Social influences on action and memory are well established. However, it is unknown how acting together affects the incidental encoding of information. The present study asked whether coactors encode information that is relevant to a partner's task, but irrelevant to their own task. In Experiment 1, participants performed a categorization task alone and together, followed by a surprise free recall test where they were asked to recall items from the categorization task. Recall was better not only for items that participants had responded to themselves, but also for items that their coactor had responded to, than for items that had not required a response. The same results were found in Experiment 2, even though financial incentives motivated participants to only encode words they had responded to themselves. Together, the findings suggest that performing tasks together can modulate how information relevant to coactors is processed. Shared task representations may act as a vehicle for establishing shared memories.

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Studies on the neural bases of action perception have largely focused on the perception of individual actions. Little is known about perception of joint actions where two or more individuals coordinate their actions based on a shared intention. In this fMRI study we asked whether observing situations where two individuals act on a shared intention elicits a different neural response than observing situations where individuals act on their independent parallel intentions. We compared the neural response to perceptually identical yet intentionally ambiguous actions observed in varying contexts. A dialog between two individuals conveyed either a shared intention or two independent parallel intentions. The dialogs were followed by an identical video clip where the two individuals performed certain actions. In one task condition participants tracked the intentions of the actors, in the other, they monitored moving colored dots placed on the same videos. We found that in the intention task versus the color task, observing joint actions based on shared intentions activated the temporal poles, precuneus, and the ventral striatum compared to observing interactions based on parallel intentions. Precuneus and the temporal poles are thought to support mental state reasoning, the latter with a more specific role in retrieving memories associated with social scripts. Activation in the ventral striatum, an area involved in reward processing, likely indicates a hedonistic response to observed shared intentional relations similarly to those experienced when personally sharing mental states with others.

Research on embodied cognition stresses that bodily and motor processes constrain how we perceive others. Regarding action perception the most prominent hypothesis is that observed actions are matched to the observer's own motor representations. Previous findings demonstrate that the motor laws that constrain one's performance also constrain one's perception of others' actions. The present neuropsychological case study asked whether neurological impairments affect a person's performance and action perception in the same way. The results showed that patient DS, who suffers from a frontal brain lesion, not only ignored target size when performing movements but also when asked to judge whether others can perform the same movements. In other words DS showed the same violation of Fitts's law when performing and observing actions. These results further support the assumption of close perception action links and the assumption that these links recruit predictive mechanisms residing in the motor system.

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