Gestural communication in parkinson's disease: language, action and cognition

Parkinson’s disease (PD) is a neurodegenerative condition which results in severe motor impairment. Deterioration in multiple domains of cognition is another hallmark of PD. Together, these motor and cognitive impairments impact substantially on language and communication. Co-speech gestures are a form of action and are also part of linguistic processes, yet have rarely been explored in PD. Gestures can provide imagistic depictions of concepts described by speech and contribute to communication in healthy individuals. They rely on visual, spatial, and motor simulations and imagery, which may be impaired in PD. It is therefore of clinical importance to evaluate how co-speech gestures might be impaired to understand the extent of communicative impairment in PD. PD can also provide a useful model to understanding the cognitive basis of co-speech gesture in healthy people. In Chapter 2, participants described isolated actions. Gesture rate did not differ between the two groups, however, the groups differed in terms of the visual perspective they adopted when depicting actions in gesture. Controls preferred a “character viewpoint” or first-person perspective where their hands represented the hands of the actor, whereas PD patients preferred an “observer viewpoint” or third-person perspective, where their hand represented a whole person. This finding was replicated and extended in Chapter 3 where low-motion and high-motion actions were described in a longer narrative task. PD patients produced fewer character viewpoint gestures when describing high-motion action events, suggesting a difficulty in simulating these events from a first-person perspective. In addition, PD patients had difficult depicting “manner” (how an action is performed) features in gesture during high but not low motion. Extending the findings of Chapter 2, whilst overall rate of gesture production was not affected, PD patients produced action gestures at a significantly lower rate than controls. Chapter 4 took a different focus by investigating gesture depictions of static spatial (rather than dynamic action) features via a house description task. Gesture rate did not differ, but the groups depicted different types of spatial properties to a different extent. Whilst both groups predominantly gestured about location and relative position information, PD patients gestured more about directions whereas controls gestures more about shape and size information. This suggests that different strategies were being employed by the two groups. Finally, testing young adults’ comprehension of these spatial gestures in Chapter 5 revealed that gestures did not significantly improve comprehension of either PD patients’ or controls’ spoken messages, though there may have been ceiling effects. However, both PD patients and controls were viewed as more competent when their messages were viewed with gestures. The findings suggest a selective action-gesture deficit in PD which complements work demonstrating action-verb impairments in these patients, and supports gesture production theories which hypothesise a role for motor simulations and imagery. Overall gesture rate appears to be largely unaffected. The effects of PD can be felt beyond changes to goal-directed action, in the realms of language and social behaviour, but gestures may be able to improve listeners’ social perceptions of PD patients.