when do infants develop stereoscopic or binocular vision
Infant visual development 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.[20] 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.[21] In other words, edges of closer objects overlap objects in the distance.[22] 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.[20]
Infant visual development One of the important discoveries of infant depth perception is thanks to researchers Eleanor J. Gibson and R.D. Walk.[18] 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 6–14 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.[18] 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.[19]
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