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).
Another method of studying infant perception relies on infants’ capacity for habituation, a gradual decline in the intensity, frequency, or duration of a response to an unchanging stimulus. For example, to examine whether an infant can discriminate between two stimuli, a researcher presents one until the infant habituates to it. Then a second stimulus is presented. If dishabituation, or the recovery of attention, occurs, it indicates that the infant detects that the second stimulus is different from the first. If the infant does not react to the new stimulus by showing dishabituation, it is assumed that the infant does not perceive the difference between the two stimuli. The habituation method is very useful in studying infant perception and cognition and underlies many of the findings discussed in this chapter.
Operant conditioning is the basis for a third method researchers use to study perception in infants. Recall from Chapter 1 that operant conditioning entails learning behaviors based on their consequences, whether they are followed by reinforcement or punishment. Behaviors increase when they are followed by reinforcement and decrease when they are followed by punishment. Research employing this method has shown that newborns will change their rate of sucking on a pacifier, increasing or decreasing the rate of sucking, in order to hear a tape recording of their mother’s voice, a reinforcer (Moon et al., 1993). Other research shows that newborns will change their rate of sucking to see visual designs or hear human voices that they find pleasing (Floccia et al., 1997). Researchers have found that premature infants and even third-trimester fetuses can be operantly conditioned (Dziewolska & Cautilli, 2006; Thoman & Ingersoll, 1993). For example, a 35-week-old fetus will change its rate of kicking in response to hearing the father talk against the mother’s abdomen, suggesting that hearing begins in the womb (Dziewolska & Cautilli, 2006).
Vision
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 is approximately 20/400 (Farroni & Menon, 2008). 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.
Face Perception
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). Face processing is influenced by experience with faces (Quinn, Lee, & Pascalis, 2018). Infants generally tend to see more female than male faces and more own- than other-race faces (Sugden, Mohamed-Ali, & Moulson, 2014). For example, between birth and 3 months, infants begin to prefer to look at and can more easily see female faces as compared with male faces, when their caregivers are female, but do not show similar preferences when their caregivers are male (Bayet et al., 2015; Rennels & Kayl, 2017).
Infants show similar preferences and abilities to discriminate same-race faces over other-race faces. That is, between approximately 3 and 9 months of age, infants tend to prefer and are better able to distinguish faces of frequently experienced groups, typically faces of members of their own race. However, differentiation of faces from within unfamiliar groups, such as other races, becomes more difficult with age (Markant & Scott, 2018). After about 9 months, infants show difficulty discriminating among unfamiliar faces, such as other-race faces. This decline in sensitivity to discriminate faces within unfamiliar groups is called perceptual narrowing (Scott, Pascalis, & Nelson, 2007). Experience influences perceptual narrowing. Specifically, babies who are extensively exposed to other-race faces (e.g., through adoption, training, or living in racially diverse communities) show less perceptual narrowing (Ellis, Xiao, Lee, & Oakes, 2017). Some researchers speculate that early perceptual differences in infancy may be associated with the emergence of implicit racial bias in childhood, but more research is needed to understand the social implications of same- and other-race face recognition (K. Lee, Quinn, & Pascalis, 2017; Quinn, Lee, & Pascalis, 2019).
Object Exploration
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. This is known as the externality effect because infants scan along the outer contours of complex visual stimuli. By 6 to 7 weeks of age, infants study the eyes and mouth, which hold more information than the hairline, as shown in Figure 4.13 (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).
Figure 4.13 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 2 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).
Color Vision
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).
Infants see color at birth, and color discrimination improves over the first few months of life.
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Depth Perception
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