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Unlike many of the other sensory systems, the visual system – from the eye to neural circuits - of a human develops largely after birth. At birth, the visual structures are fully present however immature in their potentials. For example, the distance from the cornea at the front of the infant’s eye to the retina which is at the back of eye is 16-17mm at birth, 20 to 21 mm at 1 year, and 23-25 mm in adolescence and adulthood. This results in smaller retinal images for infants. Another fundamental physical change that occurs is with pupil dimensions. In regards to pupil dimensions, newborns pupil grow from being approximately 2.2 mm compared to an adult length of 3.3 mm. From the first moment of life, there are a few innate components of their visual system. Newborns can detect changes in brightness, distinguish between stationary verses kinetic objects, and follow kinetic objects in their visual field. Due to infants inability to verbally express their visual field, growing research in this field relays heavy on non-verbal cues including infants perceived ability to detect patterns and changes. The major components of the visual system can be broken up into visual acuity, depth perception, color sensitivity, and light sensitivity.

Visual acuity 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 ciliary muscles – to be able to focus on a particular object through contraction and relaxation, but other parts of the retina such of the fovea to project a clear image on the retina. The ciliary muscles start to strength from birth to two months, at which point infants have a control of that their visual muscle. . However, images still appear unclear at two months due to other components of the visual system such as the fovea and retina and the brain are poorly developed. 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 (Dobson & Teller 1978, Norcia & Tyler 1985). The vision of infants under one month of age ranges from 20/800 to 20/200. By two months, visual acuity improves to 20/150. By 4 months, acuity improves by a factor of 2 – calculating to be 20/60 vision. As the infant grows, the acuity reaches the normal adult standard of 20/20 at 6 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 1 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 line that are closer together. Therefore, by measuring the wideth 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’s a strong defined curve or an edge. Beginning at two month old infants, infants began to direct their saccades to the interior of the object, but still focusing on strong features.

Visual Acuity and Faces

Newborns are exceptionally capable of face discrimination and recognition shortly after birth. Therefore it is not surprising that infants develop a strong facial recognition for their mother’s face. Studies have shown that a newborn have a preference for their mother’s face after 2 weeks of birth. At 2 weeks, infants would focus their visual attention on pictures of their own mother for a longer period, than a complete stranger. A more recent study has shown that even as early as 4 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 1-month old baby spans the outer contour of the face, with some interested in the eyes, while a 2-month old scans more broadly and focus on the features of the face, including the eyes and month.

When comparing facial features across species, it was found that infants of 6-months were better at distinguishing facial information of both humans and monkeys than older infants and adults. They found that both 9-month old and adults could discriminate between pictures of human faces; however, neither infant nor adult had the same capabilities when it came to pictures of money. On the other hand, 6 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 into a more broadly tuned face recognition system and, in turn, an advantage in recognizing facial identity in general (i.e., regardless of species). In contrast, healthy adults, due to their constant social interactions are human faces, have fined tuned their sensitivity to facial information of humans – which has lead to cortical specialization.

Color Sensitivity Color sensitivity improves steadily over the first year of life for humans due to strengthening of the cones of the eyes. Like adults, infants have can make chromatic discrimination using three photoreceptor types – long- (L) [reds & oranges], mid- (M) [greens & yellows] and short- (S) [ blues & violets] wavelength cones. These cones recombine in the precortical visual processing to form a luminance channel and two chromatic channels that help an infant to see color and brightness. However, there is a general debate among researchers regarding the age at which infants can detect color due to colors close relation to brightness and hue. Also the development of each type of cone varies.

It is generally accepted across all researchers that infants prefer high contrast black and white patterns over colors at their earlier stages of infancy. This is partially due to rods in the retina maturing earlier than cones. Studies have found that newborn infants looked longer at checkered patterns of white and colored stimuli (including red, than they did at uniform white color. However, this preference was not seen with