A co-actor’s intentionality has been suggested to be a key modulating factor for joint action effects like the joint Simon effect (JSE). However, in previous studies intentionality has often been confounded with agency defined as perceiving the initiator of an action as being the causal source of the action. The aim of the present study was to disentangle the role of agency and intentionality as modulating factors of the JSE. In Experiment 1, participants performed a joint go/nogo Simon task next (...) to a co-actor who either intentionally controlled a response button with own finger movements (agency+/intentionality+) or who passively placed the hand on a response button that moved up and down on its own as triggered by computer signals (agency-/intentionality-). In Experiment 2, we included a condition in which participants believed that the co-actor intentionally controlled the response button with a Brain-Computer Interface while placing the response finger clearly besides the response button, so that the causal relationship between agent and action effect was perceptually disrupted (agency-/intentionality+). As a control condition, the response button was computer controlled while the co-actor placed the response finger besides the response button (agency-/intentionality-). We observed a JSE when the co-actor responded intentionally and the causal relationship between co-actor and action effect could be perceived, but not when the co-actor did not respond intentionally and the causal relationship was disrupted (Experiment 1). When the intentionality of the co-actor was maintained but the perception of the causal relationship between co-actor and action effect destroyed, the JSE was absent (Experiment 2). Our findings clearly indicate a vital role of a co-actor’s agency for the JSE and suggest that the ascription of agency is strongly perceptually grounded. (shrink)

Numerous studies indicate that observing or knowing about another's action automatically activates the same motor representations that are active when we perform the other's action by ourselves. We investigated how affect influences this mirror mechanism. Based upon findings that positive affect encourages and negative affect impairs spreading activation, we hypothesised that positive affect should increase and negative affect decrease the automatic co-representation of other individuals' actions during jointly performed tasks. Recent research has shown that joint-action effects in a go/no-go variant (...) of the Simon task provide a good index for co-representing another's action. Participants performed such a Simon task together with another person after the induction of neutral, positive, or negative affect. Consistent with our predictions, the joint Simon-like effect was strongest after positive affect induction and absent after negative affect induction. These results represent the first evidence of affect-induced differences in the mirroring of another's action and suggest that affect influences one of our essential abilities for successful social interactions. (shrink)