Giving words a helping hand: the effects of gesture observation on verb retrieval in unimpaired speakers and in people with aphasia

Gesture and speech are intertwined in communication. However, debate remains regarding the nature of the links between them and the extent to which gestures may influence lexical retrieval (naming). In aphasia (an acquired language disorder following brain damage), it remains unclear whether gesture can specifically improve the effectiveness of treatment for naming impairments. This thesis investigated whether and how the observation of a particular meaningful gesture (i.e., pantomimes) influences verb retrieval in unimpaired speakers and people with aphasia. A combination of neuropsychological, electrophysiological and neuromodulation methodologies was used to explore what mechanisms could explain why gesturing helps lexical retrieval.  

Chapter 1 as an introduction, situates the thesis into the context of current theoretical perspectives on the links between action, gesture and language processing. The role of gesture in speech and language processing is addressed, with a focus on research findings regarding the relationship between pantomimed gestures and language processing, and research on the use of gesture in aphasia therapy.  

The study reported in Chapter 2 investigated the effects of pantomime observation on action naming in neurotypical speakers. Results showed that action naming was facilitated by the observation of a congruent pantomime relative to both an unrelated pantomime and a neutral stimulus. Moreover, priming effects were also found to be affected by gesture transparency and verb instrumentality. Possible cognitive mechanisms underpinning gesture and word processing were discussed.  

The study reported in Chapter 3 investigated the effects of gesture observation on action naming in people with aphasia. Positive effects were found at the group level from congruent gestures, but there was individual variability in the patterns found. The potential underlying mechanisms were examined by exploring participant- and item-related predictors of improvement. The results suggested that gesture observation seems more likely to facilitate action naming for individuals with spared conceptual knowledge of gestures and mild-to-moderate lexical-semantic or post-semantic impairments.  

Chapter 4 reports research investigating the time-course of neural activity associated with the effects of gesture observation on action naming, using magnetoencephalography. This study demonstrated that gesture observation affects the word production system relatively early and associated with the engagement of lexical processes.  

The last study, reported in Chapter 5, used transcranial magnetic stimulation (i.e., continuous theta burst stimulation) to examine the role of the primary motor cortex in mediating the behavioural effects of pantomimes on action naming. No evidence was found for a crucial involvement of this motor brain area in the mechanisms underlying gesture priming of action naming. In addition, this study informed the debate on the methodological challenges of using neuromodulation.  

Overall, this thesis has resulted in a better understanding of the mechanisms underlying the influence of pantomimes on lexical retrieval and provides guidance about how gestural-based therapies can be best used in aphasia treatment.