Thesis title: Modulating the Action Observation Network through transcranial magnetic stimulation: causal insights into automatic imitation phenomenon
Humans have a natural tendency to resonate with the actions and emotions of others. Since the late 1990s, cognitive neuroscience has investigated this phenomenon using neuroimaging techniques, revealing an extensive fronto-parietal network involved in action observation, execution, and imitation, known as the Action Observation Network (AON). The present thesis employs various transcranial magnetic stimulation (TMS) techniques to investigate the distinct contributions of AON regions to automatic imitation—the spontaneous tendency to reproduce observed actions, even when they are irrelevant to the current task. In the first experiment, it was verified that applying the same interstimulus interval (ISI) during cortico-cortical paired associative stimulation (ccPAS) targeting either the supplementary motor area (SMA)–primary motor cortex (M1) or the ventral premotor cortex (PMv)–M1 pathways produces comparable physiological and behavioural effects (Chapter 2). This validated protocol was subsequently used to examine the potentially dissociable roles of these two pathways within the AON. Strengthening the PMv–M1 connection enhanced automatic imitation, whereas strengthening the SMA–M1 connection suppressed it, indicating complementary facilitatory and inhibitory roles for the ventral and dorsal premotor circuits, respectively (Chapter 3). A drift-diffusion modelling approach was then applied to characterise how ccPAS influences the cognitive mechanisms underlying automatic imitation. The analyses revealed that enhancing PMv–M1 connectivity increased the bias toward imitative responses, while enhancing SMA–M1 connectivity improved the integration of contextual information, thereby promoting task-appropriate behaviour, as reflected in the drift rate parameter (Chapter 4). Collectively, these findings provide causal evidence for the dissociable contributions of ventral and dorsal premotor–motor pathways to automatic imitation, demonstrating the plasticity and functional specificity within the AON. In Chapter 5, two online repetitive TMS (rTMS) experiments were conducted to examine the roles of the PMv, SMA, inferior parietal lobule (IPL), and superior parietal lobule (SPL) during imitation inhibition and voluntary imitation tasks. The results indicate a causal role of the PMv in automatic imitation, supporting its central involvement in the automatic transformation of observed actions into motor representations, independent of intention, and functionally dissociating it from other fronto-parietal areas. Finally, a project was designed and planned to investigate the potential of single-pulse M1 stimulation to enhance visuomotor learning during counter-imitative training, as well as to determine the optimal stimulation timing to elicit such an effect (Chapter 6). Overall, this thesis provides novel causal evidence on the functional organisation and plasticity of the human AON, contributing to a deeper understanding of how motor and cognitive systems interact to shape social behaviour.