Brain histochemical studies have shown that during early and mid-childhood that there is a proliferation of synapses, particularly in cortical areas [60-62]. This process of synaptognesis, depending on the species, may last for months or years and is then followed by a period of selective elimination (“pruning”), during adolescence, which is partly experience-dependent (see [63-66]). During this same period a chronologic sequence of myelination of axons occurs with the prefrontal and association cortices being the last to develop [62]. It has been suggested that subcortical circuitry involving such structures as the ventral striatum, insula, and extended amygdala, that are involved in motivation and reward, may develop early in adolescence relative to frontal cortical circuits that sub-serve executive cognitive control functions, thus leading to the predisposition to behavioral disinhibition and risk-taking often seen in adolescence (see [67-68] for review).
