Impaired perception of human movements in Parkinson's disease.

Interacting with other individuals in a social world requires fast and accurate perception of other individuals' identity, actions, or intentions. Humans are very efficient in these social tasks, as they can extract social information even if the actor is represented only by a handful of point-lights on an otherwise invisible body. Theories have argued that efficient visual perception of actions is based on intact motor system functioning. The motor system provides visuo-motor action representations shaped by the observer's own movements or motor repertoire. If the observer's motor repertoire is impaired, this should lead to impaired visuo-motor representations and ultimately to impaired visual perception of movements. Here we tested this hypothesis in a behavioral study with patients suffering from Parkinson's disease (PD). PD patients are typically impaired in movement execution. We tested these patients and a matched control group in a visual discrimination task on human movement perception. The results showed that PD patients were significantly impaired in the perception of human movements. This impairment was most prominent for transitive (object-related) movements. The results indicate that impaired movement execution critically influences movement perception. The results support the hypothesis that the motor system plays a causal role for the visual perception of human movements.