Representation of action in Parkinson's disease: imagining, observing, and naming actions.

People with Parkinson's disease (PD) exhibit slowed movements and difficulty in initiating movements. This review addresses the issue of whether or not cognitive representations of actions in PD are affected, alongside these motor problems. In healthy people, the motor system can be involved in tasks such as observing a graspable object or another person's action, or imagining and naming actions, in the absence of overt movement. As described in this review, the fact that the slowed real movements exhibited by PD patients are coupled with slower motor imagery and verb processing provides additional evidence for the involvement of the motor system in these processes. On the other hand, PD patients can still engage in motor imagery and action observation to some extent, which is encouraging for the use of these processes in rehabilitation. Findings across the different domains of action-representation reveal several important factors. First, the nature of action is critical: patients' performance in observation and naming tasks is influenced by whether or not the action is in their repertoire and by the extent of motion required to execute the action. Second, people with PD may use alternative or compensatory mechanisms to represent actions, such as relying more on a third-person perspective or a visual strategy. Third, people with PD show a lack of specificity, responding as strongly to stimuli related and unrelated to actions. Investigating action-representation in PD has implications for our understanding of both the symptoms of PD and the cognitive representation of actions in the healthy system.