Lifespan Development. Tara L. Kuther

2009).

      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 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 4.10, 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 (E. J. 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).

      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).

      Description

      Figure 4.10 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, show a different response. In a classic study of visual perception, crawling babies moved to the shallow side of the visual cliff, even if called by their mothers. The more crawling experience infants had, the more likely they were to refuse to cross the deep side of the visual cliff.

      Source: Levine and Munsch (2010).

      Hearing

      The capacity to hear develops in the womb; in fact, hearing is the most well-developed sense at birth. Newborns are able to hear about as well as adults (Northern et al., 2014). Shortly after birth, neonates can discriminate among sounds, such as tones (Hernandez-Pavon, Sosa, Lutter, Maier, & Wakai, 2008). By 3 days of age, infants will turn their head and eyes in the general direction of a sound, and this ability to localize sound improves over the first 6 months (Clifton, Rochat, Robin, & Berthier, 1994; Litovsky & Ashmead, 1997).

      The process of learning language begins at birth, through listening. Newborns are attentive to voices and can detect their mothers’ voices. Newborns only 1 day old prefer to hear speech sounds over similar-sounding nonspeech sounds (May, Gervain, Carreiras, & Werker, 2017). Newborns can perceive and discriminate nearly all sounds in human languages, but from birth, they prefer to hear their native language (Kisilevsky, 2016). Brain activity in the temporal and left frontal cortex in response to auditory stimuli indicates that newborns can discriminate speech patterns, such as differences in cadence among languages, suggesting an early developing neurological specialization for language (Gervain, Macagno, Cogoi, Peña, & Mehler, 2008; Gervain & Mehler, 2010).

      Touch

      Compared with vision and hearing, we know much less about the sense of touch in infants. In early infancy, touch, especially with the mouth, is a critical means of learning about the world (Piaget, 1936/1952). The mouth is the first part of the body to show sensitivity to touch prenatally and remains one of the most sensitive areas to touch after birth.

      Touch, specifically a caregiver’s massage, can reduce stress responses in preterm and full-term neonates and is associated with weight gain in newborns (Diego et al., 2007; Hernandez-Reif, Diego, & Field, 2007). Skin-to-skin contact with a caregiver, as in kangaroo care (see Chapter 3), has an analgesic effect, reducing infants’ pain response to being stuck with a needle for blood testing (de Sousa Freire, Santos Garcia, & Carvalho Lamy, 2008; Ferber & Makhoul, 2008). Although it was once believed that newborns were too immature to feel pain, we now know that the capacity to feel pain develops even before birth; by at least the 30th week of gestation, a fetus responds to a pain stimulus (Benatar & Benatar, 2003). The neonate’s capacity to feel pain has influenced debates about infant circumcision, as discussed in the Lives in Context feature.

      Smell and Taste

      Smell and taste receptors functional in the fetus and preferences are well developed at birth (Bloomfield, Alexander, Muelbert, & Beker, 2017). Just hours after birth, newborns display facial expressions signifying disgust in response to odors of ammonia, fish, and other scents that adults find offensive (Steiner, 1979). Within the first days of life, newborns detect and recognize their mother’s odor (Macfarlane, 1975; Porter, Varendi, Christensson, Porter, & Winberg, 1998; Schaal et al., 1980). Infants are calmed by their mother’s scent. Newborns who smelled their mother’s odor displayed less agitation during a heel-stick test and cried less afterward than infants presented with unfamiliar odors (Rattaz, Goubet, & Bullinger, 2005). Familiar scents are reinforcing and can reduce stress responses in infants (Goubet, Strasbaugh, & Chesney, 2007; Nishitani et al., 2009; Schaal, 2017).

      Lives in Context

      Pain and Neonatal Circumcision

      An eight-day-old Jewish boy is circumcised as part of a religious ceremony called a bris.

      Dan Porges/Archive Photos/Getty Images

      Neonatal circumcision, removal of the foreskin of the penis, is the oldest known planned surgery (Alanis & Lucidi, 2004). Although it is uncommon throughout much of the world, about three quarters of males in the United States are circumcised (Morris et al., 2016). As shown in Figure 4.11, there are regional differences, with nearly twice as many infant circumcisions in the Midwest as in the West (Owings, Uddin, & Williams, 2013). In recent years, circumcision has come under increasing scrutiny within the United States as some charge that it places the newborn under great distress and confers few medical benefits.

      For decades, many scientists and physicians believed that newborns did not feel pain, leading many to perform circumcision without pain management. We now know that even the fetus feels pain (Benatar & Benatar, 2003). Newborns show many indicators of distress during circumcision, such as a high-pitched wail, flailing, grimacing, and dramatic rises in heart rate, blood pressure, palm sweating, pupil dilation, muscle tension, and cortisol levels (Paix & Peterson, 2012). Analgesia (pain relief in which the newborn remains conscious) is safe