Normal Development of Cognition and the Brain

Cognition and the brain undergo changes across the lifespan from early childhood to late life (Bremner, 2005a). Understanding these normal developmental changes is critical for determining the difference between normal development and pathology, as well as how they interact.

Normal memory formation involves encoding, consolidation, and retrieval. Encoding refers to the laying down of the memory trace, consolidation is the process by which the memory goes from short-term to long-term storage, and retrieval is the process by which long-term memories are retrieved from storage (Schacter, 1996). Memories can be divided into explicit (also known as declarative), or available for conscious recall, and implicit (also known as procedural). Explicit memory includes recall of facts or lists, while implicit memory includes memory that is not accessed by conscious recall, such as procedural memories like riding a bike, as well as conditioned responses.

Children do not develop the capacity for long-term autobiographical memory until 2 to 3 years of age (Bruce et al., 2005; Eacott & Crawley, 1998; Howe & Courage, 1993, 1997; Usher & Neisser, 1993). This coincides with the development of the ability to place events in the context of the who, what, and where of the self. Children do have memories before the age of 2, as measured by a variety of laboratory tasks, although explicit memories are not retrieved before this time period in later life (Eacott & Crawley, 1998).

Although the bulk of brain development occurs in utero, the brain continues to develop after birth (Giedd, Shaw, Wallace, Gogtay, &

Lenroot, 2006). In the first 5 years of life there is an overall expansion of brain volume related to development of both gray matter and white matter structures; however, from 7 to 17 years of age there is a progressive increase in white matter (felt to be related to ongoing myelination) and decrease in gray matter (felt to be related to neuronal pruning), while overall brain size stays the same (Casey, Giedd, & Thomas, 2000; Durston et al., 2001; Giedd et al., 1999b; Paus et al., 1999). Gray matter areas that undergo the greatest increases throughout the 7-to-17-years period of development include frontal cortex and parietal cortex (Rapoport et al., 1999; Sowell et al., 1999). Basal ganglia decrease in size, while corpus callosum (Giedd, Blumenthal, & Jeffries, 1999a; Thompson et al., 2000), hippocampus, and amygdala (Giedd, Castellanos, Rajapakse, Vaituzis, & Rapoport, 1997; Giedd et al., 1996b; Pfefferbaum et al., 1994) increase in size during early childhood, although there may be developmental-sex-laterality effects for some of these structures (Giedd et al., 1996a).

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