Parkinson’s disease disrupts both automatic and controlled processing of action verbs

► We investigated the role of the motor system in action-related language processing. ► PD patients and controls performed lexical decision and semantic judgment tasks. ► The words were either action verbs or abstract verbs. ► PD patients were more impaired in action verb processing in both tasks. ► The motor system plays a causal role in action language semantics. The problem of how word meaning is processed in the brain has been a topic of intense investigation in cognitive neuroscience. While considerable correlational evidence exists for the involvement of sensory-motor systems in conceptual processing, it is still unclear whether they play a causal role. We investigated this issue by comparing the performance of patients with Parkinson’s disease (PD) with that of age-matched controls when processing action and abstract verbs. To examine the effects of task demands, we used tasks in which semantic demands were either implicit (lexical decision and priming) or explicit (semantic similarity judgment). In both tasks, PD patients’ performance was selectively impaired for action verbs (relative to controls), indicating that the motor system plays a more central role in the processing of action verbs than in the processing of abstract verbs. These results argue for a causal role of sensory-motor systems in semantic processing.