Due to the fact that the children were required to be mobile, and this meant the minimum age was six months, Gibson Walk had to use animals as control groups to certify that the babies had not simply learnt depth perception in their first five months through experience and observation in their environment. Animals such as turtles, rats, cats, lambs, kittens, chicks and kids were used. The results of the chicks, kids and lambs were particularly important as all of these animals are precocial and would have had little opportunity in the few hours between birth and being tested to develop depth perception.
The chicks, which were all tested within 24 hours of birth all moved onto the shallow side and none crossed over to the deep side. Both kids and lambs also did not step onto the deep side once, and when placed onto the deep side, became distressed and froze still. Rats however, who use their whiskers to navigate, saw little or no preference for either side. This is because they are nocturnal and completely rely on their vibrissae to comprehend their environment. They also found that when the centre board was placed high enough that they could not reach it with their vibrissae, the rats avoided the deep side, showing much more preference for the shallow side.
Kittens at four weeks, displayed the same reactions as the lambs, chicks and kids and Gibson and Walk also used dark-reared kittens to investigate the development of the visual system. Kittens that were reared in the dark for the first 27 days of life showed no discrimination between either side. However, when kept in normal lighting conditions, they gained normal depth perception after only one week.
Turtles were also tested. Although it was hypothesised that turtles may be attracted to the shiny surface of the glass over the deep side, believing it to resemble water, the turtles also showed levels of depth perception. However, only 76% of turtles managed to avoid the deep side. This large minority suggests that turtles have poor depth perception as a species. This is probably due to their natural habitat; they are less at risk from a fall due to the fact that they spend a majority of their time under water. It is also possible that their depth perception is slightly different to land animals, as they are used to perceiving depths under water, in which distances will appear slightly different.
Kids were also used in an alternative test in which they were placed on the shallow side of the glass. The goats appeared fine to stand and move across the surface however when the pattern was lowered, the goats became distressed and froze as soon as it was more than a foot below. The kids never learnt that the glass was solid and therefore safe to stand on, no matter how long they stood on the deep side.
Six main conclusions can be draw from this study; human babies have depth perception by the time they are mobile but it is possible this is through experience in their environment before this point. This study also supports the nativist view that we are born with the ability to perceive depth and that the visual system just has to develop for this to be fully functioning (although it does not prove that depth perception is innate). Findings usually fit with the life history and ecological place of the animal being studied (for example, dark reared cats’ visual systems had not yet developed properly). All animals tested showed depth perception by the age of mobility. Habitat can affect our ability to perceive depth, for example, turtles, whose natural habitat does not require particularly accurate depth perception, show much lower levels of depth perception than other land animals. Studies with rats in which the experimenter changed the size and shape of the pattern to eliminate certain cues suggested that shape and motion cues need to develop over time whereas motion parallax is innate (supported by Yonas et al’s study into babies’ depth perception in photographs).
