Sentence comprehension in Parkinson's disease: the role of attention and memory

Sentence comprehension is a complex process involving at least attentional, memory, grammatical, and semantic components. We report three experiments designed to evaluate the impairments underlying sentence comprehension difficulties in nondemented patients with Parkinson's disease (PD). In the first experiment, we asked patients to answer simple questions about sentences which varied in terms of grammatical complexity and semantic constraint. We found that PD patients are significantly compromised in their ability to perform this task. Their difficulties became more prominent as grammatical complexity increased, but they were significantly assisted by semantic constraints that limited possible interpretations of a sentence. Analyses of individual patient profiles revealed heterogeneous performance across the group of PD patients and somewhat inconsistent performance for patients across testing sessions. In the second experiment, we tested the possibility that patients' heterogeneous performance on the sentence comprehension task is due to an impairment in memory or attention, cognitive domains known to be compromised in some PD patients. Although PD patients and control subjects differed on one memory measure, there were no significant correlations between attention and memory performance and the results of the sentence comprehension task. In the final experiment, we manipulated the sentences used in the first experiment in a fashion that stressed the need for memory and attention in a sentence. The results indicated that PD patients are significantly compromised in their ability to attend to certain critical grammatical features of a sentence. A regression analysis identified specific grammatical, semantic, and attentional mechanisms as significant contributors to PD patients' overall sentence comprehension, accounting for over 97% of the variance in their performance. We conclude that there are multiple sources of cognitive difficulty underlying PD patients' sentence comprehension impairment.