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Infancy extends from birth through 2 years of age. It has long been regarded as a special period of life in which numerous important transformations of structure take place, and the essential processes of information exchange with the world are laid down. Over this period, the infant is transformed from a condition of utter dependence on the caregiver for survival to a condition of relative independence or life-preserving agency—a transformation accompanied by marked physical, behavioral, and cognitive changes. While no characterization of this transition is entirely satisfactory, several elements set the infant apart from children of other ages. Despite disclaimers that what happens very early in life can be reversed or improved upon, most developmental psychologists celebrate the very considerable and enduring inﬂuence of early experience. This research paper paints a broad picture of infant development, its prenatal antecedents, and its relation to the behavior of older children and adults.
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1. Prenatal Development
1.1 Physical And Cognitive Development
The ‘prenatal period’ extends from conception to birth. During the ‘germinal stage’ (fertilization–2 weeks) and ‘embryonic stage’ (2–8 weeks), the fertilized egg rapidly divides and attaches to the uterine wall, the major organs and systems of the body diﬀerentiate, and interconnections form between the developing components of the central and peripheral nervous systems. During the fetal stage (8 weeks–birth), the organism grows rapidly and the physical features that identify it as human become increasingly apparent.
During the prenatal period, the mechanisms that will facilitate the organism’s adaptation postnatally become functionally mature. Infants born prematurely, for example, who readily learn responses that increase the amount of quiet sleep, reveal this anticipatory maturation. The characteristics of the mother’s voice are also learned during the prenatal period. After birth, hungry infants orient and root in the direction of their mother’s voice, thereby increasing their eﬃciency of breast-feeding irrespective of whether the mother is holding them on the left or right. Newborns will also learn to activate a tape-recording of their mother’s voice—but not of another woman’s voice—by sucking non-nutritively on a modiﬁed nipple. Moreover, ‘prenatal learning’ is highly speciﬁc: newborns will suck harder to listen to their mother reading a Dr. Seuss passage that she had read aloud daily during the last 6 weeks of gestation than to hear her reading a novel Dr. Seuss passage (DeCasper and Spence 1986). This speciﬁcity continues postnatally throughout the ﬁrst half year. Similar learning prenatally familiarizes organisms with the linguistic environment into which they eventually will be born.
1.2 The Jacksonian Principle
The development and dissolution of prenatal motor function is described by the ‘Jacksonian principle,’ which states that the last reﬂexes to develop are the ﬁrst to disappear when the organism undergoes demise, whereas the ﬁrst to develop are the last to disappear (Jackson 1884, cited in Taylor 1958). The contralateral (avoiding) reﬂex, which moves a stimulated site away from a tactile stimulus, is supplanted weeks later by an ipsilateral reﬂex (Hooker 1952). This sequence is reversed by fetal anoxia. The reversion occurs because reﬂexes that develop ﬁrst are well practiced and require less oxygen to be initiated than reﬂexes that are more recent. Similarly, the order in which sensory functions develop (tactile, vestibular, thermal, chemical, auditory, and visual) is subsequently reversed during injury, disease, or aging.
2. Mileposts Of Infant Development
2.1 Zero–6 Months
Birth marks the organism’s transition from a parasitic, waterborne existence in a thermally constant environment with constant access to nutrition to an in-dependent, land-dwelling existence in a desiccating and cold environment with discontinuous access to food. The occasion of birth is unremarkable, important primarily for triggering the onset of the ‘regulatory mechanisms’ (respiration, blood ﬂow, digestion) that are critical for this transition. These regulatory arrangements and their interactions are the major constraints on the newborn’s behavior and the environmental conditions it can tolerate (Adolph 1968). The evolutionary requirement of all newborn mammals is to grow, which necessitates maximizing energy intake and minimizing energy expenditure. Not surprisingly, therefore, newborns primarily eat and sleep. Early postnatal learning also reﬂects this necessity: Newborns readily learn where and when food is accessible. Also, because full-term newborns do not physiologically thermo regulate before 4–9 weeks of age, they will not learn energetically costly responses that compete with behavioral thermoregulation.
Over the ﬁrst 3 months of life, the birth weight of the infant doubles. During this period, brain development is very rapid and inﬂuenced appreciably by the kinds of experience to which the infant has been subjected. While most information about dendrite proliferation, the directionality of neuronal connections, cell death, and the eﬀects of deprivation on neural and behavioral development comes from mammals other than humans, the universality of the eﬀects described by neuroanatomists and neuropsychologists is impressive.
In the period following birth, much of the infant’s behavior is reﬂexive, that is, speciﬁc stimuli call forth speciﬁc reactions (e.g., touches to the mouth elicit rooting and mouthing, abrasions of the skin elicit retraction of the injured body part). This early behavior is principally under the control of the subcortical portions of the brain, and the elicited behaviors are typically swift and obligatory.
