Neurodegenerative disorders of language and speech: Non-language-dominant diseases

Abstract

Speech and language networks span vast cortical and subcortical regions in the human brain, including putative perisylvian areas but extending well beyond them. Accordingly, relevant functions can be compromised by diverse neural disruptions even in non-language-dominant neurodegenerative diseases (nldNDs) i.e., conditions that are not primarily typified by speech or language dysfunction. The present chapter offers a systematic overview of this topic, focusing on the three most prevalent nldNDs (Alzheimers disease, Parkinsons disease, and behavioral variant frontotemporal dementia). In each case, we offer detailed descriptions of spared and impaired skills across speech/language levels (phonetics, phonology, lexicosemantics, morphosyntax, discourse-level processing) and multidimensional accounts of their core neural signatures (including patterns of brain atrophy and tractographic abnormalities as well as alterations in regional activation, functional connectivity, event-related potentials, and oscillatory modulations). Next, we offer a brief contrastive summary of the core patterns in each disease. Finally, we address the main implications of the evidence and the prime challenges facing the field in the immediate future. Briefly, this chapter provides a fine-grained, multimodal, transnosological view of speech and language disruptions beyond the diseases typically associated with them.