Characteristics of Unintentional Movements by a Multijoint Effector

The authors explored the phenomenon of unintentional changes in the equilibrium state of a multijoint effector produced by transient changes in the external force. The subjects performed a position-holding task against a constant force produced by a robot and were instructed not to intervene voluntarily with movements produced by changes in the robot force. The robot produced a smooth force increase leading to a hand movement, followed by a dwell time. Then, the force dropped to its initial value leading to hand movement toward the initial position, but the hand stopped short of the initial position. The undershoot magnitude increased linearly with the peak hand displacement and exponentially with dwell time (time constant of about 1 s). For long dwell times, the hand stopped at about half the total distance to the initial position. The authors interpret the results as consequences of a drift of the referent hand coordinate. Our results provide support for back-coupling between the referent and actual body configurations during multijoint actions and produce the first quantitative analysis of this phenomenon. This mechanism can also explain the phenomena of slacking and force drop after turning visual feedback off during accurate force production task.     

Conflicting perspectives mediate the relation between parents’ and preschoolers’ self-referent mental state talk during collaboration

We examined the relations between the referent of parents and preschoolers’ mental state talk during a collaborative puzzle-solving task (N = 146 dyads; n = 81 3-year-olds, n = 65 4-year-olds). The results showed that parents’ references to their own knowledge and beliefs (self-referent cognitive talk), and references to their child’s knowledge and beliefs (child-referent cognitive talk) were both related to children’s (primarily self-referent) cognitive talk. We then tested whether any of the observed relations could be explained by the presence of conflicting perspectives within the collaborative interaction. Mediational analyses revealed that conflicting perspectives mediated the positive relation between parents’ production of self-referent cognitive talk and child cognitive talk. By contrast, the positive relation between parents’ production of child-referent cognitive talk and child cognitive talk did not depend on the presence of this type of conflict. These findings highlight an important mechanism through which parents’ references to their own mind might promote children’s developing mental state talk in collaborative contexts.