EXPLORING THE MOTOR ROOTS OF JOINT ACTION: INSIGHT FROM A NEUROPHYSIOLOGICAL INVESTIGATION

Background: Joint actions (JAs) require precise temporal and spatial coordination between co-agents and draw on both low-level reflexive mechanisms and higher-level cognitive processes, such as action co-representation and shared goal representation. The experiments in this PhD thesis focus on these processes, particularly examining how collective goals are represented in joint action and how response inhibition is modulated by co-representation. Aims: Study I: To determine whether collective goals are represented at the motor level, using transcranial magnetic stimulation with electroencephalography co-registration (TMS-EEG). We anticipate a significant modulation of corticospinal excitability during JA, indicative of motorical representation of collective goal, and no modulation in the solo condition. Study II: To investigate the role of action co-representation in JAs, specifically, in action cancellation and re-engagement. For this purpose, a variant of the Stop-Change Task (SCT), is employed during EEG, capturing reactive motor control essential for joint coordination. We anticipate longer stopping latencies, slower action re-engagement, and altered EEG patterns during joint action stopping. Methods: Study I: A turn-based joint action reaction-time task was administered, co-registered with TMS-EEG. Motor-evoked potentials (MEPs) and TMS-evoked potentials (TEPs) were recorded from solo and joint conditions to assess corticospinal and cortical excitability. Study II: Positive Control Experiment: A bimanual selective SCT was used as a proxy for action co-representation in JA and compared to an unimanual SCT, to ensure that its key features accurately capture inhibitory control processes. Study II: Joint Experiment: The validated unimanual gamified SCT was administered to naïve participants in joint and solo conditions with belief manipulation. Behavioral (partial response EMG, prEMG, and the stop-signal reaction time, SSRT) and neurophysiological (stop-signal frontocentral P3 event related potential) markers quantify the Pause process and the Cancel process. Results: Study I: No significant modulation of motor-level representation for the co-actor’s action emerged in the Joint condition. Specifically, the primary measure, MEP difference in extensor carpi ulnaris (ECU) versus flexor digitorum superficialis (FDS), showed no notable effect. Cluster-based analyses of TEPs were inconclusive. Study II: Positive Control Experiment: The greater motor-planning demands inherent in a bimanual selective SCT affected motor preparation, stopping, and re-engagement in a transversal manner. Study II: Joint Experiment: The SCT revealed longer SSRT and an later onset of the stop-P3 during the joint condition, confirming our initial hypothesis. Contrary to our expectations, prEMGs did not differ between conditions. Furthermore, reaction times to respond to the change signal (Change RTs; our measure of action re-engagement), also did not differ. Conclusion: Study I: Even after accounting for the methodological constraints outlined by previous research, we found no evidence to support our initial conjecture, as all results point away from a motoric representation of shared goals, at least with our experimental setting. Despite these null findings, our study provides valuable insights and methodological guidance for future investigations of collective goals. Study II: With our joint SCT, we advanced the understanding of action cancellation in shared-action contexts. By highlighting the contrast between collective and individual goals using belief induction and by incorporating prEMG and EEG measures, consistent with prior research, our behavioral findings suggest that co-representation during JA significantly influences reactive inhibition. However, this influence did not manifest as slower Change RTs, indicating that action reprogramming remains relatively efficient even under joint conditions.

Le azioni congiunte (Joint Actions, JA) richiedono una precisa coordinazione temporale e spaziale tra agenti. Si basano sia su meccanismi riflessi di basso livello, sia su processi cognitivi di ordine superiore, come la co-rappresentazione dell’azione e la rappresentazione di obiettivi condivisi. Obiettivi: Studio I: Determinare se gli obiettivi collettivi siano rappresentati a livello motorio, utilizzando stimolazione magnetica transcranica con co-registrazione elettroencefalografica (TMS-EEG). Studio II: Indagare il ruolo della co-rappresentazione nella cancellazione e riattivazione motoria delle JA. Metodi: Studio I: Per valutare l’eccitabilità corticospinale e corticale, abbiamo somministrato un compito di reazione a turni con co-registrazione TMS-EEG. Sono stati registrati potenziali evocati motori e da TMS (MEP, TEP) in condizioni individuali e congiunte. Studio II: I partecipanti hanno completato uno Stop-Change Task (SCT) in condizioni congiunte ed individuali, registrando marcatori comportamentali (EMG di risposta parziale, prEMG; tempo di reazione al segnale di stop, SSRT; tempi di reazione al segnale di cambio, Change-RT) e neurofisiologici (potenziale evento-correlato P3 da segnale di stop, stop-P3). Risultati: Studio I: A livello motorio non è emersa alcuna modulazione significativa della rappresentazione motoria degli obiettivi congiunti. Le analisi dei TEP sono risultate inconclusive. Studio II: È stato rilevato uno SSRT più lungo e un’insorgenza più tardiva della stop-P3 durante le JA. Contrariamente alle aspettative, né i prEMG né i Change-RT (misura della riattivazione motoria) differivano tra le condizioni. Conclusioni: Studio I: Non sono emerse evidenze a supporto della nostra ipotesi iniziale: nel nostro contesto sperimentale i risultati non suggeriscono una rappresentazione a livello motorio degli obiettivi condivisi. Studio II: I dati comportamentali mostrano che la co-rappresentazione durante la JA influenza in modo significativo l’inibizione reattiva. Tuttavia, tale effetto non si traduce in tempi di Change-RT più lunghi, a indicare che la riprogrammazione dell’azione resta relativamente efficiente anche in contesti di azione congiunta.