Global and tract-specific differences between younger and older adults in DTI measures of white matter integrity

Prior research utilizing diffusion tensor imaging (DTI) to examine cerebral white matter microstructural integrity among adults has established that increasing age is associated with poorer white matter health. While age effects on DTI measures of white matter integrity have been shown to vary in strength across different white matter tracts, tract-specific effects may be secondary to a global impact of age on white matter health. Furthermore, this global age effect could result in "homogenizing" increases in shared variance across tracts. The present study compared DTI measures in 36 white matter tracts between 71 younger adults (ages 18-37) and 129 older adults (ages 52-82), to (1) determine whether shared variance across white matter tracts increases with age, and (2) examine tract-specific variability in age-related alterations to white matter integrity. Diffusion weighted images were processed using probabilistic tractography in order to reconstruct callosal, association, and radiation tracts, from which average measures of fractional anisotropy (FA), mean diffusivity (MD), radial diffusivity (RD) and axial diffusivity (AD) were derived. In comparing inter-tract correlation matrices for each DTI measure between age groups, we found stronger inter-tract correlations for older adults relative to younger adults overall. Additionally, general factors for FA, RD and AD, derived from separate factor analyses, accounted for greater proportions of shared variance across tracts among older adults compared to younger adults. For MD, however, the amount of shared variance captured by the general factor was similar between age groups. Older adults exhibited lower FA and higher MD and RD values compared to younger adults for the majority of tracts examined, although the strength of the age effect differed across tracts. Age group differences in AD were more variable. The present findings provide support for the notion that aging exerts a global, homogenizing impact on white matter integrity, alongside tract-specific age effects.