Infant visual development
Infant vision concerns the development of visual ability in human infants from birth through the first years of life. The aspects of human vision that develop following birth include visual acuity, tracking, color perception, depth perception, and object recognition.
Unlike many other sensory systems, the human visual system – components from the eye to neural circuits – develops largely after birth, especially in the first few years of life. At birth, visual structures are fully present yet immature in their potential. From the first moment of life, there are a few innate components of an infant's visual system. Newborns can detect changes in brightness, distinguish between stationary and kinetic objects, and follow kinetic objects in their visual fields. However, many of these areas are very poorly developed. 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. The neuro-pathway and physical changes that underlie these improvements in vision remain a strong focus in research. Because of an infant's inability to verbally express their visual field, growing research in this field relies heavily on nonverbal cues, including an infant's perceived ability to detect patterns and visual changes. The major components of the visual system can be broken up into visual acuity, depth perception, color sensitivity, and light sensitivity.
By providing a better understanding of the visual system, future medical treatments for infant and pediatric ophthalmology can be established. By additionally creating a timeline on visual perception development in "normal" newborns and infants, research can shed some light on abnormalities that often arise and interfere with ideal sensory growth and change.
Development
Acuity
, the sharpness of the eye to fine detail, is a major component of a human's visual system. It requires not only the muscles of the eye – the muscles of orbit and the ciliary muscles – to be able to focus on a particular object through contraction and relaxation, but other parts of the retina such as the fovea to project a clear image on the retina. The muscles that initiate movement start to strengthen from birth to 2 months, at which point infants have control of their eye. However, images still appear unclear at two months due to other components of the visual system like the fovea and retina and the brain circuitry that are still in their developmental stages. This means that even though an infant is able to focus on a clear image on the retina, the fovea and other visual parts of the brain are too immature to transmit a clear image.Visual acuity in newborns is very limited as well compared to adults – being 12 to 25 times worse than that of a normal adult. It is important to note that the distance from the cornea at the front of the infant's eye to the retina which is at the back of eye is 16–17 mm at birth, 20 to 21 mm at one year, and 23–25 mm in adolescence and adulthood. This results in smaller retinal images for infants.
The vision of infants under one month of age ranges from 6/240 to 6/60. By two months, visual acuity improves to 6/45. By four months, acuity improves by a factor of 2 – calculated to be 6/18 vision. As the infant grows, the acuity reaches the healthy adult standard of 6/6 at six months.
One major method used to measure visual acuity during infancy is by testing an infant's sensitivity to visual details such as a set of black strip lines in a pictorial image. Studies have shown that most one-week-old infants can discriminate a gray field from a fine black stripped field at a distance of one foot away. This means that most infants will look longer at patterned visual stimuli instead of a plain, pattern-less stimuli. Gradually, infants develop the ability to distinguish strips of lines that are closer together. Therefore, by measuring the width of the strips and their distance from an infant's eye, visual acuity can be estimated, with detection of finer strips indicating better acuity. When examining an infants preferred visual stimuli, it was found that one-month-old infants often gazed mostly at prominent, sharp features of an object – whether it is a strong defined curve or an edge. Beginning at two months old, infants begin to direct their saccades to the interior of the object, but still focusing on strong features. Additionally, infants starting from one month of age have been found to prefer visual stimuli that are in motion rather than stationary.
Faces
Newborns are exceptionally capable of facial discrimination and recognition shortly after birth. Therefore, it is not surprising that infants develop strong facial recognition of their mother. Studies have shown that newborns have a preference for their mothers' faces two weeks after birth. At this stage, infants would focus their visual attention on pictures of their own mother for a longer period than a picture of complete strangers. Studies have shown that infants even as early as four days old look longer at their mothers' face than at those of strangers only when the mother is not wearing a head scarf. This may suggest that hairline and outer perimeter of the face play an integral part in the newborn's face recognition. According to Maurer and Salapateck, a one-month-old baby scans the outer contour of the face, with strong focus on the eyes, while a two-month-old scans more broadly and focuses on the features of the face, including the eyes and mouth.When comparing facial features across species, it was found that infants of six months were better at distinguishing facial information of both humans and monkeys than older infants and adults. They found that both nine-month-olds and adults could discriminate between pictures of human faces; however, neither infants nor adults had the same capabilities when it came to pictures of monkeys. On the other hand, six-month-old infants were able to discriminate both facial features on human faces and on monkey faces. This suggests that there is a narrowing in face processing, as a result of neural network changes in early cognition. Another explanation is that infants likely have no experience with monkey faces and relatively little experience with human faces. This may result in a more broadly tuned face recognition system and, in turn, an advantage in recognizing facial identity in general. In contrast, healthy adults due to their interaction with people on a frequent basis have fine tuned their sensitivity to facial information of humans – which has led to cortical specialization.
Depth perception
To perceive depth, infants as well as adults rely on several signals such as distances and kinetics. For instance, the fact that objects closer to the observer fill more space in our visual field than farther objects provides some cues into depth perception for infants. Evidence has shown that newborns' eyes do not work in the same fashion as older children or adults – mainly due to poor coordination of the eyes. Newborn's eyes move in the same direction only about half of the time. The strength of eye muscle control is positively correlated to achieve depth perception. Human eyes are formed in such a way that each eye reflects a stimulus at a slightly different angle thereby producing two images that are processed in the brain. These images provide the essential visual information regarding 3D features of the external world. Therefore, an infant's ability to control their eye movement and converge on one object is critical for developing depth perception.One of the important discoveries of infant depth perception is thanks to researchers Eleanor J. Gibson and R.D. Walk. Gibson and Walk developed an apparatus called the visual cliff that could be used to investigate visual depth perception in infants. In short, infants were placed on a centerboard to one side which contained an illusory steep drop and another which contained a platform of the centerboard. In reality, both sides, covered in glass, was safe for infants to trek. From their experiment, Gibson and Walk found that a majority of infants ranging from 6 to 14 months-old would not cross from the shallow side to the deep side due to their innate sense of fear to heights. From this experiment, Gibson and Walk concluded that by six months an infant has developed a sense of depth. However, this experiment was limited to infants that could independently crawl or walk. To overcome the limitations of testing non-locomotive infants, Campos and his colleges devised an experiment that was dependent on heart rate reactions of infants when placed in environments that reflected different depth scenarios. Campos and his colleagues placed six week-old infants on the "deep end" of the visual cliff, the six week-old infants' heart rate decreased and a sense of fascination was seen in the infants. However, when seven month-old infants were lowered down on the same "deep end" illusion, their heart rates accelerated rapidly and they started to whimper. Gibson and Walk concluded that infants had developed a sense of visual depth prior to beginning locomotion. Therefore, it could be concluded that sometime at the spark of crawling around 4–5 months, depth perception begins to strongly present itself.