Primary progressive aphasia and the evolving neurology of the language network

Primary progressive aphasia (PPA) is caused by asymmetric, selective neurodegeneration of cerebral areas involved in language. Agrammatic and semantic PPAs are typically manifestations of frontotemporal lobar degeneration, whereas the logopenic PPA is more often associated with Alzheimer disease pathology. Here, Mesulam et al. review the subclassification, clinical features and neuropathology of PPA, and discuss how increased knowledge of PPA has advanced our understanding of the neural substrates of the language network.