of the cortex on T1-weighted MRI, making it more likely to be classified as white matter. If so, the cortical changes may still reflect dendritic pruning or increased myelination. In TBM, although the gray matter is not explicitly segmented, the matching of features in the scans is based on the intensity information, although it is not assumed that corresponding features will have similar intensities. Even so, TBM provides some auxiliary information, in that it maps the growth rates for different tissue types. In Gogtay et al.(Gogtay et al., 2008) and Hua et al. (Hua et al., 2009), significant growth rates were detected for the white matter underlying the cortex, making it more plausible that ongoing myelination contributed to some of the gray matter thinning reported in prior studies. Initially, the time-lapse maps of gray matter loss were established using surface-based modeling methods that examined only the cortex (P. M. Thompson et al., 2001), but subsequently TBM was used to map growth rates in the whole brain, implicating white matter encroachment as one contributor to the observed cortical changes (Gogtay et al., 2008).
