Figure 1.

Schematic of a simple mediation model. We examined whether regional fractional anisotropy (FA) significantly mediated the effect of age on cognition. In a mediation model, the relationship between an independent variable (X) and a dependent variable (Y) is influenced by the (nonobservable) mediator variable (M). Intercepts and residuals for each equation are denoted by i and e, respectively. The total effect (c) is the sum of both the direct (c′) and mediated (ab) effects. The total effect, c, was determined with equation (1). Coefficients a and b were determined with equation (2) and equation (3), respectively. The direct effect (c′) was determined with equation (2). Mediation is determined by assessing the significance of the mediated effect (ab) with a bootstrapping approach.