MAPT and Mental deterioration: Using bootstrapped mediation analyses, we found that higher rates of global tau-PET accumulation mediated the effect of lower global florbetapir z scores in the WM on the rate of change in memory (ADNI-MEM: β=0.063 [95% CI: 0.01, 0.133], p=0.014, proportion mediated = 33.8%; Fig. 2C) and global cognition (ADAS13: β=−0.095 [95% CI: −0.180, −0.015], p=0.026, proportion mediated = 63.8%; Fig. 2D), suggesting that the effect of myelin on tau explains the association between demyelination and faster cognitive decline.