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Adolescent-Typical Behavior Patterns 13and hence eliminated synapses presumably were functional prior to their removal.The probability of a synapse being pruned during adolescence, however, varieswith type of synapse and its circuitry. For instance, excitatory input to the neocortexis particularly targeted (Bourgeois et al., 1994) and pruning is more pronouncedin prefrontal cortex (PFC) and other neocortical regions than in subcorticalareas (Rakic et al., 1994). Even within cortex, pruning more commonly involvescircuitry intrinsic to a particular cortical region than associational circuitry thatconnects different cortical regions (Woo et al., 1997). Circuitry intrinsic to particularregions of cortex is important for expression of reverberating circuits, andpruning within these intrinsic circuits has been suggested to reflect a fine-tuningof neural connectivity in these cortical regions (see Woo et al., 1997). Thus, theevidence to date supports the conclusion that the often dramatic pruning of synapsesduring adolescence largely does not reflect elimination of nonfunctionalsynapses but rather involves targeted pruning that helps to sculpt the adolescentbrain into its mature form. Although empirical data linking particular aspects ofthis neuronal sculpting to specific functional consequences is limited, this adolescent-associatedsynaptic elimination has been suggested to be a form of developmentalplasticity by which the adolescent brain may be sculpted to match theenvironmental demands and opportunities that emerge during this transition (Rakicet al., 1994).A culling of excitatory cortical synapses during adolescence, along with a declinein synaptic connections supporting reverberating circuitry in certain cortexregions, could also support a reduction or refinement in brain effort at this time.Indeed, it is interesting that during adolescence in humans (see Chugani, 1996)and other species (e.g., rats: Tyler & van Harreveld, 1942), there is an overalldecline in brain energy utilization, with the high rates of glucose metabolism, bloodflow, or oxygen utilization seen during childhood gradually declining to reachthe more moderate utilization rates typical of the adult.Levels of oxygen utilization have been used to index neural activation, giventhat activity of neurons and their associated support cells (glia) requires oxygen.Blood oxygen level dependent (BOLD) imaging can be used to noninvasivelyidentify regions of enhanced oxygen utilization, and hence presumably enhancedneuronal activation, under various stimulus conditions. Developmental studiesusing functional magnetic resonance imaging (fMRI) have revealed ontogeneticchanges during the transition from childhood to adolescence in patterns of BOLDsignals during performance of cognitive tasks thought to index various (and sometimesoverlapping) components of executive function (e.g., attention, responseinhibition, working memory, and performance on delay tasks). In some instances,children have been reported to show activation in PFC that is less specific to thetype of information, as well as less efficient, with children recruiting prefrontalterritory more diffusely during performance on a working memory task than adolescents(Casey et al., 2000) or adults (Casey et al., 1998). In other studies,