visual system to increase contrast and help see edges of surfaces better because the neurons on the edge respond more accurately than others. These processes start to develop by 2 months of age and is why visual acuity increases and is discussed later. Visual acuity is the resolving power of the eye whereby it is the ability to see two separate objects and separate. The visual acuity in an infant just born is around 6/300 which is a very low acuity which shows how little vision infants can see. In the first 12 months of life an infant’s vision develops quite rapidly figure 1 shows what vision is like for an infant and how it starts to increase over time. It is difficult to find what kind of vision an infant can see due to their lack communication therefore using methods such as a reading of a Snellen test would be impossible. There are different ways such as preferential looking technique, Teller cards and Keeler cards. The figure on the right shows data that has been collected which shows the difference between using 3 different methods of finding acuity which are Keeler cards, Teller cards and automated preferential looking. The data was collected over a number of age groups and overall 139 infants had been tested. The test was standardized by using 3 same observers for all the experiments. The data shows overall that visual acuity increases gradually over the first year of life and all methods find the same average acuity however the Keeler technique has the highest overall acuity reading. By analysing the data approximately by week 40 the acuity in cycles/degree is around 10 which when converted to a Snellen metric reading is around 6/18 compared to around week 2 which as Snellen reading of 6/180. This illustrates the development of vision over the first 12 months of life. Colour vision is an important part of vision as it provides us with another way to interact with the world and in infants, they are known to have colour blindness until two months of age where they can only see two of the primary colours but there is no data which says they can see three primary colours as discussed later. Davida Teller, started to investigate colour vision in infants and she wanted to find out if infants could see colour and if so what intensity of colour could be viewed. She first tried to see if infants could distinguish between two different colours red in a spot and a grey background (figure a.). The problem was that we cannot tell what the infant is seeing therefore Davida Teller and David Peeples tried another method where they knew that adults can see colour and distinguish between different colours therefore, they used this and created a six-fold range of intensities using adult photopic. Teller then showed the infants the pictures and found out the 2-month-old infants that were being tested could discern red stimuli with the grey background. This shows that infants may have two working photoreceptors which could be the long, mid or short wavelength cones. Adams et al (1990) has also done tests and found that newborn infants looked longer for patterns of grey but coloured red checks on a checkerboard but the same wasn’t seen when the colour was changed to blue, so this concluded that infants have colour vision first for long wavelengths not short wavelengths. Additionally, during the critical period colour vision also develops with it occurring between 3 months to 8 years of age and cones become more sensitive and functional. The human retina is not developed much in the first few months therefore the cones are not able to send signals to the brain but as it develops the signals become stronger and more colour is seen. This shows that there is lots of discrepancies and no solid evidence. Vision changes over time due to the development of the brain and retina. The brain is very important for vision because it interprets and processes the image by rotating it to make it the right way up. The brain and the retina work together via the optic nerve and when an infant is born the brain and nerve cells are still developing and vision is quite blurred. There is evidence for this when an infant’s eyes were examined who unfortunately passed away after 8 days, and under the light microscope it was found out that the macular region was immature after week 1 and it covered the ‘5 degrees of the retina’. The receptor layers were underdeveloped also with only one or two cells thick which can indicate that retina isn’t fully developed and below we discuss further. The figure on the left shows a number of retinal ganglion cells that transmit neural signals to the brain, and it shows that when an infant is born, they are light sensitive as in the newborn network diagram the sun does produce a signal from ipRGC which is in the ganglion cell layer which sends signals to other visual targets. However, because the rods and cones are not fully developed and there are no bipolar cells visible no proper signals reach the ganglion cells therefore no signals are transmitted from the retina to the brain. The ganglion cells are important because they are concerned with getting higher visual acuity and colour vision from cones. This shows why newborn infants have poor visual acuity. Therefore, as these cells start to develop and mature sensitivity to contrast and motion start to increase with increasing visual acuity as shown in figure . Also post retinal factors can affect the development of vision, with the nerve fibre myelination and development of inputs from the retina to LGN which is immature when born and increasing cell size and synaptic density changes. These all participate to increasing vision in infants. When infants are born research has found that the majority of full term babies (born after 39 weeks) have hypermetropia up to +3 dioptres. This experiment was done using 100 normal full term infants that were examined on the day of birth or after. The results show that around 81% had hypermetropia and 22% of those have hypermetropia of more than +3D and up to +10D which means that the eye was either too short or the power of the eye was too weak however as they are infants, they can accommodate. Motion perception is the process of differentiating the difference between the speed and direction of objects in an area. It is extremely important to virtually every species and even beings that are without vision rely on some kind of antenna to detect movement. For a newborn they do have functional and developing sensory systems with vision, hearing, smell and touch however they don’t have any knowledge of how to piece all this information together and this is what they gain through experience. There are a number of types of motion perception including biological motion and motion binding, these are different ways of extracting information that you see to make valid judgements. There has been a theory that infants may have a predisposition for biological motion but only been seen by chickens and lots of experiments have been undertaken which has found out by testing biological vs nonbiological point light animations and it was shown that newborn’s are able to discriminate between both and this reveals that infant’s perception is developed enough to extract information from moving objects. Infants are also born with a functioning oculomotor and brain systems to control the eye movements; however, the control of visual attention takes time to develop including smooth pursuit and saccadic eye movements. These start to develop after 2 to 3 months and they increase the ability to track moving objects and fast eye movements and they could be developed at same time as some specialised brain functions. There has been research that has been carried using mathematics and technology and have found out approximately what an infant can see and how they perceive facial expressions. In figure it shows pictures of different expressions vs different distances from the infant. This proves that infants can read facial expressions at certain distances for example at 30 cm happy and surprised can be distinguished but and 60 cm and 120 cm it can be more difficult. However, responding to these expressions usually occurs during the 1st or 2nd month and if no response occurs parents need to go to an ophthalmologist to take a look at their eyes to make sure nothing is wrong and this is important to prevent future problems. In conclusion, considering all the topics discussed there are a number of different ways to help and make sure that development of vision happens properly including making sure the infant gets the right diet and vitamins. For example, zinc is needed for growth of new cells which is essential for brain and vitamin A which is important for visual development which enables better vision in dim light and also protects the cornea. This is crucial for increasing contrast sensitivity and visual acuity. Genetical conditions is also crucial in development and this includes any diseases or conditions that the parents may pass over such as Stargardt disease or congenital cataracts as this can affect the growth of the infant’s vision and precautions can take place before birth. Also, parents that have high hypermetropia or myopia the infant will have to be monitored closely at birth to check for amblyopia and squints. So overall, all these different factors have to be taken into consideration to aid vision development.

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