Between 2 and 4 months of age, cortical maturation—especially dendritic proliferation—is increasingly evident, and neonatal reﬂexes disappear or weaken. Also during this period, activity undergoes a major transition from elicited to voluntary. The developmental sequence in which reﬂexes emerged during the prenatal period is recapitulated during this transitional period, and the sequence in which the postnatal reﬂexes appear is similarly reversed when the organism is challenged (Humphrey 1969). The postnatal repetition of fetal activity sequences is again due to the lower oxygen requirement of more primitive, hence more practiced, reﬂexes. Practicing neonatal reﬂexes reduces their oxygen requirement and speeds the transition.
The transitional process has been implicated in the sudden infant death syndrome (SIDS). McGraw (1943) characterized the period when reﬂexes are neither reliably elicited nor under voluntary control as a period of ‘disorganized behavior.’ The transitional period of the respiratory occlusion reﬂex corresponds to the time when SIDS is most likely to occur (95 percent of all crib deaths occur between 2–5 months). By 6 months, the occlusion reﬂex has come under cortically mediated voluntary control, and the infant is once again able to defend successfully against respiratory occlusion (Lipsitt 1979).
Also, over the ﬁrst 3 months of age the behavioral repertoire of the infant sharply expands. Infants can acquire both classically conditioned and operantly conditioned responses at birth and can remember what they have learned for a surprisingly long time (Lipsitt 1969). An eyeblink reﬂex that is classically conditioned at 10 days of age, for example, is remembered for at least 10 days. Newborns can habituate to a repeatedly presented stimulus and discriminate it from others that they have not previously encountered. Imitation, a particularly eﬃcient form of learning, can be demonstrated within hours of birth. Not only can newborns imitate an adult’s facial gestures immediately after they were modeled, but by 6 weeks of age, infants can imitate facial gestures after a 24-hour delay. Piaget (1962) described early facial imitation as stimulus-bound and reﬂexive and hypothesized that it disappeared along with many other neonatal reﬂexes. However, it was later shown not to disappear (e.g., Meltzoﬀ and Moore 1994). Infants’ early imitation of facial expressions foreshadows their later dyadic interactions with adults and siblings.
By 3 months of age, infants can acquire arbitrary categories, learn the serial order of an arbitrary list, and pick up incidental information about events in their visual surround—including information about correlated features (‘what goes with what’). Although some aspects of visual processing (e.g., accommodation and convergence, and feature search) are adult-like by 3–4 months, others (e.g., visual acuity and conjunction search) do not achieve adult levels for 1 year. Infants can detect depth by 2–3 months but do not fear depth until they begin to crawl. They can also recognize faces of live adults after 24 hours by 6 weeks of age and can recognize them from photos by 5 months.
Observers often remark that infants blossom into ‘social creatures’ between 2 and 4 months of age, cooing and smiling widely at individuals who interact with them, reaching to other persons, and intensely watching the behavior of others. By their fourth month, infants follow conversations between adults with their eyes, diﬀerentiate social from nonsocial objects (which they previously did not), and imitate vocalizations. Not surprisingly, social stimuli are highly eﬀective in establishing and maintaining behavior at this time. The features that deﬁne a stimulus as social, however, change with age. Although newborns diﬀerentiate between facial expressions and prefer to look at faces than at other visual stimuli, this preference is not based on a social dimension but on contrast and movement—critical features of the human face which inﬂuence infants’ scanning patterns over the ﬁrst 8 weeks. By 4 weeks of age, however, infants display greater pupillary dilation to social than nonsocial stimuli; by 6 weeks, they display diﬀerential cardiac responses to mothers and strangers; and by 12 weeks, they display diﬀerential pupillary responses to them and smile more frequently at the mother than at a stranger.
2.2 Six To 12 Months
During the second half-year, a normal infant becomes a walker, a speaker, and a social being who responds emotionally to parents and diﬀerentially to familiar persons and strangers. The simultaneous appearance of strong attachment to caregivers and wariness of unfamiliar individuals at 9–10 months of age is one of the most consistent benchmarks of this period (Ainsworth 1979). At the same time, infants begin to look at the target of a pointing ﬁnger rather than at the ﬁnger per se and begin to exhibit social referencing—a strategy by which infants assess the emotional valence of a new situation or person by checking how the caregiver has responded and responding in kind.
For a half-century, another developmental benchmark was attainment at 10 months of object permanence—the concept that an unseen object or person continues to exist. In the classic object search task, infants younger than 10 months consistently failed to retrieve an object they had just watched an adult hide. New paradigms using looking instead of reaching measures, however, have now revealed that even 3month-olds can locate where an object was hidden.
The types of learning of which infants are capable and their understanding of the physical world rapidly expands over the last half-year. Infants can individuate objects, solve problems requiring causal reasoning, form learning sets (‘learning how to learn’), comprehend some spoken speech, repeat a couple of syllables and perhaps produce a word, plan and execute a detour, and track objects through occlusion—a ﬁnding that has fueled heated debate over whether infants can count. Before infants begin to crawl (7–9 months), they have already learned what happens in what places. Once infants can locomote, they begin to form a cognitive map or representation of the spatial relationships between these places, which facilitates navigation from one place to another.
