Co-speech gestures are a window into the effects of Parkinson’s disease on action representations

Parkinson’s disease impairs motor function and cognition, which together affect language and communication. Co-speech gestures are a form of language-related actions that provide imagistic depictions of the speech content they accompany. Gestures rely on visual and motor imagery, but it is unknown whether gesture representations require the involvement of intact neural sensory and motor systems. We tested this hypothesis with a fine-grained analysis of co-speech action gestures in Parkinson’s disease. 37 people with Parkinson’s disease and 33 controls described two scenes featuring actions which varied in their inherent degree of bodily motion. In addition to the perspective of action gestures (gestural viewpoint/first- vs. third-person perspective), we analysed how Parkinson’s patients represent manner (how something/someone moves) and path information (where something/someone moves to) in gesture, depending on the degree of bodily motion involved in the action depicted. We replicated an earlier finding that people with Parkinson’s disease are less likely to gesture about actions from a first-person perspective – preferring instead to depict actions gesturally from a third-person perspective – and show that this effect is modulated by the degree of bodily motion in the actions being depicted. When describing high motion actions, the Parkinson’s group were specifically impaired in depicting manner information in gesture and their use of third-person path-only gestures was significantly increased. Gestures about low motion actions were relatively spared. These results inform our understanding of the neural and cognitive basis of gesture production by providing neuropsychological evidence that action gesture production relies on intact motor network function.