Impaired comprehension of raising-to-subject constructions in Parkinson's disease

This paper describes an experiment which shows that roughly half of nondemented patients with Parkinson's disease (PD) have impaired comprehension of subject-to-subject and object-to-subject raising constructions (e.g., Susan seems to Bill to be tall and Susan is hard for Bill to catch), but have normal comprehension of the counterpart constructions (e.g., It seems to Bill that Susan is tall and It's hard for Bill to catch Susan). Several possible explanations for this pattern of performance are considered, including a parsing disorder, a syntactic-semantic linking disorder, a reduction of working memory capacity, slowed speed of syntactic processing, and difficulty with the experimental task. Although some of these explanations are arguably more plausible than others, the exact nature of the comprehension impairment remains unclear.