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 which 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 potentials. 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, as well as 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.

Visual 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. By two months, visual acuity improves to 6/45 (20/150). By four months, acuity improves by a factor of 2 – calculated to be 6/18 (20/60) vision. As the infant grows, the acuity reaches the healthy adult standard of 6/6 (20/20) 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.

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.

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 his 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 (“deep side”) and another which contained a platform of the centerboard (“shallow side”). 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.

From an infant's standpoint, depth perception can be inferred using three means: binocular, static, and kinetic cues. As mentioned previous, humans are binocular and each eye views the external world with a different angle – providing essential information into depth. The convergence of each eye on a particular object and the stereopsis, also known as the retinal disparity among two objects, provides some information for infants older than ten weeks. With binocular vision development, infants between four and five months also develop a sense of size and shape constancy objects, regardless of the objects location and orientation in space. From static cues based upon monocular vision, infants older of five month of age have the ability to predict depth perception from pictorial position of objects. In other words, edges of closer objects overlap objects in the distance. Lastly, kinetic cues are another factor in depth perception for humans, especially young infants. Infants ranging from three to five months are able to move when an object approaches them in the intent to hit them – implying that infants have depth perception. One study found that newborn infants looked longer at checkered patterns of white and colored stimuli (including red, green, yellow) than they did at a uniform white color. However, infants failed to discriminate blue from white checkered patterns. Another study – recording the fixation time of infants to blue, green, yellow, red, and gray at two difference luminance levels – found that infants and adults differed in their color preference. Newborns and one month old infants did not show any preference among the colored stimuli, while three month old infants preferred the longer wavelength (red and yellow) stimuli to the short wavelength (blue and green) stimuli, and adults had the opposite. However, both adults and infants preferred colored stimuli over non colored stimuli. This study suggested that infants had a general preference for colored stimuli over non colored stimuli at birth, though infants were not able to distinguish the different colored stimuli prior to the age of three months. Research into the development of color vision using infant female Japanese macaques indicates that color experience is critical for normal vision development. Infant monkeys were placed in a room with monochromatic lighting limiting their access to a normal spectrum of colors for a one month period. After a one year period, the monkey's ability to distinguish colors was poorer than that of normal monkey exposed to a full spectrum of colors. Although this result directly pertains to infant monkeys and not humans, they strongly suggest that visual experience with color is critical for proper, healthy vision development in humans as well.

The threshold for light sensitivity is much higher in infants compared to adults. From birth, the pupils of an infant remain constricted to limit the amount of entering light. In regards to pupil dimensions, newborns' pupils grow from approximately 2.2 mm to an adult length of 3.3 mm.

Vision problems in infants are both common and easily treatable if addressed early by an ophthalmologist.