Читать книгу Lifespan Development - Tara L. Kuther - Страница 223

It is impossible to know whether the fetus has a sense of vision, but the fetus responds to bright light directed at the mother’s abdomen as early as 28 weeks’ gestation (Johnson & Hannon, 2015). At birth, vision is the least developed sense, but it improves rapidly. Newborn visual acuity, or sharpness of vision, is approximately 20/400 (Farroni & Menon, 2008). Researchers study visual acuity, sharpness of vision or the ability to see, in infants with the use of preferential looking tasks designed to determine whether infants prefer to look at one stimulus or another. For example, consider an array of black and white stripes. As shown in Figure 4.8, an array with more stripes (and, therefore, many more narrow stripes) tends to appear gray rather than black and white because the pattern becomes more difficult to see as the stripes become narrower. Researchers determine infants’ visual acuity by comparing infants’ responses to stimuli with different frequencies of stripes because infants who are unable to detect the stripes lose interest in the stimulus and look away from it. Preferential looking studies show that infants reach adult levels of visual acuity between 6 months and 1 year of age (Mercuri, Baranello, Romeo, Cesarini, & Ricci, 2007). Improvement in vision is due to the increasing maturation of the structures of the eye and the visual cortex, the part of the brain that processes visual stimuli.

Figure 4.8 Visual Acuity

Researchers and pediatricians use stimuli such as the Teller Acuity Cards illustrated here to determine what infants can see. Young infants attend to stimuli with wider lines and stop attending as the lines become smaller.

Source: Leat, Yadev, and Irving (2009).

Description

Figure 4.9 Externality Effect and Face Perception

The externality effect refers to a particular pattern of infant visual processing. When presented with a complex stimulus, such as a face, infants under two months of age tend to scan along the outer contours, such as along the hairline. Older infants scan the internal features of complex images and faces, thereby processing the entire stimulus.

Source: Shaffer (2002, p. 190); adapted from Salapatek (1975).

Newborns are born with preferences for particular visual stimuli. Newborns prefer to look at patterns, such as a few large squares, rather than a plain stimulus such as a black or white oval shape (Fantz, 1961). Newborns also prefer to look at faces, and the preference for faces increases with age (Frank, Vul, & Johnson, 2009; Gliga, Elsabbagh, Andravizou, & Johnson, 2009). How infants explore visual stimuli changes with age (Colombo, Brez, & Curtindale, 2015). Until about 1 month of age, infants tend to scan along the outer perimeter of stimuli. For example, when presented with a face, the infant’s gaze will scan along the hairline and not move to the eyes and mouth. By 6 to 7 weeks of age, infants study the eyes and mouth, which hold more information than the hairline, as shown in Figure 4.9 (Hunnius & Geuze, 2004). Similarly, the ability to follow an object’s movement with the eyes, known as visual tracking, is very limited at birth but improves quickly. By 2 months of age, infants can follow a slow-moving object smoothly, and by 3 to 5 months, their eyes can dart ahead to keep pace with a fast-moving object (Agyei, van der Weel, & van der Meer, 2016; Richards & Holley, 1999). The parts of the brain that process motion in adults are operative in infants by 7 months of age (Weaver, Crespi, Tosetti, & Morrone, 2015).

Newborns see color but they have trouble distinguishing among colors.

Phanie / Alamy Stock Photo

Like other aspects of vision, color vision improves with age. Newborns see color, but they have trouble distinguishing among colors. That is, although they can see both red and green, they do not perceive red as different from green. Early visual experience with color is necessary for normal color perception to develop (Colombo et al., 2015; Sugita, 2004). Habituation studies show that by 1 month of age, infants can distinguish among red, green, and white (Teller, 1997). By 2 to 3 months of age, infants are as accurate as adults in discriminating the basic colors of red, yellow, and blue (Matlin & Foley, 1997; Teller, 1998). By 3 to 4 months of age, infants can distinguish many more colors as well as distinctions among closely related colors (Bornstein & Lamb, 1992; Haith, 1993). Seven-month-old infants detect color categories similar to those of adults; they can group slightly different shades (e.g., various shades of blue) into the same basic color categories as adults do (Clifford, Franklin, Davies, & Holmes, 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).