  Object-mediated joint action is believed to be enabled by implicit
information exchange between interacting individuals using subtle haptic
signals within their interaction forces. The characteristics of these haptic
signals have, however, remained unclear. Here we analyzed the interaction
forces during an empirical dyadic interaction task using Granger-Geweke
causality analysis, which allowed us to quantify the causal influence of each
individual's forces on their partner's. We observed that the inter-partner
influence was not the same at every frequency. Specifically, in the frequency
band of [2.15-7] Hz, we observed inter-partner differences of causal influence
that were invariant of the movement frequencies in the task and present only
when information exchange was indispensable for task performance. Moreover, the
interpartner difference in this frequency band was observed to be correlated
with the task performance by the dyad. Our results suggest that forces in the
[2.15-7] Hz band constitute task related information exchange between
individuals during physical interactions.
