As we age, our brains change quite dramatically and shrinking in size and changing in composition. We conducted a study to examine one rather overlooked element of this changing composition, mainly the idea that myelin—the sheaths around our nerve cells keeping everything running smoothly—degenerate as we age. In order to study this we used diffusion tensor tractography to give us the possibility of isolating individual white matter tracts. The study yielded three main findings. First, age-related white matter deficits increased gradually from posterior to anterior segments within fiber tracts traversing frontal and parietal, but not temporal cortex. This pattern inverts the sequence of myelination during childhood and early development observed in previous studies and lends support to a “last-in-first-out” theory of the white matter health across the lifespan. Second, both the effects of aging on white matter and their impact on cognitive performance were stronger for measures thought to be sensitive to myelin integrity. Finally, the effects of aging on select white matter tracts were associated specific cognitive functions: frontal tracts were associated with executive functioning and posterior tracts were associated with visual memory. The results help to clarify how white matter decline impairs cognitive performance.
Davis, S. W., Dennis, N. A., Buchler, N. G., White, L. E., Madden, D. J., & Cabeza, R. (2009).Assessing the effects of age on long white matter tracts using diffusion tensor tractography.Neuroimage.