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Depth perception is the ability to perceive the distance of objects from each other and from ourselves. Depth perception is what permits infants to successfully reach for objects and, later, to crawl without bumping into furniture. By observing that newborns prefer to look at three-dimensional objects rather than two-dimensional figures, researchers have found that infants can perceive depth at birth (Slater, Rose, & Morison, 1984). Three- to 4-week-old infants blink their eyes when an object is moved toward their face as if to hit them, suggesting that they are sensitive to depth cues (Kayed, Farstad, & van der Meer, 2008; Náñez & Yonas, 1994). Infants learn about depth by observing and experiencing motion.

A classic series of studies using an apparatus called the visual cliff demonstrated that crawling influences how infants perceive depth. The visual cliff, as shown in Figure 4.14, is a Plexiglas-covered table bisected by a plank so that one side is shallow, with a checkerboard pattern right under the glass, and the other side is deep, with the checkerboard pattern a few feet below the glass (Gibson & Walk, 1960). In this classic study, crawling babies readily moved from the plank to the shallow side but not to the deep side, even if coaxed by their mothers, suggesting that they perceive the difference in depth (Walk, 1968). The more crawling experience infants have, the more likely they are to refuse to cross the deep side of the visual cliff (Bertenthal, Campos, & Barrett, 1984).

Description

Figure 4.14 Visual Cliff

Three-month-old infants show a change in heart rate when placed face down on the glass surface of the deep side of the visual cliff, suggesting that they perceive depth, but do not fear it. Crawling babies, however, move to the shallow side of the visual cliff and refuse to cross the deep side of the visual cliff.

Source: Levine and Munsch (2010).

Does this mean that babies cannot distinguish the shallow and deep sides of the visual cliff until they crawl? No, because even 3-month-old infants who are too young to crawl distinguish shallow from deep drops. When placed face down on the glass surface of the deep side of the visual cliff, 3-month-old infants became quieter and showed a decrease in heart rate compared with when they were placed on the shallow side of the cliff (Dahl et al., 2013). The young infants can distinguish the difference between shallow and deep drops but do not yet associate fear with deep drops.

As infants gain experience crawling, their perception of depth changes. Newly walking infants avoid the cliff’s deep side even more consistently than do crawling infants (Dahl et al., 2013; Witherington, Campos, Anderson, Lejeune, & Seah, 2005). A new perspective on the visual cliff studies argues that infants avoid the deep side of the cliff not out of fear but simply because they perceive that they are unable to successfully navigate the drop; fear might be conditioned through later experiences, but infants are not naturally fearful of heights (Adolph, Kretch, & LoBue, 2014).