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Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001.

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Neuroscience. 2nd edition.

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Critical Periods

The cellular and molecular mechanisms outlined in Chapters 22 and 23 construct a nervous system of impressive anatomical complexity. These instructions are also sufficient to create some remarkably sophisticated innate or “instinctual” behaviors (see Box A in Chapter 31). For most animals, the behavioral repertoire, including foraging, fighting, and mating strategies, relies on patterns of connectivity established by intrinsic developmental mechanisms. However, the nervous systems of complex (“higher”) animals, including humans, clearly adapt to and are influenced by the particular circumstances of an individual's environment. These environmental factors are especially influential in early life, during temporal windows called critical periods. In some cases, such as the acquisition of language, instructive influences from the environment are obviously required for the normal development of the behavior (i.e., exposure to the individual's native language). Moreover, some behaviors, such as imprinting in birds (Box A), are expressed only if animals have certain specific experiences during a sharply restricted time in early postnatal (or posthatching) development. On the other hand, critical periods for sensory and motor skills, or complex behaviors such as human language, are longer and much less well delimited.

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Box A

Built-in Behaviors.

Despite the fact that critical periods vary widely in both the behaviors affected and their duration, they all share some basic properties. A critical period is defined as the time during which a given behavior is especially susceptible to, and indeed requires, specific environmental influences to develop normally. Once this period ends, the behavior is largely unaffected by subsequent experience (or even by the complete absence of the relevant experience). Conversely, failure to be exposed to appropriate stimuli during the critical period is difficult or impossible to remedy subsequently.

While psychologists and ethologists (that is, biologists who study the natural behavior of animals) have long recognized that early postnatal or posthatching life is a period of special sensitivity to environmental influences, their studies of critical periods focused on behavior. Work in the latter part of the twentieth century has increasingly examined the underlying changes in the relevant brain circuits and their mechanisms.

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2001, Sinauer Associates, Inc.
Bookshelf ID: NBK11020

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