Language dysfunction as a primary feature of cognitive decline in neurological populations

Cognitive decline is a common feature of neurologic conditions, with language functions often affected. Word finding difficulties are commonly reported to neurologists in clinic. Receptive language dysfunction (i.e., comprehension) tends to be more difficult to recognize for both the patient and the clinician. Subtle yet pervasive decrements in language may be a key feature (and potential driver) of pathological cognitive decline inherent to neurologic diseases involving a primary or secondary neurodegenerative process. While severe language impairment such as aphasia presenting in the context of stroke or dementia has been studied in detail, mild or insidious presentations remain relatively understudied. In this review, we evaluate neural substrates and clinical manifestations of language deficits noted in four neurologic populations: Alzheimer's disease (AD), stroke, multiple sclerosis (MS), and Parkinson's disease (PD). Despite differences in etiology and pathophysiology, these four neurologic populations each present with prominent language dysfunction. For each, we describe neuroanatomical substrates and networks underlying language dysfunction. We then describe current observations of language dysfunction in each population. We incorporate a discussion of emerging speech measurement tools employing machine learning (ML) and artificial intelligence (AI). Overall, we provide evidence to support a nascent hypothesis of language dysfunction as a potential driver of cognitive decline across neurologic populations with the aim of motivating novel research insights and informing clinical care.