The neurobiological mechanisms contributing to FA increases and MD decreases during adolescence are not entirely understood, but examination of underlying diffusion dynamics point to some probable processes. For example, decreases in radial diffusivity (RD), diffusion that occurs perpendicular to white matter pathways, suggests increased myelination, axonal density, and fiber compactness (Giorgio et al. 2008; Snook et al. 2005), but have not been uniformly observed to occur during adolescence. Similarly, changes in axial diffusivity (AD), diffusion parallel to the fibers’ principle axis, show discrepant trends, with some studies documenting decreases (Eluvathingal et al. 2007; Lebel et al. 2008; Suzuki et al. 2003), and others increases in this index (Ashtari et al. 2007; Giorgio et al. 2010). Decreases in AD may be attributable to developing axon collaterals, whereas increases may reflect growth in axon diameter, processes which are both likely to occur during adolescence. Technical and demographic differences such as imaging parameters, inter-scan intervals, age range, and gender ratios may account for divergent findings.