2.3 Twelve To 24 Months
Over this period, infants’ linguistic, motoric, and cognitive skills continue to improve and broaden. Between 12 and 18 months of age, for example, infants can use landmarks to locate objects instead of using their own body as a reference point, and between 18 and 24 months, they can use geometric cues in the environment to reorient. Although 18 months was set as the minimum age for deferred imitation of actions on an object (Piaget 1962), the use of tasks particularly suited to younger infants has lowered this age to 6 months. Still, infants’ ability to imitate televised demonstrations lags well behind their ability to imitate live demonstrations. The same live demonstration that 6-month-olds can imitate after 24 hours cannot be imitated from television before 18 months, although 15-month-olds can imitate some televised actions after 24 hours if the viewing conditions are more naturalistic. Generally speaking, the pervasiveness of reports that infants’ cognitive and motoric performance is superior in natural settings calls into question most if not all minimum age limits that have been imposed by laboratory tests.
Within the last ﬁve years, research on the ontogeny of memory has revealed that retention increases linearly over the infancy period, and reminders protract retention even longer. At all ages, a single reactivation or reinstatement reminder, for example, doubles the life of a memory—a remarkable result given that memories of older infants are initially so long. The ﬁnding that a few reinstatements can maintain a memory acquired at 6 months through 2 years of age reveals that early experiences can be enduring and raises new questions about infantile amnesia (adults’ inability to recollect experiences from early child-hood). The two memory systems (declarative, non- declarative) found in older children and normal adults are also functional by 3 months (Rovee-Collier et al. 2001).
3. Risk Factors And Individual Diﬀerences
In recent years, advances in the successful delivery and survival of very small and premature babies, often born in jeopardy, have fueled new debate over the importance of early experience. Because the lower threshold for survival is very new, it is inevitable that time must pass before the numerous longitudinal studies, now under way, will yield critical information about the relation between speciﬁc prenatal deﬁcits and neonatal hazards and particular developmental outcomes. Such investigations are unlikely to rule out the possibility that some aﬀected infants will be indistinguishable from their age-matched peers in developmental competencies. It is a question of proportions and resiliency.
Although infants born 38 weeks after conception are, on average, less mature than those born at 40 weeks are, the level of physiological maturity can vary among infants born at 40 weeks by as much as 3–4 weeks. Sex diﬀerences are also present at birth, with girls being 1–2 weeks ahead of boys in neurological maturation. In normal infants, some of this variability disappears by the end of the ﬁrst year, but many individual diﬀerences persist, the full range and signiﬁcance of which are not yet fully appreciated.
Early attempts to predict later intellectual performance from infant behavior focused on sensory and motor abilities and was unsuccessful. Modest-to-good correlations have now been found between measures of information processing beginning at 5–6 months (e.g., habituation rate, novelty preference, 1-week retention ratios) and scores on various intelligence tests at 3–5 years. The Fagan Test of Infant Intelligence (Fagan and Detterman 1992), which correlates later intelligence with infant novelty preference scores, is sensitive to early exposure to environmental toxins, substances of abuse, and nutritional deﬁcits and supplements, as well as to maternal HIV infection, intraventricular hemorrhage, bronchopulmonary dysplasia, genetic anomalies, and various neurological abnormalities.
4. Past Trends And Future Trajectory
The origins of research on infant development can be traced to baby diaries published by Darwin and Preyer in the late nineteenth century. Over the ﬁrst half of the twentieth century, detailed diaries of their own children’s behavior stimulated Morgan, Stern, Guillaume, Valentine, and Piaget to propose theoretical accounts of early cognitive development (Wallace et al. 1994). At the same time, Pavlov’s seminal studies spawned longitudinal studies of classical conditioning with infants in Russia and other Eastern-block countries. These early studies provided the ﬁrst systematic data on infant learning and documented the sequential onset of sensory function from the age at which stimuli from diﬀerent modalities became eﬀective conditioned stimuli. During this period, Western researchers produced only scattered reports of infant conditioning and retention, emotional development, activity, and color vision.
In 1958, federal funding of a long-term prospective study of infant development ushered in the era of modern infancy research. Using sophisticated apparatus and innovative methods, researchers at 12 sites across the US began to systematically document the infant’s basic sensory and learning abilities. These initial inquiries sparked subsequent generations of research on all aspects of perceptual, cognitive, and behavioral development across the entire span of infancy. By the end of the twentieth century, a large amount of developmental data was amassed on infant attention, perception, cognition, motor skills, and the sequellae of many early risk conditions.
The twenty-ﬁrst century will witness intensive research that elaborates this knowledge and challenges other long-held beliefs about infant development. Also in this century, advances in cognitive and behavioral neuroscience should facilitate attempts to relate developmental changes in infant cognition to speciﬁc changes in the developing brain. This eﬀort will require the development of new, noninvasive techniques and the adaptation of current ones (e.g., functional magnetic resonance imaging, positron emission topography) for use with very young infants. Finally, methodological advances should allow new insights into prenatal development and its contribution to postnatal development.
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