The thinning associated with cortical maturation appears to occur first in primary sensorimotor areas and latest in higher order association areas such as the dorsolateral prefrontal cortex, inferior parietal, and superior temporal gyrus. Postmortem studies suggest that part of the GM changes may be related to synaptic proliferation and pruning (Huttenlocher 1994). The connection between GM decreases and synaptic pruning is indirectly supported by an MRI/quantified EEG study of 138 healthy 10–30 year old subjects that found curvelinear reductions in frontal and parietal GM were matched by similar curvilinear reductions in EEG power of the corresponding regions (Whitford et al. 2006). As EEG power reflects synaptic activity, the temporally linked EEG power and GM changes suggests that the GM volume reductions are accompanied by reductions in the number of synapses.
