The roles of sequencing and verbal working memory in sentence comprehension deWcits in Parkinson’s disease

Studies of sentence comprehension deWcits in Parkinson’s disease (PD) patients suggest that language processing involves circuits connecting subcortical and cortical regions. Anatomically segregated neural circuits appear to support diVerent cognitive and motor functions. To investigate which functions are implicated in PD comprehension deWcits, we tested comprehension, verbal working memory span, and cognitive set-switching in a non-linguistic task in 41 PD patients; we also obtained speech measurements reXecting motor sequencing processes that may be involved in articulatory rehearsal within working memory. Comprehension of sentences with centerembedded or Wnal relative clauses was impaired when they could not be understood from lexical semantic content alone. Overall comprehension error rates correlated strongly with impaired set-switching and signiWcantly with reduced working memory span and speech motor sequencing deWcits. Correlations with comprehension of diVerent sentence structures indicate that these impairments do not represent a single deWcit; rather, PD comprehension deWcits appear to arise from several independent mechanisms. DeWcits in cognitive setswitching or underlying inhibitory processes may compromise the ability to process relative clauses. DeWcits in verbal working memory appear to impair comprehension of long-distance dependencies. Speech sequencing correlated with neither set-switching nor verbal working memory span, consistent with their being supported by independent, segregated cortico-subcortical circuits. © 2005 Elsevier Inc. All rights reserved.