Gaps remain in our understanding of GM development despite the advances in finer-scale GM mapping techniques. Current MRI resolution still cannot establish the exact cellular processes underlying the GM loss. GM density on MRI is an indirect measure of a complex architecture of glia, vasculature, and neurons with dendritic and synaptic processes, and the primary cause for loss of GM density remains unknown. It may be driven by the process of synaptic overproduction followed by pruning (P. R. Huttenlocher, 1979) together with trophic glial and vascular changes and/or cell shrinkage (Morrison & Hof, 1997). This maturational synaptic pruning was also suggested by earlier studies of age-related changes in sleep physiology and EEG coherence studies (Feinberg, 1982; Feinberg, Higgins, Khaw, & Campbell, 2006; Whitford et al., 2007). The regional differences in GM maturation may thus result from the plasticity-dependent synaptic pruning in the cortex, as has been found in primate and human cerebral cortical development (Arango et al., 2008; Bourgeois, 1997; Bourgeois et al., 1994; P. R. Huttenlocher & Dabholkar, 1997; Rakic, Bourgeois, & Goldman-Rakic, 1994; Rapoport & Gogtay, 2008; Zecevic, Bourgeois, & Rakic, 1989), although this argument remains debatable (T. Paus, Keshavan, & Giedd, 2008).
