Abstract
Ideomotor accounts of joint action hold that joint actions are enabled by a common coding system in which co-actors use the predicted effects of another's actions to generate codes of the other's response. We tested this hypothesis by adapting a task developed by Hommel (1993). He found that the spatial compatibility effect could be related to the location of an after-effect, as opposed to the response location, when people were told to "generate an effect". A common coding mechanism was suggested to underlie this after-effect based compatibility. Thus, if common codes enable joint action, an after-effect based Simon effect should emerge in a joint task. Here, participants first developed the response-effect coding in a learning block where they turned on a left or right light following a high of low tone. Critically, responses and after-effects were in opposite sides of space – right button was pressed for low tones and turned on the left light, left button was pressed for high tones and turned on the right light. Participants then completed individual and joint Simon tasks in which the tones were presented from a left or right speaker. Consistent with predictions, RTs were shorter when tones were presented from the speaker beside the to-be-illuminated light than when they were ipsilateral to the response. This reversed Simon effect was present in both joint and individual tasks suggesting that common coding systems may be used in joint actions.
