Behavioural investigations of the interpersonal action

Many everyday motor tasks require interpersonal action where the goal of the joint task is shared among all those involved. The aim of this thesis was two- fold; first, to explore bottom-up and top-down factors affecting interpersonal movement and second, to examine discrepancies in motor strategy arising between intrapersonal and interpersonal task conditions. In Chapter 2, participants coordinated their wrist movements to a live model whose own actions increased in frequency over time. Motoric and spatial contributions to the stability of interpersonal coordination were teased apart; testing whether spatial congruency overrides anatomical congruency. However, both factors influenced the task. Furthermore, the representation strength of these factors was not influenced by perspective (allocentric, egocentric). Chapter 3 investigated synchronization to self-versus-other when agency was either unknown or when participants were told (correctly or falsely) who they were synchronizing to. Participants synchronized arm movements to point- light stimuli derived from either their own or another person's previous motion recordings. Performance during self-belief trials was better relative to other belief trials, indicating a significant top-down modulation of behaviour. Chapter 4 probed interpersonal adherence to Fitts' Law, specifically looking at how task workload was shared across players when efficiency was emphasized. Players placed targets down at location distances of their discretion in order for their task partner to hit them. An equity-efficiency trade-off which violated Fitts' Law was observed. Chapter 5 studied how motor strategy across intrapersonal and interpersonal conditions differed in a reach-to-target task where two vBOTs jointly controlled the movement of a single cursor. Force perturbations ensured that both participants/hands experienced on shared 'difficult' direction and one shared 'easy' direction; the other two directions were difficult for one and easy for the other. Interpersonal behaviour was characterized by an iii equity-efficiency trade-off absent in the intrapersonal task. Overall, this thesis provides behavioural evidence of how interpersonal motor behaviour is modulated by bottom-up, topdown, social-goal and movement-control variables