The Development Of Vision Over The First 12 Months Of Life

Over the first year of life, many developments in the body occur including speech advancements, fine and gross motor movements, facial expressions and the fusion of bones. One of these advancements includes the progression of our eyesight- vision is a powerful sight that allows us to protect ourselves from the environment by reacting to stimuli; there is no doubt that as we get older, we gain more independence, such as being able to walk ourselves meaning it is essential that we are able to react to our environment in order to survive.

Vision is a combination of both incoming information from the environment and our knowledge of the world, consisting of many components- this includes visual acuity, depth and colour perception all of which gradually develop over the first year of life. A newborn does not have the visual abilities required for its environment, such as accommodation, and so gains information through the interpretation of its surroundings.

The human visual system develops rapidly over the first 12 months of life, advancing both in anatomy and function. Although newborns can detect changes in brightness, distinguish between stationary and kinetic objects in their visual fields (En.wikipedia.org, 2019), the ability to do so is very weak and improves with time. The cells that make up the visual cortex in an infant are not yet isolated by function and type, as they are in adults. Additionally, most of the cells are not yet coated with myelin, a white, fatty substance that aids neural transmission and the dendrites that reach multiple layers of the cortex are still short.

Colour perception

Colour perception is the ability to distinguish between different colours or alternate wavelengths. Rods and cones are essential cells that are a part of our eyes and assist our eyes in vision; rods detect light and are responsible for vision at low levels of light. Cones are mainly used for photopic vision and are capable of colour vision. There are three types of cones known as red, blue and green. Any absence of these cones results in colour blindness- all babies are colour-blind when they are born suggesting that these cells may still be developing. This may be due to most receptors not being coated in myelin that aids neural transmission as well as the dendrites still being short, causing slower transmission. Colour vision begins to develop within a week after birth and by 6 months a baby can see every colour that an adult can (Discovery Eye Foundation, 2019).

At 1 week, infants are able to discriminate long and medium wavelengths which may be due to the development of the light sensitive cones in the retina. However, infants at 1 month can not distinguish between short wavelengths due to the absence of S cones and such mechanisms in the visual cortex which still requires some advancements (Psyc.ucalgary.ca, n.d.).

Over the first 2 months, colour perception develops rapidly and by 3 months, an infant can start to see colours due to the strengthening of rods and cones, allowing the eye to respond to different wavelengths of light. It is firmly established that two-month-old infants are at least dichromatic, but there is no clear data on whether they are trichromatic. There are a few pieces of evidence to suggest that the basic sensory capacities required for color processing are different for infants than for adults, but specifics on the ontogenetic course are as yet unknown (Werner and Wooten, 1979)

By 2 months, the S cones have now become functional, allowing the infant to better discriminate between the shorter wavelength colours. At 4 months, an infant can categorise colours in the same way as an adult, demonstrating the advancements in retina and visual cortex allowing this to be made possible.

By 6 months, advances in the visual parts of the brain have taken place, allowing an infant to see colours similar to that of an adult, including most colours of the rainbow. Testing more than 40 babies, Skelton has found that, even at four months, they, like adults, need blues and yellows to be more intense to see them than reds and greens. Research by Franklin and her team has also shed light on a surprising phenomenon: babies can categorise colours (Davis, 2019). Franklin and her colleagues familiarized 179 babies ranging from 4 months to 6 months old with a particular hue, such as blue. Then, after less than a minute, the researchers presented the baby with a familiar color alongside a new color, such as green. If the baby stayed on green, the researchers considered it novel for the baby. In the end, the researchers concluded that the babies were able to discern five color categories: red, yellow, green, blue and purple (Staedter, 2017). This in turn proposes that at 6 months, a babies colour vision is similar to that of an adult as they can make out more complex shapes as well as a wider spectrum of colours than they could pre 6 months; this implies development in photoreceptors on the retina. By 6 months, rod outer segment length, rhodopsin content has increased so that rod-mediated visual sensitivity has become similar to that of adults (Hansen and Fulton, 2006) Their sensitivity is greatest to intermediate wavelengths such as yellow and green and less for shorter wavelengths such as blue or longer wavelengths such as red.

The images above demonstrates how newborns cannot see in colour- their vision is limited to simple shapes and minute colour. It consists of a blur with a single colour, demonstrating that they may just be able to make out their parents faces. Even at 6 months, although the vision is similar to that of an adult, the colour vision is not yet fully advanced and a distinct image is not fully seen, whereas the adult can clearly see a more complicated image with brighter colours and shadows. This is most likely to be due to the lack of development in their visual cortex as well as the growth of the rods and cones.

Depth perception at birth

Depth perception is the visual ability to perceive the world in three dimensions to see if objects are closer or father away then other objects. Depth perception occurs when our brain processes information from both eyes and joins them together to form a three-dimensional image; this is a requirement of both eyes- infants do not experience depth perception, suggesting that their eyes do not work together at birth. At 3 months of age, an infant can accommodate which allows them to discriminate small differences in object distance. Therefore, sensitivity to depth from motion is present very early in infancy (Kellman and Arterberr 1998) (Sciencedirect.com, 2019)

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There are three visual cues that aid depth perception known as motion parallax, interposition and aerial perspective. Motion parallax occurs when we move our head back and forth. Objects at different distances will move at slightly different speeds. Our brains perceive this and give us cues about depth perception (Bedinghaus, 2019) In infants, dishabituation results indicate that infants may be sensitive to unambiguous depth from motion parallax by 16 weeks of age (Nawrot E, Mayo and Nawrot, 2009). Interposition is also known as overlapping and is a monocular trait, telling us which objects are closer or father away and finally, aerial perspective refers to the affect the atmosphere has on an object, causing farther away objects to seem hazy. For example, we can see clearly the keys on a laptop but if we were to look at mountains from a distance, they would seem blurry. In a newborn, aerial perspective hardly exists as the greatest they can see is a blur of the shape of their parents faces. This demonstrates that depth perception is non-existent at birth, due to the lack of visual cues present in addition to the eyes lacking the ability to work in duo.

At 2 to 3 months old, infants have some form of depth perception which is demonstrated through the ‘visual cliff’ experiment; researchers found that infants as young as 2 months showed changes in heart rate when lowered face down over the shallow and deep ends of the visual cliff. Specifically, the infants' heart rates decreased when they were lowered over the deep end, and were unchanged when over the shallow end. (Condry and Yonas, 2019) (Spillman and Werner, 1989) suggesting an interest from the infants, therefore indicating they have an idea of depth perception. However, a limitation of this experiment is that infants cannot tell the experimenter what they witness, therefore, the heart rates may be unreliable due to a number of external factors having an influence over their vision.

Recent research shows that young infants are sensitive to motion parallax in visual displays but leaves open the question of whether infants use the information to perceive spatial layout. In this experiment, 6-month-old infants were translated horizontally in front of two objects that were attached to the infant's movement. One object moved in the same direction as the motion of the infant and the other object moved in the opposite direction. This provided motion parallax information that the object that moved in the opposite direction was nearer in depth. Infants who viewed the display monocularly reached, in preference, the object that was apparently nearer. A control group of infants who viewed the display binocularly showed no such preference. These results provide the first direct evidence that young infants use the spatial information provided by motion parallax to perceive the relative distance of objects and to direct their actions accordingly. (Condry and Yonas, 2019)

Although depth perception reaches that to of an adult at 2 years, by 12 months, an infant can see the same as an adult. This development may be due to infants' eyes working together to help form a three-dimensional image as both eyes view different angles, therefore, together they can help form an image with a certain depth. With physical improvements such as increased distances between the cornea and retina, increased pupil dimensions, and strengthened cones and rods, an infant's visual ability improves drastically. (Walk, 1966)

Visual acuity at birth

Visual acuity refers to the sharpness of vision; it is usually measured by the ability to read numbers and letters from a specific distance and is affected by the refractive error of the patient. Most newborns are hyperopic therefore, their visual acuity on average is 6/240 to 6/60 (Wikipedia, 2019). The most common visual acuity test in newborns, is by testing their eye movements to a toy. However, through the use of the opticokinetic nystagmus response to striped patterns of varying width, investigators demonstrated that the newborn exhibits at least 20/150 vision (Volpe, 2008) This means that what a newborn can see at 20 feet, is what an adult can see at 150 feet, demonstrating that the vision is obviously not as sharp.

The image above demonstrates poor visual acuity in newborns. The pictures in the upper images demonstrate how groups of toys are seen by adults. The images in the lower tables show how groups of toys would be for newborns whose visual acuity is 25 times worse than that of an adult, at more than 45cm (Gwiazda et al., 2019). The cause of low visual acuity may be due to the refractive error a child is born with- most newborns are hyperopes, but the range varies to myopia as well causing visual acuity to be lower than an adult. This may be due to the shape, size and distributions of cone receptors on the retina. These changes improve the eye’s light capturing ability as well as its acuity (Psyc.ucalgary.ca, 2019). This demonstrates how visual acuity improves over the first 12 months of life.

In newborns, the cones are short and not as closely packed together than in adults- this means that light is less efficient in being absorbed. In addition to this, the cones in infants have a fat inner segment and a small outer segment, resulting in less visual pigment reducing the ability to capture light as demonstrated in figure 1 below. As the infant grows, the cells elongate, become more densely packed and migrate toward the center of the retina to form the fovea, the area of best acuity.

The image above demonstrates how acuity progresses throughout infanthood; visual acuity rapidly develops between birth and 3 months where its starts at 20/800 and rises, on average, to 20/200. After 3 months, there is a slow improvement between 3 months and 12 months where visual acuity has now reached similar to that of an adult, however, is still weak. By 3 years, an infant has now 20/20 vision due to the distribution of rods and cones being interchangeable to that of an adult.

Conclusion

To conclude, infant vision at birth is scarce with a blur and minute colour being visualised. Vision starts to develop from the day the infant is born with rods and cones strengthening, allowing an infant to interpret a wider variety of wavelengths of light; in addition to this, the visual cortex starts to develop, allowing the infant to receive impulses about its vision.

Visual acuity is extremely weak at birth with an average of 6/240 being seen. When in development, the distribution of rods and cones allow the ability of the infant to capture light to improve, which in turn, helps improve visual acuity. There is a rapid improvement between birth and 6 months, however, it slows down after 6 months but does not stop until 20/20 vision is achieved. This can be explained by the cones elongating and migrating closer together on the fovea, resulting in more defined input in the fovea. By 3 years, the visual acuity is equal to that of an adult who has perfect vision and a child can see in sync with an adult’s vision. However, there are different results from a number of different resources, making it difficult to understand the visual acuity in newborns, shining light on to whether there is a wider range of visual acuities present at birth.

Depth perception is non-existent at birth due to the infant lacking the ability to accommodate. However, depth perception begins to make its way into an infant's vision as they start to use their ability to accommodate, which in turn results in objects from a selection of distances to be perceived more accurately than they could before. This may be due to our eyes having the ability to work binocularly, which is a trait that is essential in order to start perceiving our world at different depths.