Differential vulnerability of anterior white matter in nondemented aging with minimal acceleration in dementia of the Alzheimer type: evidence from diffusion tensor imaging.

White matter microstructural integrity was assessed using diffusion tensor imaging (DTI) in 25 young adults, 25 nondemented older adults, and 25 age-matched older adults with dementia of the Alzheimer type (DAT). For each individual, measures of anisotropy and diffusivity were obtained from atlas-transformed images in the anterior and posterior callosum and in the frontal, parietal, temporal and occipital white matter. These data revealed age differences in anisotropy and diffusivity in all assessed regions. Age effects were greater in the anterior as opposed to the posterior corpus callosum and greater in the frontal white matter than in the temporal, parietal and occipital white matter, suggesting age-associated differences in white matter that exhibit a roughly anterior-to-posterior gradient. In contrast, individuals with early-stage dementia exhibited minimal, if any, additional change in anterior regions but did show greater deterioration of white matter in posterior lobar regions. Taken collectively, these results indicate that nondemented aging is characterized by significant changes in white matter most prominently in anterior brain regions. The dissociation between the regional effects of age and dementia status suggests that the mechanisms underlying age-associated cognitive decline are likely distinct from those underlying DAT.