When people perform joint actions together, task knowledge is sometimes distributed asymmetrically such that one person has information that another person lacks. In such situations, interpersonal action coordination can be achieved if the knowledgeable person modulates basic parameters of her goal-directed actions in a way that provides relevant infor- mation to the less knowledgeable partner. We investigated whether systematic violations of predicted movement duration provide a sufficient basis for such communication. Results of a joint movement task show that knowledgeable partners spontaneously and systematically violated the pre- dictions of Fitts’ law in order to communicate if their partners could not see their movements. Unknowing partners had a benefit from these violations and more so if the violations provided a good signal-to-noise ratio. Together, our findings suggest that generating and perceiving systematic deviations from the predicted duration of a goal-directed action can enable non-conventionalized forms of communication during joint action.

Eye contact serves as an important social signal and humans show a special sensitivity for detecting eyes. Here, we asked whether people’s sensitivity to eyes would enable them to overcome temporal limitations in visual attention. We used an "attentional blink" (AB) paradigm, in which the second of two visual stimuli presented in quick succession typically cannot be detected. Participants performed a rapid serial visual presentation (RSVP) task and were asked to identify, within a stream of symbols, a target and to detect whether the target was succeeded by a probe. The probe was either an image of an eye (with direct gaze) or of a star. As expected, participants' detection rate for the star was poor, demonstrating the typical attentional blink. Crucially, detection rate for the eye was significantly better. This reduced attentional blink suggests that people's sensitivity to eyes is strong enough to circumvent fundamental limitations in visuotemporal attention.

Can people interpret communicative action modulations in terms of the actor’s distal goal? We investigated situations in which the proximal goal of an action (i.e., the movement endpoint) does not overlap with its distal goal (i.e., a final location beyond the movement endpoint). Participants were presented with animations of an object being moved at different velocities towards a designated endpoint. The distal goal, however, was for the object to be moved past this endpoint, to one of two occluded final locations. Participants were asked to select the location which they considered the likely distal goal of the action. As predicted, participants detected differences in movement velocity and, based on these differences, systematically mapped the movements to the two distal goal locations. These findings extend previous research on sensorimotor communication by demonstrating that communicative action modulations are not restricted to proximal goals but can also contain information about distal goals.

When two individuals perform a task together, they combinetheir individual skills to achieve a joint goal. Previousresearch has shown that interindividual skill differencespredict a group’s collective benefit in joint perceptualdecision-making. In the present study, we tested whether thisrelationship also holds for other task domains, using adynamic object control task in which two participants eachcontrolled either the vertical or horizontal movement directionof an object. Our findings demonstrate that the difference inindividuals’ skill levels was highly predictive of the dyad’scollective benefit. Differences in individuals’ subjectiveratings of task difficulty reflected skill differences and thusalso turned out to be a predictor of collective benefit.Generally, collective benefit was modulated by spatial taskdemands. Overall, the present study shows that previousfindings in joint decision-making can be extended to dynamicmotor tasks such as joint object control.

When acting in a social context, people have an automatic tendency to represent another person’s task – to theextent that another’s task constraints may influence one’s own movement performance. Task co-representation will also affectco-actors’ performance in joint action coordination; however, how exactly movement parameters are influenced is unclear. Weinvestigated this question in four experiments. Pairs of participants performed arm movements back and forth between twotargets, instructed to synchronize their landing times while external metronome tones provided timing cues. We predicted thatactors would represent their co-actors’ task constraints such that when the co-actor moved over an obstacle the actor withoutobstacle would move higher as well. Results confirmed this prediction, suggesting that joint action partners co-representedeach other’s task constraints. Moreover, this obstacle effect increased significantly when timing cues were removed, indicatinga stronger need for co-representing the partner when demands on interpersonal coordination are amplified.