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THE HUMAN NERVOUS SYSTEM IS FUNCTIONAL at birth—newborn babies can see, hear, breathe, and suckle. However, the capabilities of human infants are quite rudimentary compared to those of other species. Wildebeest calves can stand and run within minutes of birth, and many birds can fly shortly after they hatch from their eggs. In contrast, a human baby cannot lift its head until it is 2 months old, cannot bring food to its mouth until it is 6 months old, and cannot survive without parental care for a decade.

What accounts for the delayed maturation of our motor, perceptual, and cognitive abilities? One main factor is that the embryonic connectivity of the nervous system, discussed in Chapters 45 through 48, is only a “rough draft” of the neural circuits that exist in our adult selves. Embryonic circuits are refined by sensory stimulation—our experiences. This two-part sequence—genetically determined connectivity followed by experience-dependent reorganization—is a common feature of mammalian neural development, but in humans, the second phase is especially prolonged.

At first glance, this delay in human neural development might seem dysfunctional. It does exact a toll, but it also provides an advantage. Because our mental abilities are shaped largely by experience, we gain the ability to custom fit our nervous systems to our individual bodies and unique environments. It has been argued that it is not just the large size of the human brain but also its experience-dependent maturation that makes our mental capabilities superior to those of other species.

The plasticity of the nervous system in response to experience endures throughout life. Nevertheless, periods of heightened susceptibility to modification, known as sensitive periods, occur at particular times in development. In some cases, the adverse effects of deprivation or atypical experience during circumscribed periods in early life cannot easily be reversed by providing appropriate experience at a later age. Such periods are referred to as critical periods. As we shall see, new discoveries are blurring the distinction between sensitive and critical periods, so we will use the term “critical periods” to refer to both.

Behavioral observations have helped us appreciate critical periods. Imprinting, a form of learning in birds, is one of the most striking illustrations of a lifelong behavior established during a critical period. Just after hatching, birds become indelibly attached, or imprinted, to a prominent moving object in their environment and follow it around. This is typically their mother, but it could be an experimenter who is near the newborn chick. The process of imprinting is important for the protection of the hatchling. Although the attachment is acquired rapidly and persists, imprinting can only occur during a critical period soon after hatching—in some species, only a few hours.

In humans, critical periods are evident in the ways children acquire the capacities to perceive the world around them, learn a language, or form ...

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