MAPT and Schnyder corneal dystrophy: Neuroimaging studies have shown that early AD is marked by neuronal hyperactivation detectable via fMRI, even before overt clinical symptoms, including among those with SCD, and those with in vivo AD pathology.42,43,44,45,46,47 These results underscore the potential utility of plasma ptau-181 as a marker of early tau-related pathology in individuals reporting subjective memory decline.