An emerging branch of social cognitive neuroscience attempts to unravel the critical cognitive mechanisms that enable humans to engage in joint action. In the current experiment, differences in brain activity in participants engaging in solitary action and joint action were identified using whole brain fMRI while participants performed a virtual barbalancing task either alone (S), or with the help of a partner in each of two separate joint-action conditions (isomorphic [J i ] and non-isomorphic [J n ]). Compared to the performing the task alone, BOLD signal was found to be stronger in both joint-action conditions at specific sites in the human mirror system (MNS). This activation pattern may reflect the demand on participants to simulate the actions of others, integrate their own actions with those of their partners, and compute appropriate responses. Increasing inter-dependence (complementarity) of movements being generated by cooperating individuals (J n N J i N S) was found to correlate with BOLD signal in the right anterior node of the MNS (pars opercularis), and the area around the right temporoparietal junction (TPJ). These data are relevant to current debates concerning the role of right IFG in complementary action, as well as evolving theories of joint action.

Here we report a study of joint-action coordination in transferring objects. Fourteen dyads were asked to repeatedly reposition a cylinder in a shared workspace without using dialogue. Variations in task constraints concerned the size of the two target regions in which the cylinder had to be (re)positioned and the size and weight of the transferred cylinder. Movements of the wrist, index Wnger and thumb of both actors were recorded by means of a 3D motiontracking system. Data analyses focused on the interpersonal transfer of lifting-height and movement-speed variations. Whereas the analyses of variance did not reveal any interpersonal transfer eVects targeted data comparisons demonstrated that the actor who fetched the cylinder from where the other actor had put it was systematically less surprised by cylinder-weight changes than the actor who was Wrst confronted with such changes. In addition, a moderate, accuracy-constraint independent adaptation to each other's movement speed was found. The current Wndings suggest that motor resonance plays only a moderate role in collaborative motor control and conWrm the independency between sensorimotor and cognitive processing of action-related information.

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In this study we investigated redundancy control in joint action. Ten participantpairs (dyads) performed a virtuallifting task in which isometric forces needed to be generated witb two or fom hands. The participants were not allowed to communicate but received continuous visual feedback of their performance. When the task had to be performed with fom hands, participants were confronted with a redundant situation and between-hand force synergies could, in principle, be formed. Performance timing, success rates, cross-correlations, and relative phase analyses oftbe force-time functions were scrutinized to analyze such task-dependent synergies. The results show tbat even though the dyads performed the task slower and less synchronized in tbe joint than in the solo conditions, the success rates in these conditions were identical. Moreover, correlation and relative phase analyses demonstrated that, as expected, tbe dyads formed between-participant synergies that were indicative of force sharing in redundant task conditions.

In this study, we investigate how two persons (dyads) coordinate their movements when performing cyclical motion patterns on a rocking board. In keeping with the Leading Joint Hypothesis (Dounskaia, 2005), the movement dynamics of the collaborating participants were expected to display features of a prime mover with low movement variability. Fourteen subject pairs performed the task in nine amplitude-frequency combinations that were presented in the form of a to-be-tracked stimulus on a computer display. Participants were asked to track the stimulus by jointly rocking the Board sideways while receiving continuous visual feedback of its rotations. Displacements of 28 IREDS that were attached to the rocking board, both ankles, knees, hips, shoulders and heads of both actors, were sampled at 75 Hz by means of a 3D-motion tracking system. From these data, we derived body-segment angular excursions as well as the continuous relative phase and time-lagged cross-correlations between relevant joint excursions. The results show that, at the intrapersonal level, knee rotations initially led all other joints in time while the antiphase coordination between the knees displayed relative low variability. At the interpersonal level, dyads adopted a leader-follower strategy with respect to the coordination demands of the task. We take that knee rotations create a dynamic foundation at both intra- and interpersonal levels involving subordination of individual action to joint performance thereby allowing for low-dimensional control of joint action in a high-dimensional, repetitive motor task.