MAPT and dementia: Neuropathological evidence suggests that dementias, whose onset is associated with brain atrophy as well as misfolded beta amyloid and tau protein accumulation in the grey matter, may be characterised by spreading along spared white matter tracts through prion‐like trans‐synaptic transmission of misfolded tau and beta amyloid (Brundin, Melki & Kopito, 2010; Raj et al., 2012; Clavaguera et al., 2015).