The mechanisms underlying cortical volume and thickness decline are suggested to involve selective synaptic pruning of superfluous neuronal connections, reduction in glial cells, decrease in neuropil and intra-cortical myelination (Huttenlocher and Dabholkar 1997; Paus et al. 2008; Shaw et al. 2008; Tamnes et al. 2009). Regional variations in grey matter maturation may coincide with different patterns of cortical development, with allocortex, including the piriform area, showing primarily linear growth patterns, compared to transition cortex (orbitofrontal, insular, cingulate, entorhinal, and perirhinal regions) demonstrating a combination of linear and quadratic trajectories, and isocortex (medial and lateral prefrontal, precentral motor, somatosensory, lateral temporal, and lateral occipital regions) demonstrating cubic growth curves (Shaw et al. 2008). Though the functional implications of these developmental trajectories are unclear, isocortical regions undergo more protracted development and support complex behavioral functions. Their growth curves may reflect critical periods for development of cognitive skills as well as windows of vulnerability for neurotoxic exposure or other developmental perturbations.
