Physical and Sensorimotor Development in Infancy and Childhood
The immaturity of the human infant is apparent if we examine the brain itself. Figure 14.5 shows sections of the human cortex in a newborn, a 3-month-old, and a 15-month-old child (Conel, 1939, 1947, 1955). Notice that the number of neural interconnections
grows tremendously during this period. Indeed, by one estimate, new neural connec-tions are formed in the infant’s brain, and old ones removed, at an astonishing rate of 100,000 per second (Rakic, 1995).
This growth in brain size and complexity, and, indeed, growth in all aspects of the child’s body, continues for many years, coming in spurts that each last a few months. This pattern is obvious for growth of the body (Hermanussen, 1998) but is also true for the child’s brain, with the spurts typically beginning around ages 2, 6, 10, and 14 (H. T. Epstein, 1978). Each of these spurts leaves the brain up to 10% heavier than it was when the spurt began (Kolb & Whishaw, 2009).
Even at birth, the infant’s immature brain is ready to support many activities. For exam-ple, infants’ senses function quite well from the start. Infants can discriminate between tones of different pitch and loudness, and they show an early preference for their mother’s voice over that of a strange female (Aslin, 1987; DeCasper & Fifer, 1980). Newborns’ vision is not yet fully developed. Though quite near-sighted, newborns can see objects a foot or so away (the distance to a nursing mother’s face) and readily discriminate brightness and color, and they can track a moving stimulus with their eyes (Aslin, 1987; Bornstein, 1985).
In contrast to newborns’ relatively advanced sensory capacities, infants initially have little ability to control their body movements. They thrash around awkwardly and can-not hold up their heads. But they do have a number of important reflexes (Figure 14.6), including the grasp reflex—when an object touches an infant’s palm, she closes her fist tightly around it. If the object is lifted up, the infant hangs on and is lifted along with it, supporting her whole weight for a minute or more.
Other infantile reflexes pertain to feeding. For example, the rooting reflex refers to the fact that when a baby’s cheek is lightly touched, the baby’s head turns toward the source of stimulation, his mouth opens, and his head continues to turn until the stim-ulus (usually a finger or nipple) is in his mouth. A related reflex, called the suckingreflex, then takes over and leads the child to suck automatically on whatever is placedin his mouth.
By her first birthday, the child will have gained vastly better control over her own actions, and this increase in skill continues over the next decade. These skills emerge in an orderly progression, as shown in Figure 14.7. Infants first learn to roll over (typically by 3 months), then to sit (usually by 6 months), then to creep and then to walk (usually by 12 months). However, the ages associated with these various achievements must be understood as rough approximations, because there is variation from one infant to another and from one culture to another. For example, the Kipsigis of Kenya begin training their infants to stand and walk early on (Super, 1976), whereas the Ache of Paraguay carry their children nearly everywhere, leading to delayed onset of walking (Kaplan & Dove, 1987).
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