Investigating the basis for the sentence comprehension deficit in Parkinson's disease

We sought to determine whether patients with idiopathic Parkinson's disease (PD) have sentence comprehension impairments because of a compromised grammatical parser or difficulty attending to subtle grammatical features of sentences. We assessed syntactic-thematic linking rules in sentences by asking 25 non-demented PD patients to judge whether a transitive "target" sentence entails an intransitive "probe" sentence (e.g. "The girl drowned the swimmer" → The swimmer drowned). Sentences contained either simple transitive verbs that map syntactic roles onto thematic roles in a canonical fashion, or lexical causative verbs that have an atypical syntactic-thematic linking rule that is not explicitly signaled. These verbs were set in either an active voice sentence that maps syntactic roles onto thematic roles canonically or a passive voice sentence that remaps syntactic roles onto thematic roles in a fashion that is signaled explicitly by surface sentence features. PD patients were significantly impaired on this task. This was due to their difficulty understanding sentences with lexical causative verbs, a deficit that was evident in 60% of individual PD patients. The passive voice did not interfere with sentence comprehension. In order to confirm that the deficit was due to the subtle nature of the syntactic-thematic linking rule rather than a compromised grammatical parser, we presented sentences in the periphrastic voice that is grammatically demanding but explicitly states the thematic roles associated with verbs. PD patients' comprehension of sentences with lexical causative verbs improved when cast in a periphrastic voice. We conclude that sentence processing impairments in PD are due in part to a compromised executive system that subserves attending to subtle attributes of sentence structure.