The cumulative risk of experiencing some form of central nervous system (CNS) insult increases with chronological age, even though a CNS insult may be encountered at any point in the lifespan. However, the risk of becoming cognitively impaired after a CNS insult is not the same as the risk of encountering it; the difference lies in the buffering effect of two reserves: a biological reserve, which involves plasticity, and a cognitive reserve, which involves intelligence. Biological reserve refers to the CNS tissue available to support adaptive change, or plasticity, in response to normal and abnormal events occurring throughout life. Some brain regions show little plasticity at any age, such as the frontal eye fields, the colliculi, and parts of the cerebellum, so that early and late brain lesions in these regions show similar, immediate, longlasting functional deficits (Dennis, Hetherington, Spiegler, & Barnes, 1999).
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