Contribution of language studies to the understanding of cognitive impairment and its progression over time in Parkinson's disease

Parkinson's disease is a frequent neurodegenerative disease that is mostly known for its motor symptoms. However, cognitive impairment is now recognised as an important part of the disease. Studies of cognitive impairment in Parkinson's disease reveal considerable heterogeneity in terms of which cognitive domains are impaired, and of how cognitive impairment progresses over time. In parallel, a growing body of research reports language difficulties in Parkinson's disease, more specifically in the domains of sentence processing and lexical semantic processing. In this review, the performance of patients with Parkinson's disease in these domains of language will be reviewed with a focus on the links that they have with the rest of cognition and on how they could contribute to the earlier and more precise characterization and prediction of cognitive impairment in Parkinson's disease. More specifically, the potential for modulation of complexity and sensitivity of language tasks to mild deficits and difficulties that are predictive of further decline will be emphasized. Other motivations for studying language difficulties in this disease will also be discussed.