Effects of working memory capacity on inference generation during story comprehension in adults with Parkinson's disease

A group of non-demented adults with Parkinson's disease (PD) were studied to investigate how PD affects pragmatic-language processing, and, specifically, to test the hypothesis that the ability to draw inferences from discourse in PD is critically tied to the underlying working memory (WM) capacity of individual patients [Monetta, L., & Pell, M. D. (2007). Effects of verbal working memory deficits on metaphor comprehension in patients with Parkinson's disease. Brain and Language, 101, 80–89]. Thirteen PD patients and a matched group of 16 healthy control (HC) participants performed the Discourse Comprehension Test [Brookshire, R. H., & Nicholas, L. E. (1993). Discourse comprehension test. Tucson, AZ: Communication Skill Builders], a standardized test which evaluates the ability to generate inferences based on explicit or implied information relating to main ideas or details presented in short stories. Initial analyses revealed that the PD group as a whole was significantly less accurate than the HC group when comprehension questions pertained to implied as opposed to explicit information in the stories, consistent with previous findings [Murray, L. L., & Stout, J. C. (1999). Discourse comprehension in Huntington's and Parkinson's diseases. American Journal of Speech–Language Pathology, 8, 137–148]. However, subsequent analyses showed that only a subgroup of PD patients with WM deficits, and not PD patients with WM capacity within the control group range, were significantly impaired for drawing inferences (especially predictive inferences about implied details in the stories) when compared to the control group. These results build on a growing body of literature, which demonstrates that compromise of frontal–striatal systems and subsequent reductions in processing/WM capacity in PD are a major source of pragmatic-language deficits in many PD patients